Omasal flow of nonstructural carbohydrates and nitrogenous compounds in lactating dairy cows fed diets containing timothy cut in the afternoon or morning

摘要

Shifting the cutting of grass from morning to afternoonhas been shown to increase the concentrationof nonstructural carbohydrates (NSC) in forages. Wecompared the effects of diets (66:34, forage: concentrateratio) containing a mix (% of the diet dry matter) ofbaleages (46.5%) and silages (19.3%) harvested fromtimothy cut in the afternoon (p.m.-cut TIM diet) ormorning (a.m.-cut TIM diet) on omasal flows of NSCand nitrogenous fractions, ruminal and total-tractdigestibilities of nutrients, plasma concentration ofAA, and milk yield and composition. Eight ruminallycannulated Holstein cows averaging (mean ± standarddeviation) 31.4 ± 6.13 kg/d of milk, 136 ± 17.0 d inmilk, and 611 ± 66.4 kg of body weight in the beginningof the experiment were used in a crossover designwith 21-d periods (14 d for diet adaptation and 7 d fordata and sample collection). Intake of total ethanolsolublecarbohydrates (TESC; +150 g/d), starch (+129g/d), and total NSC (TESC plus starch = +278 g/d)was greater with feeding the p.m.- than the a.m.-cutTIM diet. Likewise, the apparent ruminal digestibilitiesof TESC (+149 g/d), starch (+167 g/d), and totalNSC (+316 g/d) increased in the p.m.-cut TIM diet.Diets, however, had no effect on the omasal flows andapparent ruminal and total-tract digestibilities of drymatter and organic matter. Intake of N increased incows fed the p.m.- versus the a.m.-cut TIM diet (562and 528 g/d, respectively) despite no effect of diets ondry matter intake. Diets did not affect the omasal flowsof total nonammonia N, total bacterial nonammonia N,nonammonia and nonbacterial N, and individual AA,and the efficiency of microbial protein synthesis in therumen. Contrarily, supply of rumen-degradable proteinincreased (+9.2%) in cows fed the p.m.-cut TIM diet,with this response driven by the 6.4% increase in Nintake. Plasma concentrations of essential and nonessentialAA followed the omasal flow of AA and were notchanged by diets. Feeding the p.m.- versus the a.m.-cutdiet significantly increased yields of 4% fat-correctedmilk and milk fat, and tended to increase energy-correctedmilk, milk true protein, and milk lactose yields.Overall, feeding the p.m.-cut TIM diet to mid-lactationdairy cows did not improve microbial protein synthesisand omasal flow of AA, and these responses were inline with the lack of a treatment effect on dry matterintake.