agri 720 david doc

AGRI 720: Class Assignment number ONE, by David Mushimiyimana (Reg: AGR-4-0304-1/2014)

KENYA METHODIST UNIVERSITY

FACULTY OF EDUCATION, ARTS AND SCIENCES

PROGRAMME: DOCTOR OF PHILOSOPHY IN AGRICULTURAL AND RURAL

DEVELOPMENT

AGRI 720: ADVANCES IN ANIMAL PRODUCTION SYSTEMS

Class Assignment number ONE:

Discuss how livestock production affects biodiversity and environmental

pollution

Presented to:

Dr Mworia

By:

David Mushimiyimana (Reg: AGR-4-0304-1/2014)

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Contents

The Question Number One..........................................................................................................................1

Introduction.................................................................................................................................................1

Effect of Livestock Production on Biodiversity.............................................................................................2

Effect of Integrated Animal/Crop Production Systems on Water Pollution.................................................5

Effects of Different Animal Production Systems..........................................................................................6

Soil and water contaminants.......................................................................................................................7

Air Contaminants or Greenhouse Gases......................................................................................................8

(i) Volatile Fatty Acids........................................................................................................................10

(ii) Methane (CH4)...........................................................................................................................10

(iii) Gaseous ammonia.....................................................................................................................11

(iv) Dust...........................................................................................................................................13

Possible Interventions...............................................................................................................................13

Conclusion.................................................................................................................................................15

List of References......................................................................................................................................16

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The Question Number One

Discuss how livestock production affects biodiversity and environmental

pollution

Introduction

Thomas Malthus (1766–1834), an English economist and pioneer

demographer made a series of predictions concerning population growth

showing that while food supply increases arithmetically, population growth

increases exponentially and that the inevitable result would be hundreds of

millions of deaths by starvation in the 1960’s. Looking back today, the world

can be proud of having beaten the odds by achieving unprecedented levels

of productivity both in crop production and in livestock production.

Unfortunately, that increase in productivity came with new challenges and

agriculture is now one of the biggest contributors to environmental

degradation.

As societies continued to urbanize, plants produced food for humans and for

animals while animals provided food for humans, but only livestock recycled

nutrients back to the plants. More recently, because of the industrialization

and specialization of agriculture, the cycle that replenished the soil fertility

decreased as most producers preferred the convenience of commercial

inorganic fertilizer. In many cases, livestock production occurred as separate

specialized operations sometimes far away from plant production fields. This

change brought a major effect on the structure of livestock production, which

has focused on improving the efficiency and productivity. Economic viability

and profitability have been the primary driving forces that define the current

structure. More recently, the values of society have demanded a more

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sustainable environmental system from livestock production. The effect of

livestock production on rural communities also is being recognized as an

important consideration (Honeyman,1996). In this paper, it is my intention to

discuss how livestock production affects biodiversity and/or contributes to

environmental degradation by causing pollution.

Effect of Livestock Production on Biodiversity

The term “Biodiversity” is a contraction of “Biological diversity.” It means the

variability among living organisms from all sources including, among others,

terrestrial, marine and other aquatic ecosystems and the ecological

complexes of which they are a part; this includes diversity within species,

between species and of ecosystems. Biodiversity is found at different

hierarchical levels and spatial scales; e.g. genes within populations,

populations within species, species within communities, communities within

landscapes, landscapes within biomes, and biomes within the biosphere. As a

result, biodiversity encompasses variety of biological life at more than one

scale. It is not only the variety of species (both plant and animal) but also the

variety of genes within those species and the variety of ecosystems in which

the species reside. Having many different living things allows nature to

recover from change; too much biodiversity is lost, the remaining organisms

may not survive because all the living organisms depend on each other in a

stable ecosystem. Some of the ways biodiversity is lost are through habitat

destruction, introduced species, pollution, human population growth, over-

consumption etc. Unfortunately, as a result of human activities, ecosystems,

species and genetic diversity is being destroyed faster than nature can

create it. This damage threatens the ecological, economic, recreational and

cultural benefits that we receive from the Earth's living resources.

