The effect of a gradual or rapid dietary changeover from a grazed pasture to a conserved forage-based diet on milk yield, cow condition and rumen pH of late-lactation dairy cows

摘要

A 40-day experiment was conducted to determine the effect of a gradual versus rapid changeover from grazed pasture to grass silage on production and performance in late-lactation Holstein-Friesian cows. Eighty cows were assigned to one of the following two treatments (two groups of 20 cows each); (1) gradual changeover from grazed pasture to grass silage over a 10-day adaptation period (GRAD), or (2) immediate changeover from grazed pasture to grass silage, with no adaptation period (RAPID). In addition to grazed pasture and grass silage, cows also received equal daily amounts of supplementary concentrates throughout the 40 days (ranging from 6.6 to 7.5 kg DM/cow). The experiment was divided into three periods. In Period 1 (Days 1-12), all cows received a generous pasture allowance and no grass silage was offered. In Period 2 (Days 13-22), GRAD cows were gradually infroduced to grass silage on a stepwise basis, while still consuming grazed pasture, while RAPID cows received grazed pasture until Day17, before switching to ad libitum grass silage from Day 18 onward. In Period 3 (Days 23^0), all cows received ad libitum pasture silage and no grazed pasture. Feed intake, milk volume and composition, and rumen pH were measured. Treatment did not affect estimated dry-matter intake of grazed pasture or measured dry-matter intake of silage. Milk yield did not differ between treatments from Day 1 to Day 18 (mean 29.3 L/cow;P> 0.05), but was greater in GRAD cows from Day 19 to Day 27 (mean 25.6 vs 22.1 L/cow;P< 0.001). From Day 28 onward, no effect of freatment was detected apart from a 3-day juncture from Day 34 to Day 36, where milk yield in the GRAD freatment was greater (mean 22.8 vs 21.0 L/cow; P = 0.02). Milk fat and protein concentrations were unaffected by treatment throughout (mean 4.15% for milk fat, 3.37% for milk protein; P > 0.05). Mean rumen pH was also unaffected by freatment in periods 1 and 2 (mean 6.27; P > 0.05), but were greater in Period 3 in GRAD cows (6.34 vs 6.26 for GRAD vs RAPID; P < 0.001), while the amount of time spent under pH 6.0 did not differ between freatments (mean 2.45 h/day; P > 0.05). Changing the dietary forage source from grazed pasture to grass silage over a 10-day period increased milk yield, compared with having no dietary adaptation period, and the cumulative difference for the duration of this experiment amounted to 37 L/cow.