THE PERSISTENCE OF TRIFLURALIN RESISTANCE IN GREEN FOXTAIL (SETARIA-VIRIDIS) POPULATIONS

In the summer of 1995, green foxtail seed samples were collected from 17 fields in southwestern Manitoba where trifluralin resistance was fi rst identified in 1988. Since 1988, six of these fields had not been t reated with dinitroaniline herbicides (DNAHs), four had been treated o nce, and seven had been treated two or more times. The proportions of resistant (R) and susceptible (S) seeds in each sample were determined using a petri-dish seed bioassay. Among the samples collected from fi elds that had not been treated between 1988 and 1995, two contained > 99% R seeds. The others ranged from 40 to 83%. Among the four samples collected from fields that were treated once, two contained > 99% R se eds and the other two contained 59 and 67% R seeds. Five of the seven samples collected from fields treated two or more times contained > 97 % R seeds and the other two approximately 89 and 68% resistant seeds. There was no statistical correlation between the proportion of R seeds and the frequency of DNAH use since 1988. The proportion of R seeds p resent in 1988 and 1995 could not be compared because of the way in wh ich fields were treated prior to sampling in the 2 yr; however, the re sults clearly indicate that trifluralin resistance can persist in gree n foxtail populations for at least 7 yr. By inference, the results ind icate that trifluralin-resistant green foxtail plants are almost, if n ot equally, as fit as susceptible plants. Consequently, once resistanc e is present in a green foxtail population, DNAHs cannot be used for i ts control, even in the absence of additional selection pressure, for at least 7 yr and probably much longer.