Phytotoxicity to transplanted lettuce (Lactuca sativa) of three pre-emergence herbicides: metolachlor, pendimethalin, and propachlor

Abstract

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%.