Identifying acetolactate synthase inhibitors for potential control of quackgrass (Elytrigia repens) and Canada thistle (Cirsium arvense) in corn (Zeamays)

Control of two perennial weeds, quackgrass and Canada thistle, and the diff erential sensitivities of three imidazolinone-resistant (IMI) corn hybrids and their sensitive isolines to various acetolactate synthase (ALS)-inhibit ing herbicides were evaluated in greenhouse studies. The postemergence sulf onylurea herbicides nicosulfuron and primisulfuron controlled quackgrass > 80%. The imidazolinone herbicides imazapyr and imazamox applied postemergen ce controlled quackgrass 70 and 74%, respectively. Canada thistle control w as 89% with the sulfonylurea herbicide metsulfuron. Primisulfuron, chlorsul furon, and tribenuron controlled Canada thistle > 70%. The imidazolinone-se nsitive corn hybrids did not tolerate the imidazolinone herbicides imazetha pyr, imazaquin, imazapyr, AC 263,222, and imazamox; the sulfonylurea herbic ides chlorimuron, chlorsulfuron, metsulfuron, tribenuron, and triflusulfuro n; and the pyrimidinylthiobenzoate herbicide pyrithiobac applied postemerge nce. The 'Ciba 4393 IMR' corn hybrid, homozygous for an unknown allele, was resistant to all of the various ALS-inhibiting herbicides that injured its sensitive isoline. The magnitude of resistance for this hybrid was greater than the other IR corn hybrids. The 'Pioneer 3751 IR' corn hybrid, homozyg ous for the XA-17 ALS allele, was also resistant to a number of ALS-inhibit ing herbicides. The 'ICI 8692 IT' corn hybrid, heterozygous for the Mut2 al lele, was only resistant to the imidazolinone herbicides.