Dry conditions drive surprisingly low top-dress recommendations on corn


New technology built on 30 years of Ontario- and Quebec-specific corn data is producing some surprising nitrogen top-dress recommendations for Ontario’s corn crop. The prescription? Some dry areas of the province with a 220 bushel corn yield target are receiving total N recommendations of just 100 pounds per acre. Yes, really.

Is this even possible? Greg Stewart, agronomy lead for Maizex, says that yes, it is, but whether or not a farmer would ever risk short-changing the crop this late in the game is unlikely.

Where did this recommendation come from? Stewart says there are a few ways to determine N rates: one, on paper, using field history, yield data, soil tests, and a decent nitrogen use calculator; two, the on-paper data plus an in-season nitrate test to push or pull back the original recommendation; and, now, a third option using the Effigis FieldApex program which takes all of this into account, plus the 15 days of rainfall leading up to top-dress timing and incorporates the weather forecast.

This year, a comparison of the three methods has led to some surprisingly low added-nitrogen needs, including one field with the very low 100 pound recommendation, largely because of the very dry conditions in parts of the province.

Total rainfall during a very specific period is what’s driving the low N rates, Stewart says. It’s a relatively new way of thinking, but the data — gleaned from work done by Dr. Bill Deen with the University of Guelph, Nick Tremblay with AAFC, and Stewart himself — shows that it’s rainfall in the rapid growth period (June 10 to July 10, roughly) that strongly correlates to total nitrogen recommendations because of the relationship with soil N mineralization.

In decades of research, Deen found that the maximum economic rate of nitrogen on corn can vary from as low as 140 pounds per acre to as high as 270 pounds per acre. That’s a huge range, and speaks to the monumental task of matching N rates to growing conditions in-season.

Stewart adds, however, that as neat as new tools are for predicting total N applications, the on-paper method works surprisingly well. After all, he says, the Ontario nitrogen rate calculator is based off of 40 years of field data, much of which Stewart helped generate.

It begs the question: if you had your field tested and a program recommended only 100 pounds of total N fertilizer for the corn crop, would you do it? Even Peter Johnson, RealAgriculture’s resident agronomist, isn’t 100% comfortable going that low, however, with 140 pounds already on his own crop, he’s comfortable not adding more, if that’s what the tests showed. The key? Leave a check strip, or do a strip of low, medium, high rates and take it to yield.

“The science is telling us we need to leave a check strip,” Johnson says “The management opportunity here is in the top-dress. For anyone that puts all the N up front, you can’t ever pull back from that. A recommendation like this only has value if you’re splitting N timing.

“Even if the recommendation saves you 40 to 50 pounds of N in a dry year, that’s about $15 an acre you didn’t need to spend to achieve the same yield,” Johnson says. With corn prices down, that makes a real difference on your total profit in a year.

Johnson adds that the advancement in diagnostics and N recommendation tools could prove a real boon for farmers and the soil and water they manage, as lower N application rates don’t just save money, it also means less risk of N lost to leaching, run-off, or gassing off. And that’s a win for our environment.

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