Effect of genetic merit on the estimated partitioning of energy towards milk production or liveweight gain by Jersey cows grazing on pasture

摘要

Data from the 1999/2000 season of the Stratford demonstration farm stocking rate trial was used to estimate metabolisable energy intakes (MEI) and the amount of metabolisable energy (ME) partitioned into milk, maintenance and liveweight (Lwt) changes of Jersey cows of either high genetic merit (HGM) (average milksolids breeding value of 28 kg) or low genetic merit (LGM) (average milksolids breeding value of 11kg). Least-square mean yields for HGM and LGM cows were 327 and 289 kg of milksolids per cow respectively. HGM cows partitioned a significantly higher proportion of MEI into milk than LGM cows in early (0.67 vs 0.63; P<0.01), peak (0.59 vs 0.57; P<0.05), mid (0.58 vs 0.56; P<0.01) and fate lactation (0.54 vs 0.51; P<0.01) (HGM vs LGM respectively). In early lactation, HGM cows utilised more body reserves for milk production (-0.05 vs -0.01, for HGM and LGM respectively). These results confirm that HGM cows partition more ME to milk throughout lactation than LGM animals for a range of feedinglevels on grazed pasture. The greatest difference occurred in early lactation, resulting in increased body tissue loss and a greater negative energy balance in HGM cows.