This field trial at Washpool was undertaken to investigate factorial combinations of sowing time, seed rate and herbicides on the management of annual ryegrass in wheat.

A field trial was undertaken at Washpool in 2019 to investigate combinations of wheat sowing time, seed rate and herbicide treatments to control annual ryegrass. The average seedbank of annual ryegrass (ARG) at the site was 2249 ± 284 seeds/m2. There was no evidence at this site of any reduction in ryegrass infestation in wheat by delaying sowing by three weeks between TOS 1 (77plants/m2) and TOS (74plants/m2). This result suggests the Washpool ARG population likely has a high level of seed dormancy, reducing the rate of ryegrass germination after the opening rainfall events. Herbicide treatment had a significant influence on ARG plant density (P<0.001). Boxer gold and Sakura + Avadex reduced ARG plant density by 45% and 73% from the untreated control (125 ARG plants/m2). Wheat was much more competitive against ARG when it was sown early. This was made evident by the large increase in ARG seed production when seeding was delayed. When averaged across all other treatments TOS 1 produced 1634 ARG seeds/m2, which was 73% lower than 5974 ARG seeds/m2 produced in TOS 2 (P=0.013). Wheat seed rate had a significant influence on ARG seed production (P=0.005), with the high wheat seed rate supressing ARG seed production by 37% when compared to the lowest wheat seed rate (100 seeds/m2). Wheat grain yield at this site was significantly influenced by the time of sowing (P=0.001), seed rate (P=0.001), herbicide treatments (P=0.001), and the interaction between the time of sowing and herbicides (P=0.011). Wheat was much more tolerant to ryegrass competition when sown early (TOS 1) as shown by the small increase in grain yield in herbicide treated plots. In contrast, there was a significant increase in wheat grain yield in herbicide treatments in TOS 2. The results of this study clearly show that delayed sowing of wheat allows for greater seed set by ryegrass and is also associated with a large yield penalty.