Influence of Crop Competition and Harvest Weed Seed Control on Rigid Ryegrass (Lolium rigidum) Seed Retention Height in Wheat Crop Canopies

Harvest weed seed control (HWSC) is an Australian innovation, developed to target high proportions of weed seed retained at crop maturity by many major weed species. There is the potential, however, that a reduction in the average height of retained seed is an adaptation to the long-term use of HWSC practices. With the aim of examining the distribution of rigid ryegrass (Lolium rigidum Gaudin) seed through crop canopies, a survey of Australian wheat (Triticum aestivum L.) fields was conducted at crop maturity. Nine sites with medium to long-term HWSC use were specifically included to examine the influence of HWSC use on seed retention height. During the 2013 wheat harvest, L. rigidum and wheat plant samples were collected at five heights downward through the crop canopy (40, 30, 20, 10, and 0 cm above ground level) in 71 wheat fields. Increased crop competition resulted in higher proportions of L. rigidum seed in the upper crop canopy (>40 cm). The increase in plant height is likely a shade-intolerance response of L. rigidum plants attempting to capture more light. This plant attribute creates the opportunity to use crop competition to improve HWSC efficacy by increasing the average height of seed retention. Crop competition can, therefore, have a double impact by reducing overall L. rigidum seed production and increasing seed retention height. Examining the distribution of wheat biomass and L. rigidum seed through the crop canopy, we determined that reducing harvest height for HWSC considerably increased the collection of L. rigidum seed (25%) but to a lesser extent wheat crop biomass (14%). Comparison of + and - HWSC use at nine locations found no evidence of adaptation to this form of weed control following 5 to 10 yr of use. Although the potential for resistance to HWSC remains, these results indicate that this will not readily occur in the field.