Open-Pollinated vs. Hybrid Maize Cultivars

Abstract

The history of maize breeding methods in the USA is reviewed to examine the question of types of maize cultivars in sustainable agriculture. The yield potential of OP cultivars was much higher than national average yields prior to 1930, but hybrid cultivars today often out-yield OP cultivars by 50–100% or more. However, rates of gain for yield using recurrent selection on populations appear equal to that recorded for commercial hybrid breeding. The inbred-hybrid method, while successful, was not “the only sound basis” for maize improvement, as evidenced by later experiences in the United States and worldwide. It appears that maize breeders have practiced objective science and achieved concrete goals, although personal interests and goals clearly direct the work at times. As society looks for tools for sustainability based on achieving multiple goals, a special dedication to scientific validation and broad objectivity may be required. The potential for OP cultivars today is evaluated and research questions are identified.

Full text

Sustainability 2011, 3, 1531-1554; doi:10.3390/su3091531
sustainability
ISSN 2071-1050
www.mdpi.com/journal/sustainability
Review
Open-Pollinated vs. Hybrid Maize Cultivars
Frank Kutka
Northern Plains Sustainable Agriculture Society Farm Breeding Club, P.O. Box 194, 100 1st Ave. SW
LaMoure, ND 58458, USA; E-Mail: dakotacornman@yahoo.com; Tel.: +1-701-883-4304
Received: 30 May 2011; in revised form: 25 August 2011 / Accepted: 30 August 2011 /
Published: 22 September 2011
Abstract: The history of maize breeding methods in the USA is reviewed to examine the
question of types of maize cultivars in sustainable agriculture. The yield potential of OP
cultivars was much higher than national average yields prior to 1930, but hybrid cultivars
today often out-yield OP cultivars by 50–100% or more. However, rates of gain for yield
using recurrent selection on populations appear equal to that recorded for commercial hybrid
breeding. The inbred-hybrid method, while successful, was not “the only sound basis” for
maize improvement, as evidenced by later experiences in the United States and worldwide.
It appears that maize breeders have practiced objective science and achieved concrete goals,
although personal interests and goals clearly direct the work at times. As society looks for
tools for sustainability based on achieving multiple goals, a special dedication to scientific
validation and broad objectivity may be required. The potential for OP cultivars today is
evaluated and research questions are identified.
Keywords: breeding; composite; hybrid; inbred; maize; OP; open pollinated; synthetic
1. Introduction
Improved cultivars are a key element among practices used for integrated pest management and
other approaches to agricultural sustainability [1-3]. The focus of plant breeders on a broad conception
of sustainability has been repeatedly demonstrated. Hayes et al. said, “the primary purpose of plant
breeding is to obtain or develop varieties or hybrids that are efficient in their use of plant nutrients, that
give the greatest return of high-quality products per acre or unit area in relation to cost and ease of
production, and that are adapted to the needs of the grower and consumer. It is of great importance also
to obtain cultivars that are able to withstand extreme conditions of cold or drought or that have
OPEN ACCESS

Sustainability 2011, 3
1532
resistance to pathogenic organisms or insect pests. Such qualities help materially to stabilize yields by
controlling extreme fluctuations” [4]. This reflects the concerns of farmers and sustainability
advocates [5,6]. Plant breeding is also said to be a science, art, and business [3,7,8]. It has had great
successes with increasing yields of many species for many systems [4,9,10].
One breeding question that comes up in sustainable agriculture circles in the United States is,
“Could someone have bred a high-yielding open-pollinated corn cultivar?” Open-pollinated (OP)
maize cultivars are largely farm bred, providing yields of grain that can be saved for seed. After more
than eight decades since the commercial introduction of hybrid maize cultivars in the United States,
there is still some doubt among some farmers and scientists as to whether OP cultivars had to be given
up as inferior to hybrids. The objectivity and goals of hybrid maize breeders as “pure scientists” is
either promoted or openly doubted [11,12].
Figure 1. USDA and Agricultural Experiment Stations once actively trained farmers on
the latest breeding techniques [13]. Unfortunately, the methods promoted were not always
very successful for increasing yield.
Maize breeding and seed production by farmers was once the norm (Figure 1). Trade in maize seed
and intentional outcrossing with introduced types are ancient practices [14-16]. The result was many
OP cultivars [17-21]. However, on-farm maize breeding and efforts to improve on-farm seed
production were dropped after the 1930s in most of the United States once successful hybrids were
released [10,22-24]. An increase in yield (Figure 2) paid for these annual seed purchases and drove this
change largely on economic grounds [25], although farmers were also much impressed with improved
standability and the uniform look of the fields. Sometimes the yield increase observed in the USA in
the 1940s seems to be attributed almost entirely to the hybrid cultivars themselves [11,26].

Sustainability 2011, 3
1533
Figure 2. Average USA maize yields 1866–2010. Data are available from USDA [27].
Those interested in sustainability do not doubt the potential and utility of hybrid maize cultivars.
Modern hybrids often yield much more than current OP cultivars [9,28,29] and have often done so
since the late 1930s [22,24,30,31]. This has also been true for forage yield [32-34]. Questions remain
about the utility of other breeding methods that might allow farmers to produce their own seed, to
more effectively breed for their own systems of management, and whether these might be more
advantageous for them [35-39]. Also, the benefits of the yield boost from growing hybrid cultivars
with high input levels are still debated given the economic and environmental problems with growing
continuous maize [23,40-43]. Arguments for helping maize farmers by focusing on yield rather than
profit have continued for 100 years [44], regardless of the fact that that this focus has sometimes failed
to help farmers meet their economic, environmental or lifestyle needs [40,42,45].
Cleveland suggested that plant breeders were neither scientists in search of objective truths, nor
servants within a social construct for existing political and economic interests, but some amalgam
thereof [12]. This is a critical issue to consider given the changes to public and private plant breeding,
the centralization of seed and gene control in agriculture, and the advent of more intrusive forms of
biotechnology [23,37,46]. Were the conclusions of maize breeders presented over the last century well
defended by the data or not, as has at times been alleged?
The purpose of this paper is to revisit the history and the current state of OP cultivars and the
methods for maize improvement in order to address these questions: Were OP cultivars the cause of
low yields in the USA before 1930? Was the inbred-hybrid method the only path to follow for maize
improvement? What sort of breeding took place before and after hybrids were released? Is the story we
teach agriculture students the real and complete story? Do OP cultivars have a place today?

Sustainability 2011, 3
1534
1.1. Approach
I reviewed the maize breeding literature with a focus on the United States, delineating when key
changes occurred and by whom. The yield potential of OP cultivars in the United States from the
mid-1800s to the early 21st century was then addressed. Reports of multiple cultivar and
multi-environment trials, particularly where improved management was more likely to have been
practiced, were used to generate yield means under a variety of conditions. Averages for better-adapted
cultivars over many environments were sought to best represent the potential of this type of cultivar.
Rates of gain for methods of maize improvement were generated using published
summaries [10,47]. Data from the experiments of Weyrich et al. with the BS11 maize population were
also incorporated [48]. Very high rates of gain (>10%/cycle) observed with some tropical and other
populations were ignored for the sake of generating conservative estimates more relevant to potential
outcomes for high yielding populations in the higher latitudes.
Conservative estimates of rates of gain from selection, 0.6% annually for gridded mass selection
and 1.3% annually for modified ear-to-row selection (based on an average of 83 kg/ha applied to a
hypothetical population yielding 6270 kg/ha), were used to predict the yield of OP cultivars had they
been bred for 80 years (since 1930), 60 years (since 1950) or 40 years (since 1970) because these two
methods do not require controlled pollinations, directly provide seed for planting, and are possibly the
most adaptable to on-farm breeding [35,48]. Estimated yields of OP cultivars under modern
management, from the review of OP cultivar yields, were multiplied by these generated rates of gain.
The predicted yield gain quantity from that first cycle was then added to the base yield for successive
years to predict accumulated gains over time: steady gains can continue for many years [29]. I also
sought out literature concerning synthetic and composite populations, including tropical maize
breeding reports. Published results were used to generate predictions of the potential of this technique
to form better OP cultivars.
2. Results and Discussion
2.1. The Age of On-Farm Selection in Maize—Origins to 1935
Indigenous farmers in the Americas developed maize and methods for seed production; farmers
around the world continued this same process of mass selection wherein seeds from good ears or plants
were saved each year [14-17,20,21,49]. Selection produced widely used cultivars, such as Improved
King Philip Flint, Leaming, and Silver King [18,50,51]. Composite breeding, a technique developed
by Native and Immigrant farmers, formed the entire race of Corn Belt Dent and cultivars like Reid’s
Yellow Dent, Krug’s Yellow Dent and Falconer by crossing two racial types [16,26,48-50].
In the early 20th century, maize breeders and extension educators promoted a wide variety
of selection techniques that were considered to be an improvement over traditional
techniques [13,23,50,52-54]. Most focused on mass selection for ear and seed qualities thought to be
related to yield in “pure strains,” although some more intensive systems of selection were also being
promoted. Universities and associations of corn growers also established corn shows at fairs to exhibit
“perfect ears” that were well matured and matched for uniformity. Rist wrote that, “If we are to
estimate the value to be derived from corn shows in increased yields alone it probably would not

