Urea‐Ammonium Nitrate Management Techniques for Winter Wheat Production

Abstract

Wet soils, a consequence of poor drainage common to soils in the winter wheat ( Triticum aestivum L.) producing areas of the southern Midwest USA, frequently hamper spring top‐dress fertilizer applications and reduce N efficiency. This 2‐yr field study was an effort to identify alternative N management techniques that could be successfully implemented at either preplant or topdress application time. The effects of urea‐ammonium nitrate (UAN) time of application, method of application, and rate of dicyandiamide (DCD) nitrification inhibitor on the flag leaf N, grain N, and grain yield of winter wheat were measured in 1990 and 1991 at the University of Illinois Brownstown Agronomy Research Center on a Cisne silt loam soil (fine, montmorillonitic, mesic Mollic Albaqualf). Time of application was fall preplant and spring topdress. Method of application was broadcast and dribble, 15 in. spacings. Rates of DCD were 0, 5, 7.5, and 10% of the applied N (90 lb/acre) as DCD‐N. No factor had any significant effect on flag leaf or grain N concentrations in either year. In 1990, grain yields responded differentially to time and method of UAN application as DCD rate increased. With spring broadcast N application, yields decreased while with fall application and with spring dribble application yields increased. In 1991, grain yields were not affected by method of UAN application. As the rate of DCD increased, yields tended to increase with fall application but decreased with spring application. Overall, differences in response to all management factors were minimal, which could be attributed to winter and early spring climatic conditions that were not conducive to large N losses in either year. Spring topdress application of DCD‐treated UAN was not effective, and actually resulted in decreased yield. Although this study showed no large wheat yield increases from DCD applied with fall UAN, as a precaution against potential N loss, use of a nitrification inhibitor should be considered when the entire N requirement is fall‐applied. Research Question Winter wheat ( Triticum aestivum L.) producers in the southern Midwest USA frequently must contend with wet soil conditions at the time of spring topdress fertilizer application (shortly before regrowth begins). Saturated soils and high potential N loss ratings occur because poor drainage is common to the soils of the area. This often results in delayed N application and reduction in N efficiency. This field research was an effort to identify alternative fertilizer N management techniques that could be successfully implemented either preplant or at topdress application. These included combinations of urea‐ammonium nitrate solution (UAN) time of application, method of application, and rate of dicyandiamide (DCD — a nitrogen containing compound which research has shown to be effective in slowing the nitrification process). Literature Summary Research studies in the southern Midwest region have shown plant nutrient stress, reduced yield, and increased disease susceptibility occur with delayed spring N application. Consequently, fall application of the entire N requirement has been proposed for wheat production in this geographic area. Many years of research indicate that this is practical only when a nitrification inhibitor is used to minimize N loss. In years of excessive rainfall during the winter and early spring, use of a nitrification inhibitor has been shown to result in wheat yields either equivalent to, or slightly higher than, those with spring applied N. That research, however, had not evaluated DCD for wheat production or the potential benefit of concentrating N solution‐DCD mixed in surface band applications. Banding N solutions with DCD might allow a lower effective rate of DCD than with broadcast application. Some but not all research with fall applied DCD and urea in the southern USA indicated improvement in wheat yield. But yields were still lower than with fall‐spring split applications. In Indiana, research with DCD and urea showed yield increases in 1 of 2 yr when fall‐preplant applied. Study Description Soft red winter wheat variety ‘Cardinal’ was grown in 1990 and 1991 at the University of Illinois Brownstown Agronomy Research Center located in south‐central Illinois. Wheat production is common in this geographic area of Illinois and is common on the same, or similar, soils found in southern Illinois and adjacent states. Previous crop: Soybean Soil type: Cisne silt loam “claypan soil,” 2% organic matter N rate: 90 lb N/acre N source: UAN, 28% N solution Treatments: Broadcast and dribble (surface band), 15 in. spacings Fall preplant (before disking) and spring topdress application DCD rate of 0, 5, 7.5, and 10% of the applied N as DCD‐N A zero‐N check was included in 1991 Measurements: Flag leaf and grain N concentrations Grain yield Applied Questions Were any of the N management techniques superior for the production of winter wheat? Similar wheat grain yields were obtained in this 2‐yr field study with either fall or spring application of UAN solution and either broadcast or dribble application. Overall, with fall application, the highest rate of DCD (10% of the applied N as DCD‐N) produced only minimal increases in wheat grain production. Also, lower application rates, 5 and 7.5% of the applied N as DCD‐N, had no positive effect on yield. Dribbling UAN did not enhance productivity and did not enhance the apparent effectiveness of DCD. These results most likely can be explained by conditions not being conducive to large N loss from denitrification during the winter and early spring of the 2 yr of this study. Spring application with increasing rate of DCD resulted in reduced grain yields. Exact reasons for this were not clearly defined from our study. What would be the recommendation on the use of a nitrification inhibitor for winter wheat production? Based on the conditions present during the period of this research, use of DCD or other nitrification inhibitors with fall UAN application for winter wheat production would not be warranted on the poorly‐drained soils in south‐central Illinois. Recognizing established nitrate loss mechanisms, however, and the high N loss potential of this soil, the results obtained probably would be different in years with warmer temperatures and higher rainfall, conditions suitable for greater N loss. Under those conditions, several research studies in the southern Midwest USA have shown that use of a nitrification inhibitor reduces N loss and results in production equivalent to, or higher than, spring topdress application. Therefore, as a precaution to potential N loss conditions between the time of fall N application and rapid plant uptake in the spring, producers should consider use of a nitrification inhibitor if the entire N requirement of the wheat crop is applied in the fall. Use of DCD with spring topdress N applications should be evaluated carefully because of reduced yields found in this study. In 1990, wheat grain yield decreased with spring broadcast application of DCD but increased slightly with fall broadcast and spring dribble application. image In 1991, when averaged across method of application, wheat grain yield decreased with spring application of DCD but increased slightly with fall application of DCD. image

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