The phytotoxicity of the herbicides bromoxynil, pyridate, imazethapyr and a bromoxynil + diflufenican mixture on subterranean clover and lucerne seedlings

Abstract

Summary. The effect of the herbicides pyridate, imazethapyr and a bromoxynil + diflufenican mixture on subterranean clover (Trifolium subterraneum L.) (cvv. Trikkala and Karridale) and lucerne (Medicago sativa L.) (cv. Aurora) seedlings was examined in randomised plot field experiments in 2 successive years. Responses were compared against an unsprayed control and a standard bromoxynil application. The herbicides and the rates of product applied were: bromoxynil + diflufenican (0.5, 1.0 L/ha), imazethapyr (0.18, 0.3 L/ha), pyridate (1.0, 3.0 L/ha), and bromoxynil (1.4 L/ha). Weeds were removed by hand from the subterranean clover experiments but not the lucerne experiments. Pyridate and imazethapyr were the least phytotoxic of the herbicides applied on both subterranean clover and lucerne. The bromoxynil + diflufenican mixture was the most phytotoxic, causing severe leaf burn and a depression in herbage biomass in both species. Despite the high level of phytotoxicity by some treatments, none of the herbicides reduced lucerne seedling numbers. Lucerne herbage yields in late spring were higher in most sprayed plots compared with the unsprayed control due to the removal of weed competition. Seed yield responses in subterranean clover due to herbicide application ranged from negative responses up to –21% with pyridate to positive responses up to 92% with the bromoxynil + diflufenican treatment relative to the weed-free, unsprayed control. The positive responses were attributed to increased availability of soil water during seed set in treatments in which herbicides suppressed legume biomass. There was a good correlation in both 1992 (R2 = 0.85–0.89) and 1993 (R2 = 0.63–0.73) between the depression in herbage yield in spring and the increase in seed set relative to the control. Soil water under the subterranean clover cultivar Karridale in spring was highest in the bromoxynil and imazethapyr treatments, which produced a large reduction in biomass, and lowest in the control and pyridate treatments, which had showed the least depression in biomass 60 days after treatment application. Although some herbicides cause a high level of phytotoxicity, their use in weedy subterranean clover–lucerne mixtures is justified in view of the small negative, and potentially large positive, effects on subterranean clover seed yield and the increased lucerne yields later in the season due to weed suppression.