Are fall applications of N to winter wheat beneficial?

Carrie Laboski, Professor and Extension Soil Fertility/Nutrient Management Specialist, UW-Madison

The short answer is maybe.

Research in 2014 through 2016 demonstrated that N application at green-up generally resulted in the greatest economic return compared to application at Zadok’s growth stage 30 (GS30, approx. Feekes 5). See past articles for details: https://npketc.soils.wisc.edu/2017/03/25/tips-for-nitrogen-management-in-winter-wheat/ and https://npketc.soils.wisc.edu/2016/03/09/time-your-spring-nitrogen-applications-to-maximize-winter-wheat-yield-2/. In this same study, we had a couple of fall N application treatments. We found that 120 lb N/a applied as SuperU at planting yielded the same as 120 lb N/a as ammonium nitrate applied at green-up at five of eight sties years (Table 1). However, this rate was an over application of N at all five of these sites, so it’s not surprising that there wasn’t a yield difference between fall and green-up applications. At Chilton in 2014 and Lamartine and Pipe in 2016 fall application resulted in significantly lower yield than green-up application. These data suggest that some N loss occurred in fall and early spring.

Additional treatments evaluated the effect of N application timing and source when 60 lb N/a was applied. This rate is an under application of N relative to the economic optimum N rate for the green-up application at all sites except Pipe in 2015 and was very close to the optimum N rate at Chilton in 2014. Using suboptimal N rates is helpful to evaluate how different N sources and timings perform. Fall and fall/GU split applications yielded similarly to green-up applications at four of six sites (Table 1). At the other two sites, substantial fall N loss resulted in reduced yields with fall N applications.

Growers should consider fall application of N to winter wheat with caution. If a wet fall is forecast and/or fields are known to be wetter, waiting to apply N until green-up may be prudent. The N lost under these conditions will have a negative impact on profitability.

Growers should consider taking a pre-plant soil nitrate test (PPNT) to assist in determining a N application rate for wheat. This consists of taking a 0 to 2 foot- soil sample in 1 foot-increments at/near the time of planting. Basic sampling intensity guidelines are to collect 15 cores randomly throughout at 20-acre area and composite into one sample. Consider reducing the area represented by each sample to correspond to soil types and/or management zones. For more information on wheat N rate guidelines and how to adjust rates with the PPNT see: http://ipcm.wisc.edu/download/pubsNM/Wheat_MRTN_extended_06_2013.pdf

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