Milk production and grazing behaviour responses of dairy cows to partial mixed ration supplementation

International audienceSupplementing grazing dairy cows with a partial mixed ration (PMR) is widely used on farm. The doseresponse of grazing cows to PMR is not well known. The aim of this study was to compare the effects of three supplementation levels (0, 4 and 8 kg dry matter (DM) d-1 of PMR (85:15 maize silage/soyabean meal ratio, DM basis) on milk production and behaviour of cows grazing on perennial ryegrass pastures in spring. Pasture allowance was specific to each treatment to achieve the same post-grazing sward height between treatments. Twenty-four mid-lactation Holstein cows were used in a complete and balanced 3×3 Latin square design with three 21-day periods. Cows ate less PMR than offered (15-25% refusals). Herbage N concentration was low (119 g kg-1 DM). Milk production increased linearly by 0.52 kg kg-1 DM of PMR intake. Milk fat concentration was reduced whereas milk protein concentration was not affected by increasing the supplementation level. Daily grazing time decreased linearly by 9 min kg-1 DM of PMR intake, suggesting a low substitution rate between pasture and PMR, and explaining the great milk production response to PMR. The reduced grazing activity was mainly noticeable after the evening milking and the PMR distribution