The effects of cow genetic merit and feeding treatment on milk production, herbage intake and grazing behaviour of dairy cows

The objective of the present study was to investigate the effects of cow genetic merit and grass-based feeding treatment on milk production, herbage intake and grazing behaviour of first lactation Holstein-Friesian dairy cows. A total of 48 high genetic merit cows (HG) and 48 medium genetic merit cows (MG) were assigned, within merit group to three grass-based feeding treatments: 1) standard feeding treatment, 2) high concentrate and 3) high grass allowance. The HG cows had a higher milk yield (24.7 v. 21.2 kg/day; P < 0.001), fat yield (0.87 v. 0.82 kg/day; P < 0.05), protein yield (0.81 v. 0.73 kg/day; P < 0.001) and lactose yield (1.14 v. 1.00 kg/day; P < 0.001) than the MG cows. The milk of the HG cows had lower fat (35.6 v. 38.7 g/kg; P < 0.01), protein (33.2 v. 34.5 g/kg; P < 0.01) and lactose (46.4 v. 47.6 g/kg; P < 0.01) concentrations. The HG cows had a higher (P < 0.01) grass dry matter (DM) intake. Total grazing time was not affected by cow genetic merit but the HG cows had more (P < 0.10) grazing bouts of shorter (P < 0.05) duration. The HG cows also had higher (P < 0.001) biting rates and spent a greater (P < 0.01) proportion of time ruminating than the MG cows. Offering a higher level of concentrate (Treatment 2) significantly increased yield of milk (P < 0.001), fat (P < 0.05), protein (P < 0.001) and lactose (P < 0.001). The higher concentrate feeding treatment also resulted in a significantly (P < 0.001) higher lactose concentration with no effect on fat or protein concentrations. Allocating a higher allowance of grass (Treatment 3) had no effect on milk production except for lactose concentration which was higher (P < 0.05). Offering a higher level of concentrate reduced (P < 0.001) grass DM intake. Feeding treatments 2 and 3 both reduced (P < 0.05) the duration of grazing bouts and biting rate during the day.