Reducing the meat and livestock industry's environmental footprint.

KEY POINTS

* Grazing is the single most extensive form of land use on the planet and, therefore, livestock producers bear a major responsibility for environmentally sustainable production practices.

* The major environmental challenges facing the meat and livestock industry in Australia are not unique, but they are pressing. They include the need to limit the generation of greenhouse gases, to conserve ecosystems and biodiversity, and to ensure the efficient use of freshwater. These issues will drive change in production systems for the Australian livestock industry.

* Methods to reduce methane production from cattle and sheep are under investigation, although significant challenges remain.

* Grazing systems for both cattle and sheep need to be integrated into landscape systems to sustain biodiversity.

* The Australian livestock industry needs to calculate and environmentally cost the use of freshwater to produce food and fibre. Current methods of calculating water use in livestock production are controversial and provide widely differing estimates.

* The unique and important nutritional properties of red meat should not be lost in the debate about the environmental impact of the meat and livestock industry in Australia. Nor should the environmental costs of alternative food production.

INTRODUCTION

Global production of meat is projected to more than double from 229 million tonnes in 1999/2001 to 465 million tonnes in 2050, but the bulk of this increase is predicted to occur in China, India and Brazil. (1) The increase in demand for meat and animal products is predicted to be even stronger than for other food items, and will be driven by urbanising populations and increases in income. (1) Australia is the second largest exporter of beef and veal in the world, and these exports have continued at a record pace in 2007, despite the stronger Australian dollar and weakening demand in some overseas markets (personal communication, Meat and Livestock Australia (MLA), April 2007). In fact, beef and veal exports are the top export earner from the agricultural sector, ranking sixth in dollar value in commodity exports behind the mining commodities. Wool and dairy products are also in the top 15 export income earners. Therefore, the meat and livestock sector has a primary and growing role not only in the agricultural economy but also in the Australian economy. Meat is also an important determinant of human health and diet. Even after the projected doubling of global demand for livestock products by 2050, per capita consumption levels of meat in developing countries will not be more than half the level of developed countries. (1) The major issue with the increase in world meat consumption is the fact that the livestock sector already places stress on many ecosystems and contributes to global environmental problems. (2) The major environmental challenges facing the meat and livestock industry are those facing many industries in Australia. They are not unique, but they are pressing. The challenges include: climate change and global warming through increases in greenhouse gases, the effect of grazing on ecosystems, preserving biodiversity, the supply and efficient use of freshwater, the competition for land use and negotiation with the community for that use, and most importantly, providing good nutrition to the community, all of which must be achieved while generating an economic return through the supply chain for producers, processors and rural communities. Only by adopting a systems approach (i.e. evaluating the agricultural, environmental, economic and social dimension of any problem) will the meat and livestock industry become more environmentally sensitive, while simultaneously delivering economic profitability and social equity.

LAND USE FOR GRAZING

Currently, grazing and cropland dedicated to the production of feed for livestock is humanity's largest land use occupying more than 3.4 billion hectares or 26% of the ice-free global land surface, making it the single most extensive form of land use on the planet. (1) Growth in the livestock sector has consistently exceeded that of the crop sector. While the total demand for animal products in developing countries is expected to more than double by 2030, the demand for animal products in the industrialised countries like Australia has been growing at low rates, and livestock production in this group of countries is expected to grow only slowly over the projection period. In developing countries, extensive grazing systems have typically increased production by herd expansion rather than by substantial increases in productivity. However, globally the market share from these extensive systems is declining relative to other production systems. Moreover, the availability of rangelands is decreasing, through arable land encroachment, land degradation, conflict and so on. Hence the scope for further increasing herd numbers in these extensive rangeland systems remains limited. The five countries with most land area in grazing systems are Australia (4.4 M [km.sup.2]), China (4.0 M [km.sup.2]), the United States (2.4 M [km.sup.2]), Brazil (1.7 M [km.sup.2]) and Argentina (1.4 M [km.sup.2]). (3) However, if the measure is based on the fraction of total land area each nation uses for grazing, then Mongolia, Botswana and Uruguay lead, with 80%, 76% and 76%, respectively. (3) Countries with the highest stocking rates (based on animal units or AU) are Malaysia (320 AU/[km.sup.2]), India (272 AU/[km.sup.2]), North Korea (213 AU/[km.sup.2]) and Vietnam (184 AU/[km.sup.2]); others are found in central Europe and the Middle East. Countries containing large tracts of dryland grazing systems, such as in Australia, Argentina and the United States, all have relatively low stocking rates. (3) These figures show that Australia has the largest area of land devoted to the meat and livestock industries, grazed at a low stocking rate compared with most world systems and due in large part to the dryland grazing system used for most of that production; a system considered fragile under grazing. Therefore, the meat and livestock industry in Australia is perceived by environmental activists as the single greatest threat to the environment. Opponents of the meat and livestock industry use the Internet (e.g. Beyond Beef http://www.mcspotlight.org/reports/beyond.html) to criticise the industry on its environmental record and propose dietary alternatives to eating meat. In response, the meat and livestock industry must devise and implement strategies that reduce its environmental footprint, and if possible, enhance the environment while at all times being sensitive to urban community expectations as well as those from its rural areas.

