ABSTRACT

High input costs and lower prices have forced cotton producers to scrutinize every aspect of production. Although fertility is a vital input to achieve maximum economic return, growers often attempt to reduce nitrogen rates in hopes of lowering these costs. In Oklahoma, nitrogen is typically applied preplant as urea or anhydrous ammonia on irrigated acres, while dryland acreage may go unfertilized or receive in-season applications of nitrogen. Often the plant's ability to utilize these forms of nitrogen in-season depends upon timeliness of rainfall or irrigation. Past research shows that peak nitrogen utilization by the cotton plant occurs from flowering through boll maturity, which often differs by variety. Picker-type varieties account for most of the irrigated acreage within Oklahoma, while the majority of dryland acres are planted to stripper varieties. Typically more determinant varieties (stripper-types) respond differently to environmental stresses than do less determinant (picker) varieties. The ability to manage nitrogen inputs more closely and on an as-needed basis could be advantageous for producers. Many times an adverse weather event early in the season will severely decrease or eliminate yield potential, which also reduces or eliminates the crop's need for nitrogen. Applying more nitrogen in-season may have the potential to reduce up-front expenses in an uncertain environment. Although substituting foliar applied sources of nitrogen for traditional applications historically has had little effect on cotton lint yields, their consideration as a means of adding supplemental nitrogen (when necessary) is gaining attention. One specific consideration is the reduction of traditional pre-season applications of nitrogen by 30% with the intention of supplementing the crop through foliar applications later in the season on an as-needed basis. Prior research in the area of petiole analysis by Livingston et al., 1996 shows that in-season nitrogen needs can be accurately monitored thus providing a means for prescribed supplemental applications during the peak usage period.

The objectives of this research were as follows: (1) Diagnose in-season nitrogen needs through petiole analysis; (2) Apply prescribed rates of foliar nitrogen when petiole nitrate-N drops below a predetermined threshold; (3) Determine differences in fruiting pattern or yield between more or less determinant cotton varieties which received supplemental applications of foliar nitrogen; (4) Compare two supplemental foliar nitrogen regimes to a traditional fertility program.