NITROGEN EFFECTS ON BOLL PRODUCTION OF FIELD-GROWN COTTON

Nitrogen is an essential nutrient for cotton (Gossypium hirsutum L.) that affects plant growth, fruiting, and yield. This study was conducted to determine N effects on the development of harvestable bolls (HB) at fruiting sites among reproductive branches (sympodia). Field experiments were conducted from 1987 through 1990 on Commerce sill loam (fine-silty, mixed, thermic, nonacid, aeric Fluvaquent) to evaluate responses to preplant N rates of 0, 28, 56, 84, 112, 140, and 168 kg ha-1, and to split applications of 56 + 56 kg N ha-1. Increasing N decreased HB production at Sympodia 5 through 10, and increased HB at Sympodia 12 through 25. At Sympodia 5 through 10, production of HB at Fruiting Site 1 (first position on sympodial branch) decreased from 49% of fruiting sites at the 0 N rate, to 36% of fruiting sites, with N fertilization of 168 kg ha-1. Harvestable boll production at Sympodia 11 through 16 increased from 58% at 0 N up to a maximum of 69% at 84 kg N ha-1. At Sympodia 17 through 25, fruiting sites developing HB increased from 9% up to a mximum of 28% with application of 140 kg N ha-1. Whole-plant HB development at Fruiting Sites 1, 2, and 3 was optimized at 42, 23, and 8%, respectively, by N rates between 56 and 84 kg N ha-1. Averaged over all fruiting sites, maximum whole-plant HB development of 24% was obtained with 84 kg N ha-1. Results demonstrate that increased N increased the total number of HB per plant. Increase in yield potential in response to N was limited, however, by decrease in HB at lower sympodia as N was increased, and by the inability of plants to more effectively develop bolls at Fruiting Site 2 and 3, and those at upper sympodia.