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The Role of Productivity in
Improving the Environmental
Sustainability of Ruminant
Production Systems
Judith L. Capper
1
and Dale E. Bauman
2
1
Department of Animal Sciences, Washington State University, Pullman, Washington
99164; email: capper@wsu.edu
2
Department of Animal Science, Cornell University, Ithaca, New York 14853; email:
deb6@cornell.edu
Annu. Rev. Anim. Biosci. 2013. 1:9.1–9.21
The Annual Review of Animal Biosciences is online
at animal.annualreviews.org
This article’s doi:
10.1146/annurev-animal-031412-103727
Copyright ©2013 by Annual Reviews.
All rights reserved
Keywords
greenhouse gas emissions, dilution of maintenance, carbon footprint,
animal source foods, dairy, beef
Abstract
The global livestock industry is charged with providing sufficient
animal source foods to supply the global population while improving
the environmental sustainability of animal production. Improved pro-
ductivity within dairy and beef systems has demonstrably reduced re-
source use and greenhouse gas emissions per unit of food over the past
century through the dilution of maintenance effect. Further environ-
mental mitigation effects have been gained through the current use
of technologies and practices that enhance milk yield or growth in
ruminants; however, the social acceptability of continued intensifica-
tion and use of productivity-enhancing technologies is subject to de-
bate. As the environmental impact of food production continues to
be a significant issue for all stakeholders within the field, further re-
search is needed to ensure that comparisons among foods are made
based on both environmental impact and nutritive value to truly as-
sess the sustainability of ruminant products.
9.1
INTRODUCTION
The global agricultural industry faces a challenge in providing safe, affordable, and abundant
food to supply the growing population. According to the World Food Summit, “Food security
exists when all people, at all times, have physical and economic access to sufficient, safe, and
nutritious food to meet their dietary needs and food preferences for an active and healthy life”
(1). According to estimates from the Food and Agricultural Organization (FAO) of the United
Nations, the global population will reach 9.5 billion people by the year 2050, and food pro-
duction will need to increase by 70% to provide worldwide food security (2). Food demand for
livestock products will not rise in a linear fashion—demand is expected to double in sub-Saharan
Africa, South Asia, and other developing regions, which also will see the greatest rise in
population numbers, with relatively little change in developed countries (3). Dwindling supplies
of nonrenewable resources (e.g., fossil fuels, minerals) exacerbate this challenge, which is
complicated further by the burden that an increased population places upon renewable
resources such as land and water. Available cropland per person is predicted to decline by 25%
between 2010 and 2050 (4); thus, there is a clear need to improve yields per unit area of land as
well as total food production. However, a desire to improve agricultural sustainability runs in
tandem with the need to produce greater amounts of food per unit area, and these two needs
often are considered to conflict.
The Brundtland Report provides what is arguably the most widely used definition of sus-
tainable development, which is that it “meets the needs of the present without compromising the
ability of future generations to meet their own needs”(5, p. 5). This can be further dissected into
three components: environmental stewardship, economic viability, and social responsibility (6).
For a system or industry to be sustainable, all three components must balance—if one factor is
misaligned, the system cannot achieve long-term sustainability. Animal welfare considerations on
modern dairy farms serve as an example of this balance; optimal animal care represents the
producer’s social responsibility and is linked intrinsically to system productivity. If the animal’s
well-being and health are compromised, milk production and the efficiency of resource use
declines, which thereby has adverse effects on both the economic viability and environmental
stewardship of the dairy operation.
The environmental impact of food production has focused, to date, on greenhouse gas
(GHG) emissions, also referred to as the carbon footprint. A complete analysis, however, also
would need to consider water use as well as land- and air-quality aspects. There is no doubt that
GHG emissions are of vital importance, especially in light of food stakeholder concerns relating
to climate change, but it should be remembered that water use is likely to be the major limiting
factor, and thus a critical environmental metric in the near future. Nevertheless, the science
relating to GHG emissions is more developed at present than estimates of water or other
resources per unit of food; therefore, GHG emissions can be used as a proxy for environmental
impact. Global GHG emissions from all agriculture were estimated by Bellarby et al. (7) to
account for between 17% and 32% of all human-induced emissions, and the Environmental
Protection Agency (EPA) (8) estimated that agriculture contributes approximately 6% of the
total US carbon footprint, with animal agriculture accounting for half of the agricultural
component. A recent report by the FAO (9) concluded that animal agriculture was reported
to account for 18% of GHG emissions, and these conclusions have been adopted eagerly by
activist groups as evidence for the benefits of a vegetarian or vegan lifestyle (10, 11).
The methodology and results of the FAO reporthavebeendiscussedatlength(12,13).
Pitesky et al. (12) credited the FAO report with sound recommendations as to the importance
of improving productivity and efficiency to reduce future GHG emissions, but they documented
9.2 Capper !Bauman
discrepancies in methodology between analyses of animal agriculture and other sources of GHG
emissions that led to the marked overestimation of the global contribution made by animal
agriculture. Nonetheless, Pelletier & Tyedmers (14) suggest that animal agriculture accounted
for 14% of global GHG emissions in the year 2000, and given that a subsequent FAO report
calculated that global dairy production contributes 4% to global anthropogenic GHG emissions
(15), it is clear that animal agriculture makes a significant contribution. Given current concern
over this issue, and assuming that consumer demand for ruminant animal proteins will remain
high, this paper discusses the options available for improving the environmental sustainability of
ruminant production.
