Four experiments were conducted in central Iowa during 2001 and 2002 to determine the effects of weed emergence time and corn row spacing on common waterhemp growth and fecundity. Four common waterhemp emergence cohorts were established in each experiment and corresponded to the VE, V3, V5, and V8 stages of corn grown in rows spaced 38 and 76 cm apart. Common waterhemp mortality averaged 20, 56, 97, and 99% for the first, second, third, and fourth cohorts, respectively. Mean mature common waterhemp height for the first cohort was 140 cm, whereas plants emerging at the V8 corn stage were only 5 cm tall. Biomass of the first cohort was 20% less in 38-cm rows than in 76-cm rows, but later cohorts were not affected by row spacing. Biomass and seed production of waterhemp emerging at the V3, V5, and V8 corn stages decreased 80, 97, and 99%, respectively in comparison with the first cohort. Overall results indicate that common waterhemp biomass, survival, and fecundity decline sharply with delayed emergence relative to corn, but weeds emerging at or before the V5 corn stage may still contribute significantly to the seed bank.

Nomenclature: Common waterhemp, Amaranthus rudis Sauer AMATA; corn, Zea mays L.