Method of diet delivery to dairy cows: Effects on nutrient digestion, rumen fermentation, methane emissions from enteric fermentation and stored manure, nitrogen excretion, and milk production.

This study was conducted to examine the effect of method of diet delivery to dairy cows on enteric CH emission, milk production, rumen fermentation, nutrient digestion, N excretion, and manure CH production potential. Sixteen lactating cows were used in a crossover design (35-d period) and fed ad libitum twice daily a diet [52:48, forage:concentrate ratio; dry matter (DM) basis] provided as forages and concentrates separately (CF) or as a total mixed ration (TMR). For the CF treatment, concentrates were offered first followed by mixed forages 45 min afterward. Method of diet delivery had no effect on DM intake, but neutral detergent fiber (NDF) intake was greater when the diet was delivered as TMR as compared with CF. Apparent total-tract digestibility of DM, crude protein, and gross energy was slightly (1 percentage unit) lower when the diet was offered as TMR than when offered as CF. In contrast, NDF digestibility was greater when the cows were fed TMR versus CF. Although average daily ruminal pH was not affected by method of diet delivery, daily duration of ruminal pH <5.6 was less when the diet was delivered as TMR as compared with CF (0.9 h/d versus 3.7 h/d). Delivering the diet as TMR increased ruminal total volatile fatty acid and NH concentrations, but had no effect on acetate, propionate, or branched-chain volatile fatty acid molar proportions. Yields of milk, milk fat, or milk protein, and milk production efficiency (kg of milk/kg of DM intake or g of N milk/g of N intake) were not affected by the method of diet delivery. Daily production (g/d), yield (% gross energy intake), and emission intensity (g/kg of energy-corrected milk) of enteric CH averaged 420 g/d, 4.9%, and 9.6 g/kg and were not affected by diet delivery method. Fecal N output was greater when the diet was delivered as TMR versus CF, whereas urinary N excretion (g/d, % N intake) was not affected. Manure volatile solids excretion and maximal CH production potential were not affected by method of diet delivery. Under the conditions of this study, delivering the diet as concentrates and forages separately versus a total mixed ration had no effect on milk production, enteric CH energy losses, urinary N, or maximal manure CH emission potential. However, feeding the diet as total mixed ration compared with feeding concentrates and forages separately attenuated the extent of postprandial decrease in ruminal pH, which has contributed to improving NDF digestibility.