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Traditionally, animals and particularly ruminants were an asset to society by

converting biomass from vast grazing areas into products useful for humans,

e.g. dung, draught, milk, meat and security. However, growing human

populations cause increased and shifting demands for food and other

products. This results in the conversion of natural forests and grazing land

into arable land for crop and fodder production, thus leading to quantitative

and qualitative changes in biomass availability for human food and livestock

feed (Winrock, 1978). Where cropping is possible, it can feed more people in

terms of calories and protein than what is possible with animal production.

Apart from their inferior caloric output, compared to crops, animals are also

associated with deforestation and erosion (Durning and Brough, 1991).

However, historically, deforestation tended to start in response to the

requirement for timber for fuel and construction (Ponting, 1991). Forest was

cultivated with crops and grassland for food production through shifting

cultivation, permanent cropping or simply as a method of occupying land

(Ruthenberg, 1980). In the present day, the strong argument against

keeping of livestock is that the requirement for cropland is increasing

through expansion of grain-based beef, dairy and poultry production in the

USA, Western Europe, in peri-urban dairies of developing countries, and

recently in the Pacific Rim and China (Winrock, 1978). Combined with

changing human food patterns, this has increased the demand for crop land

relative to grazing land. As a result, even marginal grazing areas are

converted into crop land and overgrazing of the remaining areas becomes

the rule rather than the exception (Jodha, 1986). Land scarcity starts to

occur, even in pastoral areas. This upsets existing ethnic balances, and can

result in animosity between pastoralists and arable farmers who peacefully

co-existed to mutual benefit in the past (Powell and Waters-Bayer, 1985).

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The use of external inputs can increase the carrying capacity of some range-

land systems (Breman and De Wit, 1983). However, such external inputs are

not available or not affordable to all farmers. Hence, over exploitation (i.e.

mining) of land without the use of external inputs tends to be the result (Van

Der Pol, 1992). This threatens the sustainability of these systems, which is

defined here in simple terms as “the capacity to continue production”. Too

liberal use of external inputs, on the other hand, causes waste disposal

problems or increased political dependency on external supplies (De Haan et

al., 1997).

In general, animals are often considered to be the cause for unsustainability

in both high and low external input agricultural systems. In low external

input agricultural systems, animals are blamed for scavenging whatever is

left, while in high external input agricultural systems, the role of animals as

waste utilizers has been reverted to a role as polluters and converters of

prime resources. Rather than being an asset to sustainability, livestock

keeping has become a liability (Durning and Brough, 1991).

Livestock were components of systems with long term sustainability. For

example, the keeping of livestock was essential for survival in divergent

systems such as those of the pastoralists in Africa and on mountain ranges

unsuitable for cropping. Animals have long been essential in sustaining crop

yields in the infield–outfield systems of Western Europe and other parts of

the world, where dung and draught from wasteland grazing (outfields) was

used for crop cultivation on the infields around the homesteads. In a more

intricate way, animals helped to sustain crop yields by increasing the rate of

nutrient flows in the mixed crop–livestock systems or by allowing farmers to

include crops that fix atmospheric nitrogen, release immobilized phosphorus,

or enhance soil organic matter (Hoffland, 1991).

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Grazing by livestock usually follows rather than precedes deforestation

and/or cropping. In fact, animals, such as the goat, are one of the last means

of survival for large numbers of poor people on bare, exhausted, and/or arid

lands. However, in spite of the importance of animals for the poor classes of

farmers, the advocates for continued animal production on exhausted soils

should acknowledge that livestock can tip the final balance in delicate

ecosystems (Schiere and Grasman, 1997).

