Time of joining affects the performance of young Angus and Angus multiply Friesian cattle in the south-west of Western Australia. 1. Liveweight, body condition, and reproductive performance in heifers and first-calvers

A 3-year study using 183 Angus and 173 Angus x Friesian first-calf heifers joined at about 15 months of age was conducted to determine the effects of time of calving and breed on liveweight and condition, heifer reproductive performance, oestrus activity after calving, and subsequent conception. Pasture on offer and supplements were also measured. Calving times used were April (conventional calving time), March (4 weeks earlier), and May (4 weeks later). There was no significant effect of time of calving on net reproductive rate in heifers of either breed, but the number of stillbirths and dystocias in later calved Angus were greater than in Angus x Friesian. Protracted and difficult calvings were more prevalent in heifers of both breeds calving early. Highest pregnancy rates of 93-94% corresponded with the highest average liveweights at conception in nulliparous heifers of both breeds that were mated to first-calve later. Angus x Friesian first-calvers consistently returned to oestrus before Angus after their first calving. In a year when grazing pressure on depleted dry pasture residues caused significant liveweight loss in both breeds, the crossbreds commenced cycling, on average, 20-32 days before the Angus. Regression of calving date and length of the post-partum anoestrous interval (PPAI) was -0.54 days/day for Angus and -0.3 1 days/day for Angus x Friesian. There was no significant difference between breeds for time to conception. The regression of second conception date on first calving date was 0.10 days/day in Angus and 0.17 days/day in Angus x Friesian. Heavier first-calvers of both breeds returned to oestrus earlier; each extra 10 kg in liveweight was associated with shorter PPAI by 3.9 days in Angus and 1.7 days in Angus x Friesian crossbreds. Heavier first-calvers of both breeds also conceived earlier; however, about twice as much weight was required to reduce the time to conception in the larger Angus x Friesian crossbreds than in the Angus. Times to first oestrus and to conception were negatively and linearly related to condition only in Angus first-calvers. Conception rates after first calving did not differ between time-of-calving groups or breeds in 2 years, but in a difficult year and if calved later, only 73% of Angus and 61 % of Angus x Friesians were pregnant after 90 days of joining. We conclude that there is little justification on the grounds of reproductive performance for varying the time of calving of heifers from that of the main breeding herd.