A field experiment was conducted during 1977 in the Central Depression region of Rio Grande do Sul to evaluate herbicide treatments to selectively control ryegrass (Lolium multif lorum L.) in wheat (line E-7414), as well as to establish the competition levels between both grasses. The herbicides chlorbromuron, chlortoluron, cyanazine, diclofop, diuron, metoxuron, and terbutryn were compared with the control treatments: wheat without ryegrass, wheat competing with ryegrass, and ryegrass alone. All the herbicides were applied in postemergence, when wheat plants in the stage of 3-4 leaves, and ryegrass presented 1-3 leaves. It was found that, when ryegrass infestation was not controled by any means, an average reduction of 52% occurred in wheat seed yield. On the other hand, the wheat population which competed with ryegrass plants, caused a decrease in the order of 42% on ryegrass dry matter production. It was observed that all the herbicides presented phytotoxicity, causing from light to very severe injuries to wheat plants, depending on the product used; and that seed yields resulting from the chemical treatments were lower than that of the check plot free of ryegrass. However, all the compounds tested presented significant post-emergence activity, showing potential for ryegrass control. As well as ryegrass dry matter production, also visual evaluation of ryegrass control were appropriate methods to measure the herbicide effect. Among the herbicides evaluated, diclofop was the treatment that provided the most efficient ryegrass control, lower degree of phytotoxicity, and the highest wheat yield; in general, chlortoluron followed diclofop in these characteristics. The herbicides chlorbromuron and metoxuron, although showing relative selectivit y to wheat, presented insufficient post - emergence activity on ryegrass. Treatment diuron showed unsatisfactory performance in relation to ryegrass control, as well as to wheat yield. The herbicides cyanazine and terbutryn provided adequate control of ryegrass; however, their selectivities for wheat were very low; along with a decrease in the rate of wheat germination resulting from the harvest of plots treated with these herbicides.
herbicides; competition; wheat; ryegrass