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 3498 heifers on commercial properties in the south-west of Western Australia. Breed types represented were Angus, Angus x Friesian, Angus x Hereford, Devon crossbred, Hereford x Friesian, Hereford, Hereford x Simmental, Beef Shorthorn crossbred, Wokalup multibreed and Simmental. Heifers joined at 13-18 months of age were monitored throughout the growing season with emphasis on measurement of liveweight and condition score during joining (range 9-21 weeks) and subsequent reproductive performance. The timing and level of conception were examined for their relationship with liveweight, condition and their rates of change during the joining period. Mean liveweight at conception varied significantly among breeds and apparently corresponded to maturity type. Liveweights (kg) of heifers were: Angus, 255; Hereford, 281; Angus x Friesian, 290; Hereford x Friesian, 295; Simford, 326; Simmental, 416. There were large variations within breeds and between management groups in liveweight and rate of weight change at conception, the range within breed covering at least 160 kg (Angus x Hereford) and at most 240 kg (Simford). Most breeds of heifer conceived at a condition score of 2.0. Positive liveweight gains between 0.39 kglday in Angus heifers and 1.07 kgtday in Simmental heifers, and gains in condition, occurred in all breeds at conception. However, there was no significant relationship between rate of liveweight or condition score change early in the joining period, with calving rate in any breed.In 7 of 10 breeds higher calving rates were associated with heavier liveweight during the joining period. These trends were consistent throughout joining, with the strongest relationships determined by logistic regression analysis using liveweight at the beginning of joining. Statistically significant differences in calving rate were also found between light heifers, <220 kg for Herefords and Hereford x Friesian crossbreds, or <255 kg for larger or later maturing breeds including Angus x Friesian, Simford and Wokalup multibreed, and heifers in heavier liveweight categories. Similarly, the proportion of heifers pregnant was usually lower below a condition score of 1.5; highest fertility was commonly observed at condition scores of 1.5-2.5 during joining. The time from the beginning of joining to conception averaged less than 21 days for most breeds. Significant linear regressions between the interval from the beginning of joining to conception and either liveweight, condition or their rates of change in early joining showed that heifers which were heavier, in better condition, or gaining weight quickly, conceived earlier. These relationships were demonstrated in only half of the breeds in the study. The current data extend the range of cattle breeds for which there is some guide to target weights and condition scores for first conception under grazing management in southern Australia, and identify the relative importance of static and dynamic liveweight effects on heifer fertility.