EVALUATING THE ENVIRONMENTAL SUSTAINABILITY OF RUMINANT
PRODUCTION
Choosing an Environmental Sustainability Metric
Ruminant production systems are inherently variable across regions and climates, with consid-
erable diversity conferred by management systems and production practices. For example, the
majority of beef production systems in the United States are characterized by an industry com-
prising seed stock producers, cow-calf operators, backgrounders, and feedlots, with a minority of
calf inputs from the dairy industry and considerable technological inputs. By contrast, European
beef production systems tend to employ less-intensive finishing systems, with a greater reliance on
crossbred calves from the dairy industry. To attempt to determine the environmental impact of any
one of the myriad individual subsystems that comprise beef production in the United States or
Europe would render interregional comparisons meaningless. But all of these industries exist to
supply animal protein for human consumption, with associated by-products including hides,
fertilizers, and pharmaceuticals. Whole-system analyses therefore have evolved to encompass all
relevant resource inputs and waste outputs within the defined boundaries of a production system
and thus facilitate comparisons between regions and production systems.
Early environmental analyses used limiting resource availability as the denominator, for ex-
ample using carbon emissions per unit of land to compare the environmental impact of conven-
tional versus organic dairy production (16). However, such denominators again lead to difficulties
in regional comparisons and do not consider externalities—it is not obvious whether a unit of land
used for dairy production in the Netherlands has food production potential equivalent to that of
a unit of land in the Midwestern United States or arid regions of Australia. The accepted functional
unit for environmental comparisons among ruminant systems therefore has become the unit of
food (kg, t); for ruminant production this translates to mass of milk for dairy or meat for beef and
lamb. Given the wide variation in milk composition, as discussed by Bertrand & Barnett (17), this
has been further refined to units of fat and protein-corrected (or energy-corrected) milk when
assessing dairy production’s environmental impact. This corrects for production-system variation,
but it still may lead to confusion when consumers concerned about the environmental impact of
their food choices choose foods based on carbon emissions without due regard for differences in
nutrient composition.
The Relationship Between Ruminant Productivity and Environmental Impact
Resource use and GHG emissions decline with increased productivity and feed efficiency—
ruminant production systems tend to have a greater environmental impact than the vertically
integrated, monogastric production systems. Nonetheless, the US dairy and beef industries have
9.3www.annualreviews.org !Ruminant Environmental Sustainability
made significant productivity gains over the past century in tandem with increasing intensifica-
tion of ruminant production systems. The US dairy industry is considered to have originated with
the importation of European cattle to the Jamestown settlement in 1611, and the earliest recorded
US milk production data relate to a Jersey cow that produced 232 kg of milk in a 350-day lactation
in 1854 (18). Annual average milk yield per cow has increased from 1,890 kg in 1924, when USDA
dairy production record keeping began, to 9,682 kg in 2011 (19), and the current record-holding
cow (named Ever-Green-View My 1326) produced over 32,800 kg.
The introduction of genetic selection tools and the potential to produce a large number of
high–genetic merit offspring via artificial insemination have allowed dairy producers to make
informed decisions as to selection criteria when breeding replacement cattle. The greatest
progress since the early 1980s has been made in yield traits, with genetic improvement ac-
counting for approximately 55% of phenotypic gains (20). However, a negative trade-off also
has occurred between milk yield and fertility, with a 6% decline in pregnancy rate since 1980
(equivalent to a 24-d increase in calving interval), of which approximately one-third can be
attributed to genetics. VandeHaar & St-Pierre (21) noted that, although improved genetics have
played a significant role in these productivity gains, advances in nutrition and management
practices have been essential to allow dairy cattle to reach their genetic potential for milk yield.
Over the past century, the US dairy industry has shifted from extensive production systems based
entirely on forage to intensive systems with diets still founded on forage but formulated with feed
components to optimize rumen fermentation and meet the dairy cow’s nutrient requirements.
This has allowed the use of a variety of feeds, including by-products from the human food and
fiber industry, which has resulted in a total mixed ration approach that improves the efficiency of
rumen fermentation and digestibility, more adequately meets the cow’s nutrient requirements,
and allows for greater performance (21, 22). In combination with improved knowledge of dairy
cattle health and welfare, modern management practices have demonstrably improved pro-
ductivity within the dairy industry.
Productivity advances also have occurred within the US beef industry, with a regressed an-
nual increase in beef yield per animal of 2.09 kg between 1930 (average yield 239 kg/animal) and
2010 (average yield 350 kg/animal) (19). The US beef industry has followed similar trends in
improving productivity to those of the dairy industry; however, owing to regional and climatic
variation within cow-calf and backgrounder operations, and to the nutritional, labor, and
economic limitations imposed by forage-based diets within these systems, the greatest rate of
progress in terms of growth rates, technology adoption, and feed efficiency has been made within
finishing operations (23). Until the 1950s, the majority of beef consumed in the United States was
produced in pasture-based systems; however, the advent of finishing diets formulated to meet
cattle requirements and containing a significant proportion of corn and by-product feeds, with
consequent improvements in growth rate, intramuscular fat, and grading score, encouraged
producers to move toward a more intensive system (24).
The Dilution of Maintenance Concept
Every animal requires a certain amount of nutrients on a daily basis to support vital functions,
health, and minimum activities (the maintenance requirement) plus an additional nutrient re-
quirement to support production, i.e., growth, pregnancy, or lactation. The maintenance re-
quirement is dependent principally upon bodyweight and thus does not change as a function of
production (25). As this daily nutrient requirement is applicable to every animal, the maintenance
requirement of the livestock population therefore may be considered as a fixed cost of animal
production. When applied to the livestock industry, improving productivity of an individual
9.4 Capper !Bauman
animal such that a greater quantity of milk, meat, or eggs is produced in a set period of time, the
total maintenance cost per unit of food produced is reduced. Thus, the dilution of maintenance
concept is central to improvements in economic viability and environmental sustainability. The
latter is especially noteworthy because maintenance represents a cost in terms of both resource
use (including feed, land, water, and fossil fuels) and waste output (e.g., manure and GHG). An
example of the dilution of maintenance concept is illustrated in Figure 1, in which nutrient
requirements are compared for cows at two different levels of production. In 1944, when milk
yield averaged 7 kg/d, maintenance represented 69% of the metabolizable energy requirement.
