Evaluating intercropped cover crops and their compatibility with canola and wheat herbicidal weed management strategies on the Canadian Prairies

Abstract

The use of fall-seeded cover crops on the Canadian Prairies is constrained by the short growing season and limited fall precipitation. Intercropping cover crops with a cash crop can extend the establishment window and increase biomass accumulation; however, herbicide compatibility and intercrop competition remain barriers to adoption. This thesis evaluated the feasibility of intercropping cover crops within canola (Brassica napus L.) and spring wheat (Triticum aestivum L.) as well as their compatibility with herbicide management strategies. Field experiments were conducted in 2022 and 2023 at two locations (Carman, Manitoba and Kernen, Saskatchewan). In canola, three herbicide frequency treatments (pre-emergence only, pre-emergence and in-season, and a pre-emergence, in-season, and desiccant) were compared for all three herbicide tolerance systems utilized with canola hybrids (Liberty Link®, Roundup Ready®, and Clearfield®). In wheat, three in-season herbicides (bentazon, tralkoxydim, and pyroxsulam) were compared alongside a no-herbicide control. A multispecies cover crop mixture was intercropped simultaneously with both crops, and measurements included cover crop establishment, herbicide injury, weed pressure, fall biomass, and cash crop yield. Cover crop establishment and persistence were highly variable across years, locations, and cash crops, reflecting strong environmental and competitive constraints. In both cropping systems, increased seeding rates and reduced herbicide frequency improved cover crop establishment and persistence. In canola, cover crop biomass was greatest under the pre-emergence only treatment, but the increase in biomass was associated with yield penalties in some environments. Following in-season herbicide applications, cover crop performance within contrasting canola herbicide tolerance systems varied by site. In wheat, the tralkoxydim herbicide was tolerated by cover crop species and resulted in biomass amounts similar to the no herbicide control, with no effect on wheat yield. Weed suppression benefits from cover crops were limited and inconsistent, especially where cover crop establishment was poor. Overall, this research demonstrates that intercropping cover crops can partially overcome environmental constraints, but success is dependent on herbicide selection, herbicide frequency, and site-specific growing conditions. These findings highlight the need for regionally adapted agronomic strategies and further research to optimize cover crop intercropping strategies for canola and wheat that do not compromise cash crop yield.