Summary
In a five-year, collaborative project involving Queensland DPI, James Cook University, CSIRO, and NSW Agriculture, 17 research tasks were implemented. Thirteen of these tasks were incorporated in DAQ.062. These covered improving fertility and management efficiency of maiden heifers, reducing lactation anoestrus with particular emphasis on first-calf cows, and developing management systems to increase young female calf output. Heifer fertility Heifers as young as 2 months of age weaned each year (1,380 in 8 groups) were supplemented or not in the dry season post-weaning, vaccinated or not against androstenedione, and then mated initially as either yearlings or 2-year-olds. First evidence of cycling was recorded, as were subsequent conceptions and calf output.
This project has provided for the first time, estimates of age and weight at puberty for Bos indicus cross heifers in the dry tropics, how this is affected by seasons and supplementation, and the implications in relation to mating management. Though the research used 5/8 Brahman crosses, it is expected that the same principles apply to higher grade Brahman cattle. The variation in time of puberty was large.
Over several year groups within management groups, the standard deviations were about 50 kg for weight and 5 months for age. The average age at puberty was delayed in poorer seasons, with a range of 1.5 to 2.3 years recorded. A major finding was that average weight at puberty in these 5/8 Brahman cross heifers increased as age at puberty was delayed (+4-5 kg/month). For example, when the average age at puberty was 1.5 years, the average weight was about 270 kg; when the average age was delayed to 2.3 years, the average weight increased to approximately 310 kg. Dry season supplementation of heifers appeared to simply simulate improved seasonal conditions; ie, advanced age at puberty, and reduced average weight at puberty - a double bonus.
There was no long-term effect on fertility as lactating cows. Another major finding was that heifers weaned at a younger age reached puberty at the same age and weight as their older counterparts. This means that puberty was later in early weaners simply because they were younger at weaning. Again, there were no effects on lactating cow fertility. Androstenedione vaccination did not consistently affect pre-pubertal development and had no significant influences on conception rates at first mating or on subsequent fertility. Detailed hormonal studies indicated that there were no specific periods of sensitivity to undernutrition during the pre-pubertal period.
Techniques which can reduce the effects of E2- negative feedback on pre-pubertal development have yet to be developed. Maiden heifers which conceive in the first 3 months of continuous mating have higher long- term fertility (+0.1-0.2 calves under good management and continuous mating) and survival. High conception rates in this period were achieved in supplemented heifers in all years, but were as low as 55% in one unsupplemented year group in which 80% had not reached puberty till 2.6 years of age. Obviously, early weaning further exacerbated this problem, though supplementation advanced puberty sufficiently to achieve high conception rates as maiden 2- year-olds.
Conception rate in yearling-mated heifers was a function of the proportion reaching puberty; it averaged about 30% in the first few months of mating in those which were supplemented as weaners. The conception rate was 15% higher in heifers supplemented in the post-weaning dry season than in those not supplemented. Yearling mating increased long-term calf output on average by about 0.3 calves. It was found that 2-year-old heifers calving in the latter half of the growing season often experienced dystocia. Intensive studies simulating weight loss and recovery of heifers in their second dry season showed that cyclicity in post-pubertal heifers will cease when 16-17% of weight is lost, with more profound anoestrus when 23% of weight is lost. Cycling does not resume till heifers grow beyond the weight at which loss commenced.
This indicates that prevention of weight loss will achieve a rapid return to cyclicity which is necessary following breaks to prolonged dry seasons which occur around the time of mating. In detailed physiological and endocrinological studies of heifers fed energy-deficient diets, profound anoestrus was achieved. Supplementation to prevent energy deficiency and sustain reproductive activity must include an appropriate protein balance.
A large number of possible strategies for contraception for controlled periods in heifers were reviewed and one tested. None was considered suitable, though implants incorporating trenbolone acetate may be worthy of further developmental research. Lactation anoestrus Spike feeding, the basics of which were developed in a previous project, was further evaluated in detail and at a practical level. Lifetime calf output was shown to increase by 0.3 calves on average; ie, herd branding rates increased by 5%. The approach necessary to consistently achieve a profitable response to spike feeding was defined and released in a producer-targeted publication. Neither the use of melatonin implants in late-pregnant cows, nor the use of late-wet season creep feeding in lactating cows were found to produce any significant fertility responses.
Despite reports prior to this project of potentially-good responses, these strategies are not recommended for north Australian cattle producers. Protein levels in tropical pastures deteriorate rapidly in the wet season. Supplementation of lactating cows over the wet season with urea + sulphur was found to increase pre-weaning conception rates by up to 25%, even at very low intakes. Incorporation of N+S into wet season licks (generally based on phosphorus) throughout north Australia is recommended. Short-term supplementation of lactating cows with 2 kg/day of a protein meal in the late wet season was found to increase pre-weaning conception rates by at least 10%. Astute timing may increase the response to 20%. However, it was clearly shown that this type of supplement in the early-mid wet season when cows are in peak lactation will not enhance fertility, despite having significant effects on both cow and calf growth. Endocrinological studies confirmed the potency of oestrogen (E2) negative feedback (in addition to the E2-independent suckling effect) on recommencement of cycling in lactating cows. Short term supraphysiological levels of E2 were not able to counter the negative feedback. Vaccination against androstenedione (an E2 precursor) may achieve this effect and warrants future study. A profound negative effect of suckling on postpartum cycling was shown.
The period to resumption of cycling after weaning was inversely related to body condition at weaning. Cows weaned in medium-good condition resumed ovarian cycles within about 50 days. However, ovarian cyclicity was only achieved about 30 days after an improvement in nutritional conditions in cows in poor condition when their calves were weaned. This indicates appropriate nutritional management to achieve target conception rates in lactating cows. Calf output A combination of high-level supplementation and yearling mating in the Burnett region was not able to achieve target growth and fertility because of prolonged severe seasonal conditions. The actual increase in calf output achieved of 0.3 calves was not profitable. The data indicated that this strategy would be profitable if calf output could be increased by 0.5 calves or that the supplement costs were reduced by 40% to achieve a 0.3 calf output increment. This is likely to be quite possible in average-good seasons.