In contrast, the maintenance requirement was diluted out over more units of production and
represented 37% of metabolizable energy for cows averaging 29 kg/d milk in 2007. Improving
productivity consequently reduces resource use and waste output per unit of dairy. This rel-
atively simple mechanism has been the foundation for improvements in the environmental
sustainability of the US ruminant livestock industries over the past century (26–28).
RUMINANT LIVESTOCK’S ENVIRONMENTAL IMPACT: THE US
PERSPECTIVE
Dilution of Maintenance and the Environmental Sustainability of US Dairy
Production
Within dairy production, improving milk yield per cow is the most widely understood pro-
ductivity measure. The environmental impact of increased milk yield may be exemplified by
comparing resource use and GHG emissions from US dairy production in 1944 and 2007, as
described by Capperet al. (26). In 1944, the average US dairy farm had six cows fed a primarily
pasture-based diet with occasional corn or soy supplementation. Neither antibiotics nor arti-
ficial hormones were available for animal use, and animal manures provided fertilizer. In
contrast to highly mechanized modern dairy systems, 1944 farms averaged only 1.2 tractors—
69% 37%
31%
63%
0
50
100
150
200
250
1944 2007
Lactation
Maintenance
Average cow's daily energy requirement (MJ ME)
7.1 MJ ME/kg milk
15.3 MJ ME/kg milk
Year
Figure 1
The impact of increasing daily milk yield on the proportion of daily energy used for maintenance versus
lactation in dairy cows from 1944 compared with 2007—the dilution of maintenance effect. Data adapted from
Capper et al. (26). Abbreviations: ME, metabolizable energy; MJ, megajoules.
9.5www.annualreviews.org !Ruminant Environmental Sustainability
the majority of agronomical operations were achieved through draft horse work. This his-
torical system, with a national herd comprising 54% small breeds (Jersey, Guernsey, Ayrshire)
and 46% large breeds (Holstein, Brown Swiss) and an average milk yield of 2,074 kg/year, is
substantially different from modern dairying. As a result of the previously discussed gains in
nutrition, genetics, and management, the average dairy cow produced 9,193 kg milk/year in
2007 and the national herd contained more than 90% Holstein cattle. As described previously,
increasing milk yield per cow between 1944 and 2007 diluted out the maintenance cost and
reduced the energy required per kg of milk from 15.3 MJ to 7.1 MJ (Figure 1). As milk yield
increased, fewer lactating cows were required to produce a set amount of milk, and the number
of associated support animals (dry cows, replacement heifers, bulls) in the population de-
creased; thus, the total population maintenance requirement was reduced. The 2007 dairy
industry therefore produced 84.2 billion kg of milk with a national herd containing only 9.2
million dairy cattle (and 89.0 billion kg of milk from the same number of cattle in 2011)
compared with 53.0 billion kg of milk from 25.6 million head in 1944. In combination with
advances in crop productivity over this time period, feed use per unit of milk was reduced by
77%, land use by 90%, water use by 65%, and manure production by 76% (Figure 2). The
carbon footprint of a kg of milk in 2007 was 63% lower than that in 1944 (1.35 kg CO
2
-eq
compared to 3.66 kg CO
2
-eq), and the total dairy industry carbon footprint (with the boundary
of the farm gate) was reduced by 41%, despite the substantial increase in milk production (26).
Mature cow bodyweight has increased concurrently with productivity gains over the past
century; thus, although the environmental impact of a unit of milk has been reduced, daily re-
source use and GHG emissions per animal have increased. This may lead to future legislative
complications if environmental assessments are based upon the number of livestock units per
operation without consideration of productivity. Although the previous historical example
demonstrates the environmental advantages of reducing the number of animals required to
21% 23%
35%
10%
24%
37%
59%
0
10
20
30
40
50
60
Animals* Feed* Water* Land* Manure* GHG* Industry GHG
2007 resource use and carbon emissions
as proportion of 1944 values (%)
*Per unit of milk
Figure 2
Resource use and waste outputs from modern US dairy production systems typical of the year 2007,
compared with historical US dairying (characteristic of the year 1944). Data adapted from Capper et al. (26).
Abbreviations: GHG, greenhouse gas.
9.6 Capper !Bauman
produce a set quantity of milk, the population maintenance requirement is determined by the
combination of animal numbers and animal bodyweight. In a comparison of the environmental
impact of cheese production from Jersey and Holstein milk, Capper & Cady (29) demonstrated
that land use, water use, and GHG emissions were reduced by 32%, 11%, and 20% respectively
through the use of Jersey cattle. In this instance, the environmental gains were conferred by
a combination of factors including an increase in milk fat and protein content combined with
decreases in bodyweight and milk yield for Jersey cattle. Nonetheless, when breed-specific traits
were examined in isolation, the difference in bodyweight between Jersey (454 kg) and Holstein
(680 kg) cattle led to a 74% reduction in population body mass despite a 9% increase in the total
number of cattle required to produce an equivalent amount of cheese. Capper & Cady (29)
found that bodyweight was the most influential factor affecting environmental impact, with
milk composition (and thus cheese yield per unit of milk) and milk yield following closely behind
but with little effect of age at first calving, culling rate, or calving interval. These results were
echoed by Bell et al. (30), who reported that changing energy-corrected milk (ECM) yield by one
standard deviation conferred a 14.1% decrease in the carbon footprint per unit of ECM
compared with feed efficiency, calving interval, or culling rate (6.0%, 0.40%, and 0.14%
decreases, respectively).
