“’Cause nothing lasts forever, even cold November rain…”
– Guns N’ Roses

November rain — it’s more than just a classic rock anthem. In the Gwydir Valley, it’s a signal: time to make the most of those natural rainfall events that can transform crop potential without touching an irrigation valve.

As November storms roll in, they bring a golden opportunity to top dress urea — turning natural rainfall into one of the most efficient nitrogen incorporation tools we’ve got.

Nitrogen fertilising strategies for high yielding cotton crops have evolved to an increased reliance on ‘in crop’ fertilisation, to mitigate the losses from a pre plant loaded strategy. The prevalence of pre-plant fertiliser application practice was driven by historical research (Constable and Rochester 1988). This research was conducted prior to modern high yielding transgenic varieties, where N requirements have increased to over 300 kg N/ha (Baird et al. 2024).

A recent publication by John Baird illustrated that the primary movement of pre plant nitrogen is vertical, above the fertiliser placement. Highly water-soluble nitrate N travels with the irrigation water through a bed, so when we are ‘watering up’ our cotton crops we should be thinking we are ‘watering up’ our pre plant nitrogen application too. The secondary movement was laterally, towards the non-irrigated furrow. Resulting in up to 38% of pre plant nitrogen lost by first square through denitrification and volatilisation. Water up run times are often more than 24 hours this season with many fields taking longer. These losses make up the largest share of nitrogen losses in an irrigated system locally, run off accounts for about 10% and a field based lysimeter study by Macdonald et al. (2017) using 15N-labelled urea, found that vertical subsoil leaching was minor in cotton soils from a singular season of N fertiliser (<1%). 

These results and the push for higher cotton yields locally has resulted in an increasing proportion of nitrogen to be applied closer to when the crop needs it and at several points throughout the season.

The CSD and Syngenta Faststart initiative demonstrates that the faster crops are growing into first flower the higher their yield potential. With fantastic crop establishment conditions so far this season it is important that growers capitalise on the opportunity to improve their crops ability to reach their potential.  

With little or no fruit on the plant at this stage growers may be asking why would I fertilise now? It is important to remember that early in the crops development that the leaves of the plant are the biggest sink for nitrogen and by taking advantage of this opportunity to bank nitrogen into these organs of the plant it gives the crop reserves to draw on as it matures and senesces. 

The cotton crop is also moving into peak demand for nitrogen and November rainfall can yield near perfect conditions to incorporate top dress urea. Growers also need to be aware that in order to be ‘plant available’ urea needs to undergo hydrolysis (catalysed by the urease enzyme) to convert to ammonium, which plants can absorb and usually takes 2-7 days. Once converted to ammonium, further transformation to nitrate (NO₃⁻) via nitrification takes another 7–14 days, depending on microbial activity and soil temperature. 

Back the variety’s ability to convert carbohydrates into high yields and your consultant to wield the mepiquat for internode control.

Keep up to date with CSD’s work by joining the Gwydir cotton consultants WhatsApp group for more in season updates from the Gwydir Valley.

Wishing you a Faststart and plenty of crop momentum into first flower,

Stuart

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