REVIEW: Feeding Nitrogen and Phosphorus in Beef Cattle Feedlot Production to Mitigate Environmental Impacts

ABSTRACT

As a result of favorable climatic conditions, availability of feed grains, and location of beef processing facilities, the feedlot cattle industry has become increasingly concentrated in the southern and central Great Plains of the United States. These operations concentrate nutrients in a relatively small area, resulting in environmental issues associated with feedlot cattle such as nutrient pollution of ground and surface water as well as pollution of air. Excreted N and P can cause significant water pollution if directly discharged into surface water through runoff or deposited in water from aerial emissions. In addition, volatilized N in the form of NH^sub 3^ from voided urine and feces returns to the land or water via rainfall, dry precipitation, or direct absorption. The development of feeding strategies to improve the utilization of N by the animal, decreasing N lost to the environment, and improving N-to-P ratio in pen surface manure are likely to mitigate environmental impacts of feedlot cattle. Nutrient requirements of feedlot cattle change during the feeding period. Nonetheless, feedlot cattle are typically fed a constant level of CP and other nutrients from about d 24 of feeding through slaughter. Consequently, CP is often underfed early and overfed late in the feeding period. Feeding nutrients at concentrations that closely match animal requirements can prevent excess excretion of nutrients. The objectives of this review are to discuss the known environmental impacts of beef cattle feedlots and to summarize current feeding strategies to decrease these environmental impacts while maintaining animal performance.

Key words: environment, feedlot, nitrogen, phosphorus

INTRODUCTION

Concentrated animal feeding operations (CAFO) are facing increasing concern regarding environmental issues. Most beef cattle CAFO are located in the southern and central Great Plains of the United States (Albin, 1998) and concentration of nutrients (e.g., N and P) from these operations is limited to this geographical area (Cole and Greene, 1998).

Nitrogen and P are major nutrients that are important nutritionally, environmentally, and economically. They can cause significant water pollution if directly discharged into surface water through runoff (Klopfenstein et al., 2002). Moreover, volatilized N in the form of NH^sub 3^ from voided urine and feces (referred herein as manure) returns to the land or water via rainfall, dry precipitation, or direct absorption. Volatilized NH^sub 3^ is also related to the nuisance of odor and contributes to formation of particulate matter emissions that are potentially subjected to regulation. Improper nutrient disposal and management may affect water quality, odor, flies, and aerial emissions of N compounds such as ammonium (NH^sub 4^^sup +^), nitrate (NO^sub 3^^sup -^), and nitrite (NO^sub 2^^sup -^) to the atmosphere or groundwater, along with aerial emissions of methane (CH4; Van Horn et al., 1996).

The development of feeding strategies to improve the utilization of N by the animal, decrease N lost to the environment, and improve N- to-P ratio in pen surface manure is likely to mitigate environmental impacts of feedlot cattle. Reducing dietary excesses will decrease nutrient excretion in feedlot manure. Feeding nutrients at concentrations that closely match animal requirements during the feeding period can prevent excessive excretion of nutrients in feedlots (Galyean, 2000; Klopfenstein et al., 2002). The question becomes whether nutrient management planning can be successfully implemented without decreasing productivity and profitability in practical conditions. The objectives of this paper are to discuss the known impacts of beef cattle feedlots to the environment and to summarize current feeding strategies to mitigate these environmental impacts, while maintaining animal performance.

Environmental Impacts of Nitrogen

Nitrogen is continually cycled among plants, soil organisms, soil OM, water, and the atmosphere in a complex series of biochemical transformations. At any given time, most of the N in the soil is held in OM (decaying plant and animal tissue) and humus. Nitrogen is excreted in animal waste in the form of urea and other organic N forms. Urea is the major urinary end-product of N metabolism in cattle (Archibeque et al., 2001). The quantity of urea excreted is positively correlated with digestible N intake. Urea in the urine can be rapidly hydrolyzed to form (NH^sub 4^)^sub 2^CO^sub 3^. The decomposition of (NH^sub 4^)^sub 2^CO^sub 3^ frees NH^sub 4^^sup +^, which can volatilize as gaseous NH^sub 3^ as the pH of excreted waste increases. The process of urea hydrolysis is catalyzed by the enzyme urease, which is abundant in soils, plant roots, and animal feces (Anderson et al., 2003). The conversion of urinary urea to NH^sub 3^ by urease causes NH^sub 3^ to escape into the air followed by deposition and nitrification to nitrate, which can leach into water bodies (Tamminga, 1996; Krupa, 2003).

The impact of N on water quality depends on its chemical form. The most biologically important inorganic forms of N are NH^sub 4^^sup +^, NO^sub 3^^sup -^, and NO^sub 2^^sup -^. Nitrate is the nutritive form of N needed by plants; however, if produced in excess, soluble NO^sub 3^^sup -^ leaches to ground waters where it can contaminate drinking water. The NH^sub 4^^sup +^ form becomes mostly adsorbed to the soil, and it is lost primarily with eroding sediment. Even if N is not in a readily available form as it leaves the feedlot, it can be converted to an available form either during transport or after delivery to water bodies (EPA, 2003). Nitrate concentrations in ground water are associated with source availability and regional environmental factors. In general, areas with high N input, well-drained soils, and high cropland areas have the highest potential for ground water contamination by NO^sub 3^^sup -^.

Ammonia mainly originates from livestock (Perm, 1998), and the majority from this source is emitted into the atmosphere (Van Horn et al., 1996). Combined with other volatile compounds, NH^sub 3^ generates offensive odor emissions. Atmospheric NH^sub 3^ enhances the deposition of NO^sub 3^^sup -^ or N^sub 2^O, creating acid rain that acidifies soils and woodlands (McCrory and Hobbs, 2001). Depending on their diet, finishing cattle excrete approximately 60 to 80% of their N in urine and 20 to 40% in feces (Van Horn et al., 1996; Bierman et al., 1999). Urine contains up to 97% urea-N, which is readily converted by microbial urease to NH^sub 3^ shortly after excretion (Varel, 2002). The driving force for NH^sub 3^ volatilization is the difference in NH^sub 3^ partial pressure in equilibrium with the liquid phase and that in the ambient atmosphere (McCrory and Hobbs, 2001). When other ionic species are not present, this is predominately influenced by the NH^sub 4^^sup +^ concentration, pH, temperature, moisture content, air movement, and possibly other factors (Van Horn et al., 1996). Anderson et al. (2003) included factors such as the amount of N in the diet and size and species of the animal as potentially important factors in the amount of NH^sub 3^ produced. In the atmosphere, NH^sub 3^ reacts with oxides of N and S to form NH^sub 4^^sup +^ salts that occur predominantly as fine particles. Ammonia and its subsequently derived NH^sub 4^^sup +^ are removed from the atmosphere both by dry and wet deposition onto terrestrial surfaces (Krupa, 2003).

Erickson and Klopfenstein (2001) estimated N loss in feedlots by measuring the quantity of N in runoff manure remaining on the pen surface after cleaning and by using estimates of the N retention based on ADG. They reported that N volatilization was 40 to 41% during the winter and 60 to 70% during the summer. To estimate the amount of N excretion, the authors performed a simulation using a 1,000-animal feedyard assuming a DMI of 10 kg/d of a diet containing 13.5% CP. They estimated that approximately 30 kg of N are retained per animal, resulting in a total excretion of 65 kg of N/animal in 350 d (Erickson and Klopfenstein, 2001).

Meeting the Nitrogen (Protein) Requirements of Cattle

Several feeding systems are currently being used to meet protein …