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* Dairy and Swine Research and Development Centre, Agriculture and Agri-Food Canada, Sherbrooke, Québec, Canada J1M 1Z3
Soils and Crops Research and Development Centre, Agriculture and Agri-Food Canada, Québec, Québec, Canada G1V 2J3
2 Corresponding author: berthiaumer{at}agr.gc.ca
Alfalfa (Medicago sativa L.) cut at sundown has been shown to contain greater concentration of total nonstructural carbohydrates (TNC) than that cut at sunup. Fourteen multiparous (8 ruminally cannulated) and 2 primiparous lactating dairy cows were randomly assigned to 2 treatments in a crossover design (2 periods of 24 d) to investigate the effects of alfalfa daytime cutting management on ruminal metabolism, nutrient digestibility, N balance, and milk yield. Half of each alfalfa field (total of 3 fields) was cut at sundown (PM) after a sunny day, whereas the second half was cut at sunup (AM) on the following day. Both PM and AM cuts were field-wilted and harvested as baleage (531 ± 15.0 g of dry matter/kg of fresh matter). Bales (PM and AM) were ranked according to their concentrations of TNC, paired, and each pair of PM and AM baleages was then assigned to each experimental day (total of 48 d). The difference in TNC concentration between PM and AM baleages fed during the 10 d of data and sample collection varied from –10 to 50 g/kg of dry matter. Each pair of baleage was fed ad libitum to cows once daily with no concentrate. Ruminal molar proportion of acetate and total volatile fatty acid concentration were greater in animals fed the AM baleage, whereas the proportion of valerate was greater with PM baleage; no other significant changes in ruminal molar proportions of volatile fatty acids were observed between forage treatments. Digestible organic matter intake, organic matter digestibility, and plasma Lys concentration were significantly greater in cows fed PM alfalfa, suggesting that more nutrients were available for milk synthesis. Significantly lower body weight gain and retained N as a proportion of N intake were observed in cows fed PM alfalfa, thus suggesting that nutrients were channeled to milk synthesis rather than to body reserves. Intake of dry matter (+1.0 kg/d), and yields of milk (+1.0 kg/d), milk fat (+70 g/d), and milk protein (+40 g/d) were significantly greater in cows fed PM vs. AM alfalfa. Concentration of milk urea N and excretion of urea N as a proportion of total urinary N were significantly reduced, and milk N efficiency was increased when feeding PM vs. AM alfalfa, indicating an improvement in N utilization. Increasing the TNC concentration of alfalfa by shifting forage cutting from sunup to sundown improved N utilization and milk production in late-lactation dairy cows.
Key Words: alfalfa baleage • dairy cow • diurnal cutting • total nonstructural carbohydrate
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