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Revising N guidelines for wheat

Fields from a multi-year study evaluating the ‘Maximum Economic Rate of Nitrogen’ (MER-N) under current crop management regimes, spearheaded by OMAF’s cereal specialist Peter Johnson..
Photo courtesy of Shane McClure, Middlesex Soil and Crop Improvement Association.

Dec. 6, 2013 - It’s high time that nitrogen recommendations were updated for wheat, as the current suggested rates are based on research conducted 20 years ago. So says cereal specialist Peter Johnson with the Ontario Ministry of Agriculture and Food.

“What’s currently recommended may not be sufficient because so much has changed over the last two decades,” notes Johnson. “Yield potential has increased significantly since then through changes in production practices, including better genetics, fungicides and growth regulators.”

For the past three years, Johnson has been leading a study to evaluate the Maximum Economic Rate of Nitrogen (MER-N) under current crop management regimes. The research team involves summer assistants; technician Shane McClure of the Middlesex Soil and Crop Improvement Association; statistician Ken Janovicek; and University of Guelph researchers Dr. David Hooker, Scott Jay and Gerald Backx. Financial support is being provided by Agriculture and Agri-Food Canada, with assistance from the Grain Farmers of Ontario, Ontario Soil and Crop Improvement Association and industry.

As part of the study, the research team applied multiple rates of nitrogen (N) to generate N response curves and determine MER-N. “Additional treatments were included at selected sites to evaluate the impact of fall nitrogen on wheat yield and economic return,” says Johnson. “Measurements done at harvest included yield, moisture, test weight, 1,000 kernel weight and protein.

We also conducted post-harvest soil nitrate tests to determine if higher N rates increased fall residual soil-N.” Johnson notes that this could have been an environmental concern had residual nitrogen increased significantly.

As expected, yields increased dramatically with the addition of N. “There was a whopping 23-bushel gain from 0 N to 60 N,” Johnson says. “Additional N continued to increase yield.” There was a 6.7 bushel per acre (bu/ac) gain from 60 N to 90 N, a 6.1 bu/ac from 90 N to 120 N, and an average 2.7 bu/ac was gained from 120 N to 150 N. “Further analysis reveals that only four of 26 sites reached MER-N at 90 N, and surprisingly, over 60 per cent of the sites had MER-N of 150 N or greater.”

In terms of economics, Johnson and his team found that using urea at $600/tonne ($0.59/pound of actual N), with soft red wheat at $6.34/bushel means that 2.8 bushels of wheat are required to cover the cost of 30 units of N ($0.59/lb X 30 lb = $17.75/$6.34/bushel = 2.8 bushels). “At these values there is a clear financial return, on average, up to 120 N, with 150 N breaking even,” he notes. “We found that 10 of the 26 sites had a significant financial gain at 150 N. Five other locations showed a slight financial gain to 150 N but the additional returns were not significant (less than $7 per acre).”

Only five sites had a maximum economic nitrogen rate below 120 N. “It is an interesting point that of these five sites, four of them did not receive any fungicides,” Johnson explains. “The sites without any fungicide actually reached a maximum yield with 120 N and only saw a slight economic return with adding more than 90 N.” However, the sites with fungicide tell a completely different story. At these sites, there was a substantial yield increase of 7.1 bushels between 90 and 120 N. This supports other recent research results showing an interaction between nitrogen and fungicides, Johnson says. An additional 4.1 bushels was gained by increasing to 150 N.

Four of the six sites without fungicide reached MER-N at 90 N. The other two sites achieved MER-N at 120 N. “Only one of the 20 sites with fungicide required less than 120 N to reach MER-N,” says Johnson. “This site had biosolids applied, reached a maximum yield at 60 N, and MER-N was 0. Clearly the biosolids supplied sufficient N.” This site showed a potential negative impact of excess N, and lodging in the 150 N strips resulted in a loss of 6.2 bu/ac.

Fall versus spring, protein levels and more
Fall-applied N had no significant impact on yield, moisture, test weight, 1,000 kernel weight or protein. “There was a 2.6 bu/ac advantage from 30 N in the fall with 90 N in the spring, versus 90 N spring only,” explains Johnson. “However, when the full 120 N is spring applied, there is a 6.8 bu/ac gain over 90 N only. “Calculating this, it quickly becomes apparent that fall N does not pay.” Even less gain was seen with the 30+120 N treatment. Johnson said these results, added to the fact that fall nitrogen applications are a potential environmental concern, indicate that no one should apply fall N.

With an increase in N rates, protein levels in the grain rose as well. “There were no differences in protein levels between 0 and 60 N, as yield increases were so large that all additional N went to yield, rather than protein,” notes Johnson. “As N rates were increased further, protein levels consistently increased by 0.4 per cent for every additional 30 units of N.” Protein levels increased by 1.2 per cent from 60 to 150 N. The impact of this increase depends on the market being targeted, Johnson explains. He says some domestic users prefer low protein soft wheat, while other markets and export buyers prefer high protein. In general, increased protein levels would fit the majority of market opportunities somewhat better, but there’s no price premium associated with higher protein.

“Overall, these results show that there’s a great opportunity to increase wheat yields with additional N,” Johnson says. “The data also confirms other results indicating an interaction between N and fungicides. If no fungicide is applied, responses to additional N are minimal and fit the older N rate recommendations very well. If fungicides are applied, responses to N become much more significant, and new N recommendations are in order.”

While lodging has long been thought to be a major issue at high N rates, these studies don’t show it as a problem. “We warn growers, however, to proceed with caution as they increase N rates,” Johnson says. “Where lodging has not been an issue in the past several years, it’s important that growers place two test strips of an additional 30 lb N/ac to see if these results are repeated on their farm.”