Response of seeding rate and cultivar maturity with planting dates in canola

Understanding whether increasing seeding rate in canola (Brassica napus) provides a yield-compensatory mechanism with delayed planting due to weather or logistical issues is important. We aim to refine agronomic recommendations for canola production under short-season growing conditions to establish an optimal planting date and identify any interaction of spring canola cultivar and seeding rate with planting date. We conducted this study in 2014, 2015, and 2016 in Creston, MT, as split–split plot with planting dates, cultivar maturities, and seeding rates as mainplot, subplot, and sub-subplot, respectively. Except 2016, 0.06 Mg day⁻¹ yield reduction with delayed planting was observed in 2014 and likely in 2015. In 2016, daytime temperatures rose early in May, facilitating the early cessation of node production on the mainstem and, therefore, exposure to high temperatures at flower of the mid-April planted canola. Generally, the mid-April planted canola showed more nodes on the mainstem at the bolting stage. The low seeding rate increased pod number (p < 0.0001) as yield-compensatory mechanism. The late-maturing cultivar only showed yield advantage when planted in mid-April, seeded at low seeding rate, or planted during a hot and dry year. In this study, the optimal planting date for canola in this region or similar short-season growing condition is mid-April, and the probability of yield loss increases when planting is delayed. The optimal seeding rate of this study is 86 live seeds m⁻² regardless of planting date. Overall, a high seeding rate as a management strategy to compensate for yield loss with delayed planting is not recommended.