Suppression of annual bromes impacts rangeland: animal responses.

Abstract

Presence of annual bromes (Bromus spp.), introduced annual weedy grasses, can alter seasonal patterns of forage production and quality and require management changes for efficient use of infested rangelands. We determined biological impacts of the presence of brome by comparing livestock performance on brome infested rangeland to similar sites on which brome had been suppressed by autumn application of atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)- 1,3,5-triazine-2,4-diamine] at 0.56 kg ha(-1) in 1992 and 1993. Each treatment was randomly assigned to three, 12-ha pastures. Vegetation was measured for 5 months (May to September) each year from 1993 to 1995. Each pasture was stocked with 8 crossbred steers of British breed origin (Bos taurus) from mid-May to mid-September 1993 and 1995 and to mid-August 1994. Initial body weights averaged 329 kg SD = 31 in 1993, 273 kg SD = 14 in 1994, and 272 kg SD = 21 in 1995. Brome suppression and environment influenced plant species in diets, diet quality, and livestock performance. Brome suppression reduced percentage of annual grasses in diets from 14% to 10%. Annual grasses were replaced in the diet by a variety of forb and grass species [western wheatgrass [Pascopyrum smithii Rydb. (Love)], and blue grama [Bouteloua gracilis [H.B.K.] Lag. ex Griffiths]], with specific replacement depending on year and month. Steer gains were increased from 0.92 to 1.04 +/- 0.02 kg head(-1) day(-1) (P < 0.02) and from 69 to 81 +/- 2.8 kg ha(-1) (P < 0.05) with brome suppression. This experiment demonstrated that improvement in livestock performance can be expected with the suppression of annual bromes on semiarid rangelands.

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663
JOURNAL OF RANGE MANAGEMENT 54(6), November 2001
Abstract
Presence of annual bromes (Bromus spp.), introduced annual
weedy grasses, can alter seasonal patterns of forage production
and quality and require management changes for efficient use of
infested rangelands. We determined biological impacts of the
presence of brome by comparing livestock performance on
brome infested rangeland to similar sites on which brome had
been suppressed by autumn application of atrazine [6-chloro-N-
ethyl-N’-(1-methylethyl)-1,3,5-triazine-2,4-diamine] at 0.56 kg
ha-1in 1992 and 1993. Each treatment was randomly assigned to
three, 12-ha pastures. Vegetation was measured for 5 months
(May to September) each year from 1993 to 1995. Each pasture
was stocked with 8 crossbred steers of British breed origin (Bos
taurus) from mid-May to mid-September 1993 and 1995 and to
mid-August 1994. Initial body weights averaged 329 kg SD = 31
in 1993, 273 kg SD = 14 in 1994, and 272 kg SD = 21 in 1995.
Brome suppression and environment influenced plant species in
diets, diet quality, and livestock performance. Brome suppression
reduced percentage of annual grasses in diets from 14% to 10%.
Annual grasses were replaced in the diet by a variety of forb and
grass species {western wheatgrass [Pascopyrum smithii Rydb.
(Love)], and blue grama [Bouteloua gracilis [H.B.K.] Lag. ex
Griffiths]}, with specific replacement depending on year and
month. Steer gains were increased from 0.92 to 1.04 ± 0.02 kg
head-1day-1(P < 0.02) and from 69 to 81 + 2.8 kg ha-1(P < 0.05)
with brome suppression. This experiment demonstrated that
improvement in livestock performance can be expected with the
suppression of annual bromes on semiarid rangelands.
Key Words: Bromus japonicus, Pascopyrum smithii, Northern
Great Plains, crude protein yield
Annual bromes (Bromus spp.) have infested thousands of
hectares of Northern Great Plains rangelands (Haferkamp et al.
1993, Whisenant 1990, Hewlett et al. 1981). Annual bromes
affect associated plant species and nutritional quality of herbage
available to grazing livestock (Haferkamp et al. 1994, 1997,
1998). The major characteristics of annual bromes affecting live-
stock management decisions include erratic fluctuations in annual
forage production (Haferkamp et al. 2001, Haferkamp et al. 1993,
Gartner et al. 1986), reduction in perennial plant production
(Haferkamp et al. 1997, 1998, Rummell 1946), and early plant
maturation (Haferkamp et al. 1994, Vallentine and Stevens 1994).
