A hayed-off crop can make estimating available nitrogen (N) trickier than usual. Deep N testing is the best way to assess N resources.Estimates and actual results from deep soil testing were very different in recent research by Birchip Cropping Group.
After haying off there’s potential yield benefit next season from leftover nitrogen. But the benefit may not be proportional to the amount of N applied in the previous season.
The dry limited yields in 2015
Their trials in Victoria’s Wimmera region looked at soil N and wheat yields after a hayed-off barley crop. The 2015 trial had six barley varieties and five N rates (0, 30, 60, 90, 120 kg N/ha). 2015 was a very dry year – decile 1 rainfall. The trial yields averaged only 0.7 t/ha in 2015. In that year Barley did best with zero N applied. Higher N rates yielded less.
2016 yields and nitrogen rates
Clearfield wheat was sown in 2016. The 2015 N applications influenced the wheat yield. As the 2015 N rates increased, so did the wheat yield in 2016. Half of each wheat plot received 40 kg N/ha. The yield varied less between plots with different 2015 N rates where N was applied in 2016. The 2015 N rate affected yield more than N applied in 2016.
How does a hayed-off crop affect soil N?
Estimates of residual soil N, based on a simple nitrogen balance calculation, were very different to 0–70 cm soil test results. The nitrogen balance calculations overestimated available N.
Use deep N testing for an accurate picture of soil N reserves. There might be more error from the ‘N balance calculation’ as the severity of ‘haying off’ increases.
In both 2015 and 2016, there was a strong mid-season biomass response to N rates. As well as pre-season soil testing growers can watch in-crop N response. Crop signs can help guide topdressing rates to avoid over or under fertilising.