1. Introduction
Sustainability is often defined as “
meeting society’s present needs without compromising the ability of future generations to meet their own needs” and comprises three interlinked facets: environmental responsibility, economic viability and social acceptability [
1]. In this context, the sustainability of beef production comes under considerable scrutiny. Global food security and environmental issues are significant considerations for governments and policy-makers who are conscious not only of the proportion of their national population that is currently food-insecure, but also of the prediction that the global population will increase to over 9.5 billion people by the year 2050 [
2]. The greatest population increases are predicted to occur in developing regions such as Africa, China and India, and, by 2050, these nations are predicted to enjoy a per capita income similar to that currently seen within Europe and North America [
3]. As incomes increase, so does the demand for high-quality animal proteins such as meat, milk and eggs, thus the Food and Agriculture Organization of the United Nations (FAO) suggests that food requirements will increase by 70% by 2050 [
2]. In the event of considerable population growth, future competition for water, land and energy between livestock production and human activities will increase. The global beef industry will therefore face a significant challenge in fulfilling consumer demand for meat products, using a finite resource base. This issue is not confined to a future scenario—current concern over dwindling natural resources, climate change and the social acceptability of beef production practices leads to debate as to whether the U.S. beef industry should continue to intensify and improve productivity to feed the increasing population, or adopt extensive production systems often perceived by consumers to have a lower environmental impact [
4].
Advances in nutrition, genetics and management have conferred considerable advances in reducing the environmental impact of beef production over time: Capper [
5] demonstrated that compared to beef production systems characteristic of 1977, modern beef production in 2007 used 19% less feed, 12% less water, 33% less land and exhibited a 16% decrease in the carbon footprint per unit of beef. The improvements in efficiency conferred by modern management practices and technology use facilitate the production of economically-affordable beef [
6,
7]. Nonetheless, the social acceptability of specific beef production practices, specifically finishing within feedlots and the use of technology to improve growth rate, may be perceived as undesirable by the consumer due to concerns relating to animal welfare [
8,
9], human health [
10] or environmental sustainability [
11]. Beef produced without the use of growth-enhancing technology (GET; “natural” beef), or finished on a forage-based diet (“grass-fed”) may therefore gain market share [
12]. The aim of this study was to evaluate the comparative environmental impacts (defined as resource use and greenhouse gas (GHG) emissions) of conventional, natural and grass-fed beef production using a deterministic whole system model based on ruminant nutrition and metabolism.
3. Results and Discussion
Productivity is a major driver of environmental impact via the “dilution of maintenance” effect [
5]. This concept is demonstrated by the results of the current study. Animals within the CON system had an average slaughter weight of 569 kg and took a total of 444 d to raise from birth to slaughter; compared to 519 kg slaughter weight per animal after a similar time period (464 d) in the NAT system; and 486 kg after 679 d in the GFD system. As slaughter weight increases, concurrent decreases are exhibited in the number of finished beef animals required to produce a set quantity of beef, and the number of non-productive animals required to maintain the supporting beef population. Thus, the CON system required 7,046 × 10
3 animals in the population to produce 1.0 × 10
9 kg of beef compared to 8,257 × 10
3 animals (a 17.1% increase) and 12,510 × 10
3 animals (a 77.5% increase) in the NAT and GFD systems respectively (
Table 2).
Improvements in growth rate do not necessarily affect the size of the supporting beef population; however, the time elapsing from birth to slaughter has a notable effect upon the total population maintenance nutrient requirement. It is important to note that the growth rates within this study are those predicted by the AMTS Cattle Pro [
14] ration formulation software based on animal characteristics and dietary nutrient supply, and are not representative of any specific farm. Animal productivity varies considerably between and within individual systems, and it could be argued comparisons between individual farms might show differing results than those exhibited in the current study. The average time from birth to slaughter in the GFD system (679 d) is considered to be a conservative estimate as it is at the lower end of the range of finishing ages (671–915 d) quoted during personal communication with a grass-fed beef producer, Joel Salatin, Polyface Farm, Swoope, VA, USA, who is noted for a highly-successful forage-based system.
