Graduate Thesis Or Dissertation

ALS-inhibitor resistant downy brome (Bromus tectorum L.) biotypes in Oregon : Mechanism of resistance, fitness, and competition

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  • Understanding the mechanism of resistance, relative fitness and competitiveness of herbicide resistant biotypes is important to predict population dynamics and to establish resistance management strategies. This study was conducted to determine the level of resistance to acetolactate synthase (ALS) inhibitors and the mechanism of resistance of ALS-inhibitor resistant downy brome (Bromus tectorum L.) biotypes from Athena and Madras, Oregon (AR and MR). Based on the whole plant dose response test and ALS assay, the AR biotype was highly resistant to the sulfonylurea (SU) and sulfonylaminocarbonyl- triazolinone (SCT) herbicides but was not resistant to an imidazolinone (IMI) herbicide. DNA sequence analysis of the als gene in the AR biotype demonstrated a single-point mutation of the amino acid Pro₁₉₇ to Ser. The MR biotype was moderately resistant to all ALS inhibitors tested. However, no differences in ALS sensitivity and als gene sequence were observed in the MR biotype. Studies using ¹⁴C-BAY MKH 6561, a SCT herbicide, showed that the halflife of BAY MKH 6561 in the MR biotype was 8.9 h which was 40% shorter than that in the Madras susceptible (MS) biotype. These data indicate that the mutation in the als gene is responsible for the SU and SCT herbicide resistance in the AR biotype but the relatively rapid metabolism is the mechanism of resistance for the MR biotype. Seed germination, plant growth, seed production, and competitiveness of the AR and MR biotypes were investigated and compared to their respective susceptible biotypes as components of fitness. Seeds of the AR biotype germinated 27 h earlier than seeds of the Athena susceptible (AS) biotype and reached over 60% germination when the AS biotype initially germinated at 5 C. No differences in seed germination were observed between the MR and MS biotypes at any temperature tested. Growth of the AR biotype was similar to the AS biotype under competitive and noncompetitive conditions. However, the MR biotype was less fit than the MS biotype in growth, seed production, and competitive ability. Seed production of the AR and MR biotypes was 83 and 71%, respectively, when compared to the AS and MS biotypes. Although the AR biotype produced 17% less seed than the AS biotype, the AR biotype could dominate the early season weed population, because of its early germination at low temperature and large seed size. Therefore, it is very difficult to predict population dynamics of the AR biotype in the weed population. However, the MR biotype should decrease and the population shift toward higher frequency of the MS biotype in the absence of ALS inhibitors because the MR biotype is less fit and competitive than the MS biotype.
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