Supplemental feeding strategies to increase the utilization of pasture nitrogen by high producing dairy cow

Four studies were conducted to determine: (1) the extent to which ruminal and whole-animal capture of N by grazing dairy cows could be increased using carbohydrate supplements, and (2) the ability of a simulation model to predict the nutritional constraints of pasture for high milk production, and describe the biological mechanisms governing the response of grazing dairy cows to supplemental carbohydrate. Synchronous ruminal release of supplemental carbohydrate with pasture N appeared to improve capture of ruminal N, however these changes were transient and did not change the N status of the dairy cow. Continuous culture fermentation of pasture diets with increasing level of starch inclusion reduced N loss from the rumen, but did not increase microbial protein flow. The Cornell Net Carbohydrate and Protein System model was evaluated for dairy cows consuming pasture-based diets. The model gave a reasonably good prediction of the requirements and supply of metabolizable energy and protein. Milk production was first-limited by metabolizable energy supply when only high quality pasture was fed, while specific amino acids limited milk production when more than 20% of the diet consisted of a grain supplement. This model can be used in a grazing system to make realistic predictions of dairy cattle performance. In a subsequent study, cows grazing all pasture produced 33% less milk than cows fed a mixed ration in confinement. The model indicated that 61% of the decrease in milk production was a result of reduced intake. Although a high intake was achieved on spring pasture, the significant mobilization of energy reserves indicated that supplemental energy was required to achieve a milk production greater than 30 kg/d from intensive grazing systems. Improvements in N utilization will depend on the extent that supplemental carbohydrate reduces dietary N intake, and increases the availability of ruminal energy. The ability to identify specific limiting nutrients, and understand the biological reasons for these limitations, can allow milk production to be increased from high quality pasture.