Date of Graduation


Document Type



Plant species that develop resistance to one herbicide (e.g., s-triazine) may simultaneously exhibit resistance to other herbicides to which they have never been exposed. The objectives of this study were to compare the response of atrazine resistant and susceptible biotypes of redroot pigweed (Amaranthus retroflexus L.) and velvetleaf (Abutilon theophrasti Medic.) to the herbicides alachlor (2-chloro-N-(2,6-diethylphenyl-N-(methoxymethyl) acetamide), atrazine (6-chloro-N-ethyl-{dollar}N\\sp\\prime{dollar}-(1-methyl-ethyl)-1,3,5-triazine-2, 4-diamine), linuron ({dollar}N\\sp\\prime{dollar}-(3,4-dichloro-phenyl)-N-methoxy-N-methylurea), and metribuzin (4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5 (4H)-one). Cross-resistance and competitive fitness were also determined, with and without the herbicides. Three experiments were conducted with resistant and susceptible biotypes of each species under controlled conditions. First, seeds were germinated at 3 temperatures considered optimum for each species, in growth chambers. Total percent germination (TPG) and cumulative percent germination (CPG) were measured. Second, seeds were sown into pots treated with four preemergence herbicides at five concentrations and placed in a greenhouse. Biomass, height, and leaf number were measured for each plant. Third, seeds were sown in a DeWit replacement experiment series and subjected to sublethal doses of the four herbicides. Biomass, height, and seed mass were measured per plant and per pot. The atrazine-resistant biotype of A. theophrasti had a significantly higher TPG and CPG than the susceptible at 23 and 27{dollar}\\sp\\circ{dollar}C, but no difference was found at 30{dollar}\\sp\\circ{dollar}C. In A. retroflexus, however, TPG and CPG of the susceptible biotype were significantly higher than the resistant biotype at 30 and 35{dollar}\\sp\\circ{dollar}C, but no difference was observed at 40{dollar}\\sp\\circ{dollar}C. Under non-competitive conditions, the inhibition dosage (ID{dollar}\\sb{lcub}50{rcub}){dollar} based on biomass indicated that atrazine-resistant populations of A. theophrasti and A. retroflexus showed cross resistance to linuron and metribuzin but an increased sensitivity to alachlor. In the absence of herbicide, biomass of the resistant biotype of A. retroflexus was significantly less than that of the susceptible biotype; but in A. theophrasti biomass of biotypes was not different. Under competitive conditions, DeWit analysis showed that the resistant biotype of A. retroflexus was superior in biomass and seed mass to the susceptible biotype, when treated with herbicides with the same mode of actions. However, the trend was reversed in plants treated with alachlor. In A. theophrasti, the susceptible biotype exhibited fitness superiority over the resistant biotype under all herbicide treatments except atrazine. When no herbicide was applied, the susceptible biotypes of A. retroflexus and A. theophrasti showed fitness superiority over the resistant biotypes, particularly at the 50/50 mixture frequency of susceptible to resistant. Data from this research suggest that use of a herbicide with a different mode of action than atrazine or not applying any herbicide may reduce the spread of resistance in populations of these two species.