Plant Competition for Atrazine

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Abstract
Greenhouse and field studies demonstrated that plants compete for soil-applied atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine]. Oats(Avena sativaL. ‘Neal’) and soybeans [Glycine max(L.) Merr. ‘Clark 63’] were grown in greenhouse bioassay studies to study the effects of plant population on bioassay sensitivity. High plant populations were not as effective as low plant populations in detecting low levels of atrazine in soil. Conversely, by increasing plant populations or decreasing soil volumes quantitative measurement of higher atrazine concentrations can be determined. A14C-labeled atrazine study showed that by increasing soybean populations from one to six per pot, atrazine uptake per plant was decreased 50%. When soybeans were planted at a rate of six plants per 100 cm2in a field treated with atrazine at 1.1 kg/ha, soybean dry weight production was 97% of the untreated control; when planted at a rate of one plant per 100 cm2, dry weight was only 26% of the comparable untreated check. Thus, herbicide rates for optimum weed control may have to be increased with increasing weed populations or increased crop seeding rates or both.