Herbicide dose rate response curves in subterranean clover determined by a bioassay

Abstract

Summary. Herbicide dose response curves were developed for 5 herbicide treatments, MCPA, 2,4-DB, bromoxynil, MCPA + terbutryn and MCPA + diuron, when applied to 2 cultivars of subterranean clover (Trifolium subterraneum L.) in a glasshouse experiment. The effect of varying spraying time (14 May and 14 June) and leaf stage (3–4 v. 8–10 trifoliate leaves) at spraying was also examined. The dose rate multiple of the recommended rate required to reduce the clover herbage yield by 50% (I50) differed markedly between herbicide treatments. The 2 MCPA mixtures were the most phytotoxic with an I50 value between 0.45 and 0.83, bromoxynil and MCPA alone were intermediate. 2,4-DB was the least phytotoxic with an I50 value between 2.36 and 2.94 depending on cultivar and leaf stage at the time of application. The effect of leaf stage at spraying, time of spraying and cultivar on herbicide phytotoxicity was relatively small, the major factors being herbicide choice (which accounted for 83% of the variation in I50 values) and rate of herbicide applied. While there was differential cultivar tolerance to the herbicide 2,4-DB, there was no difference between the cultivars in their response to the other herbicide treatments. The phytotoxicity of the herbicides applied at either the 3–4 or 8–10 leaf stage in the glasshouse bioassay was highly correlated (r = 0.84, P<0.01) with previous estimates of phytotoxicity determined under field conditions, although slightly overestimated phytotoxicity compared with field tests. European Weed Research Council (EWRC) scores of herbicide damage in the glasshouse bioassay were found to be highly negatively correlated with dry matter yield losses in both the glasshouse (r = –0.78 to –0.98, P<0.001) and field (r = –0.72, P<0.001) and are therefore an acceptable alternative where destructive sampling is not practical. The EWRC scoring system was found to be more effective at quantifying damage where it was manifest as burning or stunting rather than as leaf distortion. The study indicated that a glasshouse herbicide bioassay is a useful technique for rapidly screening herbicides for phytotoxicity on subterranean clover.