Effect of Integrated Animal/Crop Production Systems on Water

Pollution

Viewing manure as a valuable resource rather than as a waste product to be

disposed of is a critical first step in reestablishing the nutrient recycling

process. If properly managed, livestock production can have a positive effect

on the environment as for example livestock production and the subsequent

application of manure can counteract decreasing soil fertility and soil erosion

(Baker et al., 1990). Minimizing nutrient loss in the system by applying

manure to cropland can maintain or improve water quality in most situations.

Integrated animal/crop production systems also have a financial advantage

over specialized operations. A study of Borts et al. (2004) showed that the

diversified swine-grain farm greatly decreased fertilizer costs by using

manure, had shared implement costs, and had more stable grain

pricing/costs and thereby less risk. Flora et al. (2004) also showed that

ruminant production, based on perennial forages, could enhance water

quality and decrease nutrient losses from farms.

Unfortunately, many specialized large livestock farms often lack adequate

land base for appropriate manure application and nutrients produced in the

manure far exceed crop needs, land application alone is insufficient to

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handle all generated manure economically. Changing the structure of

livestock production or adopting alternative technologies to handle manure

may be necessary to maintain water quality standards.

The long-term sustainability of agriculture requires that livestock production

must not only contribute to maintaining economy, but also sustain or

improve environmental quality. Traditionally, farmers were accountable to

their communities and the land on which they lived. If their practices

resulted in harm to the environment or community, they were held

accountable. In today’s industrial agriculture, accountability is often eclipsed

by economics (Kirschenmann, 2004). The reaction is that industrialized

agriculture no longer sustains a good balance of interests and no longer

promotes trust (Anthony, 2004). Specifically, complaints include the

magnitude, intensification, cheap food mentality, and the relocation of the

decision-making power of industrial livestock production, as well as

consumer ignorance and apathy fuelled by an attitude of resignation to the

fact that “everyone else is doing it” (Lasley, 2003).

Integration of crops and livestock on-farm can enhance equity, one of the

criteria for sustainability proposed by Conway (1986). It can also affect the

export of plant nutrients to the urban centres by providing labour

opportunity and income for the country-side, as more added value remains

on farm when crop by-products are fed on farm. Integration of several forms

of production is likely to reduce pollution problems, because waste from one

subsystem can serve as a resource for another subsystem. Thus, the

waste/losses flows can be reduced due to integration.

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Effects of Different Animal Production Systems

The type and intensity of animal production systems is an important factor in

the extent to which animal production is detrimental to the environment.

Animal production systems can broadly be classified as grazing systems,

mixed systems and landless or industrial systems (Sere and Steinfeld, 1996).

(i) Grazing systems

Grazing systems are defined as entirely land based systems with annual

stocking rates less than 10 livestock units (LU)/ha. Grazing animals are

frequently associated with overgrazing, soil degradation and deforestation.

The environmental impact of grazing systems will first of all depend on the

stocking density and further whether the livestock has to travel to find feed

(mobile), depend on local communal pasture (sedentary) or have access to

sufficient feed within the boundaries of the farm (ranching and grassland).

Examples are extensive, pasture-based beef production and most of the

dairy production systems.

(ii) Mixed livestock systems

Mixed livestock systems are systems in which a significant part of the value

of production comes from non-livestock farming activities. Mixed livestock

systems have many opportunities for nutrient recycling. The impact of these

systems on the environment depends on the source of the feed, and thus

separate systems can be described for feed provided by communal grazing,

crop residues, cut and carry processes, produced on farm or external feed.

(iii) Industrial systems

Industrial systems have average stocking rates greater than 10 LU/ha. They

depend primarily on outside supplies of feed, energy and other inputs, and

the demand for these inputs can thus have effects on the environment in

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regions other than those where production occurs. Examples are poultry

production (broilers and layers), pig production, ruminant feedlot meat

production and large-scale urban dairy production. Their impact on the

environment depends both on species and on the processing of the inputs

(feed supply) and the outputs (animal products).

