Management of a herbicide-resistant ryegrass (Lolium rigidum) population in a crop rotation using alternative herbicides, row spacing, strategic nitrogen application and RR® canola (Brassica napus)

Abul Hashem, Wendy Vance, Ross Brennan and Richard W. Bell

Keywords:

Herbicides; crop rotation, Lolium rigidum, resistant rigid ryegrass, urea ammonium nitrate (Flexi N), Roundup Ready® (RR®) canola, trifluralin, simazine, dimethenamid-p, pyroxasulfone

Abstract:

Rigid ryegrass (Lolium rigidum Gaud., henceforth, called ryegrass) is the most significant herbicideresistant weed in Australian grain cropping. Failure to adequately control ryegrass causes grain yield losses of about 36%. Therefore, new approaches for the control of ryegrass are needed in diverse crop rotations. We studied the options for managing a high-density Acetyl CoA Carboxylase (ACCase)-resistant ryegrass population in a lupin (Lupinus angustifolius L.) - wheat (Triticum aestivum L.) - canola (Brassica napus L.) rotation, under dryland conditions, at Cunderdin (31.650908 o S, 117.238906 o E), Western Australia (WA). Field trials were conducted during 2012 to 2014. In the 2012 lupin, and 2013 wheat crops, conventional herbicides (simazine in lupin, and trifluralin in wheat) and an alternative herbicide (dimethenamid-p in lupin, and pyroxasulfone in wheat) were tested. In 2014, Roundup Ready® (RR®) canola received two applications of glyphosate to control ryegrass. Three treatments of nitrogen (N) ((N1) 25 kg N ha-1 as urea; (N2) 50 kg N ha-1 as urea; and (N3) 50 kg N ha-1 as urea ammonium nitrate (UAN)) were applied to the 2013 wheat, and the 2014 RR® canola. Each crop was grown at two row spacings (22 cm, or 44 cm). None of the management factors except the herbicides significantly decreased the ryegrass density. Indeed, N3 (UAN) increased the emergence of ryegrass (more in 44 cm than 22 cm rows) compared to N1 and N2. Compared to urea N1, N3 reduced canola establishment by 28% and generally increased the grain yield of RR® canola by 11% but increased the density of ryegrass rather than controlling it. Dimethenamid-p, the alternative herbicide, decreased the ryegrass density in lupin and increased grain yield of lupin by 53%. While pyroxasulfone, the alternative herbicide, had no significant effect on the ryegrass density compared, to trifluralin in wheat, it increased the wheat grain yield by 25%. However, the 99% reduction in ryegrass by two applications of glyphosate in RR® canola was by far the most effective weed control. The inclusion of RR® canola technology in the rotation was the most effective approach to control the ACCaseresistant ryegrass, under dryland conditions of Western Australia.a