Optimized dairy grazing systems in the northeast United States and New Zealand. I. model description and evaluation

Parallels exist in the recent developments of dairy systems in the Northeas t United States and New Zealand because of greater use of pasture grazing a nd feed supplements, respectively. Lessons can be learned from each system. However, major differences exist between the regions in the patterns of pa sture production, the costs of supplementary feed, and milk prices. These d ifferences affect the optimum use of feed. In this paper, a linear programm ing model developed to determine optimum feeding strategies for dairy syste ms in each country is presented. The model optimizes grazing management (ro tation lengths) and the conservation of pasture subject to constraints on t heir use. Other feed resources include N fertilizer, grain, corn silage, an d alfalfa silage. All feeds are represented in energy terms. The substituti on of pasture intake by grain and forage supplements is included, and cow p erformance can be optimized by choosing from 73 seasonal calving herds that vary in calving date, lactation length, and daily milk production. The mod el predicts that marginal responses to grain feeding are between 1.35 and 1 .8 kg of milk/kg of grain dry matter supplement, well within the range of r esponses reported in the literature. Evaluation of the model against data f rom nine grazing system treatments in New Zealand and two in Pennsylvania s howed that model predictions averaged +3% (New Zealand) and +0.04% (Northea st) of measured milk production. The model could be used with confidence to study systems in both the Northeast United States and New Zealand.