Abstract :
The efficiency of N utilization in ruminants is typically low (around 25%) and highly variable (10% to 40%) compared with the
higher efficiency of other production animals. The low efficiency has implications for the production performance and environment.
Many efforts have been devoted to improving the efficiency of N utilization in ruminants, and while major improvements in our
understanding of N requirements and metabolism have been achieved, the overall efficiency remains low. In general, maximal
efficiency of N utilization will only occur at the expense of some losses in production performance. However, optimal production
and N utilization may be achieved through the understanding of the key mechanisms involved in the control of N metabolism. Key
factors in the rumen include the efficiency of N capture in the rumen (grams of bacterial N per grams of rumen available N) and
the modification of protein degradation. Traditionally, protein degradation has been modulated by modifying the feed (physical
and chemical treatments). Modifying the rumen microflora involved in peptide degradation and amino acid deamination offers an
alternative approach that needs to be addressed. Current evidence indicates that in typical feeding conditions there is limited net
recycling of N into the rumen (blood urea-N uptake minus ammonia-N absorption), but understanding the factors controlling urea
transport across the rumen wall may reverse the balance to take advantage of the recycling capabilities of ruminants. Finally, there
is considerable metabolism of amino acids (AA) in the portal-drained viscera (PDV) and liver. However, most of this process occurs
through the uptake of AA from the arterial blood and not during the ‘absorptive’ process. Therefore, AA are available to the
peripheral circulation and to the mammary gland before being used by PDV and the liver. In these conditions, the mammary gland
plays a key role in determining the efficiency of N utilization because the PDV and liver will use AA in excess of those required by
the mammary gland. Protein synthesis in the mammary gland appears to be tightly regulated by local and systemic signals. The
understanding of factors regulating AA supply and absorption in the mammary gland, and the synthesis of milk protein should
allow the formulation of diets that increase total AA uptake by the mammary gland and thus reduce AA utilization by PDV and the
liver. A better understanding of these key processes should allow the development of strategies to improve the efficiency of N
utilization in ruminants.