Improving management practices that have a positive effect on dairy cow productivity, for
example reducing calf mortality, improving heifer growth, or decreasing the incidence of
mastitis, also will reduce the environmental impact of dairy production. Garnsworthy (31)
reported that methane emissions could be reduced by up to 24% by improving fertility within the
UK dairy herd and thus reducing the number of heifers required to maintain milk production.
Sexed semen also has been suggested as a mechanism to improve productivity by ensuring that
heifers are born to superior animals, which thus improves the potential genetic merit and
productivity of the herd (32). Nonetheless, if Y-sorted semen were not equally available, this
potentially would affect the efficiency of beef production if a greater number of dairy heifer
calves were diverted into beef production. Further research is required on the comparative and
interactive effects of herd productivity metrics to identify the management practices that have
the greatest impact on environmental metrics within systems.
Between-system comparisons of US dairy production’s resource use and GHG emissions
again demonstrate the importance of improved productivity. A recent study from the Organic
Center (33) concluded that organic dairy production had considerable environmental advantages
over conventional systems but made the assumption that productivity was equivalent in both
systems. Consumers appear to consider that organic production systems have a lesser environ-
mental impact than their conventional counterparts (34), but the majority of studies comparing
yields in organic and conventional production reveal that yields are substantially lower in organic
systems both in the United States (35–37) and overseas (16, 38). The nutrient costs for maintenance
in an organic system are approximately 20% greater owing to increased nutrients required to
support the physical act of grazing (25), and milk yield averages 20% less across studies (39).
When the increased nutrient use for maintenance and milk-yield reductions are included in the
analysis, the increase in GHG emissions per unit of milk conferred by organic production is
approximately 13% (39).
Dilution of Maintenance and the Environmental Sustainability of US Beef
Production
In contrast to dairy production, in which lactation length has a relatively narrow range (generally
∼300–420 days) and thus the primary dilution of maintenance effect is conferred during lactation,
9.7www.annualreviews.org !Ruminant Environmental Sustainability
both beef yield and growth rate have a significant effect upon the environmental impact of beef
production. If we compare productivity gains made in conventional beef production systems
between 1977 and 2007, average slaughter weight increased over this time period from 274 kg
to 351 kg, thus reducing the number of slaughter animals and the size of the national herd
required to meet beef meat demand (27). Average growth rate was increased from 0.71 kg/d to
1.16 kg/d between 1977 and 2007, which reduced both the proportion of total energy use
apportioned to maintenance from 53% (1977) to 45% ( 2007; Figure 3) and the average number
of days required to reach slaughter weight from 609 to 485. As shown in Figure 4,thereduction
in total maintenance requirements conferred by the combination of the reduced beef population
and the lesser number of days for animals to reach slaughter weight reduced feed use by 19%,
land use by 33%, water use by 12%, fossil fuel use by 9%, and the carbon footprint per kg of
beef by 16% (27).
Yield gains within the US beef industry have been significant over the past century, but anec-
dotal evidence from the processing and retail industry suggests that slaughter weights have reached
a plateau. Consumer demand for portion sizes greater than those conferred by the current average
slaughter weight of 582 kg (40) is unlikely to increase, and the current processing infrastructure is
ill-equipped to deal with larger carcass sizes without considerable reorganization. This is un-
fortunate, because the negative correlation between beef yield and population size required to
maintain beef production means that yield gains have a substantial effect on total environmental
impact, particularly as the supporting population (i.e., cow-calf herd) contributes the greatest
proportion of GHG emissions per unit of beef (41) and is less susceptible to dietary modification of
enteric methane emissions than cattle in other sectors owing to their forage-based diets (42).
Within an efficient cow-calf system, calves should be weaned at approximately half the dam’s
mature weight, but given the nutritional limitations of pasture-based diets, this is difficult to
53% 45%
47%
55%
0
10
20
30
40
50
60
70
80
90
100
1977 2007
Growth
Maintenance
Daily energy requirement (MJ ME)
609 days birth–slaughter
128 MJ/kg beef
485 days birth–slaughter
113 MJ/kg beef
Year
Figure 3
The impact of the increasing growth rate of beef cattle on the proportion of daily energy used for
maintenance versus growth in 1977 compared with 2007—the dilution of maintenance effect. Data adapted
from Capper (27). Abbreviations: ME, metabolizable energy; MJ, megajoules.
9.8 Capper !Bauman
achieve when mature cows approach or exceed 635 kg bodyweight. Although Notter et al. (43)
reported some advantages of increased mature weight upon cow-calf system efficiency, if calf
growth rates and weaning weights can be maintained from cattle with a mature weight of less than
580 kg, environmental sustainability will be improved. Furthermore, according to US Department
of Agriculture data (44), only 89.1% of cattle produce a live calf each year in US systems; if this
were increased substantially, the consequent reduction in population size required to support beef
production would be expected to have a considerable effect upon beef’s environmental impact.
Conventional production systems contribute approximately 97% of total US domestic beef
supply and are characterized by forage-based cow-calf and backgrounder operations followed
by an intensive finishing period on corn-based diets (45). Given the reliance of these systems on
fossil fuel and fertilizer inputs for feed production and transportation, they may appear to have
an intrinsically greater environmental impact than pasture-based systems. Indeed, Sithyphone
et al. (46) conclude that increased GHG emissions per animal confer an environmental dis-
advantage for corn-finished beef cattle compared with their hay-fed compatriots in Japanese
systems. However, as discussed previously, assessing environmental impact on a per-head basis
fails to account for the productivity of the system; in this instance, the slaughter weight of corn-
fed cattle was almost double that of hay-fed cattle, which would have a considerable effect upon
GHG emissions per unit of beef.
