Effect of emergence time on growth and fecundity of Rapistrum rugosum and Brassica tournefortii in the northern region of Australia

Weeds from Brassicaceae family are a major threat in many crops including canola, chickpea, cotton and wheat. Rapistrum rugosum (L) All. and Brassica tournefortii Gouan. are two troublesome weeds in the northern region of Australia. In order to examine their phenology of these weeds, a pot study was conducted in 2018 at the Research Farm of the University of Queensland, Gatton campus with two populations sourced from high (Gatton) and medium (St George) rainfall areas of the northern grain region of Australia. Planting was carried out monthly from April to September, and the growth, flowering and seed production were evaluated. Maximum growth and seed production were observed in weeds planted in April, compared to other planting dates. Biomass of R. rugosum and B. tournefortii was reduced by 85% and 78%, respectively, as a result of the delay in planting from April to July. R. rugosum and B. tournefortii produced more than 13,000 and 3500 seeds plant(−1), respectively, when planted in April and seed production was reduced by > 84% and > 76% when planted in July. No significant differences were observed between populations of both weeds for plant height, number of leaves and biomass, however, the medium rainfall population of R. rugosum produced more seeds than the high rainfall population when planted in April. The results of this study suggest that, although R. rugosum and B. tournefortii were able to emerge in a wider time frame, the growth and seed production were greatest when both weeds were planted in April and there was concomitant reduction in growth attributes when planted in the subsequent months, indicating that management of these weeds early in the cropping season is a prerequisite to population reduction and the mitigation of crop yield losses.