The relationship between genetic merit for yield and live weight, condition score and energy balance of Holstein Friesian dairy cows on grass based systems of milk production

The objective of this study was to estimate the effects of genetic merit for milk yield on energy balance (EB), dry matter intake (DMI), and fertility for cows managed on three different grass based feeding systems, and to estimate possible interactions between genetic merit and feeding system. Individual animal intake estimates were obtained at pasture on 11 occasions across three grazing seasons. The data set contained 96 first lactation animals in 1995, 96 second lactation animals in 1996, and 72 third lactation animals in 1997. Half of these animals were of high (HG), and half of medium genetic merit (MG) for milk solids production. Genetic effects for the traits of interest were estimated as the contrast between the two genetic groups, and by the genetic regression of phenotypic performance on the estimated breeding value for fat and protein yield, based on pedigree information alone (PI). Significant effects of feeding system were observed on yields, DMI and EB, whereas there was no effect on live weight, condition score or reproductive performance. The interaction between genetic merit and feeding system was not significantly different from zero for any of the traits. Yields, grass DMI, and total DMI were all higher for HG than for MG, and also positively correlated (P<0.001) with PI. Furthermore, condition score, conception to first and second service, and pregnancy rate were significantly negatively correlated with PI. While at pasture, EB was positively (P<0.01) correlated with PI, although the contrast between HG and MG was not significantly different from zero. Condition score changes during very early lactation, demonstrated that HG had a more negative EB than MG. The results clearly illustrate the production potential of HG cows on grass based systems, however the reduced reproductive performance questions their suitability for seasonal calving systems.