Blackgrass, herbicidal resistance

In 1987, Moss first reported that a blackgrass biotype resistant to chlorotoluron and isoproturon (urea herbicides in WSSA Group 7) was also resistant to the ALS inhibitor chlorsulfuron. Menendez el al. (1997) also found that a chlorotoluron-resistant blackgrass biotype in Spain was resistant to ALS inhibitors (e.g., chlorsulfuron and imaza-methabenz), and that the resistance was due to its greater ability to metabolize the herbicides. 

Moss, S.R. (1987). Herbicide resistance in blackgrass (Alopecurus myosuroides). British Crop Protection Conference-Weeds. 879-886. 

In other weed biotypes, resistance to triazine herbicides is likely conferred by rapid metabolism of the herbicides to inactive compounds. A chlorotoluron-resistant biotype of blackgrass (slender foxtail) was cross-resistant to various other groups of herbicides, including triazines (Kemp et al., 1990). The mechanism of chlorotoluron resistance was Cyt P450-based enhanced oxidative metabolism through /V-demethylation and ring-methyl hydroxylation (Moss and Cussans, 1991). Consequently, it is likely that resistance to triazines in this blackgrass biotype is also due to enhanced herbicide detoxification. 

Multiple-resistance is when more than one mechanism conferring resistance to herbicides in different chemical classes is active in an individual weed or population of weeds. Plants with multiple resistance may possess two or more distinct resistance mechanisms. Two grass species that display both cross- and multiple-resistance are rigid (or annual) ryegrass and blackgrass (Hall et al., 1994). 

This has changed dramatically in recent years, as is apparent throughout the herbicide chapters of this book. Of particular note are the surprising speed of resistance development to acetolactate synthase inhibitors (Mazur al., Gressel, this volume), and the emergence of multiply resistant ryegrass and blackgrass biotypes noted earlier. 

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