Effects of rumen-undegradable protein on dairy cow performance: A 12-year literature review

In order to integrate and analyze knowledge on the use of protein supplemen ts and protein nutrition of lactating dairy cows, we compiled a review of 1 08 studies published throughout the world, but principally in the Journal o f Dairy Science between 1985 and 1997. In 29 comparisons from 15 metabolism trials, soybean meal was replaced by high amounts of rumen undegradable pr otein (RUP) as a supplement; the benefits were not consistently observed fo r flow to the duodenum, essential amino acids, or lysine and methionine. Hi gh RUP diets resulted in decreased microbial protein synthesis in 76% of th e comparisons. However, fish meal provided a good balance of lysine and met hionine when calculated as st percentage of total essential amino acids. In 127 comparisons from 88 lactation trials that were published fi om 1985 to 1997, researchers studied the effects of replacing soybean meal with high RUP sources, such as heated and chemically treated soybean meal, corn glute n meal, distillers grains, brewers grains, blood meal, meat and bone meal, feather meal, or blends of these sources; milk yield was significantly high er in only 17% of the comparisons. Fish meal and treated soybean meal accou nted for most of the positive effects on milk yield fi om RUP; corn gluten meal resulted in mostly negative results. The percentage of fat in milk was depressed more by fish meal than by other RUP sources. Protein percentage was decreased in 28 comparisons and increased in only 6 comparisons, probab ly reflecting the decrease in microbial protein synthesis, as was observed for diets high in RUP. The data strongly suggest that increased RUP per se in dairy COW diets, which often results in a decrease in RDP and a change i n absorbed AA profiles, does not consistently improve lactational performan ce.