This article has been peer reviewed and accepted for publication. It is in production and has not been edited, so may differ from the final published form.
Growth of Brahman cross heifers to two years of age in the dry tropics
Growth of 1,368 Brahman cross heifers from six year cohorts was monitored over the two years post-weaning in Australia’s northern forest, a low-animal-growth dry tropical environment. Heifers weighing 47-266 kg at weaning were managed in groups weighing <100 kg, 100-149 kg, 150-199 kg, and >199 kg during the post-weaning dry season. Weaner heifers were allocated to receive 300 g/d of a protein meal during the dry season or to basic nutritional management to sustain health. Heifers in three cohorts were allocated to first mating at one or two years of age, in four cohorts to vaccination against androstenedione, and in a small proportion of two cohorts to ovariectomy post-weaning. Growth was highly variable between seasons and years; average cohort live weight by the start of two-year-old mating was 256-319 kg. Heifer groups not receiving protein supplementation gained -16 to 21 kg (2 kg average) during 6-month dry seasons, and 49 to 131 kg (101 kg average) during wet seasons to reach an average of two-thirds of mature live weight (445 kg) and 95% of mature hip height (1350 mm) by the start of mating at two years. Average body condition score (1-5) fluctuated by 1-2 units between seasons. Hip height gain continued, irrespective of season, commencing at approximately 0.60 mm/day at six months of age, and decelerating by about 0.00075 mm/day through to 2.5 years of age. Standard errors of predicted means across analyses were approximately 0.015 for average daily weight gains, 0.4 mm for average monthly height gain and 0.06 score units for average seasonal body condition score change. Post-weaning dry-season supplementation increased gains in live weight, height and body condition score by an average of 0.1 kg/day, 0.1 mm/day and 0.5 units, respectively, during the supplementation period. Periods of poor nutrition or high nutritional demand secondary to reproduction suppressed daily gains in live weight and hip height, at which times body condition score was also reduced. Subsequent to this, partial to full compensation subsequently occurred for all measures. Ovariectomy had negative effects on growth. Androstenedione vaccination had no effect on growth. The main conclusion is that heifer growth in Australia’s dry tropical northern forest region is highly variable between seasons and years, thus limiting significant proportions of some cohorts from reaching target weights for mating at two years of age, even after compensatory growth.
AN17305 Accepted 21 August 2017
© CSIRO 2017