Animal Science Department


Date of this Version





In 1996, the Council for Agricultural Science and Technology (CAST) published a report entitled Integrated Animal Waste Management. One of the recommendations in that report was to “change animal diets to decrease nutrient outputs” (CAST 1996, 1). Since that time, concentration of animal production units has continued, public concern about the environmental effects of animal manure has increased, and the Environmental Protection Agency (EPA) has proposed more restrictive requirements for concentrated animal feeding operations (CAFO regulations).

Progress has been made since 1996 to decrease nutrient outputs by animals through diet modification and nutrition. The current study describes the existing technological advancements, the decrease in nutrient outputs possible, the degree of acceptance by poultry and livestock producers, and the potential for further technological advancements.

This study focuses on two nutrients and addresses two environmental concerns. The nutrients are nitrogen (N) and phosphorus (P). Nitrogen is a part of amino acids (AAs) that form proteins required by all animals; animals consume protein and AAs and then excrete various forms of N. Phosphorus is a mineral nutrient required for bone growth and many important bodily functions. But these nutrients, if directly discharged into surface water in runoff or deposited in water from aerial emissions, can cause significant water pollution.

The first environmental concern is the volatilization of N in the form of ammonia (NH3) from animal manures. Volatilized ammonia returns to the land or water via rainfall, dry precipitation, or direct absorption. Volatilized ammonia also can contribute to odor problems. Although ammonia may be beneficial as a fertilizer for agricultural fields, it may not be beneficial in other ecosystems. Manure in the form of a slurry when injected into the soil will have minimal losses of ammonia. The higher the N content of the manure, the greater the risk of ammonia loss. For example, most beef cattle are produced in open feedlots. Ammonia losses can represent as much as 70% of the N excreted by those cattle.

The second environmental concern is manure nutrient distribution. Manure is an excellent fertilizer for crop production. If manure nutrients are applied at rates equivalent to plant needs, then environmental impacts are minimal. If manure is applied at higher rates, however, N can leach into groundwater and P can build up in the soil and contaminate the surface water, harming the environment. As livestock and poultry units have increased in size, it has become more expensive to return manure to the cropland where the feed for the animals originated. The manure distribution problem can be local, regional, national, or even international. For example, approximately one-half of the corn grown in Nebraska is exported to other states or to foreign countries. Although there are many cattle feedlots in Nebraska, there is more than enough land on which to spread manure. Conversely, if midwestern corn is exported to Texas for cattle production, to North Carolina for swine production, or to Delaware for broiler production, it is difficult to return the nutrients to the land where the crops originated.

Decreasing the N and P excreted by poultry, swine, or cattle can minimize these two concerns. In the past, there has been little pressure to decrease excretion, so livestock and poultry producers have typically overfed protein (N) and P. Researchers have made key advances in this field during the past decade. Source reduction is the logical starting point to lessen the environmental impact. Significant changes are occurring, but more can be accomplished.