Establishing the Value of ALS-Inhibiting Herbicides in Fields with Confirmed Weed Resistance to ALS-Inhibiting Herbicides

Jodi Boe, Purdue University

Abstract

Acetolactate synthase (ALS) inhibitors are a widely used class of selective herbicides used to control grass and broadleaf weeds. The repeated use of ALS-inhibiting herbicides has selected for biotypes of weeds resistant to ALS inhibitors, especially in the weeds most problematic to growers in the Midwest. While ALS inhibitor use seems futile, new mechanisms of herbicide action are not predicted to be commercialized in the near future to solve this problem. This leads to the main objective of this research, determining what value ALS inhibitors provide in controlling populations of weeds with resistance to ALS inhibitors. Field experiments with soil-applied (PRE) applications of ALS inhibitors on horseweed (Erigeron canadensis) and tall waterhemp (Amaranthus tuberculatus var. rudis) exhibited higher efficacy than would be expected given the frequency of the ALS resistance trait in the population. Whereas control of these species with POST-applied applications was similar or less than the proportion of the population characterized as susceptible using molecular techniques. Soil-applied applications, therefore, resulted in relatively greater control than POST applications in populations with known ALSinhibitor-resistance mechanisms. Greenhouse experiments showed that overall resistance ratios were higher for PRE applications of ALS inhibitors in horseweed, tall waterhemp, and Palmer amaranth (Amaranthus palmeri). However, GR50 values decreased for both susceptible and resistant biotypes for the PRE applications compared to POST, suggesting the biologically effective dose of these herbicides is lower in soil residual applications. This research found that PRE applications of ALS inhibitors resulted in some level of control on horseweed and tall waterhemp classified as resistant to ALS inhibitors due to the higher efficacy of PRE herbicide applications. Genetic analysis assessing the amino acid substitutions that confer resistance to ALS inhibitors in tall waterhemp confirmed a difference in selection pressure between PRE and POST applications and between ALS active ingredients in tall waterhemp. Applications of chlorimuron PRE at 11 g ai ha-1 selected for 35% homozygous W574L genotypes and at 44 g ha-1 selected for 70% homozygous W574L genotypes. An increase of homozygous W574L individuals along with a decrease in heterozygous individuals from 65 (11 g ha-1 ) to 29% (44 g ha-1 ) suggests that W574L is semi-dominant in tall waterhemp and that high labeled rates of chlorimuron applied PRE can partially overcome the heterozygous W574L-resistance mechanism. In horseweed, no difference in selection pressure was observed between application timing or between chlorimuron or cloransulam. A new mutation conferring ALS-inhibitor resistance in horseweed was discovered, a Pro197Leu amino acid substitution, with resistance ratios of 21X to chlorimuron and 8.6X to cloransulam. These resistance ratios are slightly less than those reported for the Pro197Ala and Pro197Ser amino acid substitutions in conferring ALSinhibitor resistance in horseweed. Finally, a survey of 42 populations of tall waterhemp in Indiana counties with confirmed ALS-inhibitor resistant populations of tall waterhemp found that all populations contained at least 16% individuals with the W574L amino acid substitution, 35 populations contained at least 1% individuals with the S653N substitution, and 9 populations contained at least 1% individuals with the S653T substitution. Taking into consideration the three mutations tested, 8 of the 42 populations tested contained <50% ALS-inhibitor resistant individuals within the population.

Degree

M.Sc.

Advisors

Goldsbrough, Purdue University.

Subject Area

Agriculture|Agricultural chemistry|Agronomy|Chemistry|Organic chemistry

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