Submitted to: Journal of Dairy Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/30/2009
Publication Date: 6/1/2009
Citation: Broderick, G.A., Reynal, S.M. 2009. Effect of Source of Rumen-Degraded Protein on Production and Ruminal Metabolism in Lactating Dairy Cows. Journal of Dairy Science. 92:2822-2834. Interpretive Summary: The dairy cow obtains her needs for amino acids, the building blocks of proteins, largely from synthesis by the microbes living in the rumen, the first compartment of the cow's stomach. Usually, about two-thirds of her amino acids are obtained when microbial protein is digested in the cow’s small intestine after flowing out of the rumen. The other one-third comes from feed protein that escapes breakdown in the rumen. The rumen microbes can meet their own requirements for making protein from “degraded protein”, dietary protein that is degraded by the rumen microbes into three types of nitrogen-containing compounds: ammonia, amino acids and peptides. Ammonia is a simple chemical that can be provided by farmers to the rumens of their dairy cows by feeding urea, which is relatively inexpensive compared to typical feed proteins. However, amino acids and peptides can only be formed in the rumen by breakdown of true proteins such as those in alfalfa, soybean meal or other feeds. There is a major controversy in dairy cattle nutrition about whether urea can provide all or part of the degraded protein required by the rumen microbes; or, if it cannot provide all degraded protein, what is the degree to which microbial protein formation is impaired when urea provides some of the degraded protein? Twenty-eight lactating dairy cows were fed four different diets that were formulated from typical feedstuffs and contained equal amounts of degraded protein, but with the degraded protein coming from four different proportions of urea and soybean meal. Eight of the cows had cannulas (external openings to their rumens made surgically by veterinarians) so digesta flowing out of the rumen could be sampled and analyzed for amount of microbial protein on each of these diets. Feed consumed and yield of milk, protein and fat were all reduced as the amount of degraded protein coming from urea was increased. Sampling the digesta flowing out of the rumen revealed that increasing the proportion of degraded protein coming from urea decreased the total amount of amino acids passing out of the rumen to the small intestine. Microbial protein and amino acid supply to the cow were reduced about 20% when one-third of the degraded protein came from urea. This research indicates that replacing degraded protein from soybean meal (and probably other true proteins) with that from urea will reduce production of milk and milk components, and also farmer profits, as a direct result of depressed microbial protein formation in the cow’s rumen. Trying to save money by replacing dietary protein supplements with urea is false economy. The dairy farmer is better off purchasing true protein supplements such as soybean meal to provide the degraded protein for the rumens of his/her cows.
Technical Abstract: Twenty-eight (8 with ruminal cannulas) lactating Holstein cows were assigned to a 16-wk, 4 x 4 Latin squares study to examine the effect on production and ruminal metabolism of feeding differing proportions of rumen-degraded protein (RDP) from soybean meal and urea. Diets contained dry matter [(DM) basis] 40% corn silage, 15% alfalfa silage, 28-30% high-moisture corn, plus varying levels of ground dry shelled corn, solvent and lignosulfonate-treated soybean meal, and urea. Proportions of the soybean meals, urea, and dry corn were adjusted such that all diets contained 16.1% CP and 10.5% RDP, with urea providing 0, 1.2, 2.4, and 3.7% RDP (DM basis). As urea supplied greater proportions of RDP, there were linear decreases (P = 0.04) in DM intake, yield of milk, 3.5% FCM, fat, protein and SNF, and of weight gain. Milk content of fat, protein and SNF were not affected by source of RDP. Replacing soybean meal RDP with urea RDP resulted in linear increase (P < 0.01) in excretion of urinary urea-N and fecal N, and in concentration of MUN, blood urea-N and ruminal ammonia-N; there was also a trend for increased excretion of total urinary N. A linear increase in NDF digestibility, probably due to digestion of NDF-N from lignosulfonate-treated soybean meal, was observed with greater urea intake. Omasal sampling revealed small but significant (P = 0.05) effects of N source on measured RDP supply, which averaged 11.0% (DM basis) across diets. Increasing the proportion of RDP from urea resulted in linear decrease (P = 0.05) in omasal flow of dietary nonammonia N (NAN) and microbial NAN flow and in growth efficiency (microbial NAN/unit of organic matter truly digested in the rumen). These changes were paralleled by large linear reduction (P < 0.01) in omasal flows of essential AA, nonessential AA, and total AA. Overall, these results indicate that replacing soybean meal RDP with that from urea reduced yield of milk and milk components due to depressed microbial protein formation in the rumen, suggesting that NPN sources are not equal to RDP provided by true protein.