Harvest weed seed control: Ryegrass seed retention levels in south-eastern Australia wheat crops

Herbicide resistant weeds are a major impediment to Australian grain production and solutions to this problem are required. One of the newer methods used to reduce their impact is the collection and/or destruction of weed seeds at harvest, harvest weed seed control (HWSC). A major premise of HWSC is that the targeted weed species retain a high proportion of their total seed production at crop maturity and that this seed is subsequently collected during harvest. The adoption of HWSC systems in Western Australia has been driven by high annual ryegrass seed retention levels recorded in wheat production systems. The lack of seed retention data is likely restricting adoption of these systems across the Southern region cropping systems. To determine annual ryegrass seed retention levels in southern region, sampling was undertaken in 2013 within seven days of crop maturity (first opportunity to harvest) in 46 wheat crops located across southern NSW and Victoria. At each location wheat and annual ryegrass dry matter production, grain yield and weed seed number was determined for each of five cutting heights (0,10, 20, 30 and 40 cm) as well as from the soil surface. Overall, 80% (48-100%) of ryegrass seed was found at greater than 10 cm above the soil surface and 69% (23–100%) above 20 cm indicating that HWSC can be a valuable tool for NSW and Victorian farmers in reducing weed seed burdens. However, with the large variability shown between locations in this study the efficacy of HWSC will vary considerably.