Soil and Water Contaminants

Most important contaminants to the soil are nitrogen as NO3, P and K. They

originate primarily from N, P and K in fertilizer and in animal manure, the

latter being a mixture of feces and urine. Although all three nutrients are

important for soil fertility, excessive levels of them in the soil cause the risk

of runoff or leaching to surface and sub-soil water, causing eutrophication.

This is particularly true for P and K. Plants have relatively high K

requirements and notably forages when used as animal feeds may contain

high amounts, much higher than the animal can efficiently use and the

surplus is excreted. Accumulation in the soil does occur but only to a limited

extent and excess K leaves the system in ground and surface water.

Although maximum values of 12 mg K/l are used for drinking water, harmful

effects are not well documented. Many foods contain much higher levels, for

instance milk contains 1500 mg K/l. Other elements that cause concern,

include Cu (used as a growth promoter in pigs, but at low levels poisonous to

sheep) and Zn (included in diets of all farm livestock).

Important water contaminants are N, P and as already indicated K. Excessive

P from runoff and erosion can fertilize surface waters and cause

microscopically small algae to multiply rapidly. The algae cloud the water

and prevent larger submerged aquatic vegetation to get enough light. The

submerged aquatic vegetation may die back, reducing the available habitat

of aquatic animals.

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When the algae themselves eventually die, they decompose, during which

dissolved oxygen is removed from the water, making it difficult for other

aquatic organisms to survive. For reasons of healthy drinking water on the

one hand and avoiding eutrophication with N on the other, in 1991, the EU

has accepted the nitrate directive in which member states are required to

keep the nitrate content in so-called nitrate-sensitive zones below an often

criticized value of 50 mg/l.

Air Contaminants or Greenhouse Gases

Greenhouse gases are CO2, CH4 and N2O. They are feared for their potential

to contribute to global warming. The contribution of agriculture in general

and animal production in particular to CO2 emissions are relatively small and

even in industrialized countries with highly mechanized production systems

do usually not exceed 5% (Sauerbeck, 2001).

Methane is formed by the anaerobic decay of organic matter in the

sediments of natural marshlands and rice fields. Substantial contributions

also come from ruminants, biomass burning, decay of organic matter in

landfills, fossil fuel production etc… Because of its high global warming

potential (GWP), CH4 contributes some 55% of the GWP of a dairy cow

(Johnson et al., 1997). Nitrous oxide production arises from microbial

nitrification and/or microbial or chemical denitrification in the soil. Its

emission is considered to be on average 1.25% of the amount of N applied to

that soil. Addition of nitrogen to the soil via mineral N fertilizers, animal

manure, crop residues or sewage waste generally increases the N2O

emission. Nitrous oxide emission is influenced by land use.

Air quality is a challenging environmental standard to meet because it is

difficult to quantify and control. With the emergence of larger livestock

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operations, there has been an increasing concentration and intensity of

odors. As livestock operations increase in size, environmental risk increases

also, and different technologies will be needed to maintain air quality

standards. Manure, once viewed as a resource to be conserved and used on

cropland, is more and more considered by many a waste disposal problem.

As a result, there is a tendency for over application of nutrients to the land,

volatilization of ammonia and hydrogen sulfide gases, and excessively large

lagoons to accommodate the wastes, all which have compromised water

supplies and air quality.

Animal husbandry is a serious source of aerial pollutants. People employed in

the farming units, as well as livestock, are submitted to a wide range of

airborne contaminants that cause respiratory irritation and sensitization

(Radon et al. 2002). These kinds of bio-aerosols are consistently emitted into

the environment by the livestock-house ventilation systems, and may

consequently affect the respiratory systems of people living next to livestock

enterprises (Intergovernmental Panel on Climate Change 2007).

(i) Volatile Fatty Acids

In contrast to monogastric animals, which obtain most of their energy from

dietary starch/sugar digested in the intestines, ruminants gain energy from

volatile fatty acids (VFA) formed by microbial fermentation of plant structural

carbohydrates, starches and proteins in the rumen. After absorption, these

VFA become the principal energy substrates for the livestock. The

predominant VFA is acetic acid which constitute50–75% of the total VFA

concentration in the rumen. Propionic acid(10–30% of total production) and

to a lesser extent butyric acid.

