Prediction of Nitrogen Excretion in Dairy Farms Located in North Florida: A Comparison of Three Models1

The increasing N concentrations in surface and groundwater in north Florida emphasize the need to identify sources of N loss and ways to reduce them. The amount of N excretion produced by dairy farms and deposited into the Suwannee River agro-ecosystem is being heavily scrutinized by regulatory agencies because it is believed to contribute significantly to the high N concentrations in water. Models developed by Van Horn and the USDA-Natural Resource and Conservation Service are used to estimate N balances on dairy farms. This study explores ways to improve these estimates by using dynamic simulation of N excretion over time. The Livestock Dynamic North Florida Dairy Farm model (LiDyNoFlo), which was created for this purpose, is described. The amount of N excretion on a dairy farm depends on crude protein in the diet, milk production, the presence of mature bulls and heifers, and seasonality of production. The LiDyNoFlo considered more variables than earlier models, and estimates of N excretion differed from those of other models. Comparisons consistently showed the LiDyNoFlo predictions of N excretion were between those predicted by the Van Horn model (upper end) and the Natural Resource and Conservation Service model (lower end). The LiDyNoFlo predicted that a 1,000-cow operation produced 324 kg of N excretion/d in February and 307 kg of N excretion/d in August because of seasonal milk production and herd dynamics. Seasonal differences in N excretion are important because they determine the opportunity for N recycling in the crop fields such that total N losses into the Suwannee River agro-ecosystem may be minimized.