42 Twentieth Australasian Weeds Conference Influence of annual ryegrass seed retention height on harvest weed seed control (HWSC) and harvest efficiency Michael J. Walsh 1 , John C. Broster 2 , Charlotte Aves 3 and Stephen B. Powles 4 1 I.A. Watson Grains Research Centre, University of Sydney, 12656 Newell Highway, Narrabri, New South Wales 2390, Australia 2 Graham Centre for Agricultural Innovation (Charles Sturt University and NSW Department of Primary Industries), Charles Sturt University, Locked Bag 588, Wagga Wagga, New South Wales 2678, Australia 3 Melbourne School of Land and Environment, University of Melbourne, Nalinga Road, Dookie, Victoria 3647, Australia 4 Australian Herbicide Resistance Initiative, School of Plant Biology, University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009, Australia (m.j.walsh@sydney.edu.au) Summary 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 escalat- ing 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 m 2 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 ad- ditional 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. Keywords Herbicide resistance, harvest height. INTRODUCTION Seed retention at maturity has been identified as a weed control target for the problematic annual weed species of Australian crop production systems. The major weeds, Lolium rigidum Gaudin (annual ryegrass), Raphanus raphanistrum L. (wild radish), Bromus spp. (brome grass) and Avena spp. (wild oats) retain significant proportions of total seed production, above a low harvest height (15 cm) at wheat crop maturity (Walsh and Powles 2014). During wheat harvest, the seed of these weeds are collected and processed and then exit the harvester, predominantly in the chaff fraction, where they are evenly redistributed across the paddock. Preventing inputs to the weed seedbank has been a focus for weed control for many years and therefore, harvest is now recognized as a weed control opportunity. Subsequently, HWSC systems have been developed specifically to target weed seeds during commercial wheat crop harvest to prevent inputs to the seed bank (Walsh et al. 2013). Approaches used to target weed-seed bearing chaff include: collection and subsequent burning (chaff cart), concentration in a narrow windrow with straw residues for subsequent burning (narrow windrow burning), collection in bales along with straw residues (bale direct system) and mechanical destruction during har- vest (Harrington seed destructor). When implemented effectively, these systems all deliver similarly high levels of weed seed destruction (>85%). Therefore, growers can choose an HWSC system to suit their farming operation. The developed HWSC systems effect weed control by targeting the weed seed bearing chaff fraction and when implemented correctly are all similarly effective. The overall efficacy of these systems then is reliant on the proportion of total seed production that is col- lected during the harvest operation. The height and distribution of weed seed through the crop canopy at maturity impacts on the efficacy of HWSC. Thus the aim of this study was to establish the distribution through the canopy of mature wheat crops of retained annual ryegrass seed at widely dispersed WA wheatbelt locations.