Performance of young breeding cattle in commercial herds in the south-west of Western Australia. 2. Liveweight, body condition, timing of conception and fertility in first-calf heifers

Abstract

A 5-year study was conducted using 2531 first-calving heifers on commercial properties in the south-west of Western Australia. Breed types included Angus, Angus x Friesian, Angus x Hereford, Devon crossbred, Hereford x Friesian, Hereford, Simford, Beef Shorthorn crossbred, Wokalup multibreed and Simmental. Heifers first calving between 22 and 29 months of age were monitored from first until second calving, with the emphasis on liveweight and condition score measured during joining (range 9-21 weeks) and subsequent reproductive performance. The timing and level of reconception after first calving were tested for their relationship with liveweight, condition and their rates of change during joining. At second conception, mean liveweight varied widely both within and among breeds but apparently corresponded to maturity type. Liveweights (kg) of first-calvers were: Angus, 336; Hereford, 368; Angus x Friesian, 378; Simford, 423; Simmental, 467. At second conception, first-calvers were generally making moderate liveweight gains (0.2-0.6 kg/day) and most conceived at a condition score of 1.5-2.0. In 8 breeds with sufficient data for analysis, increased calving rates were related to heavier liveweight or better condition during joining, with the strongest relationships based on weight or condition at the beginning of joining. In general, fertility was adversely affected by liveweight <310 kg at the start of joining in early-maturing breeds such as Angus, Hereford and Beef Shorthorn crossbred and liveweight <345 kg in later maturing or larger breeds including Simford, Angus x Friesian crossbred and Wokalup multibreed. In addition, the fertility of the larger breeds was more influenced by liveweight, with greater partial regression coefficients, than was fertility in Angus or Herefords. First-calvers with condition scores less than an apparent threshold of 1.5, and those losing more than 0.6 kg/day, had lower calving rates than their counterparts in better condition or those losing less weight. In both early and later maturing breed types there was a linear relationship between higher calving rates and increasing liveweight gain. The average time of second conception for most breeds was within 5-6 weeks of the start of joining and most intercalving intervals were 370-390 days. Both time to conception and the intercalving interval were negatively and linearly related to liveweight (7-14 days reduction/100 kg), condition (7-13 days reductionlunit of condition), or weight change early in joining. Generally, heavier weights were required to effect the same reduction in time to conception in larger breeds such as Angus x Friesian, 314 Hereford x Simmental and Wokalup multibreed than in early-maturing breeds such as Angus or Hereford. Development of guidelines to producers based on second-conception weights, condition scores and the log-linear relationships derived for the breeds and crossbreeds in this work is discussed in the light of the poor reproductive performance sometimes found in this class of breeder.