Although early spring and autumn grazing are proposed as the
best ways to use annual brome infested ranges (Mayland et al.
1994, Tipton 1994, Vallentine and Stevens 1994), some of these
ranges must be used during the summer when forage nutritional
quality of annual bromes may suppress livestock performance
(Heitschmidt et al. 1993, Currie et al. 1989). The objective of this
study was to test the hypothesis that summer rates of gain of
steers grazing Northern Great Plains ranges infested with annual
bromes would be less than gains of steers grazing adjacent ranges
where annual brome abundance was suppressed.
J. Range Manage.
54: 663–668 November 2001
Suppression of annual bromes impacts rangeland: Animal
responses
MARSHALL R. HAFERKAMP, ELAINE E. GRINGS, R.K. HEITSCHMIDT, MICHAEL D. MacNEIL,
AND MICHAEL G. KARL
Authors are rangeland scientist, research animal scientist, supervisory rangeland scientist, research geneticist, and postdoctoral rangeland scientist
USDA-ARS, Fort Keogh Livestock and Range Research Laboratory, Miles City, Mont. 59301. Karl is currently rangeland management specialist,
USDI-Bureau of Land Management, Washington, DC 20240.
The authors express appreciation to Bryon Bennett, Caralee Leidholt, Cheryl
Murphy, Duane Bundy, and several summer aids for field assistance, and Mary
Ellen French and Diona Austill for assistance with graphics.
This paper is a contribution from the USDA-ARS and Montana Agr. Exp. Sta.,
Miles City, Mont.
The USDA-ARS, Northern Plains Area, is an equal opportunity/affirmative
action employer, and all agency services are available without discrimination.
Manuscript accepted Jan. 27, 01.
Resumen
La presencia de “Bromos” anuales invasores (Bromus spp.)
puede alterar los patrones estacionales de producción y calidad
de forraje y requieren cambios de manejo para hacer un uso efi-
ciente de los pastizales infestados de las Grandes Planicies del
Norte. Estudiamos los impactos biológicos de la presencia del
“Bromo” mediante la comparación de pastizales infestados con
sitios similares en los cuales el “Bromo” había sido suprimido
con aplicaciones en otoño de atrazina [6-cloro-N-etil-N'-(1-
metiletil)-1,3,5-triazina-2,4-diamina] en dosis de 0.56 kg ha-1
efectuadas en 1992 y 1993. Cada tratamiento se asignó aleatoria-
mente a tres potreros de 12 ha. De 1993 a 1995 la vegetación se
midió anualmente durante 5 meses (Mayo a Septiembre). En
1993 y 1995, de mediados de Mayo a mediados de Septiembre,
cada potrero se cargó con 8 novillos de cruzas comerciales (Bos
taurus) y en 1994 se utilizaron hasta mediados de Agosto. El for-
raje base varió temporalmente por fecha y año, pero general-
mente no fue menor de 800 kg ha-1. La supresión de “Bromo”
incrementó (P < 0.05) la concentración de proteína cruda del
“Western wheatgrass” (Pascopyrum smithii Rydb. [Love]) en
Julio (7.1 vs. 9.1%) y Agosto (6.0 vs. 7.1%). Con la variación
entre años de las poblaciones de “Bromo”, por influencia de las
condiciones durante el período de crecimiento, este experimento
demostró que al suprimir el “Bromo” anual se puede esperar un
mejoramiento en la calidad nutricional de los pastizales semiári-
dos.

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664JOURNAL OF RANGE MANAGEMENT 54(6), November 2001
Materials and Methods
Study Site
Research was conducted at the Fort
Keogh Livestock and Range Research
Laboratory (46°22'N 105°5'W) near Miles
City, Mont. Elevation ranges from 716 to
853 m. Regional topography ranges from
rolling hills to broken badlands with small
intersecting ephemeral streams flowing into
rivers in broad, nearly level valleys. Annual
precipitation averages 343 mm, with about
60% received from April through
September. Daily temperatures range from
> 38°C during summer to < -40°C during
winter. The average frost-free growing sea-
son is 150 days. Indigenous vegetation on
the 22,500-ha research station is a
grama-needlegrass-wheatgrass
(Bouteloua-Stipa-Agropyron) mix (Kuchler
1964).
Soils at the specific-study site are a com-
posite of Absher heavy clays (Fine, smec-
titic, frigid Leptic Torrertic Natrustalfs)
and Gerdrum silty clay loams (Fine, smec-
titic, frigid Torretic Natrustalfs).
