Effects of variations in herbage mass, allowance, and level of supplement on nutrient intake and milk production of dairy cows in spring and summer

Two experiments examining the effects of herbage mass and herbage allowance on the consumption of nutrients by lactating dairy cows were conducted on irrigated perennial pasture swards in northern Victoria. Experiment 1 was conducted in early lactation (spring) with a perennial ryegrass (Lolium perenne L.)-white clover (Trifolium repens L.) award at herbage masses of 3.1 (low) or 4.9 (medium) t dry matter (DM)/ha and herbage allowances of about 20, 35, 50 and 70 kg DM/cow.day. Within each herbage mass treatment, there were no significant differences between herbage allowance treatments in nutritive characteristics of pregrazing herbage. Daily DM intake increased linearly from 7.1 to 16.2 (low mass) or 9.9 to 19.3 (medium mass) kg DM/cow, as herbage allowance increased which was equivalent to 2.29 kg DM/t DM increase in herbage mass and 0.18 kg DMkg DM increase in herbage allowance. This was associated with a decrease in utilisation of herbage from 35 to 23% and from 52 to 29%. Also, milk production increased linearly from 21.8 to 27.1 (low mass) or 24.7 to 32.0 (medium mass) kg/cow.day as herbage allowance increased. Experiment 2 was conducted in mid lactation (summer) with a paspalum (Paspalum dilatatum L.)dominant sward at herbage masses of 3.0 (low) or 4.7 (medium) t DM/ha and herbage allowances of about 25 and 45 kg DM/cow.day and either 0 or 5 kg DM concentrate/cow.day. Within each herbage mass treatment, there were no significant differences between herbage allowance treatments in nutritive characteristics of pregrazed herbage. Daily DM intake increased by 0.13 kg DM for every 1 kg DM increase in herbage allowance. Over the 2 herbage masses, 2 herbage allowance treatments and 2 concentrate treatments, cows consistently selected a diet about 1.03 higher in in vitro DM digestibility and 1.24 higher in crude protein than that in the herbage on offer. Substitution rate increased from 0.20 to 0.42 (low mass) and from 0.34 to 0.44 (medium mass) kg DM reduction in herbage intake/kg DM of concentrates consumed, with increasing herbage allowance. Along with these changes, marginal returns to supplements decreased from 1.38 to 0.95 (low mass) and 1.07 to 0.97 (medium mass) kg milk/kg DM of concentrates with increasing herbage allowance. Dairy farmers should consider the effects of herbage allowance and herbage mass on intake, nutrient selection and milk production when allocating pasture to dairy cows. Herbage mass and allowance had a greater effect on intake in spring than in summer, principally due to the decline in nutritive characteristics that occurs in summer. Cows selected nutrients to varying degrees depending on the composition of the pasture sward and this selection may result in seasonal nutrient imbalances. Finally, it appears that the best use of supplements will occur when pastures are short in height because this will minimise substitution of supplement for pasture and maximise marginal returns in milk production.