ABSTRACT
This study was undertaken to understand better the mechanisms causing increased milk protein. Cows fed steam-flaked sorghum have increased milk protein 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 duodenum in two trials. The experimental design 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 Mcal/kg of NEL (on a DM basis); alfalfa hay and dry-rolled sorghum grain were the principal ingredients. Similar concentrations in feces of cows among propionate treatments 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 2.11 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 increased propionate availability for gluconeogenesis and a possible sparing of essential AA did not result in increased milk protein content, but ruminal infusion of glucose, which tended to increase microbial protein synthesis, did increase the protein percentage of milk.