Sustainability 2011, 3
1535
amount to much, as we know that ’show ears‘ do not as a rule increase the yield of corn. On the other
hand farmers of this state have paid thousands of dollars in tax money for scientific investigation along
the line of corn improvement, yet such investigations have resulted in practically no increase in the
yield of corn. Therefore, farmers could object to further investigations fully as well as could those who
are opposed to corn shows object, as their results have been about the same so far as increased yields
are concerned” [55]. In 1921, W.L. Burlison at the University of Illinois wrote that, “from the
experiments conducted at this station it appears that while selection has had some effect in increasing
the yield of corn, it has not given the results that were anticipated” [23].
Early breeding experiments with maize populations did not often yield impressive results for many
breeders at many locations, and breeding based on ear type, though wildly popular and heavily
promoted by universities, was even associated with yield reductions over time [13,52,53,56-59]. Datta
visited maize breeders across the country and reviewed their techniques [52]. What he found was a
mindset of progress through science and the land grant mission, along with a general abhorrence of
inbreeding. However, though selection had improved the uniformity and local adaptation of many
populations [49,50,60], “scientific” selection techniques promoted before 1920 appear not to have been
fully evaluated before their widespread promotion [16].
During the 1920s, the selection program of the University of Illinois Extension Service was
apparently successful in increasing yields using mass selection. J.C. Hackleman promoted selection
against male plants that appeared barren, against plants with disease, and for high rates of germination.
He reported yield gains of 300–600 kg/ha (5–15%) with this method [23]. Given the yields of the day,
these were reasonable increases, especially since this was indirect selection for yield.
2.2. Yield Potential of OP Cultivars in the United States—1847 to 2005
Reported yields of OP cultivars in university and other scientific trials were usually more than
2000 kg/ha in most parts of the United States when averaged across years, locations, and often times
cultivar as well (Table 1), however, it is possible that very poor yields were under-reported [35]. It is
clear that the yield potential of many OP cultivars with good management was often over 3000 kg/ha
before modern management practices. In recent trials, yields of better-performing OP cultivars were
often over 4400 kg/ha, a yield substantially lower than most hybrid checks [9,29,33,61-63].
Table 1. Some average yields for open-pollinated maize cultivars in the United
States [9,21,24,29,31,33,56-58,61-98]. Average state yields of open-pollinated (OP)
cultivars during each test are included when data were available from USDA [27]. Average
state yields for the period 1866–1929 for which there are data are marked with an asterisk
and are presented to demonstrate changes since this “OP era” for tests carried out after 1930.
Cultivar(s)
Yield
kg/ha
State Avg. Yield
kg/ha
Area of Evaluation
Year(s) of
evaluation
14 cultivars
4890
New York, 1 location
1847
Kingsbury
3060
2350
Vermont, 2 locations
1873
4 flint, 1 dent
3400
2290
Massachusetts, 1 location
1875

Sustainability 2011, 3
1536
Table 1. Cont.
Cultivar(s)
Yield
kg/ha
State Avg. Yield
kg/ha
Area of Evaluation
Year(s) of
evaluation
Stowe Flint
3070
2340
Vermont, 1 location
1878–1879
1 dent
5020
2590
Iowa, cultivation trials
1889
1 dent
2458
1693
Minnesota, 1 location
1889–1891
4 dents
4950
2540
Iowa, single reps, 2 exp.
1891
Leaming
4900
1910
Iowa, 6 locations
1892
3 cultivars
3894
1944
Illinois, 1 rep., 1 location
1887–1894
Minnesota 13
4070
1913
Minnesota, 1 rep., 6 trt.
1895
Silver King
3620
2090
Wisconsin, 749 environ.
1904–1909
Wis. No. 8
2510
2020
N. Wisconsin, 1 location
1907–1910
6 dents
4280
2730
Ohio, several locations
1905–1914
1 dent
4110
1410
Virginia, rotation exp.
1909–1916
2 flint, 2 dent
2870
2410
New York, 4 locations
1910
Reid
4180
2410
Iowa, breeding nursery
1910–1911
Reid
3430
2680
Iowa, Walden farm
1912–1924
Boone County
3730
1480
Virginia, several locations
1913–1916
NE White Prize
3640
1720
Nebraska, 9–14 loc. per year
1914–1917
1 dent
3260
2510
Ohio, 20 yr exp.
pre–1915
Boone County
4550
1520
Virginia, Ag Exp Sta
1916–1917
1 dent
2820
980
Kansas, 8 yr trial
pre–1918
1 dent
4870
1660
Missouri, 17 yr trial
pre–1918
Leaming
4990
2260
Conn., one location
1916–1917
3 dents
3410
2450
New York, 14 environments
1918–1920
Onondaga
4170
2600
New York, 10 environments
1919–1920
2 dents
3190
2160
Minnesota, Ag Exp Sta
1919–1920
5 dents
3270
1810*
Nebraska, coop. trials
1932
3 dents
4110
Midwest, 7 reps, 1 location
1933
3 dents
1919
1354*
E. North Dakota, 1 location
1935–1942
Minn. 13
3130
1910*
Minnesota, 12 environments
1936–1940
Murdock
3470
1910*
Minnesota, 11 environments
1938–1940
3 dents
4170
2330*
Iowa, 15 environments
1939–1941
Clarage
4410
2270*
Ohio, 19 environments
1941–1946
Foster’s White
5560
2270*
Ohio, 8 environments
1942–1946
2 dents
3490
1910*
Minnesota, 10 locations
1942–1944
5 dents
2830
1520*
S. Dakota, coop. trials
1942–1944
2 dents
3190
1480*
Kansas, coop. trials
1943–1945

Sustainability 2011, 3
1537
Table 1. Cont.
Cultivar(s)
Yield
kg/ha
State Avg. Yield
kg/ha
Area of Evaluation
Year(s) of
evaluation
Black Hills Sp.
5160
1520*
South Dakota, 1 location
1946–1948
Rainbow Flint
2310
1520*
S. Dakota, 4 environments
1951–1954
Dawes #2
4630
1810*
Nebraska, 2 locations
1953
Cornell 11
3320
2080*
New York, several locations
1953–1954
9 dents
5710
1810*
Nebraska, two locations
1955–1956
Hays Golden
5210
1810*
Nebraska, 1 loc., 20 reps
1956–1959
Reid type
5470
2330*
Iowa, 11 environments
1971–1973
Reid, Lancaster
3940
USA, 11 environments
1980–1981
Reid
5710
2330*
Iowa, 12 environments
1991–1994
Reid
6110
2330*
Iowa, 6 environments
1998, 2000
5 dents
4330
2330*
Iowa, 5 locations
2001
Krug, Reid
5370
2080*
New York, 6 environments
2001–2002
Nokomis Gold
5140
28 Midwest and NE env.
2001–2003
Wapsie Valley
5360
20 Northeast environments
2001–2004
High OP yields were familiar to many before 1930. Yields over 6200 kg/ha were recorded during
the mid-1800s [20,21]. Montgomery reported that the four largest yields of maize on record in the
United States at that time were over 12542 kg/ha for ears at husking [99], while Bowman reported the
largest yield of grain belonged to South Carolina’s Z.J. Drake in 1889 (15990 kg/ha) [50]. Hartley
wrote that “good farmers frequently raise from 75 to 100 bushels of corn per acre [4700–6270 kg/ha]”
and yields this high were regularly observed in Iowa and Wisconsin [51,54]. Cornell University
cooperative trials showed average dry grain yields over 3140 kg/ha and at times over 4390 kg/ha,
similar to yields reported by notable New York farmers in the 1840s [21,57,73,100,101].
There were two major problems for maize yields prior to 1930. Many farmers were not
producing high quality seed (not well dried, freeze damage, not well selected, inbred) and were
therefore obtaining poor stands and yields lower than the genetic potential of their
cultivars [51,53,54,59,74,102,103]. The other problem was that soil and crop management
improvements were not widely adopted, leading to unsustainable production in some cases [34,36].
Kent wrote, “By intense cultivation and proper rotation, most of the farms of Iowa would produce
from 75–80 bushels of corn per acre [4700–5020 kg/ha], and under favorable climatic conditions still
more” [70]. Yields in Missouri of 4870 vs. 740 kg/ha and in Illinois of 4000 kg/ha vs. 1690 kg/ha were
reported for rotated vs. continuous cultivation [78,104]. Still, many farmers did not rotate or fertilize
enough because: (1) fertilizers were not always economical [76,77], (2) the value of continuous maize
was higher than the value of some alternative crops [105], or (3) for some other reasons. Smith wrote
that "the land was corn sick” [104].
One example of the importance of management for yields comes from Kansas where maize yields
had fallen steadily, leading to an unsustainable situation before 1920 [79]. At that time one of the
highest yielding OP cultivars was Pride of Saline, with an average yield over 1880 kg/ha [78]. In the
1940s, Pride of Saline was yielding 3140–3760 kg/ha and after 1960 it often yielded above 6270 kg/ha

