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 we re fed twice daily a mixed diet of alfalfa silage and whole-crop oat s ilage and a concentrate consisting of primarily barley grain. A high p rotein supplement was fed at approximately 15% of the estimated dry ma tter intake of the mixed diet once daily at 0830 h, 0.5 h after the mo rning 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 f orestomach digestion of organic matter and crude protein. Ruminal conc entrations of all volatile fatty acids, except isobutyrate were higher for cows fed the protein supplement during the night. Although rumina l pH and concentrations of ammonia N did not differ between treatments , time by treatment interactions indicated that the feeding times of t he protein supplement influenced diurnal patterns of ruminal fermentat ion. The flow of nonbacterial nonammonia N at the duodenum, as a propo rtion of N intake, was lower for cows fed the protein supplement durin g the night, but production of milk fat was higher. Results were consi stent with a mechanism whereby protein fed during the night stimulated ruminal fermentation, particularly during the night, resulting in gre ater forestomach digestion of organic matter and less escape of dietar y protein from the forestomach. Clearly, the different feeding times o f this protein supplement changed the nutritional value of the overall diet.