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Effects of manipulating protein and phosphorus nutrition of feedlot cattle on nutrient management and the environment¹

Department of Animal Science, University of Nebraska, Lincoln 68583-0908

² Correspondence: E-mail: ansc825{at}unlvm.unl.edu.

Abstract

Feedlot nutrition will play a role in meeting challenges such as nutrient management. Nitrogen and phosphorus are two nutrients that are currently studied in this context. One nutritional method is formulating diets not to exceed requirements for nitrogen and phosphorus. Requirements are different for calves and yearlings. The requirements also change during the finishing period. Phosphorus requirements have not been extensively studied for feedlot cattle between 270 and 600 kg. Therefore, P requirements studies were conducted to determine the P requirement of calves (265 kg) and yearlings (385 kg). The requirement was not detected with P levels as low as 0.14 (yearlings) and 0.16% (calves) of diet DM based on performance and bone ash. Compared to NRC-predicted P requirements, P intakes ranged from 76 to 190% (calves) and 71 to 162% (yearlings). In separate nutrient balance experiments, decreasing dietary P to NRC-predicted requirements (0.22 to 0.28%) did not influence gain but decreased P input by 33 to 45% and excretion by 40 to 50% compared to the industry average (0.35% P). The metabolizable protein (MP) system was recently adopted and may allow more accurate diet formulation for protein, thereby decreasing N excretion. Compared to the industry average (13.5% CP) and formulation with the CP system, using the NRC model and phase-feeding not to exceed MP requirements over the feeding period decreased N inputs by 10 to 20% for calves and yearlings without affecting ADG. Decreasing N inputs led to a concomitant decrease in N excretion (12 to 21%) and volatilization (15 to 33%) in open-dirt feedlot pens. Nitrogen losses are variable with time of year, with averages of 60 to 70% of excreted N lost during the summer months and 40% lost from November to May feeding periods. Protein requirements are continually being refined as more research data are collected. However, formulation to meet and not exceed protein requirements and removal of P supplements are important nutritional management options to help feedlots become more environmentally sustainable.

¹ Published with the approval of the director as paper no. 13581, journal ser., Nebraska Agric. Res. Div.

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