Making late nitrogen applications work Matt McIntosh Producers should rethink best management practices for post-canopy corn fertility treatment Applying nitrogen in late season corn allows more opportunity to fine-tune fertility rates, spread out labour requirements, and rescue corn impacted by early dry conditions. ONTARIO GRAIN FARMER ENVIRONMENT 24 To get the most out of post-canopy applications, producers need to rethink some common assumptions about how nitrogen fertilizer and volatilization inhibitors interact with moisture, soil texture, and plant physiology. That’s the overall finding of a 4-year research study by a University of Guelph team trying to determine best management practices for post-canopy applications. Research team leader Dr. Joshua Nasielski says the key was to identify factors limiting or encouraging the absorption of nitrogen in corn at or past V8, while reducing the risk of losses from volatilization. Some growers broadcast, others use Y-drop systems, Nasielski says. The researchers tried to address the right split for nitrogen applied late – how much nitrogen can you delay in terms of your total nitrogen budget, what’s the best placement for urea and UAN, and what’s the best use case for volatilization inhibitors. MEASURE ATMOSPHERIC LOSSES Here’s why volatilization inhibitors are important. As urea breaks down, it turns into plant-available ammonium. That ammonium can also volatilize to gaseous ammonia though, which can be lost to the atmosphere. Volatilization is more common with insufficient soil moisture, but it’s not the only way volatilization can happen. Nasielski’s study (using dositubes as measuring devices) found average volatilization losses in Ontario were 38 pounds per acre, although the range of results was significant. Individual results vary from field to field, rate to rate, and year to year. But Nasielski says their research at field sites in Winchester, Elora and Ridgetown confirmed some long-held assumptions about volatilization risk. First, and most obvious, is that rain is required to push top-dressed nitrogen into the soil profile, and in late season applications, that nitrogen is needed by the plant as soon as possible. Researchers say findings consistently showed corn yields suffered if late season applications were combined with a prolonged no-rain period. OTHER FINDINGS WERE LESS INTUITIVE The yield benefits of late applied nitrogen were more consistent on sandier soils. No benefit was observed in other locations with different soil types, in part due to a short absorption window. Nasielski says applying nitrogen at V13 in Ontario, 10 to 12 days before silking, is too late to expect a yield response if there is no rain. Further south, in corn-producing US states such as Illinois or Indiana, where 20 or more days intercede between V13 and silking, a greater possibility exists of seeing that response. But in Ontario, Nasielski says economically you would be losing yield applying nitrogen that late. There would not be an economic payback at V13, but applications made at V10 might be more worthwhile for Ontario growers. Nasielski says the amount of rainfall required to supress volatilization late in the season was also surprising – a result of the physiology of tall, closely planted corn – as was the impact of wet soils at application time. Corn leaves channel rain away from the row middle where fertility product is placed, limiting contact with surface-applied product and the rate at which it enters the soil profile. Significantly more
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