Effect of pre-grazing herbage mass on grazing behaviour, grass dry matter intake and milk production of dairy cows

Identifying the optimum level of pre-grazing herbage mass (HM) across the grazing season is critical to maximising dry matter intake (DMI) and milk production from dairy cows. From 27 April to 17 October 2010, in Moorepark, Ireland, 45 Holstein-Friesian dairy cows (mean calving date 23 March 2010) were randomly assigned to Low (L), Medium (M) or High (H) pre-grazing HM treatments (targets 800, 1,500 or 2,200 kg DM/ha, above 4 cm, respectively) using perennial ryegrass-based pastures. Pre-grazing HM was determined twice weekly. Grazing behaviour was measured once and herbage DMI twice. Milk production was recorded daily. The experiment was divided into Period 1 (Mid-season; 27 April to 25 July) and Period 2 (Autumn; 26 July to 17 October). In the autumn, cows on L HM swards grazed for 90 minutes more and had 15% more grazing bites on a daily basis than did the cows on the other treatments. Cows grazing M HM swards tended to have highest DMI (L-15.2, M-16.5, H-15.7 kg DM/cow/day); while cows grazing H HM swards tended to have lowest milksolids yields (L-1.43, M-1.43, H-1.31 kg milksolids/cow/day). Pre-grazing HM levels of 2,200 kg DM/ha may impair DMI and milksolids production per cow, most likely due to decreased sward quality.