Charles Mitchell

     Nitrogen (N) is the most difficult nutrient to manage in cotton production. It has more impact on yields, earliness, and lint quality than any other primary plant nutrient. It is also the most costly plant nutrient applied per acre. Environmentally, potential nitrate-N leaching into ground waters is a driving force behind water quality issues and nutrient management planning policies. Because N is a primary constituent in all protein, easily transforms into several chemical forms in the soil, is biologically active, and can be mobile in the environment, farmers have problems managing it in cotton production.

      Growers have used a variety of techniques to optimize N use efficiency in cotton, such as split or multiple N applications, starter fertilizers, N fertilizer placement, foliar urea (46-0-0) applications during bloom, petiole monitoring, plant leaf analyses, water management (irrigation timing), plant growth regulators (e.g., Pix Plus®), cover crops, and computer programs and plant growth models (e.g., Gossym-Comax®, NLEAP®, EPIC®, COTMAN®)

       Since it was founded in 1883, the AAES has been studying N on cotton in an attempt to help Alabama growers better manage this nutrient. The oldest, continuous cotton experiment in the world, Alabama’s Old Rotation (circa 1896) at Auburn University, focuses on N management by rotating cotton with other crops and planting winter legumes. However the Two-year Rotation and Rates of N-P-K experiments also provide valuable information about N rates for cotton crops in Alabama. Alabama’s current standard N recommendations were developed from these and other experiments in the 1950s and 1960s, modified in the 1970s and refined in the 1980s. Therefore, if cotton responds to N rates differently today, this might reflect effects of improved varieties, higher yields, and different management.

      To take a new look at the effects of N rates on new cotton varieties with higher yield potentials, seven years of cotton yield data from these experiments were summarized. Since 1996, Bollgard® and Roundup Ready® varieties have been used at most locations. At all locations, the N source is ammonium nitrate (34-0-0) and N rates are split with half applied at planting and half as a sidedress at or near early squaring..

      Results strongly indicate that the standard N recommendations are still appropriate for maximum relative yields in these tests when cotton follows cotton (figure 1). When cotton follows soybean or peanut as in the Two-tear Rotation Experiment, predicting an optimum N rate is more difficult. A good soybean or peanut crop may contribute 20 to 30 pounds N per acre to the following cotton crop. Because as many as six months could elapse between soybean/peanut harvest in the fall and cotton planting the following April or May, much of the residual N may be lost from the soil. Data from N-rate treatments on the Two-year Rotation at five locations verify the variable nature of residual N from legumes. Since 1992, cotton on this experiment has always followed soybeans or peanuts. Clearly, cotton response to N rates was highly variable following a legume, but comments on the soil test report have always taken this into consideration.

     Promoters of fertilizer use have often espoused the concept of recommendations based on “yield goals.” This is a particularly popular and reasonable practice with grain crops such as corn, wheat, and sorghum and forages. These crops remove large quantities of N in the harvested portion of the crop. However, Alabama’s long-term N experiments do not support this practice for cotton under the conditions of these experiments (figure 2). In a disaster year when cotton yields are less than a bale per acre, very little if any N fertilizer is needed. No farmer plans on a disaster year and never fertilizes for these situations. But even in outstanding production years when yields far exceed anticipated yield goals (e.g., three or more bales per acre), data from Alabama’s research stations support the “standard” recommendation plus or minus about 30 pounds N per acre.

      Due to the boll weevil eradication program in Alabama, boll weevils have not been an economic factor for Alabama cotton farmers since 1996. With the weevil out of the way, cotton has the potential to set more late-season cotton bolls, thus increasing yields and the demand for more N. However, data from these long-term experiments have not indicated that boll weevil eradication nor the new, genetically modified varieties have had any effect on cotton yield response to N rates since 1996. In fact, highest yields were produced in 1992 or 1993 at all sites except the Brewton Experiment Field.

      Because of the rapid adoption of new, genetically modified cotton varieties, there have been few opportunities to evaluate their response to soil fertility variables. In 1996, the first year Bollgard® varieties were available to Alabama producers, the Rates of N-P-K experiment on a Lucedale soil at the Prattville Experiment Field was modified to determine if any differences existed in response to soil fertility variables between two varieties of similar genetic backgrounds. All plots were split and a Bollgard® variety, Deltapine NuCotn 35B (DP35B), was planted on half of each and a conventional variety of similar genetics, Deltapine 5690 (DP5690), was planted on the other half of each plot. This was repeated in 1997 and 1998.

     The DP35B yielded an average of 85 pounds more lint per acre per year than the conventional variety over all N rates, but the differences due to variety would not affect the standard N recommendation. This yield difference is most likely due to sub-threshold control of bollworm and budworms by the Bollgard® variety.

      Long-term, N-rate research at several Alabama locations since 1992 supports the current standard N recommendations used on soil test reports. For most sandy and loamy Alabama soils, the standard recommendation is a total of 90 (plus or minus 30) pounds N per acre during the growing season; 60 (plus or minus 30) pounds N per acre is standard for the deep, red, silt, and clay loams of the limestone valleys of North Alabama. Cotton following a good soybean or peanut crop will benefit from some residual N, but predicting this response has been difficult.

      While N recommendations based on a yield goal may apply for some crops, this clearly is not the case with non-irrigated cotton in Alabama. Producers should follow the standard N recommendation on new fields and make adjustments as experience and cropping systems dictate. Nitrogen rates do not need to be adjusted for the newer, genetically modified cotton varieties.

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