Both Capper (45) and Pelletier et al. (41) reported that GHG emissions per unit of beef meat
were greater in pasture-finished systems than in feedlot systems. Pelletier et al. (41) cite GHG
emissions of 19.2 kg CO
2
-eq/kg liveweight for pasture-finished beef compared with 16.2 kg CO
2
-
eq/kg liveweight for yearling-fed beef, whereas Capper (45) reports 26.8 kg CO
2
-eq/kg hot carcass
weight beef for pasture-finished beef and 16.0 kg CO
2
-eq/kg hot carcass weight beef for feedlot
beef. In both studies, the increased GHG emissions associated with pasture-finished beef were
a direct consequence of slower growth rates and lower slaughter weights. Pelletier et al. (41) further
showed that calf-fed beef, i.e., a system in which calves bypass the backgrounder stage and enter
70%
81%
88%
66%
80%
84%
0
10
20
30
40
50
60
70
80
90
100
Animals Feed Water Land Manure GHG
2007 resource use and carbon emissions as
proportion of 1977 values (%)
Figure 4
Comparative resource use and waste outputs per unit of beef produced in US systems characteristic of 1977
compared with 2007. Data adapted from Capper (27). Abbreviations: GHG, greenhouse gas.
9.9www.annualreviews.org !Ruminant Environmental Sustainability
the feedlot directly as weaned calves, had the lowest GHG emissions (14.8 CO
2
-eq/kg liveweight
beef) as a consequence of an increased growth rate and thus fewer days to reach slaughter weight
compared with yearling-fed cattle. Although pasture-based beef production systems predomi-
nated in the first half of the twentieth century within the United States, the increased land required
to supply sufficient beef meat for the current domestic and international market would render
whole-scale conversion of the US beef production system to pasture-fed production practically
impossible. However, if conversion did occur and annual beef production was maintained at 11.8
billion kg (40), the increase in carbon emissions would be equal to adding 25.2 million cars to the
road on an annual basis (45).
Pasture-based beef finishing systems have the potential to reduce net GHG emissions through
carbon sequestration. Pasture does not, however, sequester carbon indefinitely, nor does se-
questration occur at a constant rate (47, 48). Over time, soil carbon concentrations reach an
equilibrium beyond which no further sequestration occurs unless land is subjected to significant
management change (47,48). The present body of literature indicates that the degree to which
carbon may be sequestered by crop- or pastureland is infinitely variable among systems and is
dependent on a multitude of factors including land-use change, tillage, organic matter input, soil
type, and crop or pasture species (49, 50). Reliable data on carbon sequestration under a range
of environmental conditions and global regions are notably lacking from environmental lit-
erature, and this is one area where future research would pay dividends in bridging the current
knowledge gap.
The reliance of both conventional and pasture-based US beef systems upon pasture- and
forage-based diets for animals within the cow-calf and backgrounder systems means that any
potential mitigating effect of carbon sequestration could be attributed only to differences in the
finishing period. Pelletier et al. (41) calculated that sequestration at a rate of 0.4 tons C/ha/year
would reduce the GHG emissions from pasture-based beef production to 11.2 kg CO
2
-eq/kg
liveweight. However, when differences in dressing percentage and thus beef yield per animal
between pasture-finished and feedlot beef are considered, this would underestimate GHG
emissions per unit of beef produced. Owing to greater GHG emissions from pasture-finished
beef cattle in the study by Capper (45), sequestration would need to exceed 1.3 tons C/ha/year to
produce pasture-finished beef with GHG emissions similar to feedlot beef. Either estimate is
a lofty target, given that Bruce et al. (51) suggest that the potential for carbon sequestration in
well-managed pastureland is 200 kg/ha, whereas Conant et al. (52) estimate the potential for
sequestration at 540 kg/ha. Furthermore, complete comparisons of environmental effects need
to be based on an equivalent production of meat and account for the increased requirement for
land and feed, the greater supporting herd population (cows, calves, heifers, and bulls) conferred
by slower growth rates and reduced slaughter weights in the pasture-finished system, and the
propensity for pasture-based diets to increase ruminal methanogenesis and thus enteric GHG
emissions (53, 54). However, this is still complicated by the fact that approximately 24% of beef
production results from dairy systems in the United States (either through the provision of cull
cows or dairy calves raised for beef), thus a whole-system large ruminant analysis may be
required to properly assess the environmental impact of beef and dairy production in future.
RUMINANT LIVESTOCK’S ENVIRONMENTAL IMPACT: THE GLOBAL
PERSPECTIVE
Worldwide, ruminant livestock make a significant contribution to sustainability through pro-
vision of nutrition via animal-source foods in combination with fertilizer, draft power, social
standing, and economic income (3, 55, 56). Animal system productivity within developing regions
9.10 Capper !Bauman
generally is diminished owing to a combination of factors including inadequate nutrition (as
a consequence of both feed quality and quantity and lack of nutritionally balanced diets), lack of
access to genetic selection techniques, and increased incidence of livestock disease (3). With
reference to animal health, developed nations have seen a reduction in the burden placed by animal
disease as a result of improved veterinary services and vaccine availability and the implementation
of preventative medicine programs (57), although concern is rising with regard to the emergence
and spread of new diseases such as avian influenza H5N1 and the Schmallenberg virus (58).
