| Popis: |
I t has been known for almost a century that ruminant animals have the unique abili ty to convert nonprotein nitrogen (NPN) to protein. Becker (11) reviewed the early use of NPN compounds as protein replacers for ruminants. Weiske et al. (221) may have been the first (in 1879) to suggest using NPN compounds as protein substitutes in ruminant rations. Urea has been the most widely used NPN compound in ruminant rations. Reid (176) and Briggs (28) reviewed the literature on urea as a protein replacement for ruminants. Recent estimates are that current use of feed urea in the United States exceeds 300,000 tons annually (211), equivalent to about 20% of the high-protein meal consumption. T h e annual rate of increase in urea usage for all ruminants in the U.S. apparent ly is 25,000 to 30,000 tons. The increase results from demand for the natural supply of supplementary feed protein, the economic advantage of using urea, increased acceptance of urea by farmers, improved methods of using urea in feeds, and reduced price of urea resulting from larger urea-manufacturing plants and technological improvements. This review presents discussion of the methods that have been used to improve urea utilization by ruminants. Various aspects of urea metabolism and animal response to urea supplementation are also discussed. |
