Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/5/2006
Publication Date: 4/1/2007
Citation: Brito, A.F., Broderick, G.A., Reynal, S.M. 2007. Effects of different protein supplements on omasal nutrient flow and microbial protein synthesis in lactating dairy cows. Journal of Dairy Science. 90:1828-1841. Interpretive Summary: Protein supplements are the most costly feeds that dairy farmers have to buy for their cows. Moreover, there are substantial differences in nutritional value among the protein sources commonly fed to U.S. dairy cows. We found in a lactation experiment that milk production was much lower, and urinary nitrogen excretion substantially greater, when cows were fed supplemental protein as urea, a cheap source of dietary “crude protein”, rather than soybean meal, cottonseed meal, or canola meal. Also, the cows fed canola meal rather than cottonseed meal produced more milk protein and milk fat, while fat and protein yield was intermediate in cows fed soybean meal. Dairy nutritionists have generally assumed that the non-protein nitrogen compounds such as urea were as effective as oilseed meals to meet the protein needs of the microbes in the rumen. These rumen microbes produce most of the protein digested in the cow’s intestines but the amount and quality of the protein supplied from diet is also important. In connection with the lactation experiment, we conducted a study to see how feeding these 4 different protein supplements affected the protein supplied for digestion in the cow’s intestines. To accomplish this, the same 4 diets also were fed to 8 cows that had rumen cannulas (holes made surgically into their rumens by veterinarians) so digesta flowing out of the rumen could be sampled to measure the amounts of protein originating from the microbes in the rumen and from the diet. As expected, more dietary protein flowed from the rumen when cows were fed one of the 3 oilseed meals rather than urea. However, feeding soybean meal, cottonseed meal, or canola meal also increased the flow of microbial protein compared to urea. This explained the much greater yield of milk and milk components found in the lactation experiment when cows were supplemented with one of the 3 oilseed meals. This research means that feeding true proteins such as oilseed meals is necessary to provide protein from both the rumen microbes and the diet in amounts sufficient to meet the needs of high-producing dairy cows.
Technical Abstract: Eight ruminally cannulated Holstein cows that were part of a larger lactation trial were used in 2 replicated 4 x 4 Latin squares to quantify effects of supplementing protein as urea, solvent soybean meal (SSBM), cottonseed meal (CSM), or canola meal (CM) on omasal nutrient flows and microbial protein synthesis. All diets contained (% of dry matter) 21% alfalfa silage and 35% corn silage plus: 2% urea plus 41% high moisture shelled corn (HMSC), 12% SSBM plus 31% HMSC, 14% CSM plus 29% HMSC, or 16% CM plus 27% HMSC. Crude protein was equal across diets, averaging 16.6%. The CSM diet supplied the least rumen-degraded protein and the most rumen-undegraded protein. Microbial nonammonia N flow was similar among the true protein supplements but was 14% lower in cows fed urea. In vivo ruminal passage rate, degradation rate, and estimated escape for the 3 true proteins were, respectively, 0.044/h, 0.105/h, and 29% for SSBM; 0.051/h, 0.050/h, and 51% for CSM; and 0.039/h, 0.081/h, and 34% for CM. This indicated that CSM protein was less degraded because of both faster passage rate and slower degradation rate. Omasal flow of individual AA, branched-chain AA, essential AA, non-essential AA, and total AA all were lower in cows fed urea compared to one of the true protein supplements. Among the 3 diets supplemented with true protein, omasal flow of Arg was greatest on CSM and omasal flow of His was greater on CSM, intermediate on CM, and lowest on SSBM. Lower flows of AA and microbial nonammonia N explained lower yields of milk yield and milk components observed on the urea diet in the companion lactation trial. These results clearly showed that supplementation with true protein was necessary to obtain sufficient microbial protein and rumen-undegraded protein to meet the metabolizable AA requirements of high-producing dairy cows.