MATURITY AND LINT YIELD OF NITROGEN-DEFICIENT AND PHOSPHORUS-DEFICIENT PIMA COTTON

The Pima cotton (Gossypium barbadense L.) plant is inherently late maturing and unsuitable for production in areas where highly probable late-season rainfall coincides with boll opening. A field study was conducted between 1990 and 1992 to determine if the maturity of Pima cotton can be enhanced with applied N and P without reducing lint yield. Five preplant-applied N rates up to 180 kg ha-1 in 1990 and up to 269 kg ha-1 in 1991 and 1992 were tested. Four P rates from 0 to 45 kg ha-1 in a factorial combination with N were tested in 1991 and 1992. Nitrogen did not affect yield, days to first flower, or estimated percent open bolls in 1990. Nitrogen deficiency significantly enhanced maturity in 1991 and 1992. In 1991, >70% of the total bolls opened by 153 days after planting (DAP) where no N was applied, compared with 178 DAP where 202 or 269 kg N ha-1 was applied. In 1992, 68% of the bolls in the 0 kg N ha-1 treatment opened by 142 DAP, compared with <55% for all other treatments. The 0 kg N ha-1 treatment also significantly increased the ratio of reproductive to total plant dry weight earlier during the boll development period; however, the maturity-enhancing effect of N deficiency occurred at the expense of lint yield. Relative to the 67 kg N ha-1 treatment, zero N reduced lint yield by 277 kg ha-1 (26%) in 1991 and 214 kg ha-1 (18%) in 1992. Phosphorus deficiency reduced lint yield in 1991, but did not affect percent open bolls or reproductive efficiency in 1991 and 1992. Earliness of Pima cotton may, therefore, be managed more effectively by minimizing applied N rates than by manipulating P fertilization.