The effect of pre-graze mowing on milk production of dairy cows grazing grass-herb-legume pastures managed under contrasting spring defoliation regimes

Strategies to increase herbage dry-matter (DM) production are sought for dairy systems. One proposed strategy is tactical spring defoliation with greater pre-grazing herbage mass and higher post-grazing heights. However, there are concerns over the effects of these strategies on herbage quality, DM intake and milk production. Irrigated, diverse pastures in Canterbury, New Zealand, containing perennial ryegrass, white clover, chicory, plantain and lucerne, were managed over two grazing rotations in spring under normal (grazed to 3.5 cm) or lax management (grazed to 5 cm, allowing early ryegrass seedhead development before normal grazing at anthesis late control'). On the third grazing rotation, a milk production study was conducted. Thirty-six, mid-lactation spring calving Friesian x Jersey dairy cows were allocated to nine groups of four cows and randomly allocated to three replicates of the following three treatments: (1) normal grazing (Norm), (2) lax grazing of standing herbage (Lax) and (3) lax grazing with pre-graze mowing of herbage (Mow). Cows were offered a daily herbage allocation of 30 kg DM/cow above ground level, with milk production measured over 8 days. Pastures managed under lax management had higher pre-grazing herbage mass (4149 kg DM/ha) than did pastures managed under normal management (3105 kg DM/ha), but all treatments had similar metabolisable energy (similar to 12.26 MJ ME/kg DM). Daily milksolid (MS) production tended to be lower (P = 0.07) for cows grazing pastures managed under Lax and Mow (2.34 and 2.24 MS/cow.day respectively) than with Norm (2.43 MS/cow.day). Although there was no difference in daily MS production between mowing and greater pre-graze herbage mass, switching from a high to low grazing residual managed by either grazing or mowing in late spring is likely to have a negative impact on milk production.