Topography is gently sloping (< 2%).
Vegetation is dominated by western wheat-
grass (Pascopyrum smithii Rydb. [Love]),
blue grama (Bouteloua gracilis [H.B.K.]
Lag. ex Griffiths), Sandberg bluegrass
(Poa secunda Presl.), sand dropseed
(Sporobolus cryptandrus [Torr.] Gray),
and Japanese brome (Bromus japonicus
Thunb.). Threadleaf sedge (Carex filifolia
Nutt.) is the dominant grasslike species.
Dandelion (Taraxacum officinale Weber)
and salsify (Tragopogon dubius Scop.) are
the dominant forb species.
Treatments and Experimental
Design
Six, 12-ha pastures were used in this
study (Haferkamp et al. 2001). The 2 treat-
ments (brome suppression and control)
were assigned randomly to 6 pastures in a
completely random design. Pasture was
the experimental unit. Brome was sup-
pressed with atrazine [6-chloro-N-ethyl-
N’-(1-methylethyl)-1,3,5-triazine-2,4-
diamine] applied to dormant vegetation in
November 1992 and 1993 at 0.56 kg a.i.
ha-1. Herbicide was applied by ground in
1992 in 150 liters water ha-1and by air in
1993 in 19 liters water ha-1. No herbicide
was applied in autumn 1994.
Livestock Sampling
Each treatment was stocked annually
with 8 crossbred yearling steers of British
breed origin (Bos taurus) per 12-ha pas-
ture. Pastures were stocked continuously
from mid-May until mid-September 1993
and 1995. Limited forage production in
1994 resulted in a shortened grazing sea-
son extending from mid-May to mid-
August. Steers were handled according to
protocol approved by the Fort Keogh
Livestock and Range Research Laboratory
Animal Care Committee.
At the beginning of each grazing season
steers were weighed. Steers were stratified
by initial weight and within strata random-
ly assigned to pastures. Thereafter,
weights were obtained about every 30
days, and at the end of each annual trial.
Duration of the periods between sampling
dates ranged from 23 to 37 days. Average
daily gains were calculated from the
weights taken at adjacent sampling dates
and the duration of the respective period.
Total gain was average daily gain multi-
plied times number of steers in the pasture
and the number of days in the period. Gain
per hectare was total gain divided by the
area of the pasture. Shrunk weights on and
off pastures were initiated in 1994 due to
scale malfunction. Steers received a 200-
day estradiol implant in early May 1994
and 1995, but not 1993. Steers were treat-
ed with Ivermectin1(Merck & Co. Inc.,
Whitehouse Station, N.J.) before the study
for parasite control and received a Python1
fly repellent ear tag (Y-TEX Corp., Cody,
Wyo.) at the beginning of the grazing sea-
son. Steers were allowed ad libitum access
to a loose trace mineralized salt mix con-
taining 36% Cl, 24% Na, 6% P, 5% Ca,
240 ppm Zn, 240 ppm Mn, 60 ppm Cu, 9
ppm I, 3 ppm Co, 0.9 ppm Se, 8800 IU vit-
amin A kg-1, and 880 IU vitamin D kg-1.
Diet quality was sampled monthly in all
treatments during the week steers were
weighed. Six to 8 esophageally cannulated
crossbred heifers were used for diet sam-
ple collection. Diets were collected from
at least 3 heifers per pasture, and not all
pastures were sampled with the same
heifers. Heifers were penned at 1600 hours
with water but no feed available.
Collections were made the following
morning beginning at 0700 hours.
Collection periods lasted from 20 to 30
min and were followed immediately by a
collection in a second pasture. Esophageal
masticate samples were thoroughly mixed
by hand, freeze dried, and subsampled for
diet quality and diet composition analyses.
Sample collections began when these
heifers were about 15 months of age.
Heifers had been cannulated at 2 months
of age and were experienced with the diet
collection process. Estrous was suppressed
during diet collection periods by the use of
a Norgestomet1implant (Sanofi Animal
Health, Overland Park, Kan.). Between
sampling periods, heifers were kept in
larger pastures with a variety of range
sites. Heifers had been previously exposed
to all plant species present in the experi-
mental pastures.
