Nr. St-pierre et Cs. Thraen, Animal grouping strategies, sources of variation, and economic factors affecting nutrient balance on dairy farms, J ANIM SCI, 77, 1999, pp. 72-83
Increasing environmental concerns are forcing animal industries to reevalua
te current feeding practices and their relationships to nutrient excretion.
Previous modeling efforts have used simple budgets of nutrient flows throu
gh 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. Mo
nte 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 pot
ential, prices of inputs, and uncertainty of parameters. The simulation sho
wed that a 25% increase in milk production reduced N excretion per kilogram
of milk produced by 8%. Improved knowledge of the biology involved and fee
d composition can reduce N excretion by an additional 8%. Grouping strategi
es and number of groups used affect optimum allocation of nutrients. An opt
imum of six milking groups per production unit was derived from the simulat
ion and would reduce N excretion by 8% compared to herds fed in one group.
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