MSU Extension

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.      

Winter wheat response to topdress application of nitrogen may vary from year to year. Nitrogen response studies were conducted for three years at the MSU Agronomy Farm on a Capac loam, south of the main campus. Some of the variability in response is related to soil moisture status as it affects N availability, and to N source. In 2004, “Hopewell” soft red winter wheat responded well to nitrogen applied just prior to green-up (Table 1). Broadcasting 60 lbs N/acre resulted in an 11 bushel per acre increase. The yield difference between the 60 and 120 lbs N/acre rates was not statistically different at the 90 percent confidence level, even though the average yield was 7 bushels greater. Topdressing 60 lbs N/a (total N applied was 85 lbs/a) resulted in the best combination for economic return and minimum residual N in the soil. In that year, soil moisture conditions turned dry after N application at the Feekes 6 stage. Therefore, splitting the N between green-up and Feekes 6 did not prove beneficial (45+45 treatment).

In 2005, limited rain occurred in April and May, so movement of the surface applied N into the soil and root zone was limited. Considerable volatile N loss occurred from urea and UAN (28% liquid). Even so, the yield increase with 60 lbs N/ a broadcast was similar to that in 2004 (12 bu/a). Where ammonium sulfate was the N source, little volatile N loss occurred, and the yield with 90 lbs N/acre was 78.3 bu/a compared to 64.3 bu/acre for urea or UAN. In 2006 soil moisture was good throughout most of the spring/summer growing period, so considerable N was released from the soil organic matter. Hence, response to applied N was less. The variety “Roan” (grown in 2006) is a high yielder, but is susceptible to lodging. Heavy rains and strong winds caused the wheat to lodge about 3 weeks prior to maturity in all plots receiving 60 lbs or more of N. The severe lodging probably moderated any potential yield response to N rate. With urea and UAN applying 30 lbs N/acre resulted in as good a yield as 90 lbs N/a either all prior to green-up or by split application.

Interestingly, ESN (a controlled release polymer coated urea) applied at green-up in 2006 to supply 30, 60 or 90 lbs N/acre resulted in less severe lodging, and yields of 93.7, 106.1 and 105.7 bu/acre, respectively. Perhaps this was related to the gradual release of available N. In the dry spring of 2005, ESN applied to supply 60 and 90 lbs N/acre resulted in yields similar to those attained with ammonium sulfate, and those yields were higher than yields attained with urea or UAN. Apparently, with the warm dry soil conditions in 2005 there was significant loss of N by volatilization from urea and UAN, which did not occur with the polymer coated urea (ESN) or ammonium sulfate. Hence, soil moisture and rainfall conditions can greatly influence the release of N, and the loss of N from the various N sources.

Based on the results of these three years, it appears that wheat yield response to nitrogen is influenced by a combination of weather and N source. Under dry soil conditions the potential for volatile N loss is greatest with urea and UAN. The risk of N loss can be moderated by applying some of the N as ammonium sulfate or ESN. Across all the years, topdressing 60 lbs N/a (total of 85 lbs N/a for the crop) as urea or UAN resulted in near the best economic return. This was also the case with ESN and ammonium sulfate in 2004 and 2006. With the dry spring of 2005 applying 90 lbs topdress (total of 115 lbs N/a) as Ammonium sulfate or ESN resulted in a better economic yield. Using a combination of urea or UAN with either ammonium sulfate or ESN prior to or near green-up to supply 60 to 90 lbs N/a appears to be appropriate. This year with the higher cost of N staying closer to the 60 lbs N/a topdress rate may provide the best economic return. This assumes about 25 lbs N/a was applied at planting last fall.

Table 1. Soft red winter wheat response to nitrogen rate topdressed near green-up.

1 Average for Urea and UAN (28 % N) treatments except where indicated.
2 AS =Ammonium sulfate or ESN as N source .
Grown at the MSU Agronomy Farm, East Lansing, MI.
Capac loam. 25 lbs N/acre applied at seeding in the fall.