As shown in
Table 2, reducing slaughter weight and growth rate increases the population nutrient requirement of the CON system (228,651 × 10
6 MJ ME) by 11.5% in the NAT system (254,841 × 10
6 MJ ME) or 54.6% in the GFD system (353,484 × 10
6 MJ ME). The population maintenance nutrient requirement can be considered a proxy for both resource use and GHG emissions [
5], thus, as shown in
Table 2, environmental impact measured as a function of any measured parameter was reduced in the CON system compared to the NAT or GFD system. These results concur with those of a previous study evaluating the ecological impact of beef technology use and production system [
46], which demonstrated considerable decreases in land use and methane emissions, and increased habitat conservation in an intensive system compared to a grass-fed system. Moreover, Pelletier [
47] compared of various beef finishing systems using partial life cycle assessment, concluding that the greatest environmental impact was conferred by extensive grass-finishing systems compared to intensive feedlot-finishing systems; with the lowest impact bestowed by systems with the shortest time interval from birth to slaughter (calf-finished beef production).
Table 2.
Resource inputs, waste output and environmental impact associated with producing 1.0 × 109 kg of beef from a conventional (CON), natural (NAT) or grass-fed (GFD) system a.
Table 2.
Resource inputs, waste output and environmental impact associated with producing 1.0 × 109 kg of beef from a conventional (CON), natural (NAT) or grass-fed (GFD) system a.
System | CON | NAT | GFD |
---|
Animals | | | |
Supporting population b (×103) | 5,539 | 6,265 | 8,482 |
Stockers/Pre-finishing (×103) | 628 | 920 | 1,378 |
Finishing animals (×103) | 2,334 | 2,640 | 3,045 |
Total animals slaughtered c (×103) | 2,756 | 3,117 | 3,580 |
Total population d (×103) | 7,046 | 8,257 | 12,510 |
Nutrition resources | | | |
Population energy requirement e (MJ × 106) | 228,651 | 254,841 | 353,484 |
Feedstuffs (t × 103) | 54,476 | 67,263 | 106,166 |
Land (ha × 103) | 5,457 | 6,678 | 9,868 |
Water (liters × 106) | 485,698 | 572,477 | 1,957,224 |
Fossil fuel energy (MJ × 106) | 8,773 | 10,304 | 12,290 |
Waste output | | | |
Manure (t × 103) | 36,976 | 45,431 | 74,392 |
Nitrogen excretion (t) | 399,789 | 486,683 | 807,759 |
Phosphorus excretion (t) | 37,190 | 46,897 | 76,567 |
Greenhouse gas emissions | | | |
Methane f (t) | 501,593 | 586,729 | 854,561 |
Nitrous oxide g (t) | 7,532 | 9,078 | 13,833 |
Carbon footprint h (t CO2-eq × 103) | 15,989 | 18,772 | 26,785 |
Following established historical trends, the quantity of arable land available per capita is predicated to decrease in accordance with the global population size, reaching a nadir at 0.15 ha/person in 2050 [
48]. This is a consequence of increased demand for land used for non-agricultural purposes (e.g., industry, recreation, urban sprawl) and degradation of existing agriculture land [
49]. Efficient land use is crucial for agricultural sustainability, thus the CON system, which required 5,457 × 10
3 ha of land per 1.0 × 10
9 kg beef, appears to be more sustainable than either the NAT system which required 22.4% more land (6,678 × 10
3 ha of land per 1.0 × 10
9 kg beef) or the GFD system at 80.8% more land to produce the same quantity of beef (9,868 × 10
3 ha of land per 1.0 × 10
9 kg beef;
Table 2). Existing debate as to the validity of using grains or legumes for animal feed that could be otherwise be used for human food [
50,
51] is likely to intensify as the population increases. For example, despite its biological implausibility, a feed efficiency of 30 kg feed to one kg gain has recently been quoted as evidence of the unsustainability of beef production [
52]. Monogastric animals have an improved efficiency of feed conversion into gain compared to ruminants. However, beef production systems that utilize range and pastureland (which is generally unsuitable for human food crop production [
5]) gain a sustainability advantage over monogastric production systems that rely upon human-edible grains and legumes. This is discussed at length by Wilkinson [
53], who redefined the conventional measures of feed efficiency (7.8 kg feed per kg of gain for feedlot-finished beef) to account for the human-edible energy or protein feed inputs compared to the human-edible energy or protein output from the animal production system. Under these constraints, grass-finished beef (termed suckler beef in European systems) had a favorable human edible feed efficiency ratio whether expressed in terms of energy (1.9 MJ/MJ edible energy in animal product) or protein (0.92 kg/kg edible protein in animal product). Wilkinson’s [
53] results appear to imply that grass-fed beef would be environmentally advantageous if competition for feed/food crops is a defining criteria, however, the quantity of land required for differing production systems must be taken into consideration. If the total U.S. beef produced in 2010 (11.8 × 10
9 kg) was produced by a grass-fed system, the increase in land required compared to conventional production would be 52.2 × 10
6 hectares, equivalent to 75% the land area of Texas.