Sustainability 2011, 3
1538
(Figure 3). This is more than three times the Pride of Saline yield from university trials before 1920
with no reported breeding effort.
Figure 3. Kansas statewide yields of Pride of Saline and hybrid cultivars from 1913–1972.
Data were adapted from reports of the Kansas State University Agricultural Experiment
Station [78,79,91,106-113]. The “Best Hybrid” was not the highest performing individual
cultivar at each location but the highest yielding cultivar when averaged across
test locations.
A complete consideration of the effects of plant breeding during the 20th century includes all of the
factors that changed [22,36,114]. No more than 60–70% of the yield gain since 1930 should be
attributed to breeding and using hybrid cultivars [29,115]. Hybrid yields in Kansas tests were on
average 8–22% higher than Pride of Saline prior to 1960, and this difference increased after 1960
(Figure 2). However, while hybrids often outperformed their OP checks in the 1930s and
1940s [22,30,31,85-88], Reid’s Yellow Dent has yielded about as well as many commercial
hybrids from the 1940s and early 1950s, but substantially less than post-1960 hybrids in recent tests in
Iowa [9,29]. Also, improvements in standability in hybrids have been more dramatic still, making
for greater gains for machine harvested yields and for more flexible grain harvest
schedules [9,28-30,97,98]. Could new, more competitive OP cultivars that stand well be bred now?
2.3. Adopting the Hybrid Method—1870 to 1935
Farmers had been making crosses among cultivars for thousands of years, and then reselecting new
and more vigorous offspring from among the following generations with some success [49,50].
In Michigan, Beal suggested the use of F1 varietal hybrids in the 1870s as did Carrier in
Virginia [4,26,116,117]. These new hybrid seeds did find some commercial use in the early 20th

Sustainability 2011, 3
1539
century, although like any hybrid seeds they required annual regeneration and further improvement in
these hybrids would require ongoing improvement of each parent population.
Around 1910, some breeders began to think in new ways, to see vigorous cultivars, both uniform
and genetically diverse, that had first passed through a period of inbreeding [11,17,44,52,80,114,118].
Early data with inbred hybrids showed great potential in the method, although it would be expensive
and removed from the hands of farmers [23,49,94]. Richey said evidence suggested “pure line methods
as the only sound basis for real improvement of corn” [119].
Richey reported that F1 varietal hybrids on average did not yield more than the parent cultivars and
also noted small or no gains on average from ear-to-row breeding [119]. However, he then expounded
on the best yields from Jones’ hybridization work rather than showing that 11 of the 25 Leaming
hybrids yielded less than the OP parent [80,119]. Concerning hybridization experiments in Nebraska,
Richey wrote, “It is unlikely that all of the crosses yielded equally, and some, therefore, probably
exceeded this 0.8 bushel [50 kg/ha] increase” [119]. Had Richey applied the same focus on the best
gains from the other breeding methods he would have reported that 2.5% of F1 varietal hybrids
outyielded the better parent by 26% or more and would have noted good yield gains in some
ear-to-row breeding experiments [119]. Therefore, Richey’s conclusion about pure line methods was
only partly correct considering the data presented. On average, hybrids did not yield better than their
OP parent cultivars, but some hybrids yielded much better, as is still true today.
At the time, the difficult task of finding the best hybrid combinations provided rapid yield gains in
the short run but did not make for long term improvements. However, the method proved wildly
successful after years of intensive development when released hybrids often outyielded OP cultivars by
9–40% [10,11,17,22,23,29,31,37,86-88,91,118,120]. From on-farm ear selection and no inbreeding
with “pure” populations, breeders appear to have leapt to no population improvement and little farmer
involvement. During neither period were decisions fully evaluated through experimentation.
After 1922, most university maize programs in the USA took up inbreeding studies [89]. Some
maize breeders moved out of breeding and into different roles [22]. Hartley, the head of USDA’s
maize improvement efforts, was pushed from the USDA Bureau of Plant Industry over his
conservatism in favor of on-farm seed production and breeding [23]. This parallels changes underway
in plant science departments world-wide as molecular and transgenic techniques and courses of study
have come into favor and funding for conventional public plant breeding research and education has
dwindled even though the approach still works well [7,116,121,122].
2.4. Recurrent Selection Revisited
Interest among hybrid maize breeders in recurrent or repeated selection rebounded in the 1940s and
1950s when they discovered the need for improved populations from which to select new
inbreds [16,47,123], and this time the conclusions were very different. OP cultivars had been
thought to lack the genetic variation for successful selection and this was disproven via
experimentation [16,124]. Gardner wrote, “mass selection would appear to be as effective
as hybridization in increasing yield” [96]. Webel and Lonnquist said of modified ear-to-row
selection, “The realized gain suggests the method might be as effective as hybridization in increasing
yield” [125]. Lonnquist and Gardner found “that no critical experiments where yield was the main

Sustainability 2011, 3
1540
criterion for selection were reported during the period covered by Richey’s report” [95]. Sprague noted
“it appears that the ear-to-row method of selection for yielding ability was discredited not because of
genetic limitations of the method, but because of the inadequate field plot technique” [16]. Research
after Richey’s 1922 conclusion showed recurrent selection and varietal hybrids could be very
successful at increasing maize yields after all, although they could not provide cultivars with the yields
of the best hybrids [35,47,81,95,119,120,126-131].
The Hays Golden OP cultivar from Nebraska showed a yield increase of 40% from several years of
recurrent selection for yield alone as compared to the 9–40% yield advantage that many hybrids had
over OP cultivars with many years of development [30,47,88,123]. Average gains in yield for gridded
mass selection range from 1.8–2.6% annually [10,35,47,120,132]. Modified-ear-to-row selection has
demonstrated average gains per year of about 2.1–3.5% [10,35,47,48,133]. These and other
recurrent breeding techniques have been very successful in breeding cultivars for farmers in the
tropics [120,128,131,132]. Conservative estimates of average potential annual yield gains of 82 kg/ha
for mass selection and 83 kg/ha for modified-ear-to-row selection, largely from North American data,
are comparable to those experienced in an open-ended inbred hybrid breeding program from the
United States, where yields have increased by about 66 kg/ha each year since 1930 [9,29,35]. Both
Coors and Duvick provide lists of reasons why these results for recurrent selection might be biased
comparisons and might at times favor the results for recurrent selection over the inbred-hybrid
method [35,36]. Added to that, recurrent programs rarely if ever achieve the level of selection and
evaluation intensity described for commercial hybrid breeding [17]. Regardless of type of cultivar,
annual improvements via breeding are essentially about the same. The fastest and cheapest method
which best supports sustainable agriculture outcomes will depend on yields in the target
agroecosystems, grain prices, and farmer skills and interests.
Gardner’s modification to mass selection techniques was to focus on yield among plants in small
grids or plots across a seed selection field [96]. This reduces the environmental effects on observed
plant phenotypes and improves selection for better genotypes, although it does not allow for evaluation
in multiple agroecosystems. The method is easily adapted to farm and garden situations, although it is
better for qualitative traits than for yield and may need the two step procedure suggested by Hartley for
larger acreages [54]. Modified ear-to-row selection involves replicated evaluation of plant progenies at
three locations and is a more intensive technique that would probably require special training for
farmers or even a participating breeder [134]. Prior to 1910, some breeders had adopted replication,
check rows, detasseling and the use of remnant seed in some ear-to-row breeding trials for better plant
evaluations [13,52,58,135]. Use of the triple lattice design for progeny tests (a component of
Lonnquist’s method) was unlikely before demonstrations of its utility in accounting for finer spatial
variation of soils in the early 1940s and the use of bulk seed rows for cross-pollination appears to have
originated with Lonnquist [52,134,136,137]. The method often results in rapid gains and the
availability of computers makes it and other complex recurrent selection methods a possibility for
many farmers, especially those working in groups or when partnering with breeders.

Sustainability 2011, 3
1541
Table 2. Predicted yields of maize OP cultivars under modern management after periods of
gridded mass selection and modified ear-to-row selection using conservative rates of
gain [35,48].
Original Yield
Potential, (kg/ha)
Mass Selection
Modified Ear-to-Row
Selection
80 yrs
60 yrs
40 yrs
80 yrs
60 yrs
40 yrs
6270
9280
8530
7780
12910
11250
9590
5020
7400
6840
6210
10241
8936
7630
What would have happened if these techniques were applied in the past had recurrent selection not
been dropped prematurely? Some OP cultivars in the United States have yields of 5020–6270 kg/ha
which are not currently economically advantageous [28]. Using conservative estimates of rates of gain,
calculations of the predicted gains in yield suggest large gains were and are possible with enough
breeding effort (Table 2). Gridded mass selection could have resulted in OP cultivars yielding 9280
kg/ha or more by now had selection begun by 1930. Lonnquist’s modified ear-to-row method could
have resulted in much faster improvements and in fairly competitive OP cultivars even if it had been
employed only since its development [134]. Applying recurrent selection now, it would take many
years to develop an economically competitive OP cultivar. Competitive commercial OP cultivars for
the USA and some other regions were not developed and breeders (and farmers) would have had to
overcome the stigma of being old-fashioned and the preference of governments and many farmers for
hybrids [23]. Had higher yielding OP cultivars been released prior to 1960, would there have been an
advantage? Would many farmers use high yielding OP cultivars today? Would there be an advantage
in time, profitability, adaptation, etc.?
There are other biological problems to consider. Single plant evaluation in mass selection can
increase yield under some circumstances but often results in delayed maturity and no improvement in
stalk strength and other critical agronomic traits. Hyrkas and Carena and Bletsos and Goulas were
unable to increase yield of improved populations using mass selection and suggested using more
intensive recurrent selection techniques that could be more difficult for farmers although predicted
rates of gain can be higher [35,48,133,138]. Gardner hit a yield plateau with Hays Golden after
12 cycles of mass selection; modified ear-to-row selection reached the same plateau in only
six cycles [47]. Yield plateaus can be overcome by outcrossing, and then proceeding anew with the
resulting composite population [131,139]. Eberhart et al. promoted the idea of cooperative work with
many populations [128]. Such an approach could make available the improved populations and lines
needed for outcrossing when breeding plateaus are discovered. At any time inbreds and inbred hybrids
could be developed, making for a very comprehensive approach to maize breeding much akin to the
successful public program at Iowa State University where theoretical studies of maize breeding
technique included selection for agronomic traits as well as yield in both populations and inbreds.
There need not be an either or choice with OP and hybrid cultivars.