Laboratory Analysis
Esophageal masticate and standing crop
samples (Haferkamp et al. 2001) were
ground to pass through a 1-mm screen in a
Wiley mill1(Arthur H. Thomas Co.,
Philadelphia, Penn.). Total nitrogen (N;
organic matter basis) was determined with
a Technicon1Auto Analyzer (Technicon
Industrial Systems 1977, Tarrytown,
N.Y.). Data are presented as percentage
crude protein (CP; percentage N*6.25)
and CP yield (standing crop*decimal
equivalent CP). In vitro digestible organic
matter (IVDOM) was determined using a
modified Tilley and Terry (1963) tech-
nique (White et al. 1981).
Diet composition by plant species was
determined using a point-frame technique
on unground samples of lyophilized tissue
(Angell et al. 1986). The fragment hit at
each point was identified and recorded. If
no hit occurred, the closest fragment to the
pin point in a forward 180-degree arc was
used. Each sample was assessed at 100
points.
Data Summarization and Analysis
Analysis of variance was used to test
effects of treatments, years, dates, and
interactions among them. Treatment
effects were tested with the residual varia-
tion among pastures within treatment, and
years and the year by treatment interaction
were tested with the year by pasture within
treatment interaction. Effects of dates and
interactions of dates with other effects
were tested with the residual variation
after accounting for all other effects in the
model.
Data for the fourth sampling period in
1994 were missing, thus the marginal
means for treatment were non-estimable.
However, treatment by period subclass
means were estimable. Means were com-
pared using the Least Significant
Difference method protected by a signifi-
cant F-statistic (P ≤ 0.05) from the
analysis of variance (SAS 1990).
Comparisons for CP yield were limited
to May, June, July, and August, due to the
1Mention of a trade name or a specific proprietary
product does not constitute a guarantee or warranty
by the authors or USDA-ARS nor does it imply the
approval of these products to the exclusion of others.

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665
JOURNAL OF RANGE MANAGEMENT 54(6), November 2001
lack of September samples in 1994. Most
analyses were run on the following species
groups: Western wheatgrass + Sandberg
bluegrass, other perennial grasses, annual
grasses, sedges, forbs, total green biomass,
standing dead, and total biomass.
Results and Discussion
Brome Suppression
Standing Crop
Impact of brome suppression on stand-
ing crop (kg DM ha-1) and forage nutritive
value have been reported previously
(Haferkamp et al. 2001). Findings showed
annual brome suppression with atrazine
had the following effects: decreased stand-
ing crop of annual grasses (P ≤ 0.05), total
vegetation (P ≤ 0.05 for year by treatment
by date interaction), and total vegetation
plus standing dead (P ≤ 0.05 for year by
treatment by date interaction) and
increased forage nutritive value of western
wheatgrass (P ≤ 0.05) and Japanese brome
(P ≤ 0.05) as indicated by increases in CP.
Significant interactions with date and year
showed that these responses would vary
with the changing environmental condi-
tions both within and across years.
Crude Protein Yield
Animal performance can be limited by
either forage quality or quantity. Total
yield of nutrients is an important way to
evaluate treatment effects for their poten-
tial to impact animal gains. Crude protein
yields (kg CP ha-1) of annual grasses were
somewhat constant among dates and years
when brome was suppressed, but yields
varied among dates and years with brome
present (Fig. 1A and B). Magnitude of
treatment differences declined from spring
to late summer (Fig. 1A). Crude protein
yield of annual grass was increased (P ≤
0.01) by presence of bromes in 1993 and
1995, but not 1994 (Fig. 1B).
Steer Diets
Brome suppression decreased percent-
age of annual grasses in the diet from 14%
to 10% (P ≤ 0.01). The dietary proportion
of other perennial grass was reduced by
brome suppression in May and August but
not June or July (Table 1). With brome
suppression, much of the dietary annual
grasses were replaced by western wheat-
grass even though brome suppression did
not affect the western wheatgrass +
Sandberg bluegrass group, the dominant
component of steer diets (57%). There was
also some replacement by forbs and blue
grama at various times. Specific replace-
ment depended upon year and month.
Neither diet CP concentration (13.3%)
nor IVDOM (68.5%) was affected by
brome suppression. Moreover no interac-
tions with treatment affecting diet quality
were significant.
