NUTRITION, FEEDING, AND CALVES Effect of lsocalorlc Infusion of Glucose in the Rumen zyxw or Propionate In the Duodenum Z. WU, F. T. SLEIMAN,' C. B. THEURER, F. SANTOS, J. M. SIMAS, M. FRANCOLIN, and J. T. HUBERZ Department of Animal Sciences University of Arizona zyx Tucson 85721 ABSTRACT zyxwvutsr This zyxwvutsrqpon study was undertaken to under- stand better the mechanisms causing in- creased milk protein. Cows fed steam- flaked sorghum have increased milk pro- tein compared with that of cows fed dry- rolled sorghum because of a large shift of starch digestion from the intestine to the rumen. Five cannulated lactating cows were infused with glucose in the rumen or with propionate in the duode- num in two trials. The experimental de- sign was a 2 x 2 Latin square with 7 d of adjustment and 7 d of infusion. During the experiment, cows received a TMR containing 19.3% CP and 1.56 McaVkg of "EL (on a DM basis); alfalfa hay and dry-rolled sorghum grain were the prin- cipal ingredients. Similar concentrations in feces of cows among propionate treat- ments suggested complete absorption of infused propionate. Milk yield did not differ, but protein percentage of milk was higher (2.88 versus 2.72%) for cows infused ruminally with glucose than for those infused in the duodenum with propionate. For the respective treatments, duodenal flows were 2.1 1 and 1.76 kg/d for microbial protein and 3.44 and 2.73 kg/d for total CP (or 85 and 74% of CP intake). These data demonstrate that in- creased propionate availability for gluconeogenesis and a possible sparing of essential AA did not result in in- creased milk protein content, but ruminal infusion of glucose, which tended to in- Received November 22, 1993. Accepted March 4, 1994. *American University of Beirut, 850 Third Avenue *Reprint requests. (18th Floor), New York, NY 10022-6297. crease microbial protein synthesis, did increase the protein percentage of milk. (Key words: milk protein, starch degra- dation, propionic acid, glucose) Abbreviation key: DP = duodenal infusion of propionate, RG = ruminal infusion of glucose. INTRODUCTION Recent studies at the University of Arizona (13, 16,20,24,26) showed that percentage and yield of milk protein increased when lactating zyx dairy cows were fed steam-flaked sorghum instead of dry-rolled sorghum grain. Moreover, steam-flaking overcame decreases in milk pro- tein concentration that were associated with dietary fat supplementation (24). Compared with dry-rolling, steam-flaking of the sorghum grain increased ruminal starch degradability by lactating cows fed 40% sor- ghum grain diets from approximately 50 to 80% (15, 19, 26). Increased d n a l starch &gradation by growing steers fed 77% steam- flaked sorghum grain diets resulted in greater absorption of acetic and propionic acids (28). These VFA are more available to meet lacta- tional demands than glucose digested in the small intestine because only a small proportion of intestinally absorbed glucose reaches portal or peripheral blood (11, 27). However, quan- titative data on how much intestinally absorbed glucose spares systemic glucose is not availa- ble. Greater degradation of starch in the rumen released energy for increased synthesis of microbial protein (15, 18), which was sup- ported by more recycling of urea N to the rumen of steers fed steam-flaked sorghum than to the rumen of those fed dry-rolled sorghum (25). Kennedy (12) also reported that transfer of urea to the rumen increased from addition of fermentable carbohydrate to a hay diet for cattle. 1994 1 Dairy Sci 77:15561562 1556