Influence of time of feeding a protein meal on ruminal fermentation and forestomach digestion in dairy cows.

Four ruminally and duodenally cannulated dairy cows in midlactation were fed twice daily a mixed diet of alfalfa silage and whole-crop oat silage and a concentrate consisting of primarily barley grain. A high protein supplement was fed at approximately 15% of the estimated dry matter intake of the mixed diet once daily at 0830 h, 0.5 h after the morning meal (day), or at 0030 h, 7.5 h after the evening meal (night). Cows fed the protein supplement during the night had higher apparent forestomach digestion of organic matter and crude protein. Ruminal concentrations of all volatile fatty acids, except isobutyrate, were higher for cows fed the protein supplement during the night. Although ruminal pH and concentrations of ammonia N did not differ between treatments, time by treatment interactions indicated that the feeding times of the protein supplement influenced diurnal patterns of ruminal fermentation. The flow of nonbacterial nonammonia N at the duodenum, as a proportion of N intake, was lower for cows fed the protein supplement during the night, but production of milk fat was higher. Results were consistent with a mechanism whereby protein fed during the night stimulated ruminal fermentation, particularly during the night, resulting in greater forestomach digestion of organic matter and less escape of dietary protein from the forestomach. Clearly, the different feeding times of this protein supplement changed the nutritional value of the overall diet.