Submitted to: Journal of Dairy Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/6/2008
Publication Date: 3/1/2009
Citation: Broderick, G.A., Muck, R.E. 2009. Effect of Alfalfa Silage Storage Structure and Rumen-Protected Methionine on Production in Lactating Dairy Cows. Journal of Dairy Science. 92:1281-1289. Interpretive Summary: Alfalfa is often fed to lactating dairy cows as silage rather than hay because harvesting silage is much more rapid and requires less manual labor. Among the common structures used to ensile alfalfa are plastic bags, bunkers, and oxygen-limited silos. However, it is not known whether there is a clear advantage to using any one of these different silos for preserving the feeding value of alfalfa. There is also a problem with alfalfa silage in that its protein is extensively degraded in the rumen, the first compartment of the cow's stomach, so that the cow cannot utilize a substantial portion of the protein it eats. Because of this extensive degradation, it is often necessary to add more protein to alfalfa silage diets despite its already high protein level. Feeding too much protein to dairy cows results in excessive nitrogen excretion, especially in the urine; this form of excreted nitrogen is the most polluting to the environment. Supplementing the cow’s diet with the “limiting” essential amino acid, rather than protein, can save feed costs and help the environment at the same time. (Amino acids are the building blocks of protein.) Methionine is an essential amino acid that often limits milk production on typical rations; it is now commercially available in a rumen-protected form (coated with materials that protect the methionine from being broken down by the microbes in the rumen, but allowing the amino acid to be absorbed at the intestine when fed in the diet). This study examined two things: the difference in silage quality and milk production when using three common silo structures (plastic bag, bunker, and oxygen-limited silo); and whether the effect of adding rumen-protected methionine to the diet would vary in cows eating alfalfa silage from the three different silo structures. We found that the alfalfa silage from the oxygen-limited silo had a chemical composition that indicated it fermented better in the silo, and more of the nutrients from the original crop were preserved; therefore, it should be a better feed for dairy cows. Also, there was less wastage of silage from the oxygen-limited silo, mainly because of less spoilage, versus the other two silos. Moreover, when this silage was fed to milking cows, feed intake and fiber digestibility were greater, and cows had positive nitrogen balance (a measure of protein status) compared to cows fed silage from either bag or bunker silos. However, despite the more favorable composition and apparent nutrient utilization on the alfalfa from the oxygen-limited silo, milk production was not different from that on the other two silages. Supplementing with rumen-protected methionine increased feed and nitrogen efficiency and protein yield on all three sources of alfalfa silage; the effect of the rumen-protected methionine did not vary with silage source. This research indicates that the oxygen-limited silo is more effective for preserving nutrients during the ensiling process; it may be more profitable for dairy farmers, depending on the relative costs of the three silo structures. The results also suggest that usefulness of alfalfa silage may be limited by the supply of the essential amino acid methionine and that production and protein efficiency can be improved by supplementing rumen-protected methionine to diets based on alfalfa silage. These results show one way that U.S. dairy farmers may be able to reduce nitrogen pollution of the environment while improving farm profits.
Technical Abstract: The objective of this study was to determine whether production and nutrient utilization differed when lactating cows were fed diets based on 1 of 3 sources of alfalfa silage (AS) and whether performance was altered by feeding rumen-protected Met (RPM; fed as Mepron). Thirty-six lactating Holstein cows were blocked by parity and days-in-milk, then assigned to a randomized complete block design and fed a 3 x 2 arrangement of diets formulated from alfalfa ensiled in bag, bunker or oxygen-limited silos, and supplemented with either 0 or 9 g RPM/d. After feeding a covariate diet for 3 wk, treatment diets were fed for the remaining 12 wk of the trial. Experimental diets averaged [dry matter (DM) basis] 41% AS, 24% corn silage, 24% high moisture corn, 3.7% soybean meal, 4% roasted soybeans, 2% ground shelled corn, 1.0% minerals and vitamins, 16.7% CP and 31% NDF. Alfalfa from the oxygen-limited silo was lower in ash, higher in nonfiber carbohydrate and in vitro NDF digestibility, had lower pH, ammonia content and spoilage loss at feed-out, and gave rise to greater DM intake and ADF digestibility than silage from the other 2 silos, indicating a more effective fermentation that gave rise to greater nutrient preservation. However, the more favorable composition, intake and digestibility of alfalfa from the oxygen-limited silo were not reflected in improved milk production, which was not different among alfalfa sources. There was increased apparent N efficiency, and trends for improved feed efficiency and protein yield, with RPM supplementation across all 3 silages. The NRC (2001) model predicted that feeding RPM reduced Lys: Met ratio from 3.5 to 2.9, indicating that the diets were limiting in Met.