Green Date Analysis

About the Green Date (GD)

Beef cattle in northern Australia rely heavily on native rain-fed pasture as their primary food source. Pasture growth is primarily driven by rainfall during the wet season from October to April. While moisture is usually the main limiting factor, tropical and sub-tropical pastures generally require temperatures above 20°C for optimum growth (McCown 1981).

The start of the pasture growing season, known as the Green Date (GD), is defined as the first day after the 1st of September when a location is expected to receive 50 mm of rain over 3 consecutive days in 7 out of 10 years. The rainfall threshold for calculating the GD varies from region to region, based on a location’s geography and soil type. Locations with loamy or sandy soils typically need less rain to initiate pasture growth, while clay soils require more moisture.

Due to Northern Australia’s distinct wet and dry seasons, the GD allows for estimating the length of the slow growth season, calculated as the number of days from the end of the growing season (April/May) to the next rainfall event that stimulates pasture growth. A feed budget based on the length of the dry season, combined with factors such as total standing dry matter, pasture utilisation rate, and stock numbers, can help determine the amount of supplementary feed required. This can alleviate pressure on the pasture, and help maintain good groundcover, pasture and land condition.

In some regions, the GD can be used as a valuable guide for other livestock management decisions, such as optimal calving time and determining production date (when liveweight begins to increase between the dry and wet seasons).

Example: Charters Towers

For example, in Charters Towers (north-eastern Queensland), the historical (1900–2025) GD is calculated as 22 January. Using this date, producers can calculate the number of days between the end of the growing season to the start of the next effective rainfall event (i.e., from 30 April to 22 January), which is 268 days. This is the period during which grazing must be planned to ensure adequate feed for livestock and to maintain ground cover. This period indicates a probable growing season of 97 days (365 − 268) for Charters Towers, highlighting the need to rest pastures during this time to boost long-term productivity.

References

Balston, J., and B. English, 2009: Defining and predicting the ‘break of the season’ for north-east Queensland grazing areas. The Rangeland Journal, 31(1), 151–159. doi:10.1071/RJ08054

Bortolussi, G., J. McIvor, J. Hodgkinson, S. Coffey, and C. Holmes, 2005: The northern Australian beef industry, a snapshot. 3. Annual liveweight gains from pasture based systems. Australian Journal of Experimental Agriculture, 45(9), 1093–1108. doi:10.1071/EA03098

Brown, J. N., A. Ash, N. MacLeod, and P. McIntosh, 2019: Diagnosing the weather and climate features that influence pasture growth in Northern Australia. Climate Risk Management, 24, 1–12. doi:10.1016/j.crm.2019.01.003

Cobon, D., R. Darbyshire, J. Crean, S. Kodur, M. Simpson, and C. Jarvis, 2020: Valuing seasonal climate forecasts in the northern Australia beef industry. Weather, Climate, and Society, 12(1), 3–14. doi:10.1175/WCAS-D-19-0018.1

Cowan, T., R. Stone, M. C. Wheeler, and M. Griffiths, 2020: Improving the seasonal prediction of Northern Australian rainfall onset to help with grazing management decisions. Climate Services, 19, 100182. doi:10.1016/j.cliser.2020.100182

Lisonbee, J., J. Ribbe, J. A. Otkin, and C. Pudmenzky, 2022: Wet season rainfall onset and flash drought: The case of the northern Australian wet season. International Journal of Climatology, 42(12), 6499–6514. doi:10.1002/joc.7609

McCown, R., 1981: The climatic potential for beef cattle production in tropical Australia: Part I — Simulating the annual cycle of liveweight change. Agricultural Systems, 6(4), 303–317. doi:10.1016/0308-521X(81)90065-2

Naha, R., T. Cowan, M. C. Wheeler, J. Owens, D. Cobon, and C. Jarvis, 2026: Exploring the green break of season and green date over northern Australia. Agricultural Systems. doi:10.1016/j.agsy.2025.104526

O’Reagain, P. J., J. Bushell, L. Pahl, and J. C. Scanlan, 2018: Wambiana grazing trial Phase 3: stocking strategies for improving carrying capacity, land condition and biodiversity outcomes. Meat and Livestock Australia. CorpusID:169436715