INTERACTION BETWEEN WHEAT (TRITICUM-AESTIVUM) AND DICLOFOP TO REDUCE THE COST OF ANNUAL RYEGRASS (LOLIUM-RIGIDUM) CONTROL

The influence of wheat variety on the dose-response of annual ryegrass to diclofop-methyl (POST) was examined in the field in 1992 and 1993 in southern New South Wales, Australia, The aim was to determine if pl anting a strongly competitive variety of wheat improved control of ann ual ryegrass at reduced doses of diclofop-methyl, Suppression of ryegr ass was dependent on herbicide dose, season, and wheat variety, In the absence of herbicide, dry matter (DM) production of annual ryegrass a t 300 plants m(-2) at anthesis was 500 g ha(-1) with Dollarbird and Ka tunga compared to 1000 g ha(-1) with Rosella or Shrike in 1992, In 199 3, DM was approximately 150 g ha(-1) with Dollarbird or Katunga, and 3 50 g ha(-1) with Shrike or Rosella. Ryegrass DM was reduced by diclofo p-methyl to a greater extent, relative to the weedy unsprayed controls , with less competitive varieties Rosella and Shrike than with the mor e competitive Dollarbird or Katunga, Diclofop-methyl at 0.28 kg a.i. h a(-1) reduced DM of ryegrass growing with Katunga to less than 100 g m (-2) in 1992, compared to more than 200 g m(-2) with the other varieti es. In 1993, diclofop-methyl was more effective on ryegrass, and the s ame dose reduced ryegrass DM to almost zero in all varieties, Grain yi elds in unsprayed weedy controls of Dollarbird and Katunga were reduce d approximately 20% by annual ryegrass compared with yields achieved,v ith herbicides in both years, Yields of Rosella and Shrike in the unsp rayed controls were reduced about 40% in 1992 and 60% in 1993, Only sm all increases in grain yields of all varieties occurred from diclofop- methyl doses above 0.13 kg a.i. ha(-1). Poorly competitive varieties w ere dependent on herbicides to achieve grain yield potential and had a greater risk of weed survival when herbicide efficacy was reduced, In contrast, strongly competitive varieties, likely to retard build-up o f weed seed in the soil, are less dependent on herbicides to achieve g rain yield potential, and therefore result in reduced weed control cos t.