Should I water my wheat this spring?

PRODUCTION ADVICE - AUGUST 2020 - AGRONOMY

By John Fowler
Extension Agronomist
P: 03 5881  9933 | M: 0427  079 138 | E: john.fowler@lls.nsw.gov.au

Should I spring water my wheat – and if so, when is the best time to do it?

The decision on where to use the small amount of irrigation water currently available is not necessarily easy to make.  Basically, the question is what will give the best return per megalitre of water?  To answer that, several assumptions are required, and the conclusion will only be accurate if the assumptions are accurate.

The main assumption is predicting the amount of extra production generated from a megalitre of irrigation water applied to a crop.

Spring irrigating wheat

What extra production is it reasonable to expect from each megalitre of water used to irrigate wheat in the spring?  There is data that indicates yield will increase by at least one tonne for every megalitre of irrigation applied in the spring. However, yields of far less than 1 t/ML have often been recorded and, in the worst cases, there have been no positive impacts on yield from spring irrigations.

Budgeting on a one tonne per megalitre yield response is only reasonable in crops with good agronomy.  Basically, this means:

1. Sown early in the window – for lower frost-risk areas west of Berrigan, this means:
   • Winter wheats (e.g. Wedgetail, Longsword) before Anzac Day
   • Mid-maturity wheats (e.g. Scepter, Beckom, Suntop) before mid-May (expect about 1.0 t/ha less yield for crops sown at the end of May than those sown in early May)
   • Quick maturing wheat (e.g. Condo, Vixen) before 10 June.
2. Good plant establishment
   • At least 120 plants/m²
3. Well tillered
   • Yield potential of 3-4 t/ha requires at least 200 to 250 tillers/m²
   • Yield potential of 5 t/ha requires at least 350 tillers/m²
4. Adequate nutrition
   • Crop received at least 4 kg/ha phosphorus per tonne of target yield
   • Sufficient nitrogen to allow for at least four tillers per plant
   • Yield potential of 3-4 t/ha requires 120-150 kg N/ha
5. Good weed control
6. No root or foliar disease
7. Soil in good condition
   • Chemistry:  non-acidic (i.e. pH CaCl₂ > 5.0), low sodicity (i.e. ESP < 10)
   • Structure: soil not hard setting
8. Quick draining layout

Expected returns

If a spring irrigation gives a yield increase of about 1 t/ML, the gross margin is the value of the extra grain minus the costs associated with it.

The extra costs (apart from the water costs) are mainly any extra harvest and cartage costs plus any additional nitrogen costs. The extra harvest and cartage costs are likely to be about $25/t, while the extra nitrogen required is probably about 30 kg/ha N, which could cost about $35/t spread. If wheat is valued at $260/t delivered locally, this means the gross margin is about $200/ML (less cost of water).

Water costs

The above gross margin does not include a water cost. It does however allow you to know what price you can afford to pay for water or to compare the expected return from using your allocation on wheat with using it on other crops (e.g. rice) or even selling it on the temporary market.

When to irrigate

The most important time to avoid moisture stress is at flowering.  This means that if only one irrigation is planned, watering just prior to heading (booting or growth stage DC47) will usually give the best result.  Irrigating earlier than this may cause excessive foliar growth which uses more soil moisture and leads to greater moisture stress at flowering.

Keeping water for summer crop

The gross margin for water used on rice is probably substantially more than $200/ML. However, this depends on several factors such as rice price, water use per hectare and yield.

The best guide is to use your own costings, yield expectations, water use in the ‘Rice $cenario’ budget available online from Rice Extension.

Further resources

Two good references are:

1. GRDC fact sheet ‘Irrigated wheat in the Murray Valley’ on their website

   https://grdc.com.au/__data/assets/pdf_file/0017/142541/grdc-fs-irrigated-wheat-murrumbidgee-lr.pdf.pdf

2. ‘Key constraints to irrigated wheat yields in the southern Murray-Darling basin’ Agronomy conference paper by Sam North, Alex Schultz and Don Griffin http://agronomyaustraliaproceedings.org/images/sampledata/2019/2019ASA_North_Sam_79.pdf

Acknowledgements

Some of the technical information for this article was provided by Sam North and Alex Schultz, NSW DPI, Deniliquin.