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Factors influencing the tolerance of maize to atrazine and terbuthylazine applied post-emergence

Herbicide damage to maize seedlings reported in the 1997/98 season from certain maize-growing areas where atrazine or atrazine/terbuthylazine was applied post-emergence, prompted this investigation. Pot experiments were conducted to assess the role of growth stage, cultivar and temperature on the tolerance of maize to atrazine or atrazine/terbuthylazine applied in combination with adjuvants. Atrazine was evaluated in combination with BP Agripon Super while the atrazine/terbuthylazine mixture was evaluated in combination with AG Pene- trex. Cultivars CG4526, PAN6043 and PAN6140 were tested in the glasshouse and in growth chambers. The effect of temperature was tested in the growth chambers where the temperatures were 20/15°C or 30/25°C (day/night). Herbicide and temperature treatments were applied at the 2- and 4-leaf growth stages in Experiment I and at the 2-leaf growth stage only on the other two experiments. The rate of herbicides and adjuvants in combinations were 2.5 l ha−1 and 1 l ha−1 respectively. Significantly more damage was caused when atrazine or atrazine/terbuthylazine in combination with adjuvants was applied at the 2-leaf growth stage of PAN6043 compared to the 4-leaf growth stage. Significant differences occurred between cultivars at specific temperatures. At the low temperature regime (20/15°C) and with both herbicide treatments, CG4526 had a shoot dry mass (SDM) reduction of more than 40%, while the growth of PAN6140 was reduced by less than 10%. However, with an increase in temperature, CG4526 was the most tolerant. At the higher temperature this cultivar indicated less than 17% reduction in SDM while PAN6043 and PAN6140 indicated more than a 30% growth reduction. This study indicated that the tolerance of maize seedlings to atrazine or atrazine/terbuthylazine is dependent on growth stage, cultivar and temperature.