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(ii) Methane (CH4)

Methane (CH4) is the most significant source of greenhouse gas emissions

from agriculture, with 80 million tonnes of CH4 produced each year from

ruminant livestock globally (Beauchemin et al. 2008). Enteric methane is

produced through methanogenesis in the rumen and represents a loss of 6–

10% of gross energy intake for the livestock (Eckard et al.

2010).Quantification of methane emissions from livestock has gained

prominence in both government and public discussions of mitigation of

climate change and in reporting greenhouse gas emissions(Gerber et al.

2013).

Several scientists have published on the evaluation or the prediction of

pollutant emissions by ruminants, especially enteric methane (CH4) on one

hand (Sauvant et al., 2011) and urinary nitrogen (UN) output on the other

hand (Dijkstra et al. 2013). Methane is a major component (more than 50%

at the farm level) of greenhouse gas emissions while UN leads, among

others, to emissions of another greenhouse gas, nitrous oxide (Schils et al.,

2013), of ammonia in air, and of nitrates in water (Hristov et al., 2011). The

most important differences observed between trials are the influences of

energy and protein sources and the presence of some secondary metabolites

such as tannins (Jayanegara et al., 2012). Dijkstra et al. (2013) suggested

that decreasing N output could increase CH4 emission, depending on fibre

level, but their data were simulated from a mechanistic model. In fact, such

a relationship is a priori not obvious, because N intake, which is a major

driver of N output, is not known as a way for CH4 mitigation, and digestible

carbohydrates, which result in CH4 emission, are not directly related to N

excretion, except if energy is a limiting factor of microbial activity in the

rumen.

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(iii) Gaseous ammonia

Gaseous ammonia (NH3) is the predominant pollutant in poultry systems. It is

generated by the enzymatic decomposition of uric acid and can adversely

affect bird performance and welfare and human health, being a co-factor in

the genesis of atrophic rhinitis and bronchopneumonia (Hamilton et al.

1996).

Ammonia emission is of great environmental concern because it contributes

to soil acidification and increased N deposition in ecosystems (Pain 1998).

According to the Italian Emission Inventory of Ammonia and Greenhouse

Gases, 94% of national NH3 emissions can be attributed to agricultural

practices.

A portion of emitted NH3 reacts with acidic gases including nitric (HNO3),

hydrochloric (HCl), and sulfuric acid (H2SO4) present as aerosols, converting

the NH3 to NH4 salt particles. With significant contributions of available acid

gases from industry and transportation, their neutralization with NH3 forms

particulates that may create mist, which can be transported over long

distance before they are removed by precipitation (Asman et al., 1998).

Thus, livestock production enterprises, along with the industry and

transportation sectors, can impact very distant ecosystems.

Hatfield et al. (1993) suggested that 89 to 90% of the N inputs to anaerobic

lagoons in confined animal feeding operations were lost to the atmosphere.

These suggested NH3 emissions represented about 60% of the total feed N

input. However, Harper et al. (2005) found that only about 7.5% of N

entering into a swine production operation as feed left the area as NH3 while

another 7.3% was emitted as NH3 from the production houses and another

2% from field application of wastes to nearby. Much of the N (about 43% of

input feed) that entered the system was found to be denitrified to N2 by

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microbial and/or chemical denitrification. Consequently, emission factors

must be used with caution because of variability induced by geography and

meteorology, methodology for measurement, type and weight of animals, N

content of feedstuffs, housing and management, and other factors (Harper,

2005).

Intensive pig production leads to the release of environmental contaminants,

one of the most important being ammonia (NH3) (Zahn et al. 1997). Available

research data indicate that the crude protein (CP) level of the diets fed to

livestock have profound effects on NH3 emissions from excreted manure.

