Winter Wheat Yield Response to Nitrogen: The Latest Kentucky Research

John Grove—Research Soils Specialist
Edwin Richey—Extension Soils Specialist

Recent fertilizer retail price surveys still show ongoing N fertilizer price increases, especially for urea ammonium nitrate (UAN) solutions, which are important fertilizer N sources for Kentucky wheat producers. The UAN28 has reached $1.01/lb N and UAN32 is at $0.96/lb N. Urea is at $0.90/lb N. The time for the first application of N to winter wheat is fast approaching, but UAN might then cost as much as $1.20/lb N. So, it seems timely to share our latest field research results on wheat yield response to N rates. These studies were done with the assistance of Bill Bruening in the Wheat Variety Testing Pro-gram and Brad Wilks with Wheat Tech. We are grateful to the Kentucky Small Grain Growers Association for their financial support of this research.

The N rate response work was part of a larger effort to understand the role(s) of N, sulfur (S), boron (B) and zinc (Zn) in wheat nutrition. For the purposes of this newsletter article, we will focus on the N rate responses we observed these past two seasons, 2019-20 and 2020-21. We had trials at seven locations, three in 2019-20 and four in 2020-21 (Table 1). Soil types, wheat varieties and planting dates are given in Table 1.

Six of the seven locations were no-tilled into corn residues. The N source was Super U, a urea product that is coprilled with both a urease inhibitor (NBPT) and a nitrification inhibitor (DCD). Four rates of N (40, 80, 120, 160 lb N/A) were used. One-third of the N was applied at early greenup (Feekes 2) and the remaining two-thirds was applied at Feekes 5-6. Weed, insect and disease control were excellent at all locations. No lodging was observed in any trial, at any N rate. That said, we lost Site 9, in the 2019-20 season, to freeze damage which totally obscured any crop response to N rate (Table 2).

Table 2 gives the grain yield response information, by site, for both production seasons. Yield levels were very good (except for Site 9 in 2019-20), indicating remarkably good production seasons, as long as the late freeze damage in 2019-20 was avoided. Yield for five of the six remaining sites averaged over 100 bu/A and all were over 90 bu/A (Table 2). All six sites were very responsive to fertilizer N additions up to 120 lb N/A, and four of the six gave significant yield increases up to 160 lb N/A.

The yield increment to N addition at those six locations was high, averaging a total of about 32 bu/A when an additional 120 lb N/A was added over the lowest rate of 40 lb N/A (Table 3). The average yield increment to each 40 lb N/A added falls, from 14.4 to 6.7 bu/A, in accord with the Law of Diminishing Returns (Table 3). On average, and if wheat remains near $7/bu, 6.7 bu/A will purchase $46.90/A worth of N fertilizer, at about $1.17/lb N. The problem with this is that the average value of 6.7 bu/A is greatly influenced by the yield response (+14.9 bu/A) at one site, Site 6 in 2020-21. At the other five sites, the smaller yield response increments will not buy as much N (or will require a lower N price per pound). Further, looking at the yield increments when 120 lb N/A was applied relative to 80 lb N/A, another two sites (2019-20, site 5; 2020-2021, Site 1) may not have been responsive enough to pay for the added N. This analysis outcome will fluctuate with changing wheat and/or N prices.

UK AGR-1 recommends 90 to 120 lb N/A for no-till wheat production. This remains the ‘sweet’ range where most wheat producers will want to be in the upcoming season. Risk averse producers will want to consider N application rates in the lower end of that range.