Nitrogen budgets and losses in livestock systems
Introduction
Over the last few decades, the number of domestic animals in the world has increased faster than the human population. Between 1960 and 2000, the human population roughly doubled, while the number of domestic animals tripled [6]. This relative increase in number of domestic animals reflects an increase in the consumption of animal protein per capita [18]. Forecasts suggest further increases in animal numbers in the range of 30% to 50%, with largest increases for poultry [5]. Increases will be relatively large in the developing countries [8].
Livestock production systems can broadly be classified in (i) grazing systems, (ii) mixed systems and (iii) land-less or industrial systems [22]. Grazing systems are entirely land-based systems, with stocking rates less than one livestock unit per ha. In mixed systems, a significant part of the value of production comes from other activities than livestock production while part of the animal feed often is imported. Industrial systems have stocking rates greater than 10 livestock units per ha and they depend primarily on outside supplies of feed, energy and other inputs. In land-based animal systems, most of the N in the animal manure is ultimately returned to the land that produced the animal feed. In intensive animal production systems, the manure is disposed of elsewhere, as the land-base is missing [16]. There is a trend that pigs and poultry are increasingly held in industrial, land-less systems, also in Southeast Asia and Latin America, i.e. areas where the projected increases in animal numbers will take place [8].
The expected trend towards more (intensive) animal production systems may have important environmental and social implications. Animal production systems have a relatively large share in the emissions of ammonia (NH3), nitrous oxide (N2O) and methane (CH4) into the atmosphere [3], [12], [20]. Animal production systems also contribute to eutrophication of groundwater and surface waters as a result of N and phosphorus (P) enrichment. This holds especially for intensively managed livestock farming systems (e.g., land-less systems), where the nutrient inputs via purchased animal feed and fertilizers by far exceed the nutrient outputs in useful animal products. There is an increasing amount of evidence to suggest that these unbalances, especially for nitrogen (N) and phosphorus (P) have shifted in scale from what was once a local problem to what is now one of regional and continental dimensions. Human interference with the nutrient cycle has tremendously increased in scale and intensity during the last century, and it is indeed the shift in scale of the intensification of agricultural production that has given the impetus to use nutrient budgets as awareness raiser, as indicators for nutrient losses and as tool for improving nutrient management [17].
This paper briefly reviews N budgets and N losses in livestock farming systems. Options for improving N use efficiency in animal production systems are also discussed, using literature data and results of simple calculations and sensitivity analyses of N flows in global animal production systems.
Section snippets
Nitrogen budgeting
Nutrient budgets have a well-established basis in both the natural and socioeconomic sciences, as they rely on the principle of ‘conservation of matter’. They have been widely adopted in agronomic and environmental research, and they are increasingly incorporated in voluntary and mandatory regulations at farm, regional and continental levels [10], [17]. Nutrient budgets provide information about nutrient losses and nutrient use efficiency, and the information generated is readily communicable
Nitrogen budgets of livestock farming systems
Nitrogen budgets of some grassland-based beef and dairy farming systems in United Kingdom (UK) and The Netherlands (NL) are shown in Table 1. Commonly, total N input ranges between 50 and 500 kg/ha/year, total N output via milk and meat between 5 and 75 kg/ha/year, and the N surplus (difference between total N input and N output via milk and beef) between 50 and 450 kg/ha/year. The N surplus increases with an increase in purchased N fertilizer and animal feed. Output via animal products is
Animal N excretion and animal manure management
Only a small fraction of the N in animal feed is retained in milk, meat and eggs. The greater part (55–95%) is voided by the animals (Fig. 1). The ratio of N in urine to N in dung depends on animal species, the protein content of the animal feed and the production level of the animal. Animals supplied with high-protein diets excrete a large proportion (> 50%) of the excreted N via urine, whereas low-protein diets yield a larger proportion of the N via dung. The total amount of N excreted depends
Fate of N from animal manure
There are many opportunities and places for gaseous N compounds to escape from animal manure management systems into the wider environment. Volatilization of NH3 occurs at an early stage in the sequence of processes following the excretion of feces and urine. The estimated mean amount of NH3–N volatilized ranges from 5% to 15% of N excreted in pastures and from 5% to 30% of N excreted by confined animals in animal manure management systems (AMMSs). The wide ranges reflect the effects of
Mitigation of N losses from animal production systems
How to decrease N losses and how to increase N use efficiency in animal production systems? The key answer is most likely found in the management of the animal manure, as other factors are difficult to manage. Changes in number of animal species are primarily driven by the increasing quest for food by the increasing human population. In theory, there is opportunity for lowering per capita animal protein consumption and for replacing animal protein by plant protein in affluent countries, but
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