Author: Emily Ruth Oelker

Publisher:

Published: 2007

Total Pages: 130

ISBN-13:

Abstract: Sugar supplementation can stimulate rumen microbial growth and possibly fiber digestibility, however, increasing ruminal carbohydrate availability relative to rumen degradable protein can promote energy spilling by microbes or decrease rumen pH. Rumen degradable protein supply and rumen pH might be altered by forage source and monensin. Therefore, the objective of this study was to evaluate interactions of a sugar source (molasses) with monensin and two forage sources on rumen fermentation and milk production in lactating dairy cows. Seven ruminally cannulated lactating Holstein cows were used in five 28-day periods in a 5x7 incomplete Latin square. Four corn silage diets consisted of 1) control (C), 2) 2.6% molasses (M), 3) 2.6% molasses plus 0.45% urea (MU), 4) 2.6% molasses plus 0.45% urea plus monensin (18 mg/kg DM; MUR). Three alfalfa hay diets consisted of 1) control (C), 2) 2.6% molasses (M), 3) 2.6% molasses plus monensin (18 mg/kg DM; MR). Urea was added to corn silage diets to provide rumen degradable protein comparable to alfalfa hay diets with no urea. All diets were balanced to have 18.0% forage neutral detergent fiber and 41.0% non-fiber carbohydrates. Diets were balanced to have 16.2% crude protein except corn silage MU and MUR, which were balanced to have 17.2% crude protein. Dry matter intake was not affected by treatment but milk production was lower in alfalfa hay diets compared with corn silage diets. Monensin decreased milk fat from 3.22 to 2.72 % in corn silage diets but not in alfalfa hay diets. Milk fatty acids (% of total fat) were not affected by treatment. However, in whole rumen contents, concentrations of trans-10 cis-12 conjugated linoleic acid were increased by corn silage diets suggesting corn silage may have contributed to inhibition of biohydrogenation in the rumen. Monensin had no effect on conjugated linoleic acid isomers in either milk or rumen contents but did increase the concentration of trans-10 C18:1 in rumen contents. Alfalfa hay decreased total tract digestibilities of dry matter, organic matter and nitrogen compared to corn silage. Molasses tended to decrease neutral detergent fiber digestibility. Acetate:propionate ratio was higher in alfalfa hay than corn silage diets and monensin did not affect acetate:propionate ratio. Additionally, molasses with urea increased milk urea nitrogen in the corn silage diets (7.6 vs 12.0 mg/dl for M vs MU). Ammonia and milk urea nitrogen remained unchanged in the alfalfa hay diets. These results indicate that sugar supplementation might require urea to support microbial protein synthesis in corn silage diets balanced for moderate crude protein and perhaps especially if monensin is fed.