Effect of tillage and corn row spacing on common waterhemp growth and fecundity

Nordby, Dawn
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The acceptance of integrated weed management programs has been hindered by a lack of information on crop-weed interactions. Four experiments were conducted in central Iowa during 2001 and 2002 in corn planted in 38 and 76 cm rows to determine the response of common waterhemp (Amaranthus rudis Sauer) growth parameters to corn (Zea mays) row spacing and date of common waterhemp emergence. Four common waterhemp emergence cohorts were established in each experiment corresponding to the VE, V3, V5, and V8 stages of corn (Zea mays) development. Delayed emergence of common waterhemp in corn significantly affected all growth parameters of common waterhemp. Common waterhemp survival averaged 80, 44, 3, and 1% survival for the first, second, third and fourth cohorts respectively. Mature common waterhemp height for the first cohort was 140 cm, whereas plants emerging at the V8 corn stage were only 5 cm. Corn row spacing significantly affected biomass and fecundity of the first cohort, but later cohorts were not affected by row spacing. Biomass of the first cohort was 20% less in 38 cm rows than in 76 cm rows. Biomass and seed production of waterhemp emerging at the V3, V5, and V8 corn stages decreased 80, 97 and 99%, respectively in comparison to the first cohort. Field studies were conducted in 2001 and 2002 to evaluate the influence of tillage on common waterhemp (Amaranthus rudis Sauer) seedbank behavior. Emergence of common waterhemp was greater in no-till than chisel till. Tillage did not affect the initial time of emergence; however, the time to 50% emergence was longer in no-till than chisel till. Duration of emergence did not differ among tillage systems. Common waterhemp seed was concentrated near the soil surface in no-till, whereas seed in the chisel till were primarily found between 9 and 15 cm. The delayed and increased emergence in no-till contributes to the problems in managing common waterhemp in this system.