Grain supplementation strategies for dairy cows grazing grass pasture and their effects on milk production, microbial fermentation, and protein metabolism

摘要

Three experiments were conducted using continuous culture techniques and Holstein cows grazing Dactylis glomerata pasture to evaluate effects of supplementing grain containing differing amounts of rumen undegradable protein (RUP) and frequency of feeding (FF) on milk production and ruminal fermentation. Grazing cows in the first experiment did not increase milk yield (35 kg/d) when a high UIP grain ration was fed. However, milk protein yield was greater for multiparous cows.;In the second experiment, grain was fed twice daily at milking (Control) or offered 4X/d using a mobile computerized grain feeder (CF) on pasture. Cows on CF consumed 2 kg less grain/d than Control cows. A separate analyses was conducted using data from only those CF cows eating at least 75% of their allotted grain in 4 equal amounts compared to their respective Control pair mate. Cows fed by CF tended to produce less milk with a higher fat content. More frequent grain feeding did not alter milk production or composition. Plasma glucose and urea-N were not affected by treatment. Whereas CF cows had higher plasma NEFA concentrations than Controls. This difference in NEFA concentrations was not apparent when data from only those cows eating $ge$75% of their allotted grain was compared to their Controls.;In the third experiment, low and high UIP grain rations were fed 2 or 4 times/d using continuous culture fermenters. True DM, OM, and CP digestibilities and apparent digestibility of ADF, NDF, and TNC were not affected by treatments. Total VFA concentrations were similar and averaged 97.2 $mu$mol/ml. Feeding grain 2X/d resulted in lower valerate concentrations than feeding grain 4X/d (2.28 vs. 2.49 $mu$mol/ml, $P <$.05). Effluent total N flow was greater from fermenters fed low UIP diets. Ammonia N (NH$sb3$-N) concentration and flows were decreased by UIP supplementation ($P <$.01), and by feeding grain 4X/d ($P <$.05). Non NH$sb3$-N flows were also higher ($P <$.02) for 4X/d vs. 2X/d grain feeding (2.18 vs. 2.09 g/d). High UIP grain feeding reduced bacterial CP% ($P <$.03) and caused a slight reduction in rumen pH. Feeding grain 4X/d reduced diurnal variation in pH.