Although herbicide tolerance is not usually evaluated until the final stages of breeding programs, this trait is very important for grower adoption of new peanut cultivars. Understanding herbicide tolerance of breeding lines could help breeders develop selection strategies that maximize herbicide tolerance in new commercial cultivars. However, little is known about herbicide tolerance variability in peanut germplasm. Thirty-five randomly selected breeding lines from the peanut mini-core collection and cultivars ‘Florida-07’ and ‘Georgia-06G’ were evaluated for tolerance to 11 herbicides under greenhouse conditions. Variation among peanut lines in herbicide tolerance, measured as dry weight reductions (DWR), was similar across herbicides and was normally distributed. Florida-07 and Georgia-06G were in the lower two quartiles of injury and DWR among the evaluated peanut lines. Dose–response experiments showed that the most tolerant breeding lines had I50 (the rate required to cause 50% injury) and GR50 (the rate required to reduce dry weight 50%) values 0.4 to 2.5 times higher than the most susceptible lines, depending on the herbicide. A breeding line had a dicamba GR50 13 times higher than the most susceptible line and 2.8 and 4.7 times higher than Florida-07 and Georgia-06G, respectively. The most tolerant lines were consistently tolerant to herbicides with different mechanisms of action, suggesting that nontarget site mechanisms are more likely to be responsible for the tolerance than target-site mutations. These results confirmed peanut-breeding programs would greatly benefit from screening breeding lines for tolerance to key herbicides and developing an herbicide-tolerance catalog. This information can be used when designing new crosses to reduce the risk of developing cultivars with low herbicide tolerance especially considering that one-half of the breeding lines exhibited lower tolerance than the commercial cultivars.
Nomenclature: Dicamba; peanut, Arachis hypogaea L. ‘Florida-07’ and ‘Georgia-06G’.