Sustainability 2011, 3
1542
2.5. Composite and Synthetic Populations as OP Cultivars
Hybrid vigor has been harnessed by maize breeders for centuries [14,16,49]. Composite populations
(intermated cultivars) and synthetic populations (intermated inbreds), like any maize population with a
random mating structure, can be thought of and used as OP cultivars [140] and may offer a rapid
approach to increasing OP cultivar performance. Shull and Duvick suggested that the yield of an OP
cultivar is a result of all the possible hybrid combinations among the parents [36,44], and Wright
demonstrated that the number and yield of parents in a synthetic population determined how much of
the hybrid vigor could be retained in the F2 and later generations [141]. The equation based on his
work, which was validated for both composites and synthetics by Mochizuki, predicts that as the
number of parents increases the amount of retained hybrid vigor increases [142].
F2 Synthetic Yield = H –
(H – P)
n
(1)
H = avg yield of all F1 hybrids, P = avg yield of parents per se,
n = number of parents
While some saw the potential for synthetic maize cultivars in the USA early on [80,143], little work
was done and most results were not promising [16,144,145]. However, many breeders have had
synthetic yields at least 15% above that of common OP cultivars and up to 90% of hybrid
yields [83,93,97,118,146-149]. It appears that these high yielding synthetic populations never reached
farmers in the USA or many other regions for their consideration.
The application of Wright’s equation to historical data on maize inbreds and hybrids suggests the
possibility of synthetics that yield more than 90% of commercial hybrids if the research were carried
out (Table 3), although the work would be intensive with many questions to be answered [36,141,150].
Might such diverse populations open up new ways of dealing with diseases and pests as has worked for
multilines of rice [151]? Could synthetics reduce seed costs, provide competitive yields and be further
improved via selection on farms? Would many farmers be interested?
Table 3. Predicted F2 yields for hypothetical 8-line synthetic cultivars of maize based on
average yields of seven single cross hybrids and their inbred parents from different decades
[36].
1930s
1940s
1950s
1960s
1970s
1980s
Single Cross Mean
6717
7033
7960
8171
9098
10492
Midparent or Inbred Mean
2062
3065
3174
3493
4463
5476
Predicted Eight-line Synthetic
6135
6537
7362
7587
8519
9865
Synthetic vs. Single Cross (%)
91.3
92.9
92.5
92.9
93.6
94.0
Planting Density (1000/ha)
30
54
54
54
79
79
Some answers come from the tropics where maize breeding has embraced population improvement
and inbreeding and breeders develop inbred hybrids, varietal hybrids, synthetics and other OP cultivars
to best fit local needs [120,128,131,152-157]. In those regions synthetic cultivars have proven useful
for farmers by providing higher yields and low seed costs, although for long term improvement
synthetics may require more genetic diversity than that provided by only 8–10 lines and they may
require more intensive selection schemes for further improvement [133,155]. A few farmers in the

Sustainability 2011, 3
1543
United States, especially organic farmers, are looking for good cultivars of maize and other crops from
which they could produce their own seed as seed prices have risen sharply in recent years and the
number of commercial cultivars without transgenes has diminished [33,140,158,159]. Baltensberger et
al. suggested that dryland farming and similar regions with climatic stress may be the first
agroecosystems to consider for this approach as these lower yielding environments result in similar
economic returns for synthetics yielding only 75% of commercial hybrids [147]. Stress environments,
and those where hybrids are not economically feasible, are the targets for modern OP cultivar
development in the tropics [120,152-154]. Organic farmers may be interested as well, but high organic
grain prices could make lost yield opportunities economically detrimental unless cultivars without
recombinant DNA were otherwise unavailable [28].
Any type of hybrid is still an option for on-farm seed production. Should sustainable farmers find it
economically feasible to produce their own hybrid seed, varietal hybrids might still be a valuable
option among several (single cross, top cross, double cross, varietal cross). Improved populations,
whether synthetic or otherwise, could be crossed to produce F1 seed each year should those
populations demonstrate hybrid vigor when crossed. Advantages of this approach could be higher
yields than possible with populations per se, cheap hybrid seed given the levels of production that may
be possible with populations compared to expected yields of inbred parent lines, and the possibility for
ongoing improvement of the parent populations and their hybrids on-farm. Disadvantages could
include separate seed production requirements, likely lower yield than single cross hybrids [129], and
lower uniformity than that observed in single cross hybrids.
2.6. Maize Breeding and Society
Cleveland suggested breeders were neither pure scientists nor only servants to social constructs, and
this conclusion appears to be affirmed here [12]. Maize breeders come with their own personal
interests for the work to be undertaken and certainly make subjective decisions about specific
approaches and goals [8]. The widely promoted selection methods and shows before 1920 and the
rejection of recurrent selection from 1922 until the late 1940s show maize breeders and their
administrators sometimes got ahead of scientific validation in order to pursue exciting new options.
There were some decisions involved in promoting research and development of hybrid cultivars to
benefit specific seed businesses [23,37], and most professional maize breeders in the USA ignored the
potential of recurrent selection in favor of the task of testing tens of thousands of lines in the 1920s and
1930s [118,119]. There were, however, many promoters of hybrid cultivars who had farmers in mind,
and the science behind hybrid breeding methods eventually proved sound and useful to all forms of
maize improvement [10,22,160]. Excellent and creative work was carried out that has stood the test of
time and provided exceptional new cultivars of many types [10,16,17,89,123,161,162].
In the past thirty years maize breeding has changed further with patents, DNA marker assisted
selection, and transgenic techniques joining the process. Public breeding has been fading in favor of
breeding by major corporations around the world [116,163]. One might question whether this is once
more the promotion of the novel in place of approaches that objectivity might instead focus
upon [6,45,105]. Experience tells us that there can be some unanticipated problems with new
technologies rushed to market [164], yet sometimes full economic and ecological evaluations are still

Sustainability 2011, 3
1544
unavailable for transgenic maize cultivars until well after commercial release [165-167]. Cox reiterated
a calculation from Goodman and Carson comparing conventional hybrid breeding with the more
expensive transgenic methods which are apparently no faster to produce new cultivars than
conventional methods [168]. This situation is reminiscent of Gardener’s favorable comparison of
recurrent selection with hybrid breeding decades after the switch to hybrids [96]. At the American
Seed Trade Association conference in 2003, John Dudley, then maize breeder from the University of
Illinois, asked the audience, “Current corn yield goes up 1.9 Bu/A annually. What [trans]genes would
improve this?” Given our experiences, an open and logical discussion of the goals, means, and costs of
modern maize breeding appears in order as has always been the case.
Rather than the “March of Progress” from OP cultivars to high yielding hybrids and on to
transgenic technologies, perhaps the scientific and social successes and failures of actual maize
breeders (e.g., Darrah, Duvick, Hallauer, Hartley, Hayes, Gardner, Goodman, Lonnquist, Miranda
Filho, Pandey, Paterniani, Richey, Sevilla, Sprague, Stringfield, Troyer, Will, etc.) in meeting the
needs of farmers and consumers would be more objective and useful for students contemplating
breeding for sustainability. And rather than a competition between OP and hybrid cultivars, perhaps it
would be most sensible to consider this thought from Pandey et al.: “The critical question is not
whether hybrids are superior to OPVs, but whether a product is superior to what the farmers grow and
which new product they can afford” [152]. Breeding, like any tool with which humanity hopes to
derive a better world, requires knowledge, critical thinking, hard work, humility, cooperation,
objectivity, and a broad perspective in order to be successful [17,122]. If we are to make the most
reasonable choices for a sustainable agriculture, it appears most fruitful to attend to all of them.
4. Conclusions
OP maize cultivars were and are sometimes useful for providing low priced seeds and dependable
yields to farmers, although they usually yield less than well adapted hybrid cultivars when those are
available. In lower yielding agroecosystems and lower priced markets where OP cultivars appear to be
more competitive, farmers need to understand selection and seed production methods and their time
investment must be personally and economically satisfying to realize acceptable outcomes. This may
not be of interest to farmers with high value maize crops, large enterprises, substantial off-farm
activities, or access to hybrid cultivars that meet their needs. It appears that yield gains via breeding are
about the same for OP and hybrid cultivars, but starting at a lower yield level, most current OP
cultivars would appear to be permanently relegated to lower yield levels compared to most hybrids,
although specialty traits might help them be economically competitive anyway. The moment in history
when the yield of OP cultivars could have been improved to be competitive with hybrids via recurrent
selection alone appears to have passed, although the possibilities of forming competitive OP cultivars
using composite/synthetic methods and using more complex recurrent selection methods on-farm
remain challenging options that have not been fully investigated in the United States and Europe.
These approaches have at times been successful in the tropics. Yields of synthetics 5–10% less than
elite hybrids are still theoretically possible but have yet to be achieved. New OP cultivars could be
cost competitive in many more agroecosystems than OP cultivars are now and this process could open
up more partnerships to breed maize for non-patented traits of importance to sustainable agriculture

