Influence of annual ryegrass seed retention height on harvest weed seed control (HWSC) and harvest efficiency

HWSC is an Australian innovation where systems have been invented to specifically target the weed seed bearing chaff fraction during crop harvest. This approach to weed control is a response to escalating frequencies of herbicide resistant crop weeds that was developed because there is the opportunity to target the high seed retention levels of these weeds at crop maturity. However, seed retention is not complete and weed seeds are frequently retained at lower heights than crop seed heads. The aim of this study was to examine the seed retention height of annual ryegrass through the canopy of wheat crops at maturity. During the 2013 harvest period annual ryegrass and wheat plant material was collected at five heights (0, 10, 20, 30, and 40 cm) each within four 1.0 m2 quadrats in 25 wheat fields across the WA wheatbelt. On average 66% of annual ryegrass seed was retained above a 10 cm harvest height. However, a harvest height aimed at grain collection only (30 cm) would have reduced annual ryegrass seed collection and HWSC potential to 40%. The major proportion of wheat biomass (66%) occurs above 30 cm however, reducing harvest height from 30 to 10 cm would mean the collection of an additional 14% of the total biomass production. Thus, operational cost of harvesting lower for HWSC is the processing of just an additional 14% of crop biomass. This will impact to some extent on the speed of harvest but in contrast the gains from the adoption and use of HWSC are substantial.