Written by: Cameron Weeks | Farm Business Consultant | 0427 006 944
Can you believe it’s July already and the year is half over? My grandmother always told me things sped up as you aged so I guess that’s what’s happening given I’m 55!
Given that the growing season too is also half over it’s always a good time to reflect on the state of play at your own farm as you make remaining crop input decisions, consider when to market your grain, how much to insure the crop for, etc.
As the table below shows the season is highly variable across the state but with a clear trend to wetter in the southern half with decile 5-9 for the growing season typical whereas much of the east and north is sitting at decile 1-3.
Table: 2023 rainfall to date for a selection of Wheatbelt locations (at 3 July 2023).
Making good use of rainfall data
For rainfall data to be useful for remaining nitrogen decisions as well as grain marketing and crop insurance you of course need to use it for sensible crop yield estimates which is where I always find rainfall data more useful than a visual assessment of how the crop looks of course with rainfall data being the reality test!
A visual assessment of how the crop looks both from a timing as well as plant health perspective is important but remember it is stored water especially on better soils that really determines your yield prospects currently.
This is where the simple water use efficiency method comes into play. We apply this annually to crops to determine water use efficiency (WUE) achieved but it can also be used to forecast likely crop yield. The variation to the formula is of course the unknown amount of rainfall to come and the final WUE.
The formula
Summer rain (Jan-Mar) / 3 (to allow for summer evaporation).
plus
Growing season rain (Apr-Sep) to date plus estimate of rain to come.
minus
1/3 of growing season rain (Apr-Sep).
equals
Plant available (mm)
Plant available water (mm) x WUE estimate (kg/mm/ha) =
Estimated Crop Yield (kg/ha).
Example – Binnu
Summer rain: 56mm / 3 = 19mm
GSR to date: 43mm
GSR estimate July-Sep (based on average): 132mm
GSR Total less 1/3 for evaporation: 116mm
Plus, summer rain (19mm): 135mm
135mm x WUE 12kg (below average) = 1.6t/ha
135mm x WUE 15kg (average) = 2.0t/ha
How much more rain to assume?
When considering probabilities, I typically work on no higher than average remembering that history says achieving average or better only happens 50% of the time! There is some logic in working on decile 3-4 though if you wish to shift the odds of achieving a certain outcome in your favour.
What WUE to apply?
When it comes to what water use efficiency to apply given you have no idea how the season will finish it is logical to use your long-term average however this is where you may apply a visual assessment of how the crop is tracking. For example, if it was sown very early then struggled and plant density is sub-optimal it would make sense to assume a below average WUE. Likewise, a late sown crop most likely
will achieve a below average WUE.
When considering probabilities again, I always work on average being the highest WUE to apply with a better outcome than that a bonus.
A mixed bag!
Apply the above approach across the state and you can see that this year is shaping as a very mixed bag with >decile 5 rain to date putting many in a great position and looking at excellent yield prospects whilst others would be well pleased with anywhere near average.
As we all know though we cannot predict what will happen with rain and temperatures to come so decisions made now are merely sensible estimates.
Good luck to all of you for the second half of the year!
