Sowing date in temperate regions determines the environmental conditions to which the maize (Zea mays L.) crop is exposed during grain filling. The objective of this work was to study the effect of sowing date on individual kernel weight of irrigated, well-fertilized maize crops. Four experiments were conducted in the field from 1989-1990 to 1993-1994 with a commercial maize hybrid (DK636). The treatments consisted of different sowing dates (from mid-September to mid-December). Delays in sowing date reduced individual kernel weight. Late sowings decreased the effective rate of grain filling and shortened the effective duration of grain filling compared with earlier sowings. Plant growth rate during grain filling was slower in later plantings because of low daily incident radiation and radiation use efficiency. There were no differences in the number of endosperm cells formed among sowings; thus, the potential capacity of kernels to accumulate assimilates did not contribute to the low final weight observed in later plantings. The effective rate of grain filling was affected strongly by temperature, increasing about 0.3 mg d-1 per degrees C. The effective duration of grain filling depended on the availability of assimilates and on the rate of grain filling when sowing was delayed. Reductions in the source of assimilates at later sowing dates caused lower kernel weight at harvest.
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