Nonetheless, relatively little progress has been made in the developing world save for the erad-
ication of rinderpest (59). The World Organization for Animal Health estimates that worldwide,
more than 20% of animal protein is lost as a result of disease (60); thus, significant potential exists
to reduce the environmental impact of global livestock production through improving health.
Global livestock production has increased substantially over the past 50 years—beef pro-
duction has doubled, poultry meat production has increased almost tenfold, and both dairy and
egg yields have increased by approximately 30% (61). Major international dairy regions (in-
cluding the United States, Canada, New Zealand, and Europe) all have improved milk yield per
cow since the 1960s, the rate of improvement varying from 129 kg/year and 117 kg/year for the
United States and Canada, respectively, to 77 kg/year and 24 kg/year for Europe and New
Zealand (61, 62). A portion of the variation in the rate of improvement may be related to genetic
selection prioriti es, for example the need for smaller-framed, lower-yielding cattle in antipodean
systems compared with the trend for increasing yields in the United States and Canada (63).
Nevertheless, the environmental effects of regional productivity variation are exemplified by the
results of a recent FAO (15) report that modeled GHG emissions from dairy production using
life-cycle analysis. As intensity of production declines and the average milk yield shifts from
approximately 9,000 kg/cow for North America to ∼250 kg/cow for sub-Saharan Africa, the
carbon footprint increases from 1.3 kg CO
2
-eq/kg milk to 7.6 kg CO
2
-eq/kg milk (Figure 5). If
environmental sustainability were the only consideration, the FAO data could provoke the
conclusion that all regions should adopt North American–and Western European–style
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
9,000
10,000
Carbon footprint (kg CO2-eq/kg milk)
Annual average milk yield (kg)
North America
Western Europe
Oceania
Eastern Europe
Russian Federation
Southeast/East Asia
Central/South America
Near East/North Africa
South Asia
Sub-Saharan Africa
World
Figure 5
Average annual milk yield and carbon footprint per unit of energy-corrected milk across global regions. Data
adapted from Food Agric. Organ. (15).
9.11www.annualreviews.org !Ruminant Environmental Sustainability
production systems, or that dairying should be focused in these areas and discouraged in less-
productive regions such as sub-Saharan Africa and South Asia. However, the significant social
(both status and nutritional) and economic value of dairying in less-developed regions must not
be underestimated. The challenge for global dairy production is to improve productivity and
optimize sustainability within each region rather than prescribe one-size-fits-all production
systems or management practices.
At present, no comprehensive analyses have compared the environmental impact of beef
production across global regions. A literature scan reveals a range of GHG emissions attributed
to beef production from 9.90 kg CO
2
-eq/kg carcass weight for intensive production in Australia
(64) to 44.0 kg CO
2
-eq/kg carcass weight for Brazilian production with land-use change and
deforestation taken into account (65). Biologically, the dilution of maintenance concept would
suggest that improving productivity through intensification (as discussed previously with
reference to US systems) should reduce environmental impact on a global basis, but a pasture-
based Australian system (64) is cited as having GHG emissions per unit of beef considerably
lower than those exhibited by a range of intensive systems (41, 45, 66–69). In this instance,
a considerable amount of the variation in GHG emissions may simply be attributed to dif-
ferences in methodology and system boundaries. For example, Ogino et al. (67) examined the
environmental impact of the finishing system alone; Pelletier et al. (41) assumed that identical
cow-calf systems were used to supply calves to both calf-fed, yearling-fed, and pasture-based
finishing systems; and Capper (45) included the effect of production-enhancing technology in
the analysis. As discussed by Bertrand & Barnett (70) with reference to dairy production,
a definitive methodology for comparing the environmental impacts of beef production systems
will be required in future.
FUTURE ON-FARM STRATEGIES TO ENHANCE SUSTAINABILITY OF
RUMINANT PRODUCTION
Future gains in environmental sustainability can be achieved only through continuous im-
provement within all sectors of ruminant production, with specific emphasis placed on im-
proving productivity. Identifying the best management practices that can be applied throughout
the national and global industries, without bias toward specific systems, will be key in en-
couraging producers to maximize efficiency and thus reduce environmental impact and improve
economic viability. The use of production-enhancing technologies (PET) provides a clear op-
portunity to improve environmental sustainability. Examples include technologies that alter
nutrient partitioning toward milk production (lactating animals) or lean muscle accretion
(during growth), improve feed quality, enhance rumen fermentation and diet digestibility, in-
crease reproductive performance, improve herd health programs, and promote animal welfare
(28, 39). However, a dichotomy often exists between consumers’desire for environmentally
sustainable foods (71) and a mistrust of technology use within food production (3).
Recombinant bovine somatotropin (rbST) provides an example of PET use within dairy
production. This technology alters nutrient partitioning, which results in an increase in daily
milk yield of an average of 4.5 kg per cow (39, 72). This increase affects environmental sus-
tainability through the dilution of maintenance concept, the net effect being that rbST use
reduces the amount of land required to produce a unit of milk by 9.2%, water use by 10.4%, and
the carbon footprint by 9.1% (39). On an industry basis, rbST supplementation of 1 million
cows would therefore reduce the dairy industry’s carbon footprint by the annual equivalent of
removing ∼400,000 cars from the road. The mitigating effect of rbST use on environmental
impact has also been noted by other investigators (72–77), including Johnson et al. (78), who
9.12 Capper !Bauman
suggested that large-scale use of rbST would reduce methane emissions by approximately 9%.
Nonetheless, the political and social acceptability of rbST use within dairy production has been
a contentious issue in several countries (79–81).
