EVALUATION AND APPLICATION OF THE CORNELL NET CARBOHYDRATE AND PROTEIN SYSTEM FOR DAIRY-COWS FED DIETS BASED ON PASTURE

This study evaluated the Cornell Net Carbohydrate and Protein System f or dairy cows consuming diets based on pasture, assessed the sensitivi ty of the model to critical inputs, and demonstrated application oppor tunities. Data were obtained from four grazing experiments and four in door pasture feeding experiments (25 dietary treatments) involving dai ry cows in New Zealand and the US. The model provided a reasonably goo d estimate of changes in body condition score (r(2) = 0.78; slope not significantly different from 1), estimated energy balance (r(2) = 0.76 ; slope not significantly different from 1), blood urea N (r(2) = 0.94 ; underprediction bias of 0.5%), microbial N flow (r(2) = 0.88; slope not significantly different from 1), and milk production. The model un derpredicted dry matter intake( r(2) = 0.80; 13% bias) and overpredict ed ruminal pH (r(2) = 0.47; 1.7% bias). Predicted milk production was especially sensitive to changes :in pasture lignin content, effective fiber, rate of fiber digestion, and amino acid composition of ruminal microbes. Milk production was first-limited by the supply of metaboliz able energy when only high quality pasture was fed, but specific amino acids limited milk production when more than 20% of the diet consiste d of a grain supplement. These results indicate that the Cornell Net C arbohydrate and Protein System can be used for dairy cows in a grazing system to make realistic predictions of performance.