Resistance Development

RESISTANCE DEVELOPMENT

For genetic resistance to occur, one or more alleles (a form in which a gene may occur) for resistance must be present at some level in the field population of a weed. Typically, resistance due to a single dominant allele is found at frequencies of about one in a million plants before selection for resistance has occurred. If resistance is due to a single recessive allele, then a typical frequency for this allele is around one in 10 billion plants. Once the weed population is exposed to the herbicide to which some resistant types are present, the herbicide kills susceptible biotypes and favors resistant biotypes. With the triazine herbicides, resistance usually appeared after seven or more years of repeated application. Recently, however, resistance to the sulfonylurea herbicides has appeared after only three to five years of use.

There is no evidence that any herbicide-resistant weeds have occurred due to mutations caused by the herbicide. Resistant weed biotypes are presumed to arise from small, preexisting populations of resistant plants that build up when the herbicide selection pressure is imposed.

Researchers have used a series of mathematical considerations, simplifications, and estimations to develop a formula for predicting herbicide resistance. The formula is:

Nn = No(1+fa/b)n

where

Nn = proportion of resistant weeds in a given year n,
No = proportion of resistant weeds prior to herbicide use,
f = overall fitness of resistant biotypes relative to susceptible biotypes,
a = selection pressure (proportion of remaining susceptibles),
b = average lifetime of weed seed in soil seed bank, and
n = number of years of continuous herbicide use.

For example, hypothesize several factors for the above equation and assign values of a = 20 (no resistant weeds killed, but 95% of those susceptible are killed, that is a = 1.0 / 0.05 = 20), f = 0.75, b = 2.0, and No = 0.000001 (one in a million plants initially resistant). The equation can be solved three, four, five, and six years of continuous herbicide use. The calculation predicts the proportion of resistant weeds in the population after 3, 4, 5, and 6 years to be 0.06, 0.52, 4.44, and 37.71 percent, respectively.

The possible numbers that can be inserted into the above equation indicate that a low selection pressure has the greatest effect on reducing the rate of appearance of herbicide resistance. Consequently, the fundamental principal of any management strategy to deal with herbicide resistant weeds is to reduce the selection pressure for the evolution of resistance. Perhaps the easiest and best way of accomplishing this is through the normal use of crop and herbicide rotation. Herbicides used in rotation should differ in their mode of action. Simply changing the product used may not result in a change in mode of action; for instance all sulfonylurea and imidazolinone herbicides have the same mode of action and are used in wheat, soybeans, and corn, and one is used in potato.


Other recommendations include the use of shorter-residual herbicides; tank mixes or sequential treatments of herbicides with different mode of action; greater integration of tillage with herbicides, and preventing weed escapes from going to seed.

The following characteristics of herbicides and their use contribute to an increased probability for the evolution of herbicide resistance:

1) Single target site and specific mode of action.

2) Extremely effective killing of a wide range of weed species.

3) Long soil residual and season-long control of germinating weeds.

4) Applied frequently and over several growing seasons without rotating or combining with other types of herbicides.

 

Development of herbicide resistance 

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