Effect of bovine somototropin and protein on rumen fermentation and forestomach and whole tract digestion in dairy cows

Four lactating Holstein cows with permanent ruminal and duodenal cannulas were allocated to one of two TMR with either 17.1 or 23.6% CP at 5 to 9 d postpartum. Cows also were assigned to either bST (20.6 mg/d) or excipient (control) treatment at that time for the 84-d experiment but were switched to different protein levels every 21 d in four periods. Intake of DM and forestomach and whole tract digestion of DM were unaffected by treatments. Forestomach and whole tract apparent digestion of N was higher for cows fed the high protein diet. Forestomach NDF and ADF digestion was higher for cows fed the high protein diet but was significant for control cows only. Rumen pH, ammonia N, and isovalerate were higher for cows fed the high protein diet. Rumen ammonia N and acetate were lower for bST-treated cows. Rumen pool sizes of total ingesta, DM, NDF, and bacterial OM were not affected by either treatment. However, the total pool size of rumen NAN and nonbacterial NAN was larger for cows fed the high protein diet. Duodenal flow of AA was higher for cows fed the high protein diet. However, duodenal protein AA profiles were similar among treatments except for lysine, which was higher, and methionine and alanine, which tended to be lower, for cows fed the high protein diet. There were few bST X protein or bST X duration of treatment interactions. Results support the concept that bST-treated cows have digestive metabolism similar to untreated cows of similar actual production