As discussed previously, opportunities to improve beef system productivity through greater
slaughter weights are limited; however, considerable potential exists to further improve effi-
ciency through improving growth rate, which thereby improves environmental sustainability
(27, 45, 82). Technologies such as hormone implants, ionophores, and beta-agonists may be
used effectively to improve growth rate, but these may be perceived as undesirable by the
consumer owing to concerns relating to animal welfare (83, 84), human health (85), or envi-
ronmental impact (86). Perceived concerns may not follow through into consumer purchasing
behavior. Lusk et al. (87) demonstrated no difference in consumer valuation of beef from
hormone-treated or nontreated animals in the United States, Germany, and the United Kingdom;
however, such concerns are cited often by processors and retailers as rationales for prohibiting
PET use within the supply chain. Capper & Hayes (88) demonstrated that, through an increase
in production costs, the global competitiveness of US beef would be reduced by whole-scale PET
removal, which would increase resource use on an annual basis and cumulatively increase global
GHG emissions through a deficit in US beef production being compensated for by increased
production in less-efficient regions.
Technological approaches to directly reduce GHG emissions from ruminants include the in-
clusion of tannins in the diet to reduce the release of N
2
O from manure (89), nitrification inhibitors
to reduce N losses from pastureland (90), and use of various approaches to reduce rumen methane
production (91). Within the latter category, methane is of particular interest because, depending on
diet, ruminants lose between 2–12% of digestible energy intake as methane (54). On a global basis,
this represents ∼28% of methane production (92). Methane is a normal product of ruminal
fermentation, representing a pathway for the disposal of metabolic hydrogen produced during
microbial metabolism. Therefore, strategies to reduce ruminal methane production center on
reducing the population of microbes involved in methanogenesis or providing alternative sinks for
metabolic hydrogen. These approaches include use of high-grain diets, addition of lipid sup-
plements, chemical antimicrobials (e.g., bromoethanesulfonic acid), ionophores (e.g., monensin),
and immunization against rumen methanogens (91–94). Most of the technologies for enteric
methane abatement have shown somewhat inconsistent results. Although methane production can
be mitigated for short periods, the dynamic nature of the rumen ecology allows for adaptation as
the microbial population reverts to the initial fermentation pattern (93). Furthermore, mitigation
strategies often affect more than just methanogenesis such that rumen fermentation rates, di-
gestibility, and intake are reduced, which thereby adversely affects animal performance (94,95).
Further scientific advances are inevitable within this field, but at present, productivity advances
appear to confer a greater environmental (and economic) gain than specific technologies that
target GHG emissions.
COMPARISON OF ANIMAL SOURCE FOODS WITH PLANT SOURCE FOODS
Human diet quality is assessed by the extent to which it provides the entire compliment of
high-quality protein, energy, minerals, trace metals, and vitamins necessary to meet human
requirements. Consuming a high-quality diet is important for everyone, but children, preg-
nant and lactating women, and the elderly are most vulnerable to nutrient inadequacies. The
value of dairy products and beef meat (referred to as animal source foods) in meeting the food
security and nutritional needs of the global population is well recognized (96–98), and they
are included in dietary recommendations to promote human health and well-being by
9.13www.annualreviews.org !Ruminant Environmental Sustainability
governments and public health organizations around the world. Dairy products and meat are
nutrient-dense foods that represent the predominant, most affordable source for many es-
sential dietary nutrients (99–102). Multidisciplinary studies in developing countries have
established that schoolchildren consuming diets with little or no animal source foods have an
inadequate intake of essential micronutrients that results in negative health outcomes in-
cluding poor growth, suboptimal cognitive performance, neuromuscular deficits, psychiatric
disorders, and mortality (96, 97). Meta-analyses of prospective cohort studies with disease
events and death as the outcomes also provide convincing evidence that consumption of milk
and dairy products is associated with survival benefits in long-term health maintenance and
with the prevention of chronic diseases including diabetes, cardiovascular disease, and many
types of cancer (103–105).
The environmental sustainability of animal agriculture systems is currently under scrutiny
by both impartial scientific associations (9) and agenda-driven activist groups (11, 106). The
consensus opinion is that animal agriculture uses a significant amount of resources and makes
a significant contribution to global carbon emissions (9, 14). The majority of food-related GHG
emissions are generated during farming; thus, there has been interest in using whole-system
approaches to compare the environmental impact of animal source foods in comparison with
plant source foods. The purpose of food consumption is to supply nutrients within a balanced
diet that sustains development, health, and well-being throughout the life cycle. Unfortunately,
comparisons of the environmental sustainability among foods have not used a metric that is
functional from an environmental perspective while recognizing the nutrient value of different
foods. For example, studies comparing animal and plant food sources on the basis of GHG
emissions per unit of mass or food energy (107–111) uniformly concluded that producing plant
source foods, such as cereal grains, rather than animal source foods minimized the carbon
footprint. Similar conclusions were reached by investigators evaluating GHG emissions per unit
of selected macronutrients (e.g., protein) who concluded that the environmental impact of food
production could be mitigated by replacing dairy products and meat with plant source foods
(109, 112–115). However, the latter comparisons did not recognize that animal source proteins
have a high digestibility and a near-ideal balance of essential amino acids, whereas plant proteins
have a lower bioavailability and are typically deficient in one or more essential amino acids (97,
116). Given that food sources also differ in their composition of other essential macro- and
micronutrients, it is critical that these also be considered in comparisons of the environmental
sustainability of alternative food choices.
