Effects of feeding dried grass pasture and a grain ration differing in rumen undegradable protein at two feeding frequencies on nutrient digestibility and microbial protein synthesis during continuous culture

Four continuous culture fermenters were used in a randomized block design to evaluate the effects of grain feeding frequency and rumen undegradable protein (RUP) on nutrient utilization and microbial protein synthesis with high quality grass. Two grain rations fed two or four times per day were arranged in a 2×2 factorial with four replications. The grain rations contained 60% and 45% of CP as RUP, and similar total nonstructural carbohydrate (TNC) content (64% of DM). Grain was fed separately from freeze dried, pelleted pasture (56% of DM). Fermenters were maintained at a liquid dilution rate of 12%/h and a solids retention time of 20 h. Regardless of RUP level or grain feeding frequency, true DM, OM, and CP digestibilities averaged 72.1%, 73.3%, and 61.8%, respectively with no differences due to treatments. Apparent digestibility of ADF, NDF, and TNC were also not affected by treatments, and averaged 55.9%, 46.2%, and 80.9%, respectively. The production of the major VFA (acetic, propionic, and butyric) were not affected by treatment and averaged 56.4, 21.2 and 16.3 Mmol/ml, respectively. The interaction of grain feeding frequency and dietary RUP content was significant for isovalerate production. Increasing the grain feeding frequency significantly increased valerate production. Ammonia N (NH3–N) concentration and flows were decreased with higher RUP supplementation, and by feeding grain 4X/day (P<0.05). Non NH3–N flows were also higher with 4X/day grain feeding. Feeding a high RUP grain reduced the CP percentage in bacteria and tended to cause a reduction in rumen pH (P>0.10). The interaction of time during the day when pH was measured and feeding frequency was significant (P<0.001) reduced the diurnal variation in pH and when grain feeding frequency was measured to 4X/day.