Reducing the effect of grass weed competition

The control of crop grass weeds is a major limitation to the successful production of oats in Southern Australia. This is due to the combination of few registered herbicide options and limited knowledge on their effectiveness, coupled with effects of management, for different cultivars. Five trials with 6 cultivars were conducted between 1997 and 2001 in the southern wheatbelt of Western Australia to examine this problem. In a herbicide tolerance screen the yields of the cultivars Mortlock, Toodyay and Needilup were not reduced by any herbicide options tested and were therefore more tolerant to the chemical applications. The yields of the cultivar Hotham however, were reduced by all herbicide treatments except the safest treatments, indicating this cultivar is more sensitive. In the presence of weeds, the yields of Hotham and Needilup were not reduced by herbicides but had relatively high in crop weed biomass and therefore, can be regarded as both poor weed competitors and herbicide tolerant. The crop weed biomass of Mortlock and Toodyay was low and yield wasn t reduced by any of the herbicide options, therefore both were tolerant to the herbicides and competitive with weeds. Needilup had the strongest relationship between weed biomass and grain yield (R2=0.80) indicating that Needilup is more responsive than the other varieties tested to changes in weed biomass whilst less sensitive to herbicides. Two management practices to reduce weed competition were assessed, delayed sowing and increasing crop plant density. Utilising the delayed sowing allowed an extra knockdown herbicide to be applied and resulted in an average grain yield increase of 1.5 t/ha and weed biomass reduction of 1 t/ha. When the target plant population was increased in the presence of weeds, weed biomass was reduced and grain yield was increased. Through the utilisation of alternative herbicides and changes in management, grass weeds can be effectively controlled in oat crops without reliance on a small number of herbicides (e.g. sulfonylureas).