How Frequently Can Canola be Grown in Sustainable Cropping Systems?.

Canola (Brassica napus) is currently grown on 8 million ha in Canada. It has traditionally been grown every 3^rd or 4^th year in rotation with cereals and grain legumes in this region. However, high profit margins generated by canola in recent years have tempted farmers to grow it more frequently in their crop rotations. A multi-year field experiment (2008-2013) was conducted at five sites on the Canadian prairies to determine the merits of growing spring canola every year, every 2^nd year, or every 3^rd year. All phases of a crop rotation were grown each year to account for environmental differences over years. At the conclusion of the first 3-year rotation cycle (2010), canola yields were 21-38% greater in 2- or 3-year rotations compared with continuous canola at 3 of 5 sites. The 3-year rotation yielded more than the 2-year rotation at one site. The magnitude of differences among the rotation treatments tended to increase with time. After two 3-year rotation cycles (2013), yields were 10-86% higher in 2- or 3-year rotations compared with continuous canola at 4 of 5 sites. Somewhat surprisingly, only one site had a higher yield (23%) with a 3-year compared with a 2-year rotation. A second multi-year field experiment (2008-2013) was conducted at four sites to determine the usefulness of specific risk mitigation production practices if canola were to be grown continuously. Treatments included growing mixtures of canola cultivars and/or growing different herbicide-resistant cultivars (Roundup Ready, Liberty Link, Clearfield) in alternating years in a continuous canola cropping system. Results indicated that none of these practices were effective in reducing the negative effects of continuous canola.  Over the four sites, yields were 16-39% lower with continuous canola compared with it being grown every 3^rd year. Study findings clearly indicate that continuous canola is not a sustainable production practice. Although 2-year rotations fared reasonably well in this study, a 3-year rotation likely should still be recommended.