Nutrient density index, also referred to as nutrient profiling, is a system that allows com-
parison of foods based on their full nutrient content (117, 118). Smedman et al. (119) were the
first to use a nutrient density index in comparisons of the environmental sustainability of different
food sources. They compared the provision of required nutrients (nutrient density, ND) in relatio n
to GHG emissions (climate index, CI) for a variety of beverages, expressed as an index of the two
metrics. As shown in Figure 6, milk had a substantially more favorable NDCI index than other
beverages, including orange juice, soy drink, and oat drink. Animal source foods not only supply
high-quality and readily digested protein and energy but are also a compact and efficient source of
readily available micronutrients. Cle arly future considerations of the environmental sustainability
of food sources needs to utilize an approach similar to that of Smedman et al. (119) so that the
evaluations include a functional metric that is relevant from both a nutritional and an envi-
ronmental perspective.
The suggestion that animal agriculture should be abolished and that the global population
could subsist upon a vegetarian or vegan diet (11, 112, 120) is somewhat myopic. Most com-
parisons are based on substituting plant source foods for animal source foods on the basis of mass
9.14 Capper !Bauman
or energy (calories), and these results are extrapolated to estimates of impact on GHG emissions.
For example, the Environmental Working Group (11) ignored nutritional needs and published
a report suggesting that whole-scale adoption of a vegetarian or vegan diet by the US population
would reduce national GHG emissions by 4.5% annually, but the US EPA (8) allocates only 3.1%
of total GHG to animal agriculture (including all food animals and equines). Less-extreme
reductions in consumption of animal source foods often are proposed as a solution to re-
ducing anthropogenic carbon emissions, although Millward & Garnett (121) note that this poses
significant nutritional challenges and would result in increased fortification of foods to provide
essential nutrients. Nonetheless, the so-called Meatless Mondays campaign was invigorated by
Weber & Matthews (111), who considered only the calories available from differing food choices
and concluded that changing from red meat and dairy to fish, chicken, or vegetable sources for one
day per week’s worth of calories would mitigate GHG to a greater extent than buying all locally
sourced food. However, a simple calculation based on US EPA data for the contribution of red
meat and dairy to national GHG emissions (3.05% in total) demonstrates that if that the entire US
population removed these food sources from their diet for one day per week, it would reduce GHG
emissions by a maximum of 0.44%, without accounting for adequacy of meeting nutritional
requirements or the increased production of alternative food sources. Any attempt to reduce
anthropogenic GHG emissions is laudable; however, it appears that far greater mitigation may be
achieved through changes in other societal and industrial activities than simply by reducing meat
consumption.
Finally, the use of by-products from human feed and fiber production as animal feeds must be
considered as part of any evaluation of the environmental sustainability of animal source foods
(Table 1). Activist groups often cite ruminant production systems as being particularly in-
efficient, as the grains fed to livestock could instead be used to fulfill human energy requirements.
However, these claims are based on the assumption that all livestock feeds are human edible,
whereas Wilkinson (122) demonstrates that a considerable proportion of livestock feeds are
54%
28%
25%
7% 1% 0% 0% 0%
0
10
20
30
40
50
60
Milk Orange
juice
Soy drink Oat drink Red wine Soda Beer Mineral
water
Nutrient density/climate impact index (%)
Figure 6
Comparative nutrient density/climate change indices for a range of beverages. Data from Smedman et al. (119).
9.15www.annualreviews.org !Ruminant Environmental Sustainability
human inedible; thus, the ratio of human-edible energy or protein inputs into animal production
systems compared with human-edible energy or protein in animal source foods are more fa-
vorable than the traditional measures of feed efficiency would suggest. Furthermore, data from
the USDA’s Economic Research Service (123) indicate that only 8% of US grazed land is
sufficiently productive to be classified as cropland pasture and therefore cannot be used for
human food production aside from conversion into animal source foods via grazing. As noted by
Kock & Algeo (23) with reference to beef production, the entire ruminant industry must utilize
its competitive advantage of forage crops and by-product feeds in an arena where competition
between human food and livestock feed is ever increasing.
CONCLUSIONS AND FUTURE DIRECTIONS
The importance of productivity gains as mechanisms to improve the environmental sustain-
ability of ruminant production systems is without question. Advances in genetics, nutrition,
management, preventative medicine, and animal welfare have improved milk yields in dairy
cattle and both slaughter weights and growth rates in beef animals, which has led to reductions
in resource use and GHG emissions per unit of animal source foods. Animal source foods are
a principal source of essential nutrients, and continuous improvement in all sectors of dairy and
beef production will be of crucial importance in future food production systems to meet the dual
challenge of producing sufficient animal source foods to supply the growing population while
reducing environmental impact. Current research has focused on principal productivity metrics,
i.e., milk or meat yields, over time, but a considerable knowledge gap exists as to the contri-
bution made by other on-farm practices and herd dynamics, e.g., the specific effects of improved
health, reproduction, or animal bodyweight upon environmental impact. These knowledge gaps
must be filled for producers to make future management decisions based on economic viability
and environmental sustainability. All stakeholders within food production need to gain a
greater awareness of the multifaceted nature of sustainability to make informed production-
system and dietary choices in future. This will involve assessing food products through a com-
bination of environmental, nutritional, and economic metrics and will offer conventional
livestock producers the opportunity to reclaim the concept of sustainable food production,
which is perceived currently as applying only to niche production systems.
DISCLOSURE STATEMENT
The authors are not aware of any affiliations, memberships, funding, or financial holdings that
might be perceived as affecting the objectivity of this review.
Table 1 By-products from human food and fiber production commonly used
in ruminant diets within US production systems
Almond hulls Dust
Apple pomace Distillers grains (dry or wet)
Bakery products Feather meal
Blood meal Peanut meal
Citrus pulp Potatoes and other vegetables
Corn gluten Soybean meal
Cottonseed meal Wheat pasture and straw
9.16 Capper !Bauman
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