On soils with high residual P, N + P starter fertilizer commonly increases early-season corn (Zea mays L.) growth without increasing final plant dry weight or grain yield. This study was conducted to quantify corn growth and ontogeny as affected by starter fertilizer. In 1990 and 1991 on a Drummer silty clay loam (fine silty, mixed, mesic Typic Haplaquoll), we compared the effect of starter (liquid ammonium polyphosphate [10-15-0]) applied in a band 5 cm below and 5 cm to the side of the seed at 108 L ha-1 (15 kg N ha-1 and 23 kg P ha-1) and no starter on corn ontogeny and yield. Each year, the previous crop was soybean [Glycine mar (L.) Merr.]. Early-season plant dry weight was increased 15 to 20% by starter without affecting final plant dry weight. Early-season leaf dry weight and leaf area index were increased by starter, but the midseason maximum was not affected and the late-season decline of both leaf dry weigh and leaf area index was hastened by starter. Leaf area duration was not affected by starter. Net assimilation rate was increased during the early season by starter, but was not affected during grain fill. The normal ontological changes in crop growth rate were also hastened by starter. In both years, tassel and black-layer appearance occurred 2 d (almost-equal-to 50 growing degree days) earlier for treated than for control plants. Treated plants also had less grain moisture at harvest. Grain yield was not affected. We propose that, for this hybrid and location, the lack of an increase in final plant size and grain yield is due to the hastening of maturation, which limits the benefit from increased early-season leaf area index.