Steer Gains
Initial weight of steers averaged 329 kg
(SD = 31) in 1993, 273 kg (SD = 14) in
1994 (12-hour with water shrunk weight),
and 272 kg (SD = 21) in 1995. Steer aver-
age daily gains during the May to
September grazing season were increased
(P ≤ 0.05) from 0.92 to 1.04 ± 0.02 kg
head-1day-1, and gains per hectare were
increased (P ≤ 0.05) from 69 to 81 ± 2.8
kg ha-1by brome suppression. The treat-
ment by year interaction was not signifi-
cant for steer gains. Hart et al. (1995)
reported that with put and take stocking,
atrazine tended to increase carrying capaci-
ty and gain ha-1, but not average daily gain
or average returns to land, labor, or man-
agement on blue grama dominated range in
eastern Colorado. They suggested that with
optimum stocking rates, atrazine might
increase returns (heavy stocking early in
the season and lighter stocking later).
Environmental Effects
Crude Protein Yield
Crude protein yields of annual grasses
were greater with brome in the wetter
years of 1993 and 1995 than in 1994 (Fig.
1B). Crude protein yields varied signifi-
cantly among dates within years for west-
ern wheatgrass + Sandberg bluegrass (Fig.
2A), other perennial grasses (data not
shown), standing dead (Fig. 2B), and total
vegetation (Fig. 2C). The dominance of
western wheatgrass in this plant communi-
ty is clearly shown by the similarity of CP
yield between western wheatgrass +
Sandberg bluegrass (Fig. 2A) and total
vegetation (Fig. 2C). Crude protein yield
of standing dead (Fig. 2B) averaged 6.3 kg
Fig. 1. Least-square-means and standard errors for CP yield (kg ha-1) for significant (A) date by
brome suppression (P ≤ 0.01) and (B) year by brome suppression interactions for annual grass-
es (P ≤ 0.01).
Table 1. Least-square-means and SE for date by brome suppression treatment interaction for per-
centage of other perennial grass component in steer diets.
Brome Date
treatment MayJune
- - - - - - - - - - - - - - - - - - - - - - - - - - (%)- - - - - - - - - - - - - - - - - - - - - - - - - -
Undisturbed 17BCa1
12Ca
Suppressed 11Cb
15BCa
SE
JulyAugust
18Ba
24Aa
25Aa
19ABb
1.9
1Comparison of date effect for each brome suppression treatment (within a row) is indicated by uppercase superscripts.
Comparison of treatment effect for each date (within a column) is indicated by lowercase superscripts. Means with simi-
lar superscripts are not significantly different (P ≤ 0.05).

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666JOURNAL OF RANGE MANAGEMENT 54(6), November 2001
ha-1among dates and changed little in
1993, but decreased 44 and 16 kg ha-1with
advancing date in 1994 and 1995. The rel-
atively small CP yield for standing dead in
1993 was partially related to grazing
standing crop to a short stubble in late
summer 1992 prior to applying atrazine. In
contrast ample precipitation produced
large amounts of herbage in 1993 that was
converted to standing dead in 1994. An
intermediate amount of herbage was con-
verted to standing dead in 1995.
Steer Diets
Percentages of plant species groups con-
tained in steer diets varied with date
among year (Fig. 3). Maximum propor-
tions of annual grasses in the diet occurred
in May 1993. Although dietary content of
annual grasses declined (P ≤ 0.01) from
May to June 1993, percentages were simi-
lar among remaining dates in 1993 and all
dates in 1994 and 1995. In addition to
May, distinct year differences were also
noted in July when only 1% annual grass
was present in diets in 1994 compared
with 14% in 1993 and 1995. This is a
reflection of the small amount of annual
grass available in the 1994 standing crop
(< 30 kg ha-1). Cool-season perennial
grasses (western wheatgrass and Sandberg
bluegrass) in the diet increased (P ≤ 0.01)
from May each year, with dietary propor-
tions being similar during the remainder of
the grazing season. The amount of other
perennial grasses in the diet increased (P ≤
0.01) from May 1993, but differences
among dates were not as distinct in 1994
and 1995.
In 1993, CP concentration in diets
declined from May to July but increased in
August to levels similar to June (Fig. 4B).
Crude protein of diets declined from May
to August in 1994 and 1995. Digestibility
decreased with advancing date each year
(Fig. 4A). The year differences, higher
quality in 1993, clearly show the influence
of growing-season precipitation on diet
quality.
