370 Predicting nitrogen and phosphorous flows in beef open lots

This research summarizes a large existing data base collected over fifteen years to examine origin and fate of nitrogen and phosphorous in an open lot beef system, as impacted by seasonal changes, diets, and management practices. Data from 15 winter trials, including 200 pens, and 15 summer trials, including 216 pens were reviewed for nutrient mass balance, which was then used to determine the correlations of season, diet, and management with nitrogen and phosphorous intake, retention, manure harvested, loss, and runoff. All relationships were evaluated using linear regression SAS. Results suggested N in harvested manure for winter and summer averaged 90 and 51 g/steer daily, respectively (compared to ASABE standard of 88.5) and P harvested in manure for winter and summer averaged 26 and 17.7 g/steer daily, respectively (compared to ASABE standard of 37.5). Results of this summary show that the amount of nitrogen lost and retained in the manure is correlated to season (winter versus summer feeding period). Nitrogen in the manure (R2=0.21) and amount lost (R2=0.65) correlations with N intake in the summer were greater compared to winter (R2=0.06) and (R2=0.22). There is a greater correlation of phosphorous intake with amount harvested in manure (R2=0.32) in the winter versus the summer (R2=0.19). Results of the mass balance data compared with ASABE standards suggest greater observed dietary nutrient intakes than assumed by these standards. For example, average winter and summer nitrogen intakes of 220 and 250 g/steer daily exceeds the ASABE standard of 192 g/steer daily. These unique data describe the variability of nutrient recovery and losses from beef systems—and the importance of basing standard planning estimates on dietary concentration and evolving with current practices.