Shading and Nitrogen Effects on Cotton Earliness Assessed by Boll Yield Distribution

Shading and N fertilization affect fruit distribution in cotton (Gossypium hirsutum L.), but there is no detailed information on earliness of crop maturity according to phenological development. The aim of this work was to evaluate the effect of shading at early flowering and N top-dressing rates on relative cotton earliness using plant mapping. Field experiments were conducted in Itapeva (16 d of shading; and 0, 60, 120, and 180 kg N ha(-1)) and Chapadao do Sul (17 d of shading; 0, 80, and 160 kg N ha(-1); and early-and full-season cultivars), Brazil. Seed cotton yield was grouped by phenological position (PP) according to the standard phenological scale, for interpolation calculation at each 10% increment in accumulated harvestable yield (A(c)), weighted average PP (PPwa) determination, and logistic-regression analysis. Crop maturity earliness was predicted based on the reduction in PP wa and in PP. Shading increased PP up to 10 and 30% of A(c) due to a decrease of 33 and 40% in the number of bolls on early fruiting sites in Itapeva and Chapadao do Sul, respectively, but did not affect PPwa. Increases in PP up to high A(c) percentages and in PPwa values were observed at the two higher N rates in both experiments, mainly due to lower and higher boll number at earlier and later fruiting sites, respectively. Short-term shading during early flowering of cotton changed yield distribution by decreasing boll number on early fruiting sites, but did not affect the earliness of crop maturity. Earliness was decreased by high N rates due to higher cumulative seed cotton yield at later fruiting sites.