Post-grazing sward height imposed during the first 10 weeks of lactation: Influence on early and total lactation dairy cow production, and spring and annual sward characteristics

Increasing the proportion of grazed grass in the diet of the dairy cow is the main target of grass-based milk production systems. Imposing a severe post-grazing sward height (PGSH) in early lactation is one strategy to increase grass utilisation. A grazing experiment was undertaken to investigate the direct and carryover effects of PGSH imposed in early lactation on sward and dairy cow lactation performance. Ninety Holstein–Friesian dairy cows (mean calving date: February 13) were randomly assigned to one of three target PGSH treatments: 2.7cm (severe; S), 3.5cm (low; L) and 4.2cm (moderate; M) from February 14 to April 24, 2011 (period 1; P1). This was followed by a carryover period (period 2; P2) during which cows were randomly re-assigned within their P1 treatment across two PGSH treatments: 3.5 or 4.5cm until November 13. Sward utilisation (>2.7cm) during P1 was significantly improved by decreasing PGSH from M (0.74) to L (0.82) and further to S (0.94). At the end of the entire grazing season, the M treatment swards had produced +1.4t dry matter (DM)/ha than the S and L treatment swards which had similar total DM yields (14.1t DM/ha). Treatment had no immediate or carryover effect on the proportions of leaf, stem and senescent material in the sward or the quality of herbage selected by the animals. During P1 the cows in S had greater bodyweight loss (−18kg), reduced milk (−2.1kg/day) and milk solids yields (−0.21kg/day) as well as lower grass DM intake (−1.7kg DM/day) compared with the cows in L and M, which performed similarly (−5kg, 24.1kg/day, 1.94kg/day, and 13.0kg DM/day, respectively). There was no carryover effect of early lactation PGSH on milk and milk solids yields, fat and protein concentrations during P2. This indicates that cows restricted in P1 were able to adjust production in accordance with the higher PGSH imposed during the remainder of the lactation. The S treatment had numerically lower, though not significantly lower, total lactation milk and milk solids yields, reflecting their significantly reduced yields in early lactation. It was concluded that grazing to 2.7cm in early lactation is too restrictive for dairy cows. Cows in L and M had very similar total cumulative production performance. As a result, grazing to 3.5cm during the first 10 weeks of lactation contributes to achieving both high milk output from pasture and high grass utilisation.