Sustainability 2011, 3
1545
(e.g., stress tolerance, insect and disease resistance, nutrient use efficiency, grain quality). There are
also existing opportunities for more on-farm development and production of hybrid cultivars. We can
approach questions about maize improvement and cultivar choice for sustainable agroecosystems
objectively with the most sustainable outcomes for society in mind, an approach often used in maize
breeding throughout much of the world.
Acknowledgments
The author would like to thank Margaret Smith, Jane Mt. Pleasant, Laurie Drinkwater, Ross Welch,
Theresa Podoll, unnamed reviewers, and the editors of Sustainability for comments on various versions
of this manuscript. Thanks to A.F. Troyer for inspiration and copies of his publications. Thanks also to
Leonard Borries, Marcelo Carena, David Christensen, James Christensen, Vaughn Emo, Walter
Goldstein, Victor Kucyk, Kendall Lamkey, Jack Lazor, Klaas Martens, David Podoll, Zeno Wicks,
Steve Zwinger and many others for many discussions of this topic and to the library staff at Cornell
University who made much of this research possible. Funding for the on-farm and graduate work that
led to this review was provided in part by the USDA Sustainable Agriculture Research and Education
program, the Organic Farming Research Foundation, the Bush Foundation, Pioneer HiBred, and the
National Council of Commercial Plant Breeders.
Conflict of Interest
The author declares no conflict of interest.
References and Notes
1. Delate, K. Organic grains, oilseeds, and other specialty crops. In Organic Farming: The
Ecological System; Francis, C., Ed.; American Society of Agronomy, Crop Science Society of
America, Soil Science Society of America: Madison, WI, USA, 2009; Volume: Agron. Monogr.
54, pp. 113-136.
2. Liebman, M.; Davis, A.S. Managing weeds in organic farming systems: An ecological approach.
In Organic Farming: The Ecological System; Francis, C., Ed.; American Society of Agronomy,
Crop Science Society of America, Soil Science Society of America: Madison, WI, USA, 2009;
Volume: Agron. Monogr. 54, pp. 173-195.
3. Acquaah, G. Principles of Crop Production: Theory, Techniques, and Technology, 2nd ed.;
Pearson Education, Prentice Hall: Upper Saddle River, NJ, USA, 2004.
4. Hayes, H.K.; Immer, F.R.; Smith, D.C. Methods of Plant Breeding, 2nd ed.; McGraw-Hill Book
Co.: New York, NY, USA, 1955.
5. Dobbs, T.L. Multifunctional economic analysis. In Agroecosystems Analysis; Rickerl, D.,
Francis, C., Eds.; American Society of Agronomy, Crop Science Society of America, Soil
Science Society of America: Madison, WI, USA, 2004; Agron. Series No. 43, pp. 75-92.
6. Cleveland, D.A.; Soleri, D.; Smith, S.E. Do folk varieties have a role in sustainable agriculture?
Biosci. 1994, 44, 740-751.

Sustainability 2011, 3
1546
7. Tracy, W.F. What is plant breeding? In Proceedings of Summit on Seeds and Breeds for 21st
Century Agriculture, Washington, DC, USA, 6–8 September 2003; Sligh, M., Lauffer, L., Eds.;
Rural Advancement Foundation International—USA: Pittsboro, NC, USA, 2004; pp. 23-30.
8. Bernardo, R. Breeding for Quantitative Traits in Plant; Stemma Press: Woodbury, MN,
USA, 2002.
9. Duvick, D.N.; Cassman, K.G. Post-green revolution trends in yield potential of temperate maize
in the north-central United States. Crop Sci. 1999, 39, 1622-1630.
10. Hallauer, A.R.; Miranda, J.B. Quantitative Genetics in Maize Breeding, 2nd ed.; Iowa State
University Press: Ames, IA, USA, 1988.
11. Crabb, R. The Hybrid Corn-Makers, 2nd ed.; West Chicago Publishing Co.: Chicago, IL,
USA, 1992.
12. Cleveland, D.A. Is plant breeding science objective truth or social construction? The case of
yield stability. Agric. Hum. Values 2001, 18, 251-270.
13. Webber, H.J. Plant-breeding for farmers. Cornell Univ. Agr. Exp. Sta. Bull. 1908, 251, 282-332.
14. Johannessen, S.; Hastorf, C.A., Eds. Corn and Culture in the Prehistoric New World; Westview
Press: Boulder, CO, USA, 1994.
15. Wilson, G.L. Buffalo Bird Woman’s Garden: Agriculture of the Hidatsa Indians; Minnesota
Historical Soc. Press: St. Paul, MN, USA, 1987.
16. Sprague, G.F. Corn breeding. In Corn and Corn Improvement; Sprague, G.F., Ed.; Academic
Press: New York, NY, USA, 1955.
17. Troyer, A.F. Temperate corn—Background, behavior and breeding. In Specialty Corns, 2nd ed.;
Hallauer, A., Ed.; CRC Press: New York, NY, USA, 2000.
18. Woods, T.A. A short history of King Philip corn. Seed Savers Exchange Newslett. 1988, Summer
Edition, 44-48.
19. Hinebauch, T.D. Corn Culture in North and Northwest; Self Published: Tower City, ND,
USA, 1902.
20. Emerson, W.D. History and Incidents of Indian Corn and Its Culture, Including Statistical,
Analytical and Other Tables; Also, Illustrations and Diagrams; Wrightson and Company:
Cincinnati, OH, USA, 1878 (reprinted by Scholarly Resources, Inc.: Wilmington, DE, USA, 1973).
21. Salisbury, J.H. Analysis of Indian corn. Trans. N.Y. State Agric. Soc. 1848, 8, 678-873.
22. Pratt, R.C. An historical examination of the development and adoption of hybrid corn: A case
study in Ohio. Maydica 2004, 49, 155-172.
23. Fitzgerald, D. The Business of Breeding: Hybrid Corn in Illinois, 1890–1940; Cornell Univ.
Press: Ithaca, NY, USA, 1990.
24. Wiidakas, W. Early North Dakota corn hybrids. Bimonthly Bull. North Dakota Agric. Exp. Stn.
1942, 4, 13-15.
25. Griliches, Z. Hybrid corn and the economics of innovation. Science 1960, 132, 275-280.
26. Wallace, H.A.; Brown, W.L. Corn and Its Early Fathers, rev. ed.; Iowa State Univ. Press: Ames,
IA, USA, 1988.
27. Anonymous; Quick Stats; USDA National Agricultural Statistics Service: Washington DC, USA,
2011; Available online: http://quickstats.nass.usda.gov/ (accessed on 15 September 2011).