GREENHOUSE EMISSIONS AND CLIMATE CHANGE

In Australia, the major environmental issue of current community concern is climate change. The scientific evidence for climate change is now overwhelming: climate change is a serious global threat, and it demands an urgent global response. The evidence about global warming associated with climate change gathered by the Stern Report (4) presented a simple conclusion: the benefits of strong and early action far outweigh the economic costs of not acting. The Stern Report also concluded that climate change will affect the basic elements of life for people around the world--access to water, food production, health and the environment. The meat and livestock industry will not be exempt from either a review of its operations or the impact of climate change on those enterprises. Irrespective of which climate model prediction is used, a rise of average temperature of at least 0.6[degrees]C by 2030 is already predicted for southern Australia, with the increase expected to be less in coastal regions, but more inland. Considering emissions along the entire commodity chain, livestock currently contribute about 18% to the global warming effect made up of about 9% of total carbon-dioxide emissions, but 37% of methane, and 65% of nitrous oxide. (2) Therefore, the meat and livestock industry is a major contributor to global climate change. In Australia, 15.7% of the greenhouse gas inventory arises from agriculture, which is a very large proportion. (5) Agriculture is the most significant contributor of methane (59.5%), with the bulk of this produced from ruminant livestock as a consequence of rumen metabolism. (5) As methane is 23 times more warming than C[O.sub.2], any reductions in methane would consequently have a large proportional impact in reducing the rate of C[O.sub.2] equivalent increase in atmosphere. Australia has been at the forefront of mapping agriculture's contribution and in devising methods to reduce the impact of methane from agriculture. However, methane production is an integral part of the hydrogen economy of the rumen, which drives microbial energy metabolism. Moreover, molecular studies of rumen methane metabolism have indicated a much more complex ecology and speciation than previously realised. (6) So it is unlikely that the greenhouse gas contribution from Australian livestock will be reduced in the near future via reduction of methanogens in the rumen ecology, and more likely is a reduction of total methane production through rapid growth paths as recommended under MSA (Meat Standards Australia) and SMEQ (Sheep Meats Eating Quality) protocols for beef and lamb, respectively.

The rise in mean temperatures in southern and eastern Australia through the early years of this century, has been associated with an observed decrease in rainfall in these temperate zones of Australia. At the same time, rainfall has increased in the pastoral zones of central and north-western Australia (Figure 1).

It is tempting to propose a shift of beef production to these northern western zones, where there are already grasslands and storage of water for irrigation of fodder crops, which will enhance the advantage of potential rainfall increases. Currently, Western Australia has only 2.1 million head of cattle out of the national herd of 27.78 million, that is, less than 8% and in proportion to its relative population base. Most of the herd (71% of the beef and veal production) is located in central and southern-western Queensland and northern and western NSW, where climate models predict that there will be only marginal effects on rainfall. Thus, any changes in rainfall in Queensland and NSW as a result of climate change should not lead to the need for large-scale changes to livestock operations in those zones. While increasing or shifting some beef production to the northwest of Western Australia might seem to be appropriate strategic response to global warming, the issues that lie behind the shift contain the environmental problems inherent in all production zones. Moving north will restrict the breed mix and necessarily increase the proportion of Bos indicus in the national red meat profile, primarily because of the superior adaptation and health under tropical climates of indicus-based breeds. Genetic improvement was the cornerstone of research during the first two Beef Cooperative Research Centres (CRCs), but this centred on taurus-based breeds, particularly Angus. There will be accelerated molecular genetic selection for quality traits in the current CRC for Beef Genetic Technologies, much of which will focus on B. indicus mainly to increase the eating quality of Brahman-type cattle but also, in part, to accommodate the changes in the beef industry with climate change. The rapid increase in the understanding of the genetic make-up of these animals is likely to have a major impact on animal breeding. Genetic improvement could be accelerated by direct identification of genes affecting not only important traits for economic performance or disease resistance, but also for adaptation to adverse environmental conditions, such as climate change and global warming.

