Summary
Native pastures of northern Australia are seasonally deficient in nutrients for the purposes of cattle breeding (Fordyce and Entwistle 1992), and nutrition is considered one of the key factors influencing reproduction (Entwistle 1983). Nutrient supplementation improves reproductive performance (McCosker et al. 1991). However, there is a need to improve knowledge regarding timing and level of supplementation (Dixon 1994) under commercial conditions (Miller et al. 1997). The only previously published nutritional research in the Barkly Tableland district (Hart and Michell 1965) examined phosphate-only delivery during the dry season, a practice now considered unviable (Winks 1990).
The current study involved the investigation of over 5000 individually identified cattle that were fed over 100 tonnes of supplement. The cattle were managed as four herds, grazing an area of 740km2 of native pastures on a commercial cattle station in the Northern Territory of Australia. The extensive nature of this study proved to be both its greatest strength and greatest challenge. The outcome of this study has changed the way beef producers in the Barkly Tableland region of the Northern Territory (an area covering 145,000 km2, running 500,000 head of cattle) manage nutrient supplementation and has established new production benchmarks for extensive beef breeding herds in northern Australia. More specifically, producers have re-evaluated their approach to determining the timing of supplementation by adopting objective approaches such as faecal sampling, pasture sampling and breeder body condition scoring.
To address the issues regarding amount and timing of delivery of breeder supplementation, measurement of current breeder herd productivity and the means by which animal nutrient status can be estimated was necessary, as this information has not been previously investigated in the Barkly Tableland region. Pregnancy rates, duration of anoestrus, calf growth and weaning weight were analysed for the influence of breeder body condition, breeder weight and breeder age-group. The relationship between nitrogen and phosphorus concentration in monthly samples of pasture and faeces were described as a means of estimating animal nutrient status.
Due to the large size of paddocks and number of cattle investigated, adequate replication of supplementation regimes and paddock rotation was impossible and therefore there was the potential for confounding effects between variables. To strengthen the discussion of commercial implications, measurement of potential confounding variables was undertaken. Satellite imagery, aerial photography and assessment of pasture composition and yield were used to determine uniform paddock selection for the study. Pasture quality, water and soil nutrient concentrations and daily rainfall in all paddocks were recorded. All animals were of the same breeding and were stocked in paddocks to achieve as similar as possible utilisation rates. However, a statistical comparison of the supplementation regimes is inappropriate and is therefore neither undertaken nor the focus of the discussion. The models developed from the current study provide wider application of the findings than any comparison of selected supplement recipes. The models are applicable to beef breeding operations in the Barkly Tableland, regardless of nutritional management practice.
The annual rainfall amounts for the years 1996 to 2001 were above average every year, with 2000 registering the highest annual rainfall recorded in over 100 years. In contrast to previous research, mean second-calf heifer pregnancy rate (89%) was not significantly different to heifer pregnancy rates (90%), with both being less than mature cow pregnancy rates (95%). It was found that as breeder weight and condition increase, the probability of pregnancy increases. As breeders proceed through the first three or four years of reproductive activity, they are more likely to be pregnant and will take longer to conceive. Conversely, as breeder weight and condition increase (for breeders with condition scores 4 to 9), the growth from birth to weaning and their weaning weights were found to decrease. Calf growth from birth to weaning is highest for the progeny of mature cows.
A significant association between monthly pasture samples of Barley Mitchell grass (Astrebla pectinata) and faeces for nitrogen was found. More specifically, average Mitchell grass protein concentration of 5%, is associated with faecal nitrogen concentration of approximately 8% and therefore below this level (Winks et al. 1979), a response to supplementation can be expected. This coupled with the known consistency of pasture sampling comparative to faecal sampling and relative monotony of Barkly Tableland pastures, lead to the conclusion that monthly pasture sampling using the described ‘indicator species’ method can be used to estimate faecal nitrogen concentration.
