Nitrogen is an essential element in animal production. Largequantities of N are required for the growth of feed crops. CropN, primarily in the form of protein, is then an essential feedcomponent for animal growth and development. Most of the N consumedis excreted by animals, providing manure nutrients needed forcrop growth. The problem in this cycling of N is that largelosses normally occur that contribute to the degradation ofour environment (Figure 1). The challenge is to manage the animals,crops, and other farm components to efficiently use availablemanure N, and thus reduce the potential loss to the environment.
Figure 1. Major nitrogen flows in animal production within the farm and between the farm and its environment.
The primary pathways of N loss are volatile emissions into the
atmosphere and leaching and runoff losses to ground and surface
waters (de Vries et al., 2001
). In the recent past, leaching
of nitrates into groundwater has been the major N loss concern.
Groundwater concentrations in excess of 50 mg NO3
/L are potentially
harmful to human health, particularly for infants and small
children (Di and Cameron, 2002a
). Concern is now developing,
though, for N emissions to the atmosphere. Atmospheric ammonia
emissions contribute to ecosystem fertilization, acidification,
and eutrophication (NRC, 2003
). Microbial processes of nitrification
and denitrification also emit nitrous oxide into the atmosphere
). Nitrous oxide is a potent greenhouse gas that is
contributing to the concern over global warming. Minor amounts
of nitric oxide and nitrogen dioxide may also be emitted, which
are often limiting precursors in tropospheric ozone production
). Surface runoff of N, primarily as nitrate, is a
lesser concern, but it also contributes to the eutrophication
of surface waters along with soil drainage loss and acid rain.
Reducing N loss from the farm must begin with proper animalfeeding and management to reduce N excretion. Even with goodmanagement though, large quantities of N are in the manure.A major portion of this excreted N can quickly transform intoammonia, which may readily volatilize and move into the atmosphere.Volatile loss begins soon after excretion, and it continuesthrough all manure handling processes until the manure nutrientsare incorporated into soil. If steps are taken to maintain theN until it is incorporated, leaching and denitrification lossesof soil N will increase if that N is not applied at the appropriateamount and time for crop uptake. Only by properly managing allfarm components can we truly increase N use efficiency in animalproduction and reduce N escape into our environment (Figure1; Rotz et al., 1999b).
Management processes are available or under development thatreduce N loss, but implementation often remains a challengedue to various constraints. The major constraint is usuallyeconomic. New management practices often require large investmentsor greater operating costs that are difficult to justify. Currentprofit margins are low in most animal operations, and the directeconomic return for saved nutrients is small considering theirfertilizer replacement value. Labor can also be a constraint.Available labor is often heavily used in animal production,so changes that require more time, particularly for the farmmanager, will not be readily accepted. Environmental issuesthemselves may also be a constraint. For example, reducing ammoniaemissions may lead to greater emission of nitrous oxide. Dueto the potential long-term impact of this greenhouse gas, thenet benefit to society may not be positive, and implementationof the ammonia-conserving technology will be constrained.
The goal of this review is to quantify N losses for variousanimal production strategies and to discuss the major managementoptions available for reducing these losses. Although the materialpresented generally applies to all animal species, the emphasiswill be placed upon dairy, beef, swine, and poultry production.