About 60–70% of the nitrogen (N) from the diet is excreted in the faeces and

urine (Dourmad et al. 1999). Nitrogen excreted via faeces is predominantly

incorporated in bacterial protein, which is less susceptible to rapid

decomposition, but N excreted via urine is mainly in the form of urea, which

is easily hydrolyzed and catalyzed by the urease present in feces to NH3

(van der Peet-Schwering et al. 1999). Reducing CP in pig diets can reduce N

excretion in the manure mainly due to decreased N excretion in urine (Canh

et al. 1998; Hernández et al. 2011). Several researchers have tried to

decrease NH3 by reducing dietary CP. O’Connell et al. (2006) observed

decreased NH3 emissions from slurry from pigs fed a 160 g/kg CP diet

compared with a 220 g/kg CP diet. Hayes et al. (2004) observed that NH3

emissions from pig manure can be reduced by decreasing dietary CP from

220 to 130 g/kg, from 200 to 120 g/kg and from 180 to 120 g/kg,

respectively.

(iv) Dust

Dust represents another important aerial contaminant of livestock houses,

since it is often coupled to inorganic compounds, gases, bacteria and viable

endotoxins, becoming a potentially hazardous agent (Nimmermark et al.

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2009). Dust can play a role in the prevalence of human respiratory diseases

as gathered from occupational health reports on farm workers in livestock

houses.

Possible Interventions

It is now widely accepted that undesired nutrient losses should be avoided or

at least kept to a minimum. This needs a combination of technical, legal and

mental interventions, the success of which will depend on the quality of

legislation and the acceptability of its implementation by stakeholders. Many

types of interventions at many sites in the production chain (soil–plant–

animal–manure) are. Important technical tools are a reduction in the use of

NPK fertilizer, nutrition and manure management.

(i) Tanging into consideration the carrying capacity of the soil

To ensure a sustainable animal production system, animal densities should

not exceed the carrying capacity of the soil. A crucial question then becomes

what the carrying capacity of the soil is. Carrying capacity is nowadays often

expressed in terms of the maximum acceptable application rates of N and P,

instead of in LU/ha or (milk) production per hectare. Livestock (LU) or animal

unit (AU) was originally defined by the FAO in 1974 on the basis of body

weight (330–500 kg), with buffaloes, horses and mules as the standard (1

AU). Later, at least in the US, this was replaced by the amount of feed

consumed and a cow was taken as standard (Ensminger et al., 1990).

(ii) Reduce external inputs

Staying within the maximum allowed limits of N and P application and losses

and maintaining a high production per hectare is only possible by limiting

inputs. Reductions in external inputs can be achieved through reduction in

fertilizer use (N, P, K) and by reducing the amount and composition of

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concentrates. A reduced use of inorganic fertilizers is possible without

dramatic reduction in yield (Valk et al., 2000), particularly when organic

fertilizer is applied at the right moment. The reduction will almost

automatically lead to a reduction in the loss of the greenhouse gases NH3

and N2O.

(iii) Optimizing the nutrition

Optimizing the nutrition has also been recommended as possibility to reduce

CH4 losses of ruminants. Various nutritional interventions have been

suggested to reduce CH4 production of ruminants. Examples are the use of

starch rich diets promoting the production of propionic acid, supplementation

with ionophores, the addition of fats, notably poly-unsaturated ones. Such

interventions may be feasible and effective in industrial animal production

systems, but for the vast majority of ruminants this is hardly a solution,

because the options very much depend on the production system.