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MAINTAINING BIODIVERSITY

Maintaining biodiversity through preservation of wilderness or high-value conservation areas is a priority for government conservation policy, but land use for meat and livestock production should not simply mean habitat destruction. Although agricultural land holds much of the Australia's biodiversity, the relative contribution of agricultural practices to conservation or reduction of biodiversity is little known. Biodiversity conservation will not work without protecting what remains of pristine habitats, but the community also needs to recognise the contribution of agricultural land and management practices. There are different responses in broadly different regions of Australia. The magnitude of grazing pressure and the region of Australia are both significant determinants of preservation of plant biodiversity. For instance, in arid rangeland zones of South Australia, pastoral development had a predominantly negative effect on the abundance of species at the regional level: 16 species were less abundant in paddocks than in lands that had never been developed, and only one species was more abundant. (7) However, localised trends within paddocks were more positive: significantly more species showed trends of increasing abundance with increasing proximity to watering points and associated grazing activity. (7) So location and functioning of water points and grazing pressure are all important to preserving biodiversity in these zones. In the subtropical grasslands of Queensland, a range of grazing intensities at the landscape scale helped optimise the conservation of herbaceous plants. (8) However, economic pressure to increase grazing intensity can decrease the biodiversity of plants, because under non-selective grazing, larger tussock grasses tended to be replaced by a 'grazing lawn', which was a lesser habitat for vertebrates and many invertebrates. (8) On the other hand, medium levels of grazing maintained the biodiversity and ecosystem function. (8) Fires in the subtropical zones are a known significant contributor to greenhouse gas emissions as well as loss of biodiversity, so grazing management of subtropical grasslands to reduce fire risk is a key to preserving biodiversity. Much of the meat and livestock industry is located around rivers and riparian zones, either near the coast or inland. Riparian habitats are the boundary of terrestrial and aquatic systems, and are important in supporting high levels of biodiversity and are powerful indicators of catchment quality. In these riparian zones, the grazing and trampling activities of livestock have had major negative impacts on the vegetation and soils of river banks such as in the Murrumbidgee River and its tributary streams in the Murray-Darling Basin of south-eastern Australia, particularly during droughts or even hot, dry periods of the year. (9) While not advocating exclusion of the grazing industry from the riparian zone, Jansen and Robertson 2001 proposed that lowered stocking rates, particularly in the upper parts of the catchment, resting of paddocks to allow recovery from grazing, and the provision of off-river watering points could all be used to improve riparian habitats. (10) Thus, the meat and livestock industry in Australia has the background information in natural resource management, and should be able to implement the appropriate grazing management to reduce environmental impact on biodiversity. The gap in knowledge here is more comprehensive mapping of the biodiversity, and how grazing practices in the meat and livestock industry can enhance or sustain biodiversity in conjunction with conservation zones. The meat and livestock industry should aim to collaborate with community and environmental groups already involved in preserving and mapping biodiversity, such as the Gondwana Link Project located in one of the prime international biodiversity 'hotspots' in the south-west Western Australia (Figure 2).

Similarly, an Eastern Wildlife Conservation Corridor that aims to provide an almost continuous link from the Alps to Atherton, is being established along the Great Australian Divide. The Eastern Wildlife Conservation Corridor will allow animals and plants to adapt to climate change by moving or establishing at their optimum location within this corridor. Again this project has the support of community groups, environmental groups, several state governments and the Federal Government. Farmers in the areas of these two projects should offer their skills and experience to ensure sustainable systems for both grazing and preservation of biodiversity. Some farmers may see these projects as a threat to their land tenure and farming operations, especially because 'green' groups may have increased purchasing power to acquire land in these areas as a result of the 'green' groups becoming sponsors or beneficiaries of carbon-trading schemes. This latter perspective will not help with reducing livestock's environmental footprint or with preserving biodiversity under threat of global warming.

