Effect of time of initial grazing date and subsequent stocking rate on pasture production and dairy cow performance

The objective of this study was to investigate the effects of contrasting first spring grazing dates (GD) and subsequent stocking rate (SR) on milk production performance of dairy cows. In total, 48 autumn calving Holstein cows (160 ± 35 days in milk) were assigned to one of four (n = 12) different grazing treatments. Two swards were created by grazing in March (early grazing; E) or not grazing until April (late grazing; L). Two stocking rates, high (H) and medium (M), were applied across each sward, beginning on April 17th and finishing after 2 grazing rotations on June 20th. Cows grazing at the high and medium stocking rate were stocked at 6.3 and 5.0 cows·ha^-1 in rotation 1. In rotation 2, the stocking rates were 4.6 and 4.0 cows·ha^-1 for both high and medium stocking rates, respectively. Daily herbage allowance (> 50 mm) were on average 12.9, 15.7, 18.2 and 21.0 kg DM·cow^-1 for EH, EM, LH and LM in both rotations, respectively. There was a significant interaction between date of first grazing and stocking rate for milk (P < 0.001), fat (P < 0.01), protein yield (P < 0.001 ) in both rotations and fat corrected milk yield (P < 0.001 ) in rotation 1. During the first rotation, cows grazing the EM treatment yielded 24.1 kg of milk, 894 g fat and 688 g protein. The difference in milk production (cow per day) between EM and EH treatments was +3.0 kg milk, +86 g fat and +91 g protein in the first rotation whereas the difference between LM and LH treatments was only +0.9 kg milk, +15 g fat and protein. During the second rotation, the milk production difference between EM and EH (cow per day) increased and reached +4.0 kg milk, +128 g fat and +118 g protein. The production difference between cows grazing the LM and LH treatments was only +0.9 kg milk, +14 g fat and 30 g protein, respectively. Stocking rate had no effect on the milk production performance from late grazed swards. Grass offered above 50 mm had a higher level of grass utilisation (102 vs. 84%) with early grazing, these swards had also higher grass quality, which persisted in subsequent rotations. Early grazed swards should be grazed at a medium-stocking rate to obtain high individual milk production performance.