Wet, saturated soils and variable environmental conditions certainly increase the difficulty associated with nitrogen fertilization of wheat fields in the Mid-South. In order to implement proper management, we must evaluate wheat health and growth stage to determine current nutritional needs and appropriate timing that suit what the crop needs in each field. Variables like planting date and variety may greatly affect crop condition and maturity affecting wheat management needs. Likewise, environmental conditions can vary considerably from year to year, so reliance on calendar date is not particularly reliable or optimal for management timing. You will achieve much better response by managing nitrogen application according to wheat development and health, than by pre-determined calendar dates.
If a field shows poor tiller development or signs of nitrogen deficiency, you need to apply enough nitrogen in timely fashion to meet its demand or the crop will suffer yield loss. Nitrogen nutrition primarily influences vegetative biomass and plant heath, rather than maturity, so it is not as critical as other factors affecting wheat maturation. Maturation is primarily determined by planting date and variety response to temperatures and photoperiod. Thus, delaying nitrogen application will likely reduce wheat yield potential by depriving plants of nutritional needs.
Keys for Wheat Fertilization Success – There are several keys to successful wheat nitrogen fertilization in Mississippi. Split application of nitrogen fertilizer is likely more important for wheat than any other crop, including corn. This is because wheat nitrogen fertilization occurs during the wettest months of the year in a high rainfall, warm regional climate, which can be very conducive to nitrogen loss. Thus, nitrogen loss may be high and limit productivity, when we experience prolonged, soggy weather during the early spring. Likewise, nitrogen application timing can be very important, particularly for the first spring application. Optimal nitrogen timing can vary substantially from year to year and field to field because seasonal weather and temperature fluctuation primarily affects wheat development (along with specific planting date and variety). Therefore, you can make better fertility decisions, by closely monitoring wheat health and development, and evaluating the weather forecast. This knowledge can help you make more appropriate fertility decisions, rather than exclusively relying on specific calendar dates and rate guidelines. Using best management practices can substantially improve fertilizer efficiency, crop yields and profitability – especially since input and commodity prices are high.
Split Application Guide – I believe the most prudent method to apply nitrogen to southern wheat is either a 2-way or a 3-way split with at least 2/3 of the nitrogen applied in the late split(s). Using split nitrogen applications with the majority of fertilizer applied late will satisfy crop demand without subjecting a substantial amount of expensive N to losses during wet, saturated conditions typical during the early spring. Only a small amount of the total N is theoretically needed in the first topdress application (20-30 lbs. N/a.), because wheat nitrogen needs are quite modest, until stem elongation and rapid growth begins. The initial topdress of a split application should be applied when dormancy breaks in late winter while wheat is in prostrate, tillering stages (Feekes growth stage 3 or 4 – normally early February). Neglecting wheat nutritional needs during tillering stages limits the number of tillers which will produce viable heads. Thus, proper nitrogen timing is essential to produce high wheat yields. Thereafter, N can be applied according to crop needs, which increase with vegetative development. A second nitrogen application should occur when plants become strongly erect and stem elongation begins, and again prior to boot stage, if you choose to make a third application.
Nitrogen Sources – Urea is the most commonly used nitrogen source on wheat because it is generally the most economical nitrogen source, it can be applied by air, and volatility is less likely to be substantial during the wheat season (because temperatures are cool and rain is frequent), than during the summer. Likewise, the need for urease inhibitors containing the active ingredient NBPT, such as Agrotain, is less during the early spring, compared to late-spring or summer applications of urea-based nitrogen sources. Ammonium sulfate or other sulfate forms of sulfur fertilizer should generally be applied in an early spring application, if sulfur may be needed, because sulfur deficiency will stunt vegetative development and will likely substantially limit wheat grain yield. Sulfate forms of sulfur fertilizer are strongly recommended to address seasonal crop needs and deficiency cases, because they are immediately available for crop uptake, while elemental sulfur is not. Liquid nitrogen solution (UAN) can potentially burn leaf tissue, especially if high rates are broadcast on erect wheat, so granular nitrogen sources are generally preferred, particularly for single, or the latter split applications.
Nitrogen Rates – Our wet southern climate may influence nitrogen use efficiency considerably depending upon seasonal rainfall frequency and amount. Thus, specific nitrogen rate suggestions based solely upon crop yield goal are not very reliable for wheat production in the South. Since soil texture significantly influences soil-water relations and potential nitrogen loss during typical wet springs, our general recommended spring nitrogen rates vary depending on soil texture. We normally suggest from 90 – 130 lbs. N/a. on light-textured soils and 120 – 150 lbs. N/a. on heavy clay soils. However, monitoring crop response to nutrition, culture and environmental conditions offers producers substantial opportunity to fine tune your rate more appropriately.