Sustainability 2011, 3
1547
28. Kutka, F.J. New and Historical Issues Concerning Open-Pollinated Maize Cultivars in the United
States. Ph.D. Thesis, Cornell University, Ithaca, NY, USA, 2005.
29. Duvick, D.N.; Smith, J.S.C.; Cooper, M. Changes in performance, parentage, and genetic
diversity of successful corn hybrids 1930–2000. In Corn: Origin, History, Technology and
Production; Smith, C.W., Betran, J., Runge, E.C.A., Eds.; John Wiley and Sons: Hoboken, NJ,
USA, 2004.
30. Wiidakas, W. Comparative yield and maturity of corn hybrids. Bimonthly Bull. North Dakota
Agric. Exp. Stn. 1943, 5, 32-34.
31. Wiidakas, W. Corn variety and hybrid performance. Bimonthly Bull. North Dakota Agric. Exp.
Stn. 1942, 4, 24-28.
32. Darby, H.; Cummings, E.; Madden, R.; Gervais, A. 2009 Vermont organic corn silage
performance trial results. University of Vermont Extension: Burlington, VT, USA, 2009;
Available online: http://www.uvm.edu/pss/vtcrops/articles/CornReports/organiccorntrials09.pdf
(accessed on 14 January 2011).
33. Smith, M.; Seiter, S.; Mt. Pleasant, J.; Kutka, F. Performance of open-pollinated corn varieties
for grain and silage production. In Proceedings of the 58th Northeast Corn Improvement
Conference, Ottawa, ON, Canada, 13–14 February 2003; Reid, L., Ed.; Agriculture and
Agri-Food Canada: Ottawa, Canada, 2003.
34. Lauer, J.G.; Coors, J.G.; Flannery, P.J. Forage yield and quality of corn cultivars developed in
different eras. Crop Sci. 2001, 41, 1449-1455.
35. Coors, J.G. Selection methodology and heterosis. In The Genetics and Exploitation of Heterosis
in Crops; Coors, J.G., Pandey, S., Eds.; American Society of Agronomy, Crop Science Society
of America, Soil Science Society of America: Madison, WI, USA, 1999.
36. Duvick, D.N. Heterosis: Feeding people and protecting natural resources. In The Genetics and
Exploitation of Heterosis in Crops; Coors, J.G., Pandey, S., Eds.; American Society of
Agronomy, Crop Science Society of America, Soil Science Society of America: Madison, WI,
USA, 1999.
37. Kloppenburg, J.R. First the Seed: The Political Economy of Plant Biotechnology, 1492–2000;
Cambridge Univ. Press: New York, NY, USA, 1988.
38. Berlan, J.P.; Lewontin, R. The political economy of hybrid corn. Monthly Rev. 1986, 38, 35-47.
39. Simmonds, N.W. Principles of Crop Improvement; Longman: London, UK, 1979.
40. Brummer, E.C. Breeding for sustainable cropping systems. In Proceedings of Summit on Seeds
and Breeds for 21st Century Agriculture, Washington, DC, 6–8 September 2003; Sligh, M.,
Lauffer, L., Eds.; Rural Advancement Foundation International—USA: Pittsboro, NC, USA,
2004; pp. 63-67.
41. Francis, C. 2004. Soil dynamics, plant nutrition, and soil quality. In Agroecosystems Analysis;
Rickerl, D., Francis, C., Eds.; American Society of Agronomy, Crop Science Society of America,
Soil Science Society of America: Madison, WI, USA, 2004; Agron. Series No. 43, pp. 31-47.

Sustainability 2011, 3
1548
42. Kirschenmann, F. What would 21st Century breeding programs look like if they were geared
toward a more sustainable agriculture—Objectives, goals. In Proceedings of Summit on Seeds
and Breeds for 21st Century Agriculture, Washington, DC, 6–8 September 2003; Sligh, M.,
Lauffer, L., Eds.; Rural Advancement Foundation International—USA: Pittsboro, NC, USA,
2004; pp. 45-54.
43. Keller, D.R.; Brummer, E.C. Putting food production in context: Toward a postmechanistic
agricultural ethic. Biosci. 2002, 52, 264-271.
44. Shull, G.H. The genotypes of maize. Am. Nat. 1911, 45, 234-252.
45. The New American Farmer, 2nd ed.; Berton, V., Ed.; USDA Sustainable Agriculture Network:
Beltsville, MD, USA, 2005.
46. Hendrickson, M.; Heffernan, W. Lessons for public breeding from structural changes in the
agricultural marketplace. In Proceedings of Summit on Seeds and Breeds for 21st Century
Agriculture, Washington, DC, 6–8 September 2003; Sligh, M., Lauffer, L., Eds.; Rural
Advancement Foundation International—USA: Pittsboro, NC, USA, 2004; pp. 11-22.
47. Gardner, C.O. Population improvement in maize. In Maize Breeding and Genetics; Walden,
D.B., Ed.; John Wiley and Sons: New York, NY, USA, 1978.
48. Weyrich, R.A.; Lamkey, K.R.; Hallauer, A.R. Responses to seven methods of recurrent selection
in the BS11 maize population. Crop Sci. 1998, 38, 308-321.
49. Will, G.F. Corn for the Northwest; Webb Publishing: St. Paul, MN, USA, 1930.
50. Bowman, M.L. Corn: Growing, Judging, Breeding, Feeding, Marketing; Waterloo Publishing
Co.: Waterloo, IA, USA, 1915.
51. Hughes, H.D. Silver King—A corn for northern Iowa. Iowa State College Agric. Exp. Stn. Bull.
1913, 138, 75-95.
52. Datta, D. Corn Breeding. M.S. Thesis, Cornell University: Ithaca, NY, USA, 1908.
53. Holden, P.G., Atkinson, A.; Stevenson, W.H.; Olin, W.H. Selecting and preparing seed corn.
Iowa State College Exp. Stn. Bull. 1903, 68, 273-286.
54. Hartley, C.P. Improvement of corn by seed selection. In 1902 USDA Yearbook in Agriculture;
US Dept. of Agriculture: Washington, DC, USA, 1902; pp. 539-552.
55. Rist, F.J. The value of continuing competitive corn exhibits. In Ninth Annual Report of the
Nebraska Corn Improvers Association; Kiesselbach, T.A., Ed.; Nebraska Corn Improveers
Association: Lincoln, NE, USA, 1918; pp. 48-51; Available online: http://www.usgennet.org/
usa/ne/topic/resources/OLLibrary/Journals/ncia/index.htm (accessed on 25 May 2011).
56. Kiesselbach, T.A. Ear type selection and yield of dent corn. J. Am. Soc. Agron. 1922, 14, 27-48.
57. Myers, C.H.; Love, H.H.; Bussell, F.P. Production of new strains of corn for New York. Cornell
Univ. Agric. Exp. Stn. Bull. 1922, 408, 205-268.
58. Williams, C.G.; Welton, F.A. Corn experiments. Bull. Ohio Agric. Exp. Stn. 1915, 282, 71-109.
59. Davis, J.P. Seed corn, its selection, care and testing. Michigan State Farmer Institutes Bull. 1909,
15, 85-97.
60. Kirkpatrick, C.D. Experience in developing a high-yielding strain of corn. J. Am. Soc. Agron.
1925, 17, 487-488.

Sustainability 2011, 3
1549
61. Belsito, A.M. Open pollinated corn variety trials and a discussion of the practical implications for
open pollinated corn in small scale whiskey production. M.S. Thesis, Cornell University: Ithaca,
NY, USA, 2004.
62. Kutka, F.; Conway, P.; Christensen, J. The Heritage Maize Project (FNC00-301) Study of
Open-Pollinated Corn: Final Report to the North Central Region Sustainable Agriculture
Research and Education Program, US Dept. of Agriculture SARE Program, 2004; USDA SARE
program: College Park, MD, USA, 2004; Available online: http://mysare.sare.org/mySARE/
ProjectReport.aspx?do=viewRept&pn=FNC00-301&y=2000&t=1 (accessed on 9 September
2011).
63. Russell, W.A. Comparative performance for maize hybrids representing different eras of maize
breeding. Proc. Ann. Corn and Sorghum Res. Conf. 1974, 29, 81-101.
64. Towle, E.R. Fertilizers. In Second Biennial Report of the State of Vermont State Board of
Agriculture, Manufactures and Mining for the Years 1873–1874; Collier, P., Ed.; Freeman Steam
Printing House and Bindery: Montpelier, VT, USA, 1874; pp. 146-157.
65. Child, W. Grain culture—Does it pay to raise corn in Vermont? In Third Biennial Report of the
Vermont State Board of Agriculture, Manufactures and Mining for the Years 1875–1876;
Seeley, H.M., Ed.; Tuttle and Co.: Rutland, VT, USA, 1876; pp. 306-311.
66. Goodwin, E.M. Indian corn and its cultivation. In Eighth Vermont Agricultural Report by the
State Board of Agriculture for the Years 1883–1884; Cutting, H.A., Ed.; Watchman and Journal
Press: Montpelier, VT, USA, 1884; pp. 278-286.
67. Speer, R.P. Experiments with corn. Iowa Agric. Exp. Stn. Bull. 1889, 7, 247-259.
68. Curtiss, C.F. Corn growing. Iowa Agric. Exp. Stn. Bull. 1892, 16, 312-314.
69. Curtiss, C.F. Corn growing. Iowa Agric. Exp. Stn. Bull. 1892, 19, 605-609.
70. Kent, D.A. Crop report of the farm department. Iowa Agric. Exp. Stn. Bull. 1892, 16, 303-308.
71. Gardner, F.D. Corn experiments, 1894. Illinois Agric. Exp. Stn. Bull. 1895, 37, 1-24.
72. Delwiche, E.J. Opportunities for profitable farming in northern Wisconsin. Univ. Wisconsin
Agric. Exp. Stn. Bull. 1910, 96, 3-34.
73. Hutcheson, T.B.; Hodgson, E.R.; Wolfe, T.K. Corn culture. Virginia Agric. Exp. Stn. Bull. 1917,
214, 3-12.
74. Minns, E.R. Cooperative tests of corn varieties. Cornell Univ. Agric. Exp. Stn. Bull. 1912, 314,
395-410.
75. Hughes, H.D. The germination test of seed corn. Iowa State College Agric. Exp. Stn. Bull. 1913,
135, 307-379.
76. Hutcheson, T.B.; Wolfe, T.K. Fertilizers and their relation to crop production in Virginia.
Virginia Agric. Exp. Stn. Bull. 1919, 221, 5-74.
77. Drinkard, A.W. Annual Report of the Virginia Polytechnic Institute Agricultural Experiment
Station, 1916–1917; Brown-Morrison Co.: Lynchburg, VA, USA, 1918.
78. Cunningham, C.C.; Wilson, B.S. Varieties of corn in Kansas. Kansas Agric. Exp. Stn. Bull. 1921,
227, 5-40.
79. Call, L.E.; Throckmorton, R.I. Soil fertility. Kansas Agric. Exp. Stn. Bull. 1918, 220.
80. Jones, D.F. The effects of inbreeding and crossbreeding upon development. Connecticut Agric.
Exp. Stn. Bull. 1918, 207, 5-100.

