The Latest on Corn Nitrogen Response: Weather, N Uptake in Modern Hybrids and Late Application
Research in much of Ontario has shown a clear benefit to sidedressing nitrogen in corn. By applying N closer to the period of maximum uptake, you provide the nutrient when it's needed and minimize the risk of losses from leaching or denitrification.
There may now be several other reasons to move away from an "all upfront" approach and to split your nitrogen application in corn.
Weather impacts on nitrogen response
Recent research has shown that growing season precipitation can drive yield potential and response to nitrogen in corn. Dr. Nicolas Tremblay of Agriculture and Agri-Food Canada (AAFC), analyzed data from 51 nitrogen response studies across North America, including Ohio and Quebec. He concluded that "abundant and well-distributed rainfall" is a major driver of nitrogen response of corn in both fine (e.g. clay loam) and medium textured soils. Abundant and well spread out rainfall in the weeks prior to and following sidedressing resulted in a greater response of corn to nitrogen.
Dr. Bill Deen at the University of Guelph has found that optimal corn N rates from 2000-2017 at Elora have been affected by mid-June to mid-July rainfall. Generally speaking, greater amounts of rainfall have resulted in higher maximum economic rates of nitrogen (MERN) values. Interestingly, this relationship was not observed for trials prior to 2000. Recent results from the long-term corn nitrogen response trial at Elora (2009-present) have shown MERN values as low as 118 lbs/acre, up to 230 lbs/acre N. Growing season rainfall - and crop nitrogen demand - has been the main determining factor.
Together, these results suggest that an in-season rate adjustment, based on current and forecasted conditions, may be a wise approach.
Post-silking nitrogen uptake
Nitrogen uptake in corn accelerates starting around V5 (Figure 1). As corn moves into reproductive growth following tasseling, a portion of nitrogen is moved, or remobilized, from plant tissue to meet the grain demand. Soil nitrogen uptake also continues during reproductive growth stages.
Figure 1. Corn nitrogen uptake (Adapted with permission from Iowa State University Extension)
It's also important to consider how modern corn hybrids take up nitrogen. Newer hybrids take up a greater proportion of their total nitrogen following silking - 36% vs. 30% in older hybrids (before 1991). Also, a larger percent of grain nitrogen in newer hybrids comes from soil uptake after silking compared to N remobilization from stalks and leaves. In fact, as N stress increases, newer hybrids are able to increase nitrogen uptake after silking - something that older hybrids weren't able to do.
Research from 2014-16 at Purdue found that delaying the last 40 lbs/acre of N to the V12 stage did not negatively impact yield, but did increase the efficiency of nitrogen uptake. Josh Nasielski, a PhD student with Bill Deen at the University of Guelph, found that waiting until V13 to sidedress the full N rate for corn did not hurt yield in 2017 at Elora. In his trial, a generous top-up at V13 of a pre-plant rate of 70 lbs N/acre supplied enough N in time to maximize yield compared to an all upfront application.
A new approach to nitrogen
With high clearance equipment available and later nitrogen uptake in modern corn hybrids, Ontario farmers now have the flexibility to apply N later into the season.
A recently completed OSCIA Tier Two project in Eastern Ontario found that late N application increased corn yield at some sites compared to earlier application in 2017, likely due to very wet growing season conditions. In 2016, however, there was no yield advantage to late applied nitrogen.
Despite the mixed results to date on late N applications, the practice provides the opportunity to more accurately assess in season rainfall and growing conditions. By doing so, you can make rate adjustments to better reflect the crop's yield potential and nutrient demand.
Tried and true
In Ontario, the Corn Nitrogen Calculator is an excellent starting point to determine your N rate. It provides a recommendation based on decades of Ontario N trials and accounts for background factors such as soil type and previous crop. By combining it with a split application that allows you to fine-tune rates for in-season conditions, you're on your way to better matching crop demand and maximizing your nitrogen dollar.
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