Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/26/2008
Publication Date: 12/1/2008
Citation: Broderick, G.A., Luchini, N.D., Reynal, S.M., Varga, G.A., Ishler, V.A. 2008. Effect on Production of Replacing Dietary Starch with Sucrose in Lactating Dairy Cows. Journal of Dairy Science. 91(12):4801-4810.
Interpretive Summary: Diets based on alfalfa silage and other hay-crop silages contain large amounts of protein that already have been broken down to nonprotein nitrogen (NPN). When such diets are fed to dairy cows, fermentation of carbohydrates (sugars and starch) and protein in the rumen (the first compartment of the cow's stomach) may be out of synchrony. That is, energy may be released too slowly to allow the microbes in the rumen to use that energy to form protein from the NPN in alfalfa silage. Normally, dairy cows are fed carbohydrates as starch from grains such as corn. Replacing some of dietary starch with sugar might result in more effective capture of NPN as microbial protein, a form that is very useful to the cow. Not only will this improve the cow’s protein supply, but it will also improve nitrogen efficiency and help the environment at the same time. The objective of this research was to test whether replacing part of the dietary cornstarch with sugar would improve milk production and nitrogen efficiency of dairy cows consuming alfalfa silage. Forty-eight Holstein dairy cows were fed diets containing normal amounts of alfalfa silage plus other typical ingredients except that part of the corn was replaced with cornstarch or sucrose (normal table sugar). The diets contained 7.5% cornstarch and 0% sucrose; 5.0% starch and 2.5% sucrose; 2.5% starch and 5.0% sucrose; or 0% starch and 7.5% sucrose. Replacing cornstarch with sucrose resulted in greater feed intake by the cows and also increased milk fat content and fat production. There also were changes in the rumen indicating that the microbes were making more protein. However, no differences were found when direct measurements were made of the amount of microbial protein leaving the rumen. This research indicates that the main positive effect of replacing some of the starch in the diet with sugar is for the cows to eat more feed and, as a result, to make more milk fat. This research also confirms earlier work showing that adding enough molasses to increase dietary sugar by 2.5% was the optimal level for the dairy cow. The results indicate that the effects of sugar supplementation on protein are small and mean that U.S. dairy farmers will not be able to improve the usefulness of NPN in alfalfa silage by adding sugar to the diets fed to their cows.
Technical Abstract: Replacing dietary starch with sugar has been reported to improve production in dairy cows. Two sets of 24 Holstein cows averaging 41 kg/d of milk were fed a covariate diet and then blocked by DIM and randomly assigned in two phases to four groups of 6 cows each. Cows were fed experimental diets containing [dry matter (DM) basis]: 39% alfalfa silage, 21% corn silage, 21% rolled high moisture shelled corn, 9% soybean meal, 2% fat, 1% vitamin-mineral supplement, 7.5% supplemental non-structural carbohydrate (NSC), 16.7% CP, and 30% NDF. Nonstructural carbohydrates added to the 4 diets were: 7.5% cornstarch, 0% sucrose; 5.0% starch, 2.5% sucrose; 2.5% starch, 5.0% sucrose; or 0% starch, 7.5% sucrose. Cows were fed the experimental diets for 8 wk. There were linear increases (P = 0.05) in DM intake and milk fat content and yield, and linear decreases in ruminal concentrations of ammonia and branched-VFA, and urinary excretion of urea-N and total N, and urinary urea-N as a proportion of total N, as sucrose replaced cornstarch in the diet. Despite these changes, there was no effect of diet on microbial protein formation, estimated from total purine flow at the omasum or purine derivative excretion in the urine, and there were linear decreases (P = 0.04) in both milk/DM intake and milk N/N-intake when sucrose replaced dietary starch. However, expressing efficiency as fat-corrected milk/DM intake or solids-corrected milk/DM intake indicated there was no effect (P = 0.78) of sucrose addition on nutrient utilization. Replacing dietary starch with sucrose increased fat secretion, apparently via increased energy supply due to greater intake; positive responses normally correlated with improved N efficiency that were altered by sucrose feeding were not associated with increased protein secretion in this trial.