Sulfonylurea (SU) herbicides registered for use in other crops in Australia may leave residues in the soil interrupting rotations involving soybean. However, two unlinked genes (ALS1 and ALS2) confer SU tolerance in soybean. Here, we report experiments that explored the effectiveness of these ALS genes in Australian soybean germplasm. The first two experiments assessed the tolerance to halosulfuron‐methyl or metsulfuron‐methyl of lines with either zero ALS gene (ALS1) or both ALS1 and ALS2 in five different Australian genetic backgrounds. The presence of both ALS1 and ALS2 conferred strong SU tolerance at herbicide application rates as high as four times (4×) the recommended field rate, with evidence of interaction with the genetic background; lines carrying only ALS1 also performed well but demonstrated less tolerance. For the third experiment, the five lines homozygous for both ALS genes were intercrossed in a half‐diallel mating design. The resulting F2 population were screened for tolerance to metsulfuron‐methyl herbicide. In all instances, the presence of the two ALS genes provided absolute protection for the growth tip. Analyses of seedling dry weight and purple leaf discoloration demonstrated significant differences among the crosses. For dry weight, high and significant heritability estimates indicated significant additional genetic variation for SU tolerance in five of the 10 F2 crosses (p ≤ 0.05). The results of these three studies confirm that two ALS genes confer tolerance to SU herbicides when crossed into Australian soybean germplasm, but suggest possible epistatic effects within different backgrounds, and the possibility of other (minor) genes conferring additional SU tolerance.

In soybean cropping, sulfonylurea (SU) herbicides are a potentially useful alternative to glyphosate-based herbicides. Normally, soybeans are susceptible to SU herbicides, but two unlinked non-GMO genes (ALS1 and ALS2), have been identified that confer SU tolerance. In this project, we explored the effectiveness of these genes in Australian soybean genetic backgrounds. Four lines, carrying both ALS1 and ALS2, were derived by backcrossing different Australian genotypes to a SU-tolerance donor line ‘W4-4’ and then using molecular markers, plants homozygous for both ALS genes were selected. The W4-4 donor and the four derived lines were evaluated in a hydroponic system at increments up to 4× the recommended field rate of metsulfuron-methyl. The ALS genes provided high levels of tolerance, with evidence of some minor interaction with the genetic background. To further test whether there was an effect of background, the five lines were crossed together in a half-diallel mating design and the resulting ten F2 populations were screened hydroponically for tolerance to metsulfuron-methyl herbicide at 4× the recommended field rate. Analyses of seedling dry weight of the five parental lines and their F2 progeny in response to the herbicide, identified differences among the crosses. These results indicated that the development of commercial varieties with maximum herbicide tolerance requires incorporation of both ALS genes, and if combined with selection in segregating populations in the presence of SU herbicide may capture additional tolerance from background genes of minor effect.