Steer Gains
Most of the variation in steer gains
exhibited in Table 2 can be explained by
utilization of shrunk weight on and off
pastures in 1994 and previously described
variation in amount and distribution of
precipitation and the subsequent impact on
the quantity and nutritional quality of for-
age (Haferkamp et al. 2001). Average
daily gains were consistently greater in the
first half than the second half of the graz-
ing season, and due to the shrink, gains
decreased more rapidly in the 1994 season
than in the wetter 1993 and 1995 seasons
(Table 2). May to June daily gains were
least in 1993 when May precipitation was
below average, whereas 1993 late season
gains were highest or equal to gains in
1995 in response to cool temperatures and
above average precipitation. Gain aver-
Fig. 2. Least-square-means and standard errors for CP yield (kg ha-1) for significant year by date
interaction for (A) western wheatgrass + Sandberg bluegrass (P ≤ 0.01), (B) standing dead (P ≤
0.01), and (C) total biomass (P ≤ 0.01). September data for 1993 and 1995 are included for visu-
al comparisons, but these data were not included in statistical analyses.
Fig. 3. Least-square-means for percentage diet composition for significant (P ≤ 0.05) year by date
interactions. September data for 1993 and 1995 are included for visual comparisons, but these
data were not included in statistical analyses.

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667
JOURNAL OF RANGE MANAGEMENT 54(6), November 2001
aged 82 kg ha-1in 1993 and 1995 but was
less (P ≤ 0.01) in 1994 (60 kg ha-1) due to
the truncated grazing season. This differ-
ence (P ≤ 0.01) was shown in the May to
August gain data collected for the 3 years.
May to August gains averaged 70 ± 1.3 kg
ha-1in 1993, 60 ± 1.3 kg ha-1in 1994, and
74 ± 1.3 kg ha-1in 1995. Daily gains were
similar to those reported by Grings et al.
(1996) for steers grazing other pastures at
Fort Keogh during the same time period.
They found steers gained from 1.3 to 1.5
kg head-1day-1during May to September
1993, May to July 1994, and May to
August 1995. In contrast, steers gained 0.6
to 1 kg head-1day-1when grazing
September to October 1993, July to
September 1994, and August to
September 1995.
Conclusions
Findings reported here and in
Haferkamp et al. (2001) show annual
brome suppression with atrazine has the
following effects: decreased standing
crops of annual grasses, total vegetation
and total vegetation plus standing dead;
increased forage nutritive value of western
wheatgrass and Japanese brome; and
decreased proportions of annual grasses
and other perennial grasses in diets.
Interactions with date and year suggest
that these responses will vary with the
changing environmental conditions both
within and among years.
Gains of stocker cattle were increased
about 16% by suppression of annual
brome on Northern Great Plains ranges.
The untreated pastures were uniformly
infested by annual bromes, but standing
crop of annual grasses was small relative
to other years on this site (Haferkamp et al.
2001). Thus, the increase in livestock per-
formance with annual brome suppression
might be greater with larger brome stand-
ing crop or higher composition of annual
bromes, but it might be smaller when
steers graze large pastures with spotty dis-
tribution of annual bromes. The botanical
composition of pastures used for summer
grazing yearling cattle should be evaluated
before devising management strategies to
maximize efficiency of utilization of
Northern Great Plains rangelands.
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Table 2. Least-square-means and SE for significant year by date interaction for steer gains and
average duration of each grazing period.
Year Date and average length of grazing periods
May June
JuneJuly
(25 days) (30 days)
- - - - - - - - - - - - - - - - - - - - - - (kg head-1day-1) - - - - - - - - - - - - - - - - - - - -
19931.41Ac
1.38Aa
1994 2.05Aa
1.17Bb
19951.86Ab
1.19Bb
SE
1Comparison of year effect for each grazing period (within a row) is indicated by upper case superscripts. Comparison
of date effect for each year (within a column) is indicated by lower case superscripts. Means with similar superscripts
are not significantly different (P ≤ 0.05).
July
August
(33.3 days)
August
September
(34.5 days)
0.79Ba
0.27Cb
0.80Ca
0.61Ca
—
0.29Db
0.06
Fig. 4. Least-square-means and standard errors for percentage diet CP concentration (P ≤ 0.01)
and percentage IVDOM (P ≤ 0.01) for significant year by date interaction. September data for
1993 and 1995 are included for visual comparisons, but these data were not included in statistical
analyses.