Water use for agriculture is an area of growing concern within many regions and is predicted to be the primary limiting factor affecting agricultural productivity in future [
54] as agricultural requirements conflict with industrial and urban use, and the rate of withdrawal from aquifers exceeds replenishment. Within beef production, water is used within two major sub-systems: the animal sub-system in terms of voluntary water intake, and the cropping sub-system, in which water is used for crop and pastureland irrigation. As with other environmental measures, animal productivity has a considerable effect on water consumption as a maintenance requirement for water may be partitioned out for each individual animal. Thus increased growth rate and slaughter weight in the CON system reduces water consumption to 485,689 × 10
6 liters (CON) compared to a 17.9% increase in the NAT system (572,477 × 10
6 liters per 1.0 × 10
9 kg beef) or a 302% increase in the GFD system (1,957,224 × 10
6 liters per 1.0 × 10
9 kg beef;
Table 2). However, irrigation water is the major contributor to total water consumption, thus the magnitude of the difference in water use between the CON and GFD systems (compared to the proportional differences in other environmental measures) is due to the assumption within the model that 50% of grassland used to finish cattle in the GFD system is irrigated. This is an area of uncertainty compared to the irrigation data for the feed crop (corn, soy, alfalfa) components of the model. USDA irrigation surveys [
22] provide data upon average water use per pastureland unit area and the percentage of pastureland irrigated on a national basis, yet there is no data available as to how much irrigated pastureland is allocated to beef. If we change the original assumption (50% of pastureland used to finish cattle is irrigated) and run the model with 25%, 15% or 5% of land being irrigated, the total quantity of water used by the GFD system declines from 1,957,224 × 10
6 liters to 1,044,070 × 10
6 liters (25%), 678,808 × 10
6 liters (15%) or 313,547 × 10
6 liters (5%). Thus, the model is sensitive to irrigation water use to the extent that if greater than 9.7% of land used to finish beef is irrigated (while holding irrigation water use within the CON system constant), the GFD system is less environmentally-desirable than the CON system.
Nutrient (N and P) excretion was primarily affected by animal productivity (
Table 2), with minor effects of nutrient supply
vs. requirements. The quantities of N and P excreted from the population per 1.0 × 10
9 kg kg beef were reduced in the CON system (399,789 t N/kg beef and 37.190 t P/kg beef) compared to the NAT system (486,683 t N/kg beef and 46,897 t P/kg beef) or GFD system (807,759 t N/kg beef and 76,567 t P/kg beef). Nutrient run-off into waster courses is a primary concern relating to P excretion, and N excretion is also associated with ammonia emissions to the atmosphere, particularly in confined animal systems. Variation in manure application rate, storage characteristics, climatic conditions and pasture-based/housed animal management will have a considerable effect upon both nutrient run-off [
55] and ammonia emissions [
56]. It should therefore be noted that neither P nor N excretion provides a direct measure of nutrient run-off or ammonia emissions, but simply act as a comparative measure for the potential for run-off or gaseous emissions to occur.
