Resistance of Amaranthus hybridus to atrazine

Resistance of weeds to triazine herbicides has been recorded in many countries. The extent of the problem in South Africa is uncertain. In a pilot study, the atrazine resistance of Amaranthus hybridus L. (smooth pigweed) was investigated. Suspected resistant (R) and susceptible (S) biotypes were treated with commercially formulated atrazine. After post- and pre-emergence applications under tunnel conditions, it was found that the suspected R biotype plants were not affected at herbicide dosages of between 1.25 and 25.0 kg a.i. ha(-1), i.e. up to 20 times greater than the lowest recommended dosage. However, the S biotype plants were killed by the lowest dosage. In the field, mortalities in the R biotype were not observed after post-emergence applications of 1.25-10.00 kg a.i. ha(-1). In contrast, all S biotype plants were killed. In tunnel experiments, the R biotype was also found to be resistant to cyanazine and cyanazine+atrazine, while slight tolerance to linuron was observed. All these treatments resulted in 100% mortality of the S biotype. Although S biotype seeds of A. hybridus were found to germinate slightly sooner under controlled conditions than R biotype seeds,preliminary results suggest that there are no major differences. Indications are that, although the growth of the S biotype may be greater than that of the R biotype, the competitive effect of the two biotypes on crop seedlings may well be similar.