HERBICIDE DOSE-RATE RESPONSE CURVES IN SUBTERRANEAN CLOVER DETERMINEDBY A BIOASSAY

Herbicide dose response curves were developed for 5 herbicide treatmen ts, 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% (I-50) differe d markedly between herbicide treatments. The 2 MCPA mixtures were the most phytotoxic with an I-50 value between 0.45 and 0.83, bromoxynil a nd MCPA alone were intermediate. 2,4-DB was the least phytotoxic with an I-50 value between 2.36 and 2.94 depending on cultivar and leaf sta ge at the time of application. The effect of leaf stage at spraying, t ime of spraying and cultivar on herbicide phytotoxicity was relatively small, the major factors being herbicide choice (which accounted for 83% of the variation in I-50 values) and rate of herbicide applied. Wh ile there was differential cultivar tolerance to the herbicide 2,4-DB, there was no difference between the cultivars in their response to th e other herbicide treatments. The phytotoxicity of the herbicides appl ied at either the 3-4 or 8-10 leaf stage in the glasshouse bioassay wa s highly correlated (r = 0.84, P<0.01) with previous estimates of phyt otoxicity determined under field conditions, although slightly overest imated 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 lo sses 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 d estructive sampling is not practical. The EWRC scoring system was foun d to be more effective at quantifying damage where it was manifest as burning or stunting rather than as leaf distortion. The study indicate d that a glasshouse herbicide bioassay is a useful technique for rapid ly screening herbicides for phytotoxicity on subterranean clover.