Of resistant weeds

Table 11.2 Occurrence of resistant weed biotypes to different herbicide groups up to 2006...

Although the ALS inhibitor herbicides have been used for approximately 20 years, the number of resistant weed biotypes for this group now exceeds those for all other types of herbicides. Singh and Shaner (1995) and Boutsalis (2001) reported that a total of five chemical families or herbicide classes are commercially marketed as inhibitors of ALS, and that these herbicides comprise more than 50 active ingredients for selective use in many different crops. They include sulfonylureas, imidazolinones, triazolopyrimidines, sulfonylamino-carbonyl-triazolinones, and pyrimidinyl (thio)benzoates. 

The rate of evolution of resistant weeds is based on several factors, including characteristics of the weed and herbicide, gene frequency, size and viability of the soil seedbank, weed fitness, herbicide potency, frequency and rate of application, and persistence in soil. Various attempts have been made to use modeling to determine the relative importance of these factors and to predict the probability of resistance, as well as to evaluate how to avoid, delay, or solve the problem (Gressel and Segel, 1990). 

Farmers continue to manage resistance retroactively, responding to the presence of resistant weeds by using different herbicides or alternative control practices, perhaps accompanied by changes in cropping practices. 

With the first invasion of resistant weed populations, prompt action has been essential to avoid serious and permanent problems. Preventive action to avoid herbicide resistant weeds from evolving in the first place is definitely the best strategy. It is virtually essential in all cases of herbicide resistance to have other classes or types of herbicides, with alternate sites, modes of action and degradation available. In some countries, because of failure to respond promptly, the lack of suitable alternatives, or for other reasons, triazine resistant weeds have not been controlled, resulting in rapid spread,.leading to the almost total loss of effective weed control with these herbicides (1). 

This approach should be used to enhance herbicide selectivity in crop varieties, to avoid carry-over injury, for specific and limited special problems, and for minor acreage and high value crops. A major objective of developing herbicide resistant crops should be to provide more flexibility in control of resistant weeds. 

Various parameters appear in the simple algebraic Eq. 1 for the frequency (proportion) of resistant weeds in a species that is preponderantly sensitive to a given herbicide. N is the very low frequency of resistant individuals in the population before it is exposed to the herbicide. In the absence of herbicide, resistance is sustained in the population by a balance between mutation and depletion of the proportion of resistant individuals by their lesser fitness in the absence of selection, resulting in a resistance fraction somewhat lower than the mutation frequency. If mutants were more fit than the wild type, they would be the wild type. Mutant fitness can be near neutrality, and the mutants would then be found in different proportions at various geographical areas due to drift". 

The properties of the herbicides that target ALS can also contribute to the emergence of resistant weeds. There are several classes of compounds which target ALS, including the sulfonylureas, the imidazolinones and the triazolopyrimidines. Within these classes are a number of herbicides that are used at rates that kill a high proportion of the weeds, thus increasing the likelihood that resistant biotypes will... 

M. Heap, in International Survey of Resistant Weeds, www.weedresearch. com, assessed March, 2006. 

Once resistance is confirmed it will always be preserrt on the farm. Recently there has been a reduction in the number of different herbicides available and few new herbicides are coming on the market, so management of resistant weed populations is becoming more difficult. 

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