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COMPETITION FOR LAND USE

One of the major issues facing meat and livestock producers in southern Australia is the competition for land use. This is a common environmental problem for producers in the USA, Canada and Europe. Access to coastal regions or lakes and national parks is another common feature of the desirable locations for wealthy urbanites, so-called sea-changers or tree-changers, who seek respite from the city. Unfortunately, meat and livestock enterprises, while once part of the attractiveness of these locales, rapidly become subject to criticism of their environmental stewardship by the new arrivals. With increasing settlement, and higher wealth of those arriving, new amenities and activities are developed that sometimes are at odds with some farming practices, such as spraying, meulsing or even weaning. Rural people, including farmers, who hold land in these competing zones may in fact require more help with complex social, political, legal and economic problems than with technical aspects of agricultural production. The existence of an urban middle class that has income to spend on enjoying the 'natural amenities' of rural areas near capital cities adds another difficult dimension to particularly the environmental issues faced by those rural communities. All over Australia, there is growing interest in preserving natural environments for recreation, and this creates tension with those who use these regions for traditional agricultural practices. In particular, more citizens are concerned about the preservation of biodiversity and controlling nutrients in waterways.

WATER SCARCITY

Australia is the driest inhabited continent, so water shortages and drought are not a new phenomenon. However, the past decade has seen prolonged and severe drought over much of the eastern and southern Australia. In fact, significant parts of the Southern Hemisphere, including southern Australia, have become drier since the 1960s. In south-west Australia, annual total rainfall has declined by some 15-20%. In southern Victoria and adjacent parts of South Australia, annual rainfall has been particularly low since the mid-1990s. There has been a significant decline in the April-July rainfall across southern Australia since the 1950s. The rainfall changes have happened abruptly, and coincide with a strengthening of the high-pressure belt over the mid-latitudes of the Southern Hemisphere, during autumn and early winter.

In the south-west of Western Australia, a decrease of 10% in rainfall since 1970 has resulted in a 40% decrease in stream flow (Figure 3). (11) Of concern is the sharp, second decrease since 1998 (Figure 3). Consequently, the southwest of Western Australia has seen decreasing storage quantities, and an increasing reliance on underground sources, and desalination while the northern zones of Western Australia, in keeping with many subequatorial and equatorial zones, have experienced increased rainfall. The southwest of Western Australia has livestock production systems based on annual pasture. Increasingly, these systems have degraded, particularly through rising water tables and increased salinisation. More sustainable livestock production systems are being developed that can address land degradation through the profitable use of perennial pastures. Ever-Graze is such a project that aims to increase farm profit by 50% while reducing groundwater recharge by 50% in the high-rainfall zone (>600 mm) of southern Australia. Whole-farm models for the Albany Eastern Hinterland in Western Australia suggested that the project aims could be met through a combination of a meat Merino enterprise, deep-rooted perennials, increased winter pasture production and higher weaning percentage. (12) The Future Farm CRC is planning other new pasture systems in the mixed cropping and livestock production zone of south-west Western Australia in response to drought, climate change and the poor economic prospects for crops. The new systems will be based on native perennials, which will mean a substantial change in farming systems in this zone, where perennials currently play no substantial role. The name for the target system is 'Enrich' and aims to develop permanent perennial pasture based on fodder shrubs (preferably domestication of Australian native species, e.g. Atriplex, Rhagodia) for low-input pasture production from poorer soils).

