Cotton growth and yield as influenced by different timing of late-season foliar nitrogen fertilization

Foliar fertilization to meet the nitrogen (N) requirement of the cotton crop during the latter fruiting stages is a production practice that is not well understood. The objective of this study was to investigate the benefits of late-season foliar-N fertilization on growth and yield of cotton in relation to soil-N level and timing based on weeks after first flower (WAFF) and nodes above white flower (NAWF). A 4-year field study was conducted with four foliar-N treatments consisting of a control with no foliar-N, and one, two, or three foliar-N sprays under different soil-N regimes. In 1990, the foliar-N treatments were based on WAFF sprayed during fifth, sixth and seventh WAFF. Foliar-N significantly increased nodes above white flower (NAWF) over the control with no significant differences among the three foliar-N treatments. A negative relationship (r2=0.98) existed between NAWF and days after planting (DAP). Foliar-N also significantly increased plant height, leaf number, leaf area, leaf dry weight, boll number, boll dry weight and yield. The same foliar-N treatments were applied on low and high soil-N regimes in 1991 and 1992, and in 1993 on four different soil-N regimes, 0, 55, 82, and 110 kg N ha-1 at NAWF = 7, 6 and 5. No significant difference was found in NAWF among the four foliar-N treatments within each soil-N level during 1991. Significant differences between the control and the three foliar-N sprays were found for leaf area, boll number, and boll dry weight. In 1992, the NAWF of control plants showed a similar response to the 1990 control plants. In contrast, the foliar-N sprayed plants extended the highest NAWF for an additional week, after which it steadily declined below 5. Foliar-N significantly increased yield in 1990, yield and yield components in 1991 and 1992, and yield in 1993. Neither WAFF nor NAWF appear to be good indicators for timing late-season foliar-N fertilization. The study clearly demonstrated, however, that late-season foliar-N fertilization is beneficial to cotton plants, although the precise timing of such N application is still unclear.