Rate of seedling emergence, relative growth rate (RGR), and phenological development were compared in several accessions of rigid ryegrass belonging to three distinct resistance classes. The aryloxyphenoxypropionate resistant (AOPP-R) class had a faster and less variable seedling emergence than the sulfonylurea resistant (SU-R) and susceptible (S) classes. However, even the fastest of the AOPP-R accessions was within the range of the S and SU-R classes. No significant differences were detected among the resistant classes in seed dormancy, RGR, and the rate of phenological development. The rate of spike emergence, irrespective of the resistance class, was related to the latitude of the origin of the accessions, suggesting adaptation to the local climates since introduction. Due to considerable variation among weed populations for most biological attributes, the need to include several R and S accessions, in studies similar to the one reported here, is of vital importance. Because of the means and variances of the three resistance classes, at least four accessions from each resistance class would have been required to detect the observed differences between emergence rates of the AOPP-R and S classes with a confidence of 95%.

A seedling bioassay was developed for the rapid diagnosis of resistance to clethodim and fluazifop-P in johnsongrass. The assay was based on differences in the coleoptile length of susceptible (S) and resistant (R) seedlings exposed to clethodim and fluazifop-P in petri dishes for 5 d. Bioassay concentrations of 0.09 mg/L clethodim and 0.18 mg/L fluazifop-P were chosen as discriminant based on rate responses of each biotype to increasing herbicide dose. At 5 d after treatment (DAT), the amounts of clethodim required to reduce coleoptile length by 50% (GR50) for the R and S seedlings were 462.5 and 24.8 mg/L, respectively, resulting in an R:S ratio of 18.7. The fluazifop GR50 values for the R and S seedlings were 618.7 and 17.5 mg/L, respectively, resulting in a R:S ratio of 35.4.

Reports of resistance to grass-selective herbicides, including johnsongrass (Sorghum halepense) resistant to fluazifop-P, have become quite common. Experiments were conducted to determine if fluazifop-P resistance could be transferred from johnsongrass to crop sorghum (Sorghum bicolor) and to identify the heritability of resistance in the progeny. A population of male sterile, fluazifop-P–sensitive sorghum, AKS-82, was interplanted with fluazifop-P–resistant johnsongrass and progeny treated with 0.105 kg ai/ha fluazifop-P postemergence (POST). Surviving seedlings were backcrossed to fluazifop-P–resistant johnsongrass. Viable plants from this successful cross were then testcrossed to another sensitive sorghum, TX622. These testcross progeny were then screened for herbicide resistance by again applying fluazifop-P POST at 0.105 kg /ha. Chi-square analysis revealed that resistance to fluazifop-P was inherited by a single, dominant gene. Natural hybridization of johnsongrass and sorghum is possible, and transfer of herbicide resistance between species can occur.