Sustainability 2011, 3
1550
81. Griffee, F. First generation corn varietal crosses. J. Am. Soc. Agron. 1922, 14, 18-27.
82. University of Nebraska News Service. Hybrid corn varieties produce highest yields. J. Heredity
1933, 24, 64.
83. Richey, F.D.; Stringfield, G.H.; Sprague, G.F. The loss in yield that may be expected from
planting second generation double crossed seed corn. J. Am. Soc. Agron. 1934, 26, 196-199.
84. Hayes, H.K.; Murphy, R.P.; Rinke, E.H.; Borgeson, C. Minhybrid corn varieties for Minnesota.
Agric. Exp. Stn. Univ. Minnesota Bull. 1941, 354.
85. Stringfield, G.H.; Lewis, R.D.; Pfaff, H.L. Ohio corn performance tests and
recommendations—1942. Ohio Agric. Exp. Stn. Circ. 1942, 64.
86. Stringfield, G.H.; Lewis, R.D.; Pfaff, H.L. Ohio corn performance tests and
recommendations—1942. Ohio Agric. Exp. Stn. Circ. 1943, 66.
87. Stringfield, G.H.; Lewis, R.D.; Pfaff, H.L. Ohio corn performance tests and
recommendations—1943 and 1944. Ohio Agric. Exp. Stn. Circ. 1946, 71.
88. Stringfield, G.H.; Pfaff, H.L. Ohio corn performance tests: 1945 and 1946. Ohio Agric. Exp. Stn.
Circ. 1948, 77.
89. Hayes, H.K. A Professor’s Story of Hybrid Corn; Burgess Publishing Co.: Minneapolis, MN,
USA, 1963.
90. Manke, K.F.; Grafius, J.E. South Dakota corn performance test, 1944. South Dakota Agric. Exp.
Stn. Circ. 1945, 55.
91. Heyne, E.G.; Clapp, A.L.; Porter, C.R.; Scott, W.O.; Davis, C.D. Kansas corn tests, 1945.
Kansas State Univ. Agric. Exp. Stn. Bull. 1946, 329.
92. Shank, D.B. 1948 corn performance tests. South Dakota Agric. Exp. Stn. Circ. 1949, 76.
93. Lonnquist, J.H.; McGill, D.P. Performance of corn synthetics in advanced generation of
synthesis and after two cycles of recurrent selection. Agron. J. 1956, 48, 249-253.
94. Everett, H.L.; Crowder, L.V. Cornell corn breeding program. Cornell Univ. Agric. Exp. Stn. Bull.
1965, 1000.
95. Lonnquist, J.H.; Gardner, C.O. Heterosis in intervarietal crosses in maize and its implication in
breeding procedures. Crop Sci. 1961, 1, 179-183.
96. Gardner, C.O. An evaluation of effects of mass selection and seed irradiation with thermal
neutrons on yield of corn. Crop Sci. 1961, 1, 241-245.
97. Castleberry, R.M.; Crum, C.W.; Krull, C.F. Genetic yield improvement of U.S. maize cultivars
under varying fertility and climatic environments. Crop Sci. 1984, 24, 33-36.
98. Delate, K.; Lamkey, K.; Burcham, B. Plant Population Effects on Open-Pollinated Corn, Report
ISR F00-12 from Armstrong Research and Demonstration Farm; Iowa State Univ.: Ames, IA,
USA, 2001.
99. Montgomery, E.G. The Corn Crops; The Macmillan Company: New York, NY, USA, 1915.
100. Bliss, E.C. E.C. Bliss’ farm, westfield, chatauque. Trans. N.Y. State Agric. Soc. 1848, 8, 202-210.
101. Kirtland, B.B. 1848. Management of the Cantonement Farm, Greenbush, Rensselaer County.
Trans. N.Y. State Agric. Soc. 1848, 8, 210-218.
102. Kline, J. Breeding and selecting seed corn. Michigan State Farmer Institutes Bull. 1914. 20, 34-37.
103. Norgard, C.P. Crop demonstrations on state and county farms. Univ. Wisconsin Agric. Exp. Stn.
Bull. 1911, 208.

Sustainability 2011, 3
1551
104. Smith, C.B. Rotations in the corn belt. In 1911 USDA Yearbook in Agriculture; US Dept. of
Agriculture: Washington, DC, USA, 1911; Reprinted in Missouri Farm 1989, 6, 37-39.
105. Karlen, D.L.; Varvel, G.E.; Bullock, D.G.; Cruse, R.M. Crop rotations for the 21st Century. Adv.
Agron. 1994, 53, 1-43.
106. Clapp, A.L.; Tatum, L.A. Kansas corn tests—1949. Kansas State Univ. Agric. Exp. Stn. Bull.
1950, 342.
107. Clapp, A.L.; Tatum, L.A.; Burkhardt, C.C. Kansas corn tests—1954. Kansas State Univ. Agric.
Exp. Stn. Bull. 1955, 373.
108. Clapp, A.L., Findley; W.R. Kansas corn tests—1959. Kansas State Univ. Agric. Exp. Stn. Bull.
1960, 419.
109. Walter, T. Kansas corn performance tests—1965. Kansas State Univ. Agric. Exp. Stn. Bull. 1966,
490.
110. Walter, T. 1966 report on Kansas corn performance tests. Kansas State Univ. Agric. Exp. Stn.
Bull. 1967, 503.
111. Walter, T. 1967 report on Kansas corn performance tests. Kansas State Univ. Agric. Exp. Stn.
Bull. 1967, 514.
112. Walter, T. 1969 report on Kansas corn performance tests. Kansas State Univ. Agric. Exp. Stn.
Bull. 1969, 525.
113. Walter, T.L. Report on 1972 Kansas corn performance tests. Kansas State Univ. Agric. Exp. Stn.
Bull. 1973, 567.
114. Troyer, A.F. Champaign County, Illinois, and the origin of hybrid corn. Plant Breeding Rev.
2004, 24, 41-59.
115. Duvick, D.N. Genetic contributions to advances in yield of U.S. maize. Maydica 1992, 37, 69-79.
116. Murphy, D. Plant Breeding and Biotechnology: Societal Context and the Future of Agriculture;
Cambridge University Press: New York, NY, USA, 2007.
117. Carrier, L. The immediate effect on yield of crossing strains of corn. Virginia Agric. Exp. Stn.
Bull. 1913, 202, 3-11.
118. Hayes, H.K. Present-day problems of corn breeding. J. Am. Soc. Agron. 1926, 18, 344-363.
119. Richey, F.D. The experimental basis for the present status of corn breeding. J. Am. Soc. Agron.
1922, 14, 1-17.
120. Paterniani, E. Maize breeding in the tropics. Crit. Rev. Plant Sci. 1990, 9, 125-154.
121. Jones, S.S. A system out of balance—The privatization of the land grant university breeding
programs. In Proceedings of Summit on Seeds and Breeds for 21st Century Agriculture,
Washington, DC, September 6–8, 2003; Sligh, M., Lauffer, L., Eds.; Rural Advancement
Foundation International—USA: Pittsboro, NC, USA, 2004; pp. 109-110.
122. Lamkey, K.R. Plant breeding: research and education agenda. In Proceedings of Summit on
Seeds and Breeds for 21st Century Agriculture, Washington, DC, September 6–8, 2003; Sligh,
M., Lauffer, L., Eds.; Rural Advancement Foundation International—USA: Pittsboro, NC, USA,
2004; pp. 129-142.
123. Jenkins, M.T. 1978. Maize breeding during the development and early years of hybrid maize. In
Maize Breeding and Genetics; Walden, D.B., Ed.; John Wiley and Sons: New York, NY,
USA, 1978.

