The phytotoxicity of the pre-emergence herbicides metolachlor, pendimethalin, and propachlor to heading lettuce was investigated in 5 experiments conducted in southern Queensland during 1990-91. Metolachlor was applied before transplanting lettuce seedlings in 2 experiments. On a krasnozem soil at Toowoomba, spraying >1.75 kg a.i./ha reduced lettuce growth and the number of marketable lettuce heads. When applied on a black earth at Gatton, 1.44 kg a.i./ha stunted the lettuce and reduced the number of marketable heads by 30% compared with hand-weeded treatments. On both soils, 2.2-2.9 kg a.i./ha of metolachlor is recommended for broadleaf weed control in other crops. The risks of phytotoxicity from such rates preclude its use in transplanted lettuce in this environment. Lettuce were severely stunted from application of 1 kg a.i./ha of pendimethalin after transplanting. Spraying at the same rate before transplanting initially reduced lettuce width in 1 of 4 experiments, but the lettuce recovered. Lettuce yields from areas treated with 1 kg a.i./ha before transplanting were unaffected. The number and size of harvested heads tended to decline where >1.3 kg a.i./ha was applied. Higher rates (up to 1.5 kg a.i./ha) may be acceptable on krasnozem soils. Use of pendimethalin at 1-1.3 kg a.i./ha would control or suppress many important broadleaf weeds in lettuce, as well as a range of grasses. Propachlor was less phytotoxic when applied immediately after transplanting the lettuce seedlings than when sprayed before transplanting. Yields were unaffected from application of 2-2.5 kg a.i./ha after transplanting on black earth and sandy soils. When sprayed before transplanting, >2 kg a.i./ha caused significant yield reductions in several experiments, although lettuce grown on a krasnozem soil appeared more tolerant. Use of 2 kg a.i./ha of propachlor would give a suppressive effect against several broadleaf weeds, but another herbicide would also be required to achieve effective weed control. A spraying strategy involving pendimethalin and propachlor herbicides in transplanted lettuce has the potential to reduce weed control costs by up to 80%.

The phytotoxicity to green beans (Phaseolus vulgaris) of the herbicides metolachlor, pendimethalin, cyanazine, acifluorfen, diflufenican, bentazone, metribuzin, prometryn, terbutryn, methabenzthiazuron, and oxyfluorfen was investigated in 4 experiments on a black earth soil (clay content 40-60%) at Gatton Research Station in southern Queensland during 1990-91. Metolachlor was applied post-sowing and pre-emergence; up to 4 kg a.i./ha did not significantly (P>0.05) affect growth or yields, indicating a considerable safety margin for this herbicide when used at commercial rates. Pendimethalin did not cause significant crop damage when applied in the same manner at rates up to 2.7 kg a.i./ha. Acifluorfen and diflufenican were each applied at 3 or 4 weeks after sowing in 3 experiments. Sensitivity of the bean crop to acifluorfen varied: 0.5 kg a.i./ha did not significantly reduce bean growth or yield in 2 experiments, but 0.11 kg a.i./ha reduced yields by 20% in a third experiment. Application of 0.1-0.12 kg a.i./ha of diflufenican had no adverse effect on beans in 2 experiments, although significant damage was observed in an initial screening experiment. Bentazone applied 3 weeks after sowing had no significant effect on bean yield or growth in 1 experiment; in another, the maximum label rate of 0.96 kg a.i./ha significantly reduced bean growth and yield. Post-emergence application of cyanazine, metribuzin, prometryn, terbutryn, methabenzthiazuron, or oxyfluorfen at rates required for acceptable weed control either killed the bean plants within a few days or resulted in complete yield loss. Levels of damage from these herbicides preclude their use in green beans. Although green beans showed some tolerance to postsowing, pre-emergence application of cyanazine, low rates of 0.75-1 kg a.i./ha reduced yields by 35%. Both metolachlor and pendimethalin appear suitable for pre-emergence use in green beans. Further work on factors affecting phytotoxicity of acifluorfen, diflufenican, and bentazone to green beans is required.