REVIEW: Does Protein in Alfalfa Need Protection from Rumen Microbes?

Alfalfa (Medicago sativa), commonly fed to ruminants as hay or silage, is a highly-nutritious and high-yielding forage legume. Pure stands of alfalfa are seldom grazed due to the risk of pasture bloat. Alfalfa is high in CP content, and approximately 80% of the protein is degraded in the rumen. This high degradability potentially leads to poor N utilization by the rumen microorganisms and a loss of excess ammonia via absorption, energy dependent conversion to urea, and eventual excretion as urinary N, contaminating the environment. Excessive degradation of protein in the rumen results in reduced efficiency of dietary N utilization, particularly in high-producing dairy cows. There has been a growing interest in developing novel ways to reduce the rate and extent of alfalfa protein degradation in ruminants. The rationale for reducing rumen degradability of dietary proteins are 1) protein requirements of high-producing animals cannot solely be met from rumen microbial sources; 2) protein is the most expensive component of ruminant animal diets; 3) excessive degradation of proteins leads to greater amounts of nitrogenous compounds excreted in the urine and feces, which is a major environmental concern; 4) high plasma urea associated with extensive protein degradation in the rumen can cause reproductive problems in dairy cows; and 5) a rapid and excessive ruminal degradation of fresh alfalfa protein can cause bloat, which in severe case causes death of the animal. The available data strongly suggest that alfalfa protein is extensively degraded in rumen and, therefore, is not optimally utilized in ruminants. Alfalfa constitutes a significant portion of the diet of high-producing cows in the United States and any approach that reduces alfalfa protein degradation in the rumen will improve the efficiency of N utilization. This paper reviews the available literature on alfalfa protein degradability and summarizes the approaches that have a potential to increase alfalfa protein utilization in ruminants.