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The major effect of the decrease in rainfall in southern Australia has been to put pressure on the use of freshwater for agriculture. If the meat and livestock industry is to respond adequately to scarcity of freshwater, then it must have a reasonable basis for calculating the cost in freshwater to produce animal products. Therefore, calculating the cost in freshwater for each kilogram of beefsteak or clean wool has never been more urgent, or more controversial. The amount of daily drinking water required by beef cattle is listed in the range of 5-10% of body weight per day, that is, 20-40 L per day for a 400-kg steer, (13) while that for a 50-kg sheep is 1.5-3 L per day. The current debate about the amount of water required for livestock production cites values such as 50000 L (14) or 100000 L (15) per kilogram of beef, or 170000 L per kg clean wool. (16) The calculations of Meyer (1998) are based on the annual evapotranspiration rate from pasture measured as 15 ML (or 15000000 L) per hectare, (16) and then assessed against a nominal but reasonable annual weight of dressed steak (300 kg) or yield of clean wool (88 kg) per hectare. Clearly, these calculations would place livestock products such as beef and wool well above cereal (e.g. 1000 L per kg of maize grain) in terms of water requirements. However, one has to question whether it is biologically and environmentally reasonable to calculate water consumption of grazing livestock by Meyer's methodology. If the calculation is based on just the water consumed by a 300-kg steer on a daily basis (i.e. 15-30 L), then the annual intake is 5400-10800 L. This steer should yield approximately 150 kg carcase weight that might dress out at 65%, which means a total of 97.5 kg of steak. By this methodology the water used per kg steak would be 5400 or 10800/97.5 = approximately 55-111 L per kg steak, that is, as much as three orders of magnitude less than values cited by Pearce or Meyer. Naturally this type of methodology does not account for the total water used by the enterprise or the water returned as urine, nor the water that enters metabolism or is produced during energy metabolism, or the water that enters lean tissue turnover pool. At an environmental level rather than production level, the water diverted from flows to livestock production should be taken into account. Obviously, net water use by livestock is a major gap in the knowledge of assessing the environmental footprint of the meat and livestock industry. This issue must be addressed in future research via modelling water use in livestock, as future water scarcity will be determined in large measure by the way that water is managed and used in agriculture. Vegans argue that, in a country like Australia, producing a vegetarian diet requires about half the consumption of water than a typical omnivorous diet containing meat. The future debate will centre on the type of agriculture that best mitigates the rate of greenhouse gas increase. This debate will have efficiency of water use as one of the prime mitigating factors.

SUSTAINABILITY OF FEEDING SYSTEMS

Nutritional efficiency of protein use by livestock also affects the sustainability of grazing or feeding systems. Protein nutrition influences productivity, profitability and the efficiency of nitrogen (N) use. The inefficiency of N use in animal agriculture is becoming a major environmental concern in the USA. (17) In Australia, N use is a major environmental concern during live export of cattle and sheep. (18) Increasing the efficiency of N use, especially the efficiency of absorbed N use, per unit of meat produced may be a major challenge for livestock enterprises in the future. From a producer's perspective, the economic cost of losing production due to underfeeding protein greatly exceeds the cost of feeding excess protein as a margin of safety. However, the excess N impacts directly on the environment via eutrophication of rivers and wetlands. Phosphorus from fertilisers for pastures and crops has a similar eutrophying effect specifically polluting rivers and estuaries. These are major environmental concerns that require further research to optimise nutritional efficiency in intensive livestock systems in particular, and may require additional legislative intervention as the final driver for reduction of the environmental impact.

BALANCING NUTRITIONAL AND ENVIRONMENTAL CONCERNS

The nutritional quality of meat in the human diet should not be lost in the debate about the environmental impact of the meat and livestock industry in Australia. Meat is a high-quality protein source and an excellent source of bioavailable iron and zinc. These two essential minerals are very difficult to source from cereals and vegetables, because they are either present in low amounts or poorly available, or both. For instance, the relative bioavailability of zinc from animal sources was about 100%, while availability from grain or legume sources was highly variable and averaged about 60-70%. (19) Furthermore, any consideration of reducing red meat production (and consumption) in Australia for environmental reasons should acknowledge our nutritional need for these minerals. In this consideration, one needs to take into account the environmental impact of producing alternative foods to fill this nutritional gap. This is likely to be comparable, if not greater than from red meat production. Second, the practicality of changing land use is problematic--grazing country is typically unsuitable for cropping primarily due to topography among other factors. Were this land not used for grazing, it would put even greater food production and environmental pressure on more arable land areas. Finally, as global meat production is predicted to double over the next 50 years, the environmental impact from Australia's meat production systems and technologies needs to be considered against that of the less sophisticated meat production systems of the developing world.

FUTURE RESEARCH

The meat and livestock industry's potential for reducing its environmental footprint depends on the major funders of rural research and the rural community leaders prioritizing environmental research by instigating dedicated programs dealing with the issue of livestock and the environment. Funding organisations like MLA should assess the scope of livestock-environment interactions and regional variations as to their nature and extent. Concurrently, MLA should identify trade-offs and priority areas for research development. MLA should also assess the extent to which it can lead meat and livestock research linked to national environmental issues in the area of climate change, biodiversity protection, water and land degradation that cross state boundaries. With this approach, MLA can be the national leader in proposing guidelines, policy papers and decision-support tools for mitigating environmental impacts of the livestock sector; specifically MLA could explore opportunities to facilitate national certification schemes in support of sustainable livestock production. Finally, MLA and other rural livestock organisations should explore links with other organisations, for example Greening Australia, who are already planning their business opportunities in carbon trading (e.g. the Gondwana Link) to explore new business opportunities in sustainable livestock production. Reducing the environmental impact of the meat and livestock industry could lead to new ways of sustaining the profitability of the industry.

