Modeling and Integrating Metabolizable Energy and Protein Supply and Requirements in Dry and Lactating Dairy Cattle to OptimizeNitrogen Utilization

摘要

The dairy cow has two requirements, one for rumen microbial function and growth that can be supplied by ammonia, small peptides and amino acids (AA) whereas the other requirement is for post-ruminal AA. Ruminai requirements are met from feed nitrogen(N), recycled urea and endogenous protein. Current data on urea recycling indicates the cow converts 50 to 70% of intake N into ammonia, which is converted to urea and of that 35% of intake N via urea is transferred into the rumen. The remaining urea N is excreted in the urine and milk. In addition to urea, the cow recycles a significant amount of endogenous protein that is utilized by the microbes or flows to the small intestine as high quality AA. The endogenous protein can constitute 20% of the totalAA supply to the cow, has a high intestinal digestibility (ID) and until recently this has been ignored and considered only as a maintenance requirement. Further, to ensure adequate N availability at the small intestine, the digestibility in the intestine needs to be considered. Recent data suggest feeds like blood meal can vary from 40% to 90% intestinal digestibility and not knowing the ID would create a situation where productivity falls, thus reducing the efficiency of use of absorbed N. Finally, energy drives the efficiency of use of absorbed AA and the AA are best described as a function of the metabolizable energy (ME) intake of the cow as ME drives protein synthesis, thus creating the demand for AA.