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Determine variability in your pasture growth

Georgia January 21, 2022

Pasture supply varies from year to year and between different locations. The variability of pasture growth between years is an indicator of risk and affects the ability to set an optimal stocking rate target or the reliability of a particular time of year for lambing (see figure 8.6).

Figure 8.6 Variability in annual pasture growth rates for Kojonup, WA (from DAFWA Woolpro sites and Pastures from Space) and native redgrass/sub clover pasture, Armidale, NSW. Source: GrassGro®.

In figure 8.6, the 10% line represents the one-in-ten bad years (only 10% of all pasture growth records would fall below this line). The 50% line represents an “average” year (half of the recorded years were above and half were below this line), while the 90% line represents the boom seasons (90% of all pasture growth records would fall below this line) – only 10% of recorded year’s pasture growth would be above this year.

The closer all three lines are together in any one month, the less variability there is between years. In figure 8.6, the most reliable months at Kojonup are July-September and at Armidale December-February and June-July are most reliable. Autumn growth in annual pastures is variable at Kojonup, whilst spring is the most variable growth period for native pastures at Armidale.

To better understand your feed supply, construct pasture supply curves for your farm and assess the variability between years.

The simplest approach to constructing a pasture supply curve is to draw the growth curves for the main pasture types on a single graph.

Use the relevant tables in tool 8.1 to construct simple pasture supply curves for your farm (all tools available on the MMFS website). The estimates presented are only intended as a guide to help plan your annual feed availability. As an example, a pasture supply curve has been constructed for Dubbo NSW (figure 8.7) using the tables in tool 8.1. 

Figure 8.7 Pasture supply curves for Dubbo, NSW. Summer active pastures in winter rainfall zones can extend the growing season and provide land management benefits. Source: GrassGro™.

On your whole-farm pasture supply curve, draw estimates of how much higher these lines might be above the average (the 1-in-5 best years) and how much lower the lines might be than average (the 1-in-5 worst years), using:

  • Your experience, and/or that of your advisor.
  • The Bureau of Meteorology Climate Data Online website provides historical weather details (see instructions for looking up your local data at the beginning of this chapter).
  • The data in tool 8.1 (all tools available on the MMFS website).

The MLA Feed demand calculator describes how best to match animal demand to your pasture supply and minimise your exposure to risk from seasonal variability. The feed supply curve is generated as well as an inventory of stock demand across the year (all classes considering purchase, sale and lambing dates). Compare supply with demand to get a better understanding of when pasture will be in surplus or deficit and by how much. Use this information to set up your enterprises to better match available feed with animal demand.