REFERENCES

1 Food and Agriculture Organisation (FAO) Committee on Agriculture. Managing Livestock--Environment Interactions. COAG 2007/4. Rome: FAO, 2007.

2 Food and Agriculture Organisation (FAO). Livestock's Long Shadow. Environmental Issues and Options. Rome: FAO, 2006.

3 Asner GP, Elmore AJ, Olander LP, Martin RE, Harris AT. Grazing systems, ecosystems responses, and global change. Ann Rev Environ 2004; 29: 261-99.

4 Stern N. The Economics of Climate Change: The Stern Review. Cambridge: Cambridge University Press, 2006. ISBN-13: 9780521700801.

5 Australian Greenhouse Office. National Greenhouse Gas Inventory 2005. Canberra: Commonwealth of Australia, 2007. ISBN: 978-1-921297-22-9.

6 Wright A-DG, Williams AJ, Winder B, Christophersen CT, Rodgers SL, Smith KD. Molecular diversity of rumen methanogens from sheep in Western Australia. Appl Environ Microbiol 2004; 70: 1263-70.

7 Landsberg J, James CD, Maconochie J, Nicholls AO, Stol L, Tynan R. Scale-related effects of grazing on native plant communities in an arid rangeland region of South Australia. J Appl Ecol 2002; 39: 427-44.

8 McIntyre SK, Heard M, Martin TG. The relative importance of cattle grazing in subtropical grasslands: does it reduce or enhance plant biodiversity? J Appl Ecol 2003; 40: 445-57.

9 Robertson AI, Rowling RW. Effects of livestock on riparian zone vegetation in an Australian dryland river. Regulated Rivers Res Manage 2000; 16: 527-41.

10 Jansen A, Robertson AI. Relationships between livestock management and the ecological condition of riparian habitats along an Australian floodplain river. J Appl Ecol 2001; 38: 63-75.

11 Australian Greenhouse Office. Topic 9: what is causing the rainfall declines over southern Australia--ozone, climate variability or climate change? In: Hot Topics in Climate Change Science. Australian Greenhouse Office, Department of the Environment and Heritage, 2005; 000-00. (Cited 6 June 2007.) Available from URL: http://www.greenhouse.gov.au/science/hottopics.

12 Sanford R, Young J, Ryan J. EverGraze--Development of Profitable and Sustainable Livestock Systems for the High Rainfall Zone of Western Australia. Proceedings of the Australian Agronomy Conference, Australian Society of Agronomy, 13th Australian Agronomy Conference 10-15 September 2006, Perth, WA.

13 Beatty DT, Barnes A, Taylor E, Pethick D, McCarthy M, Maloney SK. Physiological responses of Bos taurus and Bos indicus cattle to prolonged, continuous heat and humidity. J Anim Sci 2006; 84: 972-85.

14 Meyer WS. Water for Food--the Continuing Debate, 1998. (Cited 6 June 2007.) Available from URL: http://www.clw.csiro.au/publications/water_for_food.pdf

15 Pearce F. Thirsty meals that suck the world dry. New Sci 1997; 2067: 7.

16 Meyer WS, Dugas WA, Barrs HD, Smith RCG, Fleetwood RJ. Effects of soil type on soybean crop water use in weighing lysimeters. 1. Evaporation. Irrig Sci 1990; 11: 69-75.

17 VandeHaar MJ, St-Pierre N. Major advances in nutrition: relevance to the sustainability of the dairy industry. J Dairy Sci 2006; 89: 1280-91.

18 Costa ND, Accioly J, Cake M. Determining Critical Atmospheric Ammonia Levels for Cattle, Sheep and Coats. Brisbane: Meat & Livestock Australia Ltd, 2003. ISBN 1 74036 296 9.

19 Baker DH, Ammerman CB. Zinc Bioavailability. In: Ammerman CB, Baker DH, Lewis AJ, eds. Bioavailability of Nutrients for Animals. San Diego, CA: Academic Press, 1995; 375.

Nick D COSTA

School of Environmental Science, Murdoch University, Murdoch, Western Australia, Australia