Researchers at the University of Arizona completed an extensive literature review of the use of protein supplements and protein nutrition of lactating dairy cows. The review involved 108 studies published during the period 1985 to 1997. In 29 comparisons from 15 metabolism trials, soybean meal was replaced with feed ingredients that provided high amounts of rumen undegradable protein. The high rumen undegradable protein diets resulted in decreased microbial protein synthesis in 76 percent of the comparisons. The rumen undegradable protein ingredients did not produce a consistent benefit in the flow of essential amino acid to the duodenum. In 127 comparisons, from 88 lactation studies, the effects of replacing soybean meal with high rumen undegradable protein ingredients - such as heat and chemically treated soybean meal, corn gluten meal, distillers grains, brewers grains, fish meal, blood meal, meat and bone meal, feather meal, or blends of these ingredients - significantly increased milk yields in only 17 percent of the comparisons. Fish meal and treated soybean meal accounted for the most of the positive effects on milk yield. The percent fat in milk was depressed more by fish meal than the other protein sources. Protein content of the milk was depressed in 28 comparisons and increased in only 6 comparisons, probably reflecting decreased microbial protein synthesis, as was observed in the diets high in rumen undegrabable protein.

These results suggest that high levels of rumen degradable proteins in dairy cow diets change the absorption pattern of amino acids and do not consistently improve milk production. Soybean meal, or soybean meals that have been treated to improve rumen by-pass, continue to provide a valuable, cost-effective protein source for dairymen.

Santos, F. and co-workers. 1998. Effects of rumen degradable protein on dairy cow performance: A 12-year literature review. J. Dairy Sci:81(12):3182-3213.