Reliance on synthetic auxin herbicides (HG 4) has increased following the introduction of dicamba and 2,4-D resistant soybean. A common question among users of these products is: “If I use one trait repeatedly and select for resistance to that herbicide, will I be able to switch to the other trait to control the resistant population?”
I did a quick, non-comprehensive literature search for papers that evaluated cross-resistance to 2,4-D and dicamba (Tables 1 and 2). Biotypes that were selected by one of the herbicides had cross-resistance to the other about 50% of the time.
When cross-resistance was present, in most biotypes the level of resistance (R:S) was lower to the herbicide not responsible for selection than for the herbicide that selected the resistance (i.e. a population selected by repeated 2,4-D use had a lower level of resistance to dicamba).
Numerous mechanisms are known to provide resistance to synthetic auxin herbicides, including insensitive target sites, enhanced metabolism, and altered translocation. The specific mechanism found in a weed probably influences the likelihood of cross-resistance.
Several populations of waterhemp and Palmer amaranth resistant to synthetic herbicides have been identified in soybean producing states. While cross-resistance between 2,4-D and dicamba is not a given, it occurs frequently enough to reinforce the need for integrated management to sustain the value of these herbicides.