Response of common ragweed (Ambrosia artemisiifolia) and giant ragweed (Ambrosia trifida) to postemergence imazethapyr

A study was conducted to evaluate the response of common and giant ragweed to postemergence applications of imazethapyr using relative growth rate parameters, The relative growth rate was the same for untreated common and giant ragweed through the 21 d harvest interval, Relative growth rates of treated common and giant ragweed were 50% lower than the relative growth rates of untreated ragweeds 21 d after treatment, Between 21 and 56 d after treatment, the relative growth rate of common ragweed declined an additional 13%, while the relative growth rate of giant ragweed declined an additional 38%. The sharp continued decline in the relative growth rate of giant ragweed indicated plant death, The moderation and slight increase in the relative growth rate of common ragweed between 21 and 56 d corresponded with the initiation of lateral axillary buds and the regeneration of plant growth, Relative growth rate parameters identified differences in the response of common and giant ragweed to imazethapyr as early as 21 d after treatment, Relative growth rate demonstrated utility by objectively measuring differences in the growth response of these two weeds that are moderately susceptible to imazethapyr under laboratory conditions.