Meta-analyses of the effects of pre-grazing herbage mass and herbage allowance on pasture intake, milk production and composition, and grazing behaviour of rotationally grazind dairy cows

Abstract

Grazing management is known to affect dairy cow performance. Two meta-analyses of the literature were performed for reviewing the effects of either herbage allowance (kg DM/cow/day, studied at a given herbage mass) or pre-grazing herbage mass (kg DM/ha, studied at a given herbage allowance) on pasture intake, milk production and grazing behaviour of dairy cows under rotational grazing management. The height at which pasture is cut (0 vs 3 cm vs 5 cm above ground level) to estimate pre-grazing herbage mass was considered as a primary factor explaining the variability of cow responses. The curvilinear relationships describing the effects of herbage allowance on herbage intake and milk production were conceptually similar whatever the methodology of herbage sampling (cutting height), even if equations parameters are specific to each cutting height. Herbage intake and milk production variations were related to herbage intake rate rather than to grazing time variations. The effect of pre-grazing herbage mass on herbage intake, herbage intake rate and milk production depends on the methodology of herbage sampling: it is positive, null or negative depending on whether the cutting height is at ground level, above 3 cm or above 5 cm from ground level, respectively. Herbage allowance measured above 3 cm is the best predictor of cow performance because it is independent of pre-grazing herbage mass. These results allow better understanding and predicting of the effects of grazing management on performance of rotationally grazing dairy cows.