Animal grouping strategies, sources of variation, and economic factors affecting nutrient balance on dairy farms.

Increasing environmental concerns are forcing animal industries to reevaluate current feeding practices and their relationships to nutrient excretion. Previous modeling efforts have used simple budgets of nutrient flows through animals, assuming a constant productivity level. This assumption is not valid if animals are not in a steady state. A response model of dairy cow production to levels of net energy for lactation (NEL) and crude protein (CP) was derived from an abrupt threshold and plateau model of individuals. Monte Carlo techniques were used to simulate populations of cows fed diets of various NEL and CP concentrations, to derive the optimum allocation of NEL and CP, and to estimate how the optimum is affected by herd production potential, prices of inputs, and uncertainty of parameters. The simulation showed that a 25% increase in milk production reduced N excretion per kilogram of milk produced by 8%. Improved knowledge of the biology involved and feed composition can reduce N excretion by an additional 8%. Grouping strategies and number of groups used affect optimum allocation of nutrients. An optimum of six milking groups per production unit was derived from the simulation and would reduce N excretion by 8% compared to herds fed in one group.