The effect of grazing pressure on rotationally grazed pastures in spring/early summer on the performance of dairy cows in the summer/autumn period

Abstract

Two experiments (E1 and E2) were carried out to examine the effects of sward type (ST) on dairy cow performance. Applying grazing pressures (GP) in spring/early summer of 6.35, 4.24 and 3.53 cows/ha in E1, and 6.06, 5.05 and 4.03 cows/ha in E2, created the different ST. From summer to autumn, two stocking rates (SR) were applied to each sward, i.e., high (HR) and low (LR). As GP was reduced, the swards were characterised by progressively higher herbage mass of lower organic matter digestibility (OMD) and live leaf (LL) proportion, termed high (HQ), medium (MQ) and low (LQ) quality. There was no interaction between ST and SR for any animal performance variables except for grazing time. Mean diet OMD was 0.816, 0.803 and 0.794 (s.e. 0.0029) in E1, and 0.793, 0.780 and 0.772 (s.e. 0.0021) in E2, for HQ, MQ and LQ, respectively. The corresponding values for LL were 0.785, 0.740 and 0.709 (s.e. 0.0121) in E1, and 0.825, 0.790 and 0.759 (s.e. 0.0095) in E2. Milk yield per cow was 13.2, 12.2 and 10.6 (s.e. 0.55) kg in E1, and 18.4, 17.5 and 16.2 (s.e. 0.32) kg in E2, for HQ, MQ and LQ, respectively. Milk yields were 11.1 and 12.9 (s.e. 0.46) kg in E1, and 16.4 and 18.3 (s.e. 0.26) kg in E2, for HR and LR, respectively. There was no effect of ST or SR on milk composition or body weight gain. Herbage organic matter intake was 12.8, 12.5 and 11.1 (s.e. 0.28) kg in E2, for HQ, MQ and LQ, respectively. The corresponding values were 11.4 and 12.9 (s.e. 0.23) kg for HR and LR, respectively. The results show that milk yield of springcalving dairy cows is higher in summer when high rather than low stocking rates are applied in spring/early summer. The increased milk production is attributed to higher intake of herbage of higher nutritive value.