WEED SCIENCE Starter Fertilizer Effects On Cotton Development And Weed Interference

Application of postemergence-directed herbi-cides in cotton (Gossypium hirsutum L.) requires a height differential between the crop and weeds. Weeds may respond more to starter fertilizer than cotton, and the enhanced weed growth could adversely affect the height differential, herbicide effectiveness, and reduce lint yields. A field experi-ment was conducted to determine the effect of type and placement of starter fertilizer and timing of postemergence-directed herbicide applications on growth of cotton, sicklepod (Senna obtusifolia L. Irwin and Barneby), and pitted morningglory (Ipomoea lacunosa L.) and cotton yield. Starter fertilizer treatments included urea-ammonium nitrate (UAN) and ammonium polyphosphate (AMP) applied either in a 10-cm band over the surface of the crop row or 5 cm below and 5 cm to the side of the crop row. Methazole plus MSMA at 0.8 plus 2.2 kg a.i. ha-1 were applied early or late postemergence-directed to cotton 9 to 10 or 15 to 18 cm tall, respectively. When early season condi-tions were dry, optimal cotton yields were obtained with AMP starter fertilizer and early herbicide application. When early season soil moisture was adequate, weed control and cotton yield were gen-erally better with late herbicide application, and starter fertilizer did not affect cotton yield. Cotton maintained sufficient height differential over weeds to allow directed herbicide application, even when starter fertilizer increased sicklepod growth. These results indicate that the benefit of starter fertilizer and timing of directed herbicide application to optimize cotton yield is dependent on soil moisture conditions following planting. Cotton response to starter fertilizer has been variable in the southeastern United State