The carbon footprint (expressed as total GHG emissions in CO
2-equivalents per unit of beef) of livestock production systems is one of the most debated issues relating to environmental impact. Previous research has demonstrated that improving productivity demonstrably reduces the carbon footprint of beef production [
5,
47,
57,
58,
59,
60], which concurs with the results revealed by the 17.4% increase in NAT system carbon emissions (18,772 t CO
2-eq per 1.0 × 10
9 kg beef) compared to the CON system (15,989 t × 10
3 CO
2-eq per 1.0 × 10
9 kg beef;
Table 2) within the current study. Nonetheless, the perception remains that extensive, grass-based systems have a lower carbon footprint than intensive, confined systems. This is exemplified by a report from the Environmental Working Group [
61] that states “Meat, eggs and dairy products that are certified organic, humane and/or grass-fed are generally the least environmentally damaging.” Within the current study, the GFD system had a carbon footprint of 26,785 t CO
2-eq per 1.0 × 10
9 kg beef, which, is an increase of 67.5% compared to the CON system and would be equivalent to adding 25.2 × 10
6 US cars to the road on an annual basis based on average mileages and carbon emissions per mid-sized automobile from US EPA [
24]. The increase in carbon emissions was primarily affected by the increase in population size and time elapsed from birth to slaughter in the GFD population, however, provision of a forage-based diet also increased daily methane emissions per animal as noted by Johnson and Johnson [
62] and Pinares-Patiño
et al. [
63].
The potential for carbon sequestration by well-managed pastureland may be a mitigating factor for carbon emissions within the GFD system, yet it was not accounted for throughout the current study due to a lack of sustentative data. Although the GFD system is forage-based throughout, the cow-calf and stocker sub-systems within the CON and NAT production systems were also forage-based. In the absence of significant differences in land conversion or management in these sub-systems, potential for carbon sequestration could therefore only be considered to be a mitigating factor within the grass-finishing system compared to the feedlot-finishing sub-system. Partitioning out the carbon emissions from sub-systems reveals that the grass-finishing sub-system accounted for 6,868 t × 10
3 CO
2-eq per 1.0 × 10
9 kg beef. With a total land use of 1,392 × 10
3 ha in the grass-finishing sub-system and assuming carbon equilibrium for land used by the feedlot-finishing system, the pastureland used to finish cattle in the GFD system would need to sequester 4.93 t CO
2 per ha/yr, equivalent to 1.35 t C per ha/yr, in order to produce a finishing sub-system with a similar carbon footprint to that of the CON system. This appears to be a lofty target, given that Bruce
et al. [
64] suggest that the potential for carbon sequestration in well-managed pastureland is 200 kg/ha, whereas Conant
et al. [
65] report 540 kg/ha. Moreover, this does not take into consideration the increased land use and carbon emissions from cow-calf and stocker populations in the GFD compared to the CON system. As cow-calf and stocker operations tend to be located on unimproved rangeland or forage crops that do not achieve significant carbon sequestration [
64], the estimate of the amount of carbon needed for the GFD system to reach equal carbon emissions per unit of beef should be regarded as a considerable underestimate. Well-managed rotational grazing systems within the cow-calf operation would lessen the impact of the cow-calf sub-system on total carbon emissions per unit of beef, however, this mitigation is not confined to GFD systems and could equally be practiced within the CON or NAT systems.
Feed and animal transportation are often considered to be a major factor affecting fossil fuel use in CON or NAT beef production systems, yet within the current study transport accounted for 0.87% of the carbon footprint from the CON system, 0.83% of the NAT system’s carbon emissions and 0.24% of total carbon emitted from the GFD system, a result which is in agreement with the results published by Capper [
5]. The increased contribution of transportation to the CON and NAT systems’ carbon footprints resulted from the greater reliance upon feeds imported into the feedlot system, compared to increased proportional contributions of CH
4 emissions in the GFD system. Fossil fuel use within the three systems followed a similar pattern to the previously discussed resources, with CON system using less fossil fuel energy per 1.0 × 10
9 kg beef (8,773 × 10
6 MJ) compared to the NAT (10,304 × 10
6 MJ, an increase of 17.5%) or GFD (12,290 × 10
6 MJ, an increase of 40%) systems. This is contrary to the popular belief that lesser fossil fuel use is a major environmental advantage of extensive beef production systems. Within the current study, cropping and harvesting practices are the major contributors to fossil fuel use: decreases in total feed use and therefore cropping inputs and feed transportation resulting from improved animal productivity are demonstrated by the difference in fossil fuel energy between the CON and NAT systems. The greater use of fossil fuel energy in the GFD system results from cropping and harvesting practices for conserved forages to support animals during winter months.