|Valadares Filho, S|
Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/25/1999
Publication Date: N/A
Citation: N/A Interpretive Summary: More than half of the protein in alfalfa silage, a major forage fed to dairy cows in the U.S., gets broken down to nonprotein nitrogen (NPN) in the silo. Improved use of this NPN can be obtained by feeding high energy feeds such as grains. Grains stimulate growth of the microbes in the rumen (the first compartment of the cow's stomach) to form protein from the NPN; this protein can in turn be used by the cow to meet its own protein requirements. However, the amount of grain that can be fed is limited. Over-feeding of grain results in over-production of acid in the rumen which leads to digestive upset and health problems for the cow. The main objective of this trial was to find the best level of high moisture corn to feed with alfalfa silage to get the greatest amount of protein formed from NPN by the rumen microbes. This experiment was conducted at the same time as another trial whose purpose was to find the best level of grain to feed for maximum production of milk and milk components. Four diets were fed containing: 80, 65, 50 or 35% alfalfa silage, supplemented with 20, 35, 50 or 65% grain (mainly from high moisture corn). Protein formation by rumen microbes was determined indirectly from urinary excretion of purine breakdown products. Purines absorbed by cows come largely from RNA and DNA in rumen microbes; therefore, urinary excretion of purine breakdown products is a reliable measure of the amount of microbe protein formed in the rumen. The maximum amount of microbial protein was produced at 48% grain in the diet; this would be the point of optimum utilization of the NPN in alfalfa silage. Farmers can use this information to determine how much grain to feed with alfalfa silage to get maximum utilization of its NPN and to minimize excretion of nitrogen from the dairy cow.
Technical Abstract: Twenty-four multiparous lactating Holstein cows were blocked by days in milk and assigned to treatment sequences in six 4X4 Latin squares with 21-d periods. Diets were formulated from alfalfa silage plus a concentrate mix based on ground high moisture ear corn, solvent soybean meal and urea. The four diets fed in the trial contained [dry matter (DM) basis]: 1) 20% concentrate, 80% alfalfa silage, 2) 35% concentrate, 65% alfalfa silage, 3 50% concentrate, 50% alfalfa silage, or 4) 65% concentrate, 35% alfalfa silage; diets were isonitrogenous and contained similar proportions of NPN. Intake of DM and milk yield indicated that adaptation was complete within 7 d of changing the diets within the Latin square. There were linear increases in apparent digestibility of DM and organic matter, and a linear decrease in apparent digestibility NDF, with increasing dietary concentrate. Solutions of significant quadratic equations yielded estimated dmaxima for intake of DM, organic matter, digestible organic matter, and ND at, respectively, 51, 54, 71, and 27% dietary concentrate. There were linear increases in yield of milk, protein, lactose and SNF, and a linear decrease in milk fat content, with increasing dietary concentrate. Maxima for yield of FCM and fat were estimated to occur at, respectively, 57 and 43% dietary concentrate. Our results suggested that 2-wk periods (total trial 8-wk) would have given the same statistical inferences as using 3-wk periods (with a third less cost and time) within the Latin square design. In this short-term trial, maximal feed intake and FCM yield indicated that the optimum concentrate level in dairy cows fed alfalfa silage as the only forage was between 51 and 57%; however, yield of milk, protein and SNF was still increasing at 65% dietary concentrate.