| Field studies were conducted to confirm glyphosate tolerance of horseweed accessions from Fayette (Spindletop research farm), Trigg, Hardin, Henderson, and Calloway counties. Seeds of these accessions were grown in a greenhouse and then transplanted in the field. Glyphosate was applied at 1.26 kg ae/ha or 2.52 kg ae/ha. Glyphosate at 1.26 kg ae/ha or 2.52 kg ae/ha did not kill horseweed plants from Trigg, Hardin, Henderson, or Calloway counties; confirming that these plants exhibited tolerance. Glyphosate at 1.26 kg ae/ha or 2.52 kg ae/ha killed Spindletop horseweed populations, confirming that they were susceptible.;A greenhouse experiment evaluated glyphosate resistant horseweed accessions for cross-resistance to other herbicides. Herbicides evaluated were glyphosate (1.26 kg ae/ha, 2.52 kg ae/ha), chlorimuron (0.052 kg ai /ha, 0.105 kg ai /ha), cloransulam (0.021 kg ai /ha, 0.042 kg ai /ha), paraquat (0.56 kg ai /ha, 1.12 kg ai /ha) and atrazine (2.24 kg ai /ha, 4.48 kg ai /ha). The Trigg, Hardin, Henderson and Calloway accessions were tolerant to glyphosate at 1.26 kg ae/ha and 2.52 kg ae/ha. However, the Spindletop accession was susceptible. Chlorimuron, cloransulam, paraquat and atrazine at normal rates killed Spindletop, Trigg, Hardin, Henderson and Calloway horseweed populations, confirming that they were not cross-resistant to the herbicides evaluated.;A field experiment was conducted to evaluate horseweed seedling emergence from fall and spring plantings under Kentucky conditions. Two biotypes (Spindletop and Trigg) were used in this experiment. There were 8 planting dates: October, November, December, January, February, March, April and May. Horseweed height and diameter were not significantly different at any planting dates. A conclusion was made that horseweeds can emerge any time of year, and timing of emergence depends on suitable soil and environmental conditions. Horseweed is not a true winter annual, because of its germination in summer. Horseweed's life cycle may be regulated by timing of seed shed. Horseweed plants from seeds that germinated in spring did not form a rosette. However, they did mature and produce seeds.;KEYWORDS: Horseweed Conyza canadensis (L.) Cornq., Glyphosate tolerance, Cross resistance, Germination, Weed tolerance in Kentucky. |
