ONTARIO GRAIN FARMER ENVIRONMENT 26 “Leaf scorching has to be very significant to matter,” Nasielski says, adding crop insurance assessments for hail-induced leaf loss account for a similar yield penalty model. “The takeaway is broadcast urea rarely caused burn severe enough to cause yield loss. Only UAN consistently caused enough burn to reduce yield potential. I think that is the big take away: urea is safe to top-dress. Even with observable burn, it won’t cause yield loss.” Cation exchange capacity (CEC) – a soil property, measured in cmol (100 mol) per kilogram, used to predict plant nutrient availability and retention – was also found to significantly impact the net returns of inhibitor additives. Nasielski says when soil CEC is above 23 cmol(+)/kg, volatilization losses are already going to be low. So, the chance of positive net returns to using an inhibitor on surface-applied UAN or urea on a soil with CEC greater than 23 are less frequent (33 per cent chance of positive net return). On soils with CEC less than 23 cmol(+)/ kg, volatilization losses are often higher, so the chance of positive net returns to an inhibitor were 90 per cent when used on surface-applied urea or UAN. “The bottom line is that in heavy clay, an inhibitor is much less likely to make a difference late in the season. You’re not losing a ton of money, but in terms of profitability you have a lower chance,” he says. Finally, Nasielski and his colleagues determined “luxury nitrogen” – nitrogen taken up by the plant and stored in the lower stems for potential later use – can benefit the plant during grain fill, if occurring under dry conditions. “Plants use the luxury nitrogen in place of taking up more nitrogen from roots,” he says. “It’s also good in very high yielding conditions because you’re getting extra yield with bigger kernels, not just more kernels per plant. Those kernels are helping to contribute to high yield.” Overall, Nasielski says split nitrogen applications late in the season can work. Whether he recommends the approach, though, is another matter. “As a scientist, you have to look at what the data says, and the data makes me more cautious about recommending more aggressive splits going later than V10 in Ontario. I still think late applications have a place, if you’re adjusting your rates, because it gives the farmer more control. Something like adding the last 50 pounds of nitrogen at V10 is likely no problem if you have 150 pounds up front.” • In-field technology helped researchers determine which late nitrogen applications work. ACKNOWLEDGEMENTS: This research was funded by Agriculture and Agri-Food Canada and the Ontario Agri-Food Innovation Alliance, a collaboration between the Government of Ontario and the University of Guelph. The support of the Natural Sciences and Engineering Research Council of Canada is also acknowledged.
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