Effect of herbage allowance on grazing behavior and productive performance of early lactation primiparous Holstein cows

Effects of daily herbage allowance (DHA) on milk production and composition, body condition score (BCS) and grazing behavior of primiparous Holstein cows during early lactation were investigated. Cows were blocked by body weight (BW), age and BCS, and randomly assigned within block to one of three grazing treatments (n = 11 cows each) being: high (HA, 30 kg DM/cow/d), medium (MA, 15 kg DM/cow/d) and low (LA, 7.5 kg DM/cow/d) DHA. Cows were grazed in 8 consecutive grazing paddocks (GP) of 7 d of occupation each. The experiment was a completely randomized block design and data were analyzed as repeated measurement in time. Milk production was higher in HA and MA groups than in LA (24.3 and 22.7 vs. 19.2 L, P<0.01 respectively). Milk response to extra DHA was 0.43 L of milk/kg DHA between LA and MA treatments, which dropped to 0.19 L of milk/kg between LA and HA treatments. Cows in the HA treatment had higher BCS than cows in MA and LA treatments (3.18 vs. 3.05 and 3.07; P<0.05). Cows on all treatments mobilized BCS during the first 3 weeks after calving but, while HA cows lost ∼0.5 points of BCS, LA cows lost 1 point. The probability of a cow grazing at any time increased (P<0.01) as the experiment progressed, being 54.5, 61.3, 66.8 and 68.7 min/100 min for GP 3, 5, 7 and 8, respectively. Probability of grazing increased linearly for cows receiving HA (0.39 min/100 min/d) and MA (0.44 min/100 min/d) treatments at higher rate than cows receiving LA (0.22 min/100 min/d) treatment. A linear effect of days in milk (DIM) on bite rate as well as an interaction of DIM by treatment occurred. At DIM = 0, the mean value for bite rate was ∼15 bites/min, and the slope relative to DIM was higher for HA and MA cows (0.54 and 0.69 bites/min/d, respectively) than for LA cows (0.29 bites/min/d). The result of the present experiment evidence the major role of DHA on milk production in primiparous dairy cows during early lactation, being more important at lower levels of DHA. The lack of response on milk production to higher levels of DHA might have been related to the low effective grazing time and bite rate exhibited by early lactation grazing primiparous cows.