The Effect of Slow-Release N Fertilizer Rate and Timing on Wheat Grain Yield Compared to Other N Sources

Management strategies to reduce soil N loss include improved timing of N fertilizer applications, better use of soil and plant testing procedures to determine N availability, application of nitrification or urease inhibitors, and use of N fertilizer sources that are suitable for local environmental conditions (Dinnes et al., 2002). Use of slow-release urea fertilizer sources is a common strategy to reduce N leaching losses in horticultural crops, but its agronomic performance and cost-effectiveness has not been well established. Recent research at over 42 research sites in the Midwest indicated that average corn yield response to polymer-coated urea was 5 bu/acre higher compared to conventional urea or UAN solution, especially during years of normal or high rainfall (Blaylock, 2003). Use of slow-release N fertilizer for wheat may be a cost-effective management practice to improve crop performance and possibly allow a single fertilizer application in the fall for wheat. The extra cost of this N source (approx. 10˘ more per lb urea N) must be evaluated relative to its potential benefits under field conditions in Missouri due to wet fall and spring conditions commonly encountered by Missouri farmers. Limited research has examined the agronomic performance of wheat in response to polymer-coated urea. The objectives of this research were to 1) determine the impact of polymer-coated urea rates and application timings on wheat grain and frost-seeded clover forage yields and 2) evaluate the costeffectiveness of polymer-coated urea compared with other commonly used nitrogen sources for wheat production in Northeast Missouri.

Research was conducted at the Greenley Research Center near Novelty, MO in 2004, 2005, and 2006. This research was arranged as a randomized complete block design with four replications in plots 10 by 45 ft in 2004 and 2005, and six replications in plots 10 by 35 ft in 2006. ‘Ernie’ was no-till drilled on 23 October 2003, 8 November 2004, and 1 October 2005 at 150 lbs/acre in 7.5 in. rows. N fertilizer rates were applied at fall and split-application timings (Tables 1, 2 and 3). Fall N treatments were applied 1 November 2003, 3 December 2004, and 11 October 2005 while the spring N treatment was applied 19 March 2004, 3 March 2005, and 28 February 2006. Red clover was also frost-seeded to the entire plot area on 19 March 2004, 8 March 2005, and 27 February 2006 to simulate a broadcast application of fertilizer and clover seed. Wheat fresh weights were measured at physiological maturity (data not presented) and plots harvested with a small-plot combine. Grain moisture was adjusted to 13% prior to analysis. All data were subjected to analysis of variance and means separated using Fisher’s Protected LSD (P=0.05).

Wheat grain yield was similar among nitrogen source application rates and timings in 2004 (Table 1). Wheat treated with fall applied ESN at 50 lb N/acre had grain yields 6 and 4 bu/a greater than 32% urea ammonium nitrate at 50 lb N/acre and ammonium nitrate at 50 lb N/acre, respectively, in 2004.

In 2005, fall applied ESN increased wheat grain yield 8 to 22 bu/a when compared to urea or urea plus Agrotain applied in the fall (Table 2). Ammonium nitrate, 32% urea ammonium nitrate, and ESN fall applied had similar grain yields at 50 and 75 lbs N/a; however, ESN fall applied at 100 lbs/a had grain yields greater than all other fall applied N sources in 2005. Grain yield was similar among ESN, urea, urea plus Agrotain, 32% urea ammonium nitrate, or ammonium nitrate when these treatments were split-applied in 2005.

Fall applied ammonium nitrate at 100 lb N/acre had the greatest grain yield at 88 bu/a in 2006 which was similar to urea + Agrotain or 32% urea ammonium nitrate split-applied at 50 fb 50 lb N/acre (Table 3). Wheat grain yield was greater when ESN and ammonium nitrate were fall applied at 100 lbs N/acre compared to a split application while grain yields were greater when 32% urea ammonium nitrate was split applied at 50 fb 50 lbs N/acre when compared to a fall applied only application. However, wheat grain yields with urea + Agrotain or urea alone at 100 lbs N/acre were similar when fall or split-applied.

There was no effect of Agrotain + urea on wheat grain yields when compared to urea applied alone in the fall or split-applied in 2004 or 2005, and when split-applied in 2006 (Tables 1 and 2). However, Agrotain + urea increased grain yield 5-7 bu/a at the 75 and 100 lb N/a rates when compared to urea alone in 2006. Wheat grain yield with ESN was equal 10 of 15, greater than 3 of 15 (fall applied in 2005), and less than 2 of 15 (split-applied in 2006) rate-application timingyears when compared with urea alone from 2004 to 2006. Differences in crop response may be related to surface drainage issues since 2004, 2005, and 2006 had moderate, poor, and excellent surface drainage, respectively. Nitrogen sources may need to vary depending on the amount of surface and internal drainage.

The authors would like to thank CPS at LaPlata for supplying ESN in 2004, ammonium nitrate in 2004 and 2005, and urea in 2006; Morgan’s Fertilizer at Edina for supplying 32% urea ammonium nitrate in 2004, 2005, and 2006; and Agrium for supplying ESN in 2005 and 2006 for this trial.

References:
Blaylock, A. 2003. Controlled release fertilizer: Research summary 2000-2002. Agrium U.S. Inc. Denver, CO.

Dinnes, D.L., D.L. Karlen, D.B. Janes, T.C. Kaspar, J.L. Hatfield, T.S. Colvin, and C.A. Cambardella. 2002. Nitrogen management strategies to reduce nitrate leaching in tile-drained Midwestern soils. Agron. J. 94:153- 171.

Table 1. The effect of N fertilizer source, rate, and application timing on no-till wheat grain yield in 2004 (moderate surface drainage)^a.

Table 2. The effect of N fertilizer source, rate, and application timing on no-till wheat grain yield in 2005 (poor surface drainage)^a.

Table 3. The effect of N fertilizer source, rate, and application timing on no-till wheat grain yield in 2006 (excellent surface drainage)^a.