Seasonal variation in acetohydroxy acid synthase inhibition by imazapyr in Cynodon dactylon

Cynodon dactylon (cynodon) is a weed that is particularly difficult to control due to its ability to reproduce readily using stolons and rhizomes. Although glyphosate is the most commonly used herbicide to control cynodon, current recommendations involve repetitive applications, which appear likely to select for resistance. Rotating glyphosate with herbicides that have different modes of action will be a more sustainable strategy for cynodon control. Earlier work suggested that the growth of cynodon is strongly inhibited by imazapyr, an herbicide that inhibits the acetohydroxy acid synthase (AHAS) enzyme. The optimum time for application of imazapyr on cynodon, and the subsequent inhibition of AHAS activity in cynodon are unknown. The aims of this study were firstly, to determine the effectiveness of inhibiting AHAS activity when applied at different times of the year, and secondly to compare the efficacy of foliar and root applications. Results showed that application time has a significant effect on the efficacy of imazapyr to inhibit enzyme activity, with mid-late summer being optimal, and furthermore that the herbicide can be absorbed by both the shoots and the roots.