Tolerance of maize to Atrazine and Terbuthylazine applied post-emergence

Abstract

Recent reports of damage to maize seedlings where atrazine or atrazine/terbuthylazine was applied post-emergence again raised the issue of maize tolerance to triazines. The potential of atrazine or atrazine/terbuthylazine to injure maize when applied post¬emergence was investigated under controlled conditions. Factors considered were growth stage, surfactants, cultivar tolerance, temperature and crop recovery from herbicide injury. The first experiment was conducted to determine the effect of atrazine and terbuthylazine on maize seedlings, and the second one was carried out to identify whether stage of maize development has an influence on atrazine or terbuthylazine phytotoxicity. In Experiment 1, herbicide treatments were applied at the 2-leaf stage of cultivar PAN 6043, and in Experiment 2 at the 2-leaf and 4-leaf stages. The third experiment was carried out to determine cultivar tolerance to atrazine or atrazine/terbuthylazine. The three cultivars used were CG 4526, PAN 6043 and PAN 6140. The fourth and fifth experiments were carried out in growth chambers where the role of temperature and cultivars were investigated. The temperature levels were 20/15 and 30/25°C (day/night). Herbicide and temperature treatments were applied at the 2-leaf stage of cultivars CG 4526, CAN 3891, PAN 6043 and PAN 6140. Finally, an experiment was conducted to investigate the recovery of maize seedlings from the initial atrazine/terbuthylazine phytotoxicity. The cultivar PAN 6043 was used and the herbicide rates were the recommended and double the recommended rate. The rates of the herbicides and associated activity enhancers were 2.5 L ha-1and 1 L ha-1 respectively. The following products were used: Atrazine 500 SC (485 g L-1 atrazine) + BP Agripon (940 g L-1 mineral oil); Gesaprim Super 600 SC (291 g L-1 atrazine and 291 g L-1 terbuthylazine) + AG Penetrex (363 g L-1); Gesaprim Super 600 SC (291 g L-1 atrazine and 291 g L-1 terbuthylazine) + Complement (363 g L-1). Maize seedlings were tolerant when atrazine or atrazine/terbuthylazine was applied at the 4-leaf stage, but significant damage was caused when application was done at the 2-leaf stage. No significant differences were found between surfactants. Significant differences occurred with cultivars at specific temperatures. At the low temperature regime (20/15°C), CG 4526 showed shoot dry mass (SDM) reduction of > 40 %, while the growth of PAN 6140 was reduced by less than 10 % by both herbicides. However, with an increase in temperature, CG 4526 was the most tolerant. It showed <17 % reduction in SDM while others showed >30 % reduction. It is suggested that the tolerance of maize (specifically at the two-leaf stage) to atrazine or atrazine/terbuthy1azine is dependent on cultivar and temperature. Although initial phytotoxic symptoms were visible several weeks after spraying, the growth inhibition of the atrazine/terbuthylazine-treated maize plants was transient. At five weeks after application, leaf area index (LAI) had recovered 100% and 95% at the recommended and double the recommended rate, respectively. Recovery in SDM and root dry mass (RDM) were similar, with 100% recovery reached after eight weeks following application. It was found that although growth stage plays an important role in the tolerance of maize seedlings when atrazine or atrazine/terbuthylazine is applied post-emergence, surfactants, cultivars, and temperature also have an influence on the tolerance of maize seedlings towards these herbicides.