(iv) Control of the composition and quality of manure

In principle, animal manure is a commodity, in many areas a valuable one,

but in surplus areas often with a negative value because of its role in

emissions to the environment. Despite this, animal manure still has a value

as fertilizer, be it that its suitability as organic fertilizer in relation to soil

quality varies. It is thought that a high carbon to nitrogen (C/N) ratio in

manure may increase its value for the soil. Hence, an increased interest in

composition and quality of manure has developed. Quality aspects are its

content of water, organic matter, N, P and K. Being a mixture of excreted

urine and fecal matter, manure is composed of end products of metabolism,

undigested dietary components, newly added endogenous components and

fermentation end products as well as biomass from endogenous

microorganisms. It was estimated (Larsen et al., 2001) that, in dairy cows,

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fecal N consisted of 16% undigested feed, 55% of microbial nature and 29%

of endogenous origin. It has also been demonstrated that in pigs the

partition of N between feces and urine can be manipulated by nutrition

without retarding protein gain (Canh et al., 1997).

Conclusion

Changes in resource/demand patterns cause changes in the behaviour of

(livestock) production systems. This implies that livestock can be essential

for the sustainability of one system in one context and detrimental for the

same or another system in a context elsewhere with other resource flows. It

is possible to identify contexts and systems where livestock can be useful for

increased sustainability and the generalized claims that livestock are

detrimental is not supported. The complexity of decision making increases

when more factors are involved, i.e. when more criteria for sustainability are

used. It is a form of experimentation and data handling that is alien to the

traditional approaches in reductionist research that separates all factors to

study only a few at a time.

Producers should be encouraged to stop making decisions based on

production efficiency and profitability alone. Instead, their decisions need to

include the impact on the environment and how they contribute to the local

community. Animal scientists also need to consider the environmental and

community effects of research and management programs.

It is important to develop production systems that integrate and respect

local community values and consider environmental impacts. There is a need

to evaluate, refine, and demonstrate these technologies and create business

systems that minimize external costs and effects on society.

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In the future, we will need to apply technologies that are consistent with the

values of society. We also need to direct research efforts to develop systems

of animal production that value and preserve natural resources. One solution

is to develop performance standards or expected outcomes for

environmental quality for the livestock industry.

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Borts, L.; May, G. and Lawrence, J. D. 2004. Diversified versus specialized swine and grain operations. ASL-R1959. ISU Anim. Ind. Rep. Dept. of Anim. Sci., Iowa State University, Ames.

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De Haan, C.; Steinfeld, H. and Blackburn, H. 1997. Livestock and the Environment: Finding a Balance. WREN-Media, Fressingfield, Eye,

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Suffolk, UK, 115 pp.

Dijkstra, J.; Oenema, A., van Groeningen, J.W.; Spek, J.W.; van Vuuren, A.M. and Bannink, A. 2013. Diet effects on urine composition of cattle and N2O emissions. Animal 7, 292–302.

Dourmad, J.Y.; Sève, B.; Latimier, P.; Boisen, S.; Fernandez, J.; van der Peet-Schwering, C. and Jongbloed, A.W. 1999. Nitrogen consumption, utilisation and losses in pig production in France, The Netherlands and Denmark. Livestock Production Science 58, 261–264.

Durning, A.B. and Brough, H.B. 1991. Taking stock: animal farming and the environment. Worldwatch Paper 103, 64–67.

Ensminger, M.E.; Oldfield, J.E. and Heinemann, W.W. 1990. Feeds and Nutrition. The Ensminger Publishing, Clovis, CA, USA, p. 1524.

Flora, C.; Black, J. R.; Dalton, T. J.; Kershegan,R.; Liebman, M.; Smith, S. N.; Sapp, S. S. and White, G. K. 2004. Reintegrating crop and livestock enterprises in three northern states. Preliminary Report on USDA-IFAFS Project 2001-52101-11308. Iowa State Univ., Ames.

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Hamilton, T.D.; Roe, J.M. and Webster, A.J. (1996) Synergistic role of gaseous ammonia in etiology of Pasteurella multocida-induced atrophic rhinitis in swine. Journal of Clinical Microbiology 34, 2185–2190.

Harper, L.A. 2005. Ammonia: Measurement issues. p. 345–379. In J. Hatfield (ed.) Micrometeorological measurements in agricultural systems. ASA, CSSA, and SSSA, Madison, WI.

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