Sustainability 2011, 3
1552
124. Robinson, H.F.; Comstock, R.E.; Harvey, P.H. Genetic variances in open pollinated varieties of
corn. Genetics 1955, 40, 45-60.
125. Webel, O.D.; Lonnquist, J.H. An evaluation of modified ear-to-row selection in a population of
corn. Crop Sci. 1967, 7, 651-655.
126. Carena, M.J. Maize commercial hybrids compared to improved population hybrids for grain
yield and agronomic performance. Euphytica 2005, 141, 201-208.
127. Carena, M.J.; Hallauer, A.R. Response to inbred progeny selection in Leaming and Midland
yellow dent maize populations. Maydica 2001, 46, 1-10.
128. Eberhart, S.A.; Salhuana, W.; Sevilla, R.; Taba S. Principles for tropical maize breeding.
Maydica 1995, 40, 339-355.
129. Mungoma, C.; Pollak, L.M. Heterotic patterns among ten corn belt and exotic maize populations.
Crop Sci. 1988, 28, 500-504.
130. Stringfield, G.H. Developing Heterozygous Parent Stocks for Maize Hybrids; DeKalb
AgResearch, Inc.: DeKalb, IL, USA, 1974.
131. Darrah, L.L.; Eberhart, S.A.; Penny, L.H. A maize breeding methods study in Kenya. Crop Sci.
1972, 12, 605-608.
132. Darrah, L.L.; Mukuru, S.Z. Recurrent Selection Methods for Maize Improvement: The East
Africa Experience; East African Agriculture and Forestry Research Organization: Muguga,
Nairobi, Kenya, 1977.
133. Hyrkas, A.; Carena, M.J. Response to long-term selection in early maturing maize synthetic
varieties. Euphytica 2005, 143, 43-49.
134. Lonnquist, J.H. A modification of the ear-to-row procedure for the improvement of maize
populations. Crop Sci. 1964, 4, 227-228.
135. Bussell, F.P. Improving the Corn Crop by Selection and Breeding. The Cornell Reading Course
for the Farm, Lesson 129; Cornell University: Ithaca, NY, USA, 1917; pp. 111-128.
136. Wellhausen, E.J. The accuracy of incomplete block designs in varietal trials in West Virginia.
J. Am Soc. Agron. 1943, 35, 66-76.
137. Zuber, M.S. Relative efficiency of incomplete block designs using corn uniformity trial data.
J. Am. Soc. Agron. 1942, 34, 30-47.
138. Bletsos, E.A.; Goulas, C.K. Mass selection for improvement of grain yield and protein in a maize
population. Crop Sci. 1999, 39, 1302-1305.
139. Comstock, R.E. Quantitative genetics in maize breeding. In Maize Breeding and Genetics;
Walden, D.B., Ed.; John Wiley and Sons: New York, NY, USA, 1978.
140. Lamkey, K.R. Breeding and evaluating open pollinated varieties of corn. In Report of the North
Central Corn Breeding Research Committee, Proceedings of NCR-167 Annual Meeting, Ames,
IA, USA, 5–6 February 2001; Hallauer, A.R., Ed.; Iowa State University: Ames, IA, USA, 2001;
p. 20; Available online: http://www.agron.iastate.edu/corn/ncr167/Minutes/2001_NCR167_
Minutes.pdf (accessed on 15 September 2011).
141. Wright, S. The effects of inbreeding and crossbreeding on guinea pigs. USDA. Bull. 1922, 1121.
142. Mochizuki, N. Theoretical approach for the choice of parents and their number to develop a
highly productive synthetic variety in maize. Jpn. J. Breeding 1970, 20, 105-109.

Sustainability 2011, 3
1553
143. Hayes, H.K.; Garber, R.J. Synthetic production of high protein corn in relation to breeding.
Agron. J. 1919, 11, 309-318.
144. Kinman, M.L.; Sprague, G.F. Relation between number of parental lines and theoretical
performance of synthetic varieties of corn. J. Am. Soc. Agron. 1945, 37, 341-351.
145. Sprague, G.F.; Jenkins, M.T. A comparison of synthetic varieties, multiple crosses, and double
crosses in corn. J. Am. Soc. Agron. 1943, 35, 137-147.
146. Burger, H.; Schmidt, W.; Hartwig, H. Comparison of Methods for the Development of Optimal
Maize Varieties for Organic Farming (in Deutsch); Report to Bundesprogramm Okologischer
Landbau, 2006; BLE: Bonn, Germany, 2006; Available online: http://orgprints.org/16714/1/
16714-03OE651-kws-burger-2006-maissorten.pdf (accessed on 19 August 2011).
147. Baltensperger, D.; Frickel, G.; Russell, W.K.; Guillen-Portal, F.; Nelson, L. Development of
dryland corn populations for the high plains. In Proceedings of ASA-CSSA-SSSA International
Annual Meetings, Salt Lake City, UT, USA, Nov 6–10, 2005; Abstract for Poster 608a;
Available online: http://acs.confex.com/crops/2005am/techprogram/P8839.HTM (accessed on
19 August 2011).
148. Kiesselbach, T.A. Performance of advanced generation corn hybrids. Agron. J. 1960, 52, 29-32.
149. Hayes, H.K.; Rinke, E.H.; Tsiang, Y.S. The development of a synthetic variety of corn from
inbred lines. J. Am. Soc. Agron. 1944, 36, 998-1000.
150. Kutka, F.J.; Smith, M.E. How many parents give the highest yield in predicted synthetic and
composite populations of maize? Crop Sci. 2007, 47, 1905-1913.
151. Zhu, Y.; Fen, H.; Wang, Y.; Li, Y.; Chen, J.; Hu, L.; Mundt, C.C. Genetic diversity and disease
control in rice. Nature 2000, 406, 718-772.
152. Pandey, S.; Narro Leon, L.A.; Friesen, D.K.; Waddington, S.R. Breeding maize for tolerance to
soil acidity. Plant Breeding Rev. 2007, 28, 59-100.
153. Muraya, M.M.; Ndirangu, C.M.; Omolo, E.O. Heterosis and combining ability in diallel crosses
involving maize (Zea mays) S1 lines. Austr. J. Exp. Agric. 2006, 46, 387-394.
154. Impact of Public- and Private-Sector Maize Breeding Research in Asia, 1966–1997/1998;
Gerpacio, R.V., Ed.; CIMMYT: Mexico, D.F., Mexico, 2001; Available online:
http//libcatalog.cimmyt.org/download/cim/75341.pdf (accessed on 9 September 2011).
155. Development, Maintenance, and Seed Multiplication of Open-Pollinated Maize Varieties, 2nd
ed.; CIMMYT: Mexico, D.F., Mexico, 1999.
156. Chaves, L.J.; Miranda Filho, J.B. Predicting variety composite means without diallel crossing.
Braz. J. Genet. 1997, 20; Available online: http://www.scielo.br/scielo.php?script=sci_arttext
&pid=S0100-84551997000300023 (accessed on 9 September 2011).
157. Logrono, M.L.; Lantin, M.M. Genetic effects for nine characters from variety diallel cross of six
maize populations. Philipp. J. Crop Sci. 1985, 10, 87-92.
158. Podoll, T. 2003. Farm Breeding Club—Seeds for the Future. In Proceedings of Summit on Seeds
and Breeds for 21st Century Agriculture, Washington, DC, 6–8 September 2003; Sligh, M.,
Lauffer, L., Eds.; Rural Advancement Foundation International—USA: Pittsboro, NC, USA,
2004; pp. 165-170.

Sustainability 2011, 3
1554
159. Goldstein, W. Developing open-pollinated corn varieties for organic farmers. Org. Farming Res.
Found. Inform. Bull. 2002, 11, 22-24; Available online: http://ofrf.org/publications/ib/ib11.pdf
(accessed on 9 September 2011).
160. Troyer, A.F. Phenotypic selection and evaluation of maize inbreds for adaptedness.
Plant Breeding Rev. 2007, 28, 101-123.
161. Johnson, M.W.; Ayers, J.E. Registration of eight maize germplasm sources for Gray Leaf Spot
(GLS) resistance. Crop Sci. 1988, 28, 871-872.
162. Richey, F.D. Hybrid vigor and corn breeding. J. Am. Soc. Agron. 1946, 38, 833-841.
163. Soleri, D.; Cleveland, D.A. Breeding for quantitative variables. Part 1: Farmers’ and scientists’
knowledge and practice in variety choice and plant selection. In Plant breeding and farmer
participation; Ceccarelli, S., Guimaraes, E.P., Weltzien, E., Eds.; Food and Agriculture
Organization of the United Nations: Rome, Italy, 2009.
164. Carson, R. Silent Spring; Houghton Mifflin: Boston, MA, USA, 1962.
165. Hellmich, R.L.; Siegfried, B.D.; Sears, M.K.; Stanley-Horn, D.E.; Daniels, M.J.; Mattila, H.R.;
Spencer, T.; Bidne, K.G.; Lewis, L.C. Monarch larvae sensitivity to Bacillus
thuringiensis-purified proteins and pollen. Proc. Natl. Acad. Sci. USA 2001, 98, 11925-11930.
166. Scriber, J.M. Bt or not Bt: Is that the question? Proc. Natl. Acad. Sci. USA 2001, 98, 12328-12330.
167. Zangerl, A.R.; McKenna, D.; Wright, C.L.; Carroll, M.; Ficarello, P.; Warner, R.;
Berenbaum, M.R. Effects of exposure to event 176 Bacillus thuringiensis corn pollen on
Monarch and Black Swallowtail caterpillars under field conditions. Proc. Natl. Acad. Sci. USA
2001, 98, 11908-11912.
168. Cox, S. The mirage of genetic engineering. Am. J. Alt. Agric. 2002, 17, 41-43.
© 2011 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article
distributed under the terms and conditions of the Creative Commons Attribution license
(http://creativecommons.org/licenses/by/3.0/).