VARIABLES ASSOCIATED WITH THE CALVING PATTERN OF PRIMIPAROUS HOLSTEIN-FRIESIAN HEIFERS IN SEASONALLY CALVING HERDS

摘要

The aim of this study was to identify some variables associated with the calving pattern of primiparous Holstein-Friesian heifers as these variables may help understand determinants of reproductive performance of nulliparous heifers. The InCalf Project included a large, prospective, observational field study that described the reproductive performance of 29,462 dairy cows in 168 herds in 9 regions in Australia. In the study described here, a subset of animals from the original study population was selected for further analysis. We selected all primiparous Holstein-Friesian heifers (n=918) in seasonal calving herds (n=35) in the InCalf Project that used AI exclusively for the first 6 weeks of breeding, and that had milk production, sire Australian Breeding Value (ABV) and pre-calving liveweight records. In seasonal calving herds, heifers are mated in groups; the planned start of calving (PSC) date for a group is 282 days after the date that breeding commenced. Thus, the interval between the herd PSCdate and each animal’s actual calving date (PSC-CI) reflects time to conception. In herds where PSC date was not known, it was estimated based on gestation length distributions in herds where PSC was known. Heifers were categorised into quartiles for each independent variable, and multivariable statistical analyses were performed with herd as a random effect using the PROC MIXED procedure of SAS (SAS 1996).