Sweet Spot: improving breeder herd performance through optimal pasture utilisation
Optimising pasture utilisation is fundamental to sustainable beef production in northern Australia. In these systems, cattle nutrition is almost entirely derived from grazing native rangelands, and addressing any mineral deficiencies with targeted supplementation strategies. The quality and availability of pasture vary considerably between regions, reflecting differences in rainfall patterns, soil type, and land condition.
Extensive knowledge exists on the influence of genetics, nutrition, and environmental conditions on reproductive outcomes in beef breeding herds (Burns et al., 2010; McGowan et al., 2015; Bell & Sangster, 2022). However, the direct effects of pasture utilisation on the reproductive performance of beef breeding females remains largely unquantified. Most of the evidence to date has been extrapolated from studies involving steers or nonbreeding females (O’Reagain et al. 2011). Furthermore, reviews have shown that while total beef production per hectare and enterprise level with larger herd sizes, individual animal performance typically declines at higher grazing pressures (Ash & Stafford Smith 1996; Hunt et al. 2014). While this trade-off is well recognised, its implications for reproductive performance of breeding females and overall economic impacts are less well understood.
The “Sweet Spot” project was established to address this knowledge gap. Conducting a meta-analysis of existing datasets, the project examined the relationship between pasture utilisation and four key measures of reproductive performance in northern Australian beef herds:
- annual pregnancy rates
- pregnancy within four months of calving while lactating
- weaning percentage
- foetal and calf loss.
Methods
There is an untapped gold mine of breeding herd production data from sites across northern Australia. We used existing datasets to ask new questions, getting the most out of research that has already occurred.
We have collated, analysed and modelled breeder data to:
- identify the pasture utilisation rates which optimise breeder productivity
- improve existing models to predict breeder performance and economics of herd management
- develop tools to improve breeder productivity and maximise profitability.
Project objectives
- Develop tools to predict the impact of pasture utilisation on reproduction.
- Help producers optimise pasture use to maximise the kilograms turned off/ha (increase weaning rates and reduce breeder mortality) while maintaining the resource base.
- Increase beef production efficiency with a reduced cost of production.
Project locations
We are analysing and modelling 60 breeders herds from across northern Australia.
| Region | Sites |
|---|---|
| Alice | 9 |
| Barkly | 15 |
| Central Queensland | 1 |
| Far North Queensland | 1 |
| Katherine & Sturt Plateau | 4 |
| North Queensland | 10 |
| Victoria River District | 21 |
When
Results
The key findings from the empirical analyses were:
- Pasture utilisation relative to recommended safe levels was strongly associated with breeder reproductive performance.
- Increasing utilisation rate corresponded with reduced performance, with annual pregnancy rates declining by around 10% and pregnancy within four months decreasing by 15–30% when comparing low to high utilisation.
- Foetal and calf loss rose by approximately 2.3% per incremental increase in utilisation.
Stocking rate and breeder performance
These findings align with the widely accepted concept that lower stocking rates allow for greater dietary selection and quality, supporting higher body condition and fertility, whereas higher utilisation intensifies competition and reduces animal performance. However, these effects were moderated by Bos indicus content, with herds classified as containing less Bos indicus having the greatest magnitude of effect, while high Bos indicus content groups being comparatively resilient. Importantly, however, in herds with >75% Bos indicus content, an increased response tended to occur lower utilisation levels, possibly suggesting that competition for pasture at much lower levels.
The role of diet quality during key reproductive periods
In addition to the above findings, the project highlighted the critical role of diet quality during key reproductive periods, particularly around calving, in determining reproductive performance. Several mechanisms were identified.
Annual pregnancy rates were positively associated with increased pasture growth in the surrounding 3 months of calving in the previous production cycle.
The likelihood of cows reconceiving within four months while lactating increased with increasing proportion of green pasture available in the three months around calving. A 25% increase in green proportion was predicted to correspond with a 20% increase in pregnancy within 4 months of calving — highlighting the importance of aligning calving periods with times of higher-quality pasture.
Weaning percentage also improved when pasture had higher nitrogen content around calving in the previous year and when early dry-season pasture growth was strong. This appears to support cows that have weaned a calf but not yet conceived by the first annual muster, enabling them to regain condition, resume ovarian activity, and subsequently be identified as pregnant at the second muster.
Heat stress and foetal and calf loss
The study also explored the role of heat stress using several climate indices, including the Temperature-Humidity Index (THI), Heat Load Index (HLI), and the Comprehensive Climate Index (CCI). Of these, the CCI was identified to be the most informative. This index integrates temperature, humidity, wind speed, and solar radiation into a single measure of animal heat load. Somewhat surprisingly, the current study found no association between average daily maximum Comprehensive Climate Index (CCI) and foetal or calf loss. This result could reflect the coarse resolution of monthly averages, which may overlook acute heat events critical to calf survival, as well as unaccounted factors such as breed differences and the timing of calving. These results highlight the need for future research using finer-scale data that can link calving events more precisely with climatic extremes.
If it’s happened once, it’s more likely to happen again
Patterns in foetal and calf loss provided further insights. Losses were nearly 4% lower in cows that had lactated in the previous production cycle compared with those that did not. This finding aligns closely with results from Fordyce et al. (2023) and suggests that calf loss maybe a slightly repeatable trait within individual cows. This result raises the possibility that calf survival may have a maternal heritable component and warrants further investigation.
The role of soil phosphorus
Soil phosphorus availability was also examined, given its recognised role in supporting fertility and performance in northern beef systems (Quiglely et al., 2015). Although no significant effect was detected in this study, there was a trend toward improved efficiency at higher soil phosphorus levels. However, this analysis relied on mapped soil phosphorus data rather than direct measures of animal status, and results may have been confounded by factors such as supplementation practices, soil variability within paddocks, and mobilisation of body reserves. These limitations illustrate the complexity of isolating the role of phosphorus in extensive grazing systems.
Mating management
Finally, mating management emerged as an important factor. Controlled mating groups showed a 9% higher rate of cows becoming pregnant within four months compared with continuous mating systems. Interestingly, this advantage was not reflected in annual pregnancy or weaning rates, suggesting that some of the observed effect may be linked to broader herd management or regional differences.
Conclusion
Effective management of northern beef herds requires an integrated approach that balances pasture utilisation, nutrition, breed resilience, and reproductive strategies. Across all reproductive measures, pasture utilisation was a significant determinant, with higher utilisation consistently associated with reduced performance. Managing pasture quality and availability, particularly around calving and early dry-season periods, supports good body condition, reproductive performance and calf survival. While supplementation, including phosphorus, may provide additional benefits, responses can vary depending on soil and management conditions. Furthermore, the potential impacts of acute heat stress on reproductive outcomes warrant further investigation, as current data may not fully capture the effects of short-term extreme events on cow and calf performance.
Contacts
- Dr Robyn Cowley, Department of Industry, Tourism and Trade, Berrimah NT, 0419 829 493 or robyn.cowley@nt.gov.au
- Dr Kieren McCosker, The University of Queensland, k.mccosker@uq.edu.au
Project Team: Northern Territory Department of Agriculture and Fisheries, University of Queensland, Queensland Department of Primary Industries, Queensland Department of Environment and Science, Karingal Systems and Trudi Oxley.
Download the papers presented at the 2025 International Rangeland Congress:
- Factors influencing pasture utilisation in northern Australian rangelands (PDF, 671KB)
- Feeling the heat a retrospective investigation of thermal load impacts on calf loss (PDF, 220KB)