RESISTANCE OF AMARANTHUS-HYBRIDUS TO ATRAZINE

Resistance of weeds to triazine herbicides has been recorded in many c ountries. The extent of the problem in South Africa is uncertain. In a pilot study, the atrazine resistance of Amaranthus hybridus L. (smoot h 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 f ound that the suspected R biotype plants were not affected at herbicid e 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 pl ants were killed by the lowest dosage. In the field, mortalities in th e R biotype were not observed after post-emergence applications of 1.2 5-10.00 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 t o 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 germin ate slightly sooner under controlled conditions than R biotype seeds,p reliminary results suggest that there are no major differences. Indica tions are that, although the growth of the S biotype may be greater th an that of the R biotype, the competitive effect of the two biotypes o n crop seedlings may well be similar.