Break of season rules for forage budgets

Forage budgets are a way of ensuring you stock a paddock at a rate that leaves you with enough feed for your cattle and enough stubble on the ground for your pastures.

At best, dry season forage budgets are a guesstimate. One of the big variables is the date you nominate as the end of the budget period, usually a date by which you can reasonably expect sufficient rain to give significant grass growth. For most areas it is later than we might expect. This may be because we tend to remember those few years when 100 mm falls in spring. We remember them because they are exceptions to the rule rather than the rule itself.

So what constitutes a break in the season and what are the chances of it occurring by a certain date? How effective rain is depends on how quickly it falls and how dry conditions were before the rain. The resultant pasture response is also variable, depending on the soil type and the land condition. Generally, pasture on lighter soils responds more quickly and to smaller rainfall events than pasture on heavy clays.

There is variability in timing and quantity of rainfall across northern Australia. Even producers with the same rainfall will have different ideas about what constitutes ‘break of season rules’ – this is related to their attitude to risk. For example, let’s look at Gin Gin in the north Burnett region of Queensland.

Many producers consider 100 mm of rain over a two-week period by the end of September as an effective break (and it certainly is!). Others nominate 50 mm in a week by the end of November. How often do we see these sorts of rainfall events?

Packages such as Rainman enable us to view the rainfall records for many districts over many years to determine safe break of season rules.

If we look at the past 113 years of rainfall records for Gin Gin (Queensland), we can determine the chances of meeting three ‘break of season rules’; 30 mm in 3 days, 50 mm in a week, or 100 mm in a fortnight, by the end of each month from September to January.

Percentage of years where the beak of season rule is met by the end of the given month
30 mm in 3 days 50 mm in a week 100 mm in a fortnight
September 30% 12% 4%
October 61% 42% 15%
November 80% 61% 28%
December 94% 85% 53%
January 98% 96% 79%

At Gin Gin, you cannot rely on getting 100 mm of rain over a fortnight by the end of September. It has only happened four times in the last 113 years; i.e. 1906, 1933, 1947 and 1998!

The table highlights the unreliability of a spring break. In setting safe break of season rules for forage budgeting you need to consider your attitude to risk. A break of season rule that is met about 70–80 % of the time (seven or eight years in ten) is relatively safe.

In this case, 30 mm in 3 days by the end of November will be a safe rule for most land types. So when you calculate your forage budget in the area around Gin Gin, you would want the available feed to last until the end of November. For land types with heavy clay soils, 50 mm over a week by the end of December would be a safer bet.

Remember, forage budgeting is an inexact science. Be realistic about when you can reasonably expect a break to the season.

For more information see Dry season pasture budget – a guide for stocking rates.

Written by Bill Schulke, formerly Queensland Government.

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