Split nitrogen application could boost protein in spring wheat, but it might be hard to gauge whether that boost adds actual financial value in a given season.
Amy Mangin and Don Flaten, both crop nutrition researchers with the University of Manitoba, have been hoping to update nitrogen management in spring wheat, given the new genetics on the market.
Flaten has called the established guideline of 2.5 pounds of nitrogen per bushel into question, arguing that yields have essentially doubled since the ’70s, and that making nitrogen applications at that rate increases the economic and environmental risk.
“If we’re wanting 80-bushel-an-acre wheat, we found that you still needed around 160 pounds of nitrogen available for that crop to get that and have adequate protein content,” Mangin said.
Why it matters: Changing spring wheat varieties have left previous fertility guidelines in the dust. Now researchers are looking at split applications to better manage that pile of nitrogen required by today’s high-yielding wheat.
Trials at two main sites (Brunkild and Carman) suggested an ideal 140 pounds an acre of added nitrogen for about 180-187 pounds of available nitrogen total. Trials took place in 2016 and 2017 and averaged about two pounds of nitrogen needed per bushel of yield, Mangin said.
Some other sites took far less added nitrogen to reach their best economic yield (Carberry only needed an extra 60 pounds an acre in 2016), something the two researchers attribute to soil mineralization differences.
The study tested the benefit of split application at five locations in those two years. Researchers applied either 30 or 60 pounds an acre of extra nitrogen at stem elongation or flag leaf, on top of a base 80 pounds per acre applied at seeding. That was compared to 110 and 140 pounds an acre put down entirely at seeding.
Extra fertilizer gave an extra 3.3 bushels of wheat an acre when applied at stem elongation, at least at the two main sites. There was no yield difference between the higher and lower split applications. Likewise, there was no yield difference between the two rates when they were applied later, at flag leaf.
The delay did pay off in protein, however. Protein averaged 0.3 per cent higher when extra fertilizer was applied at stem elongation compared to seeding, and 0.7 per cent higher when an extra shot of fertilizer was applied at flag leaf.
“We had similar yield with our split applications compared to entirely at planting, but we actually found that the later we applied it in the season, the higher the protein content was, so we weren’t sacrificing yield by holding back some of our nitrogen but we did see increases in protein content,” Mangin said.
The study also found protein was 1.8 per cent higher when fertilized after anthesis. Researchers compared a split application of 80 pounds an acre, plus 30 pounds an acre of UAN or urea solution after anthesis, with 80 and 110 pounds an acre supplied up front. Urea-treated plots also returned 0.6 per cent higher protein compared to UAN.
The UAN produced more leaf burn, Mangin said, a definite risk in post-anthesis application.
No crystal ball
It may be hard for a farmer to gauge if they need that split application, even if it does boost protein.
Manitoba’s wheat is, “some of the last wheat to come off in North America,” Mangin said. “If in the U.S., southern U.S., they’re having wheat yields that are having good protein, high protein, we might not have the protein premiums that we have some years, but if they’re pulling off lower-protein wheat earlier in the season, we might be getting that protein premium.”
That foreknowledge will not help farmers gauge how close they are to that protein threshold while the crop is still in the field, however. Late application might tip a crop into premium territory, or it could be wasted money if wheat has too far to climb to meet that threshold, or if its protein is already high enough without an extra shot of nitrogen.
Mangin and Flaten tested tools like NDVI, the near-infrared imaging used to gauge crop health and growth in precision agriculture, to see if the technology could predict yield and protein in season.
Higher NDVI values showed good correlation with yield, Mangin said, but no relationship with protein levels.
“With every tool that we tested, it was not able to predict protein content,” she said. “That is just because of the variability in protein with management, as well as soil and weather. There’s just too much variability site to site with protein content.”
Mid-season nitrogen application might still be useful if a farmer is looking for insurance on a potential protein premium, however, she noted.