By Haiying Tao, Washington State University Extension, Washington State University Farmers Network
Integrating a new crop into an existing cropping system requires a comprehensive understanding of the new crop. For example, a grower must assess the feasibility of producing the new crop under the new agroecological conditions and must understand the crop’s rotational benefits, market value, and best management practices. Canola was introduced in Washington State (WA) in the 1970s. However, canola acreage did not start booming until approximately 10 years ago when Washington State University (WSU) partnered with the WA Department of Agriculture to initiate the Washington Oilseed Cropping Systems Research and Extension Program (WOCS). Lower transportation costs, as a result of a seed crushing facility built in Warden, WA in 2015, also likely played a role in the expansion of canola acreage. Prior to this in-state facility, WA canola growers incurred extara costs to transport canola seed longer distances for crushing, primarily to Saskatchewan, Canada.
In 2018, a total of 66,000 acres of canola were harvested in WA. Canola has been successfully grown in irrigated farming regions and in low, moderate, and high rainfall zones of eastern WA’s dryland farming region. In the high-rainfall (annual cropping) zone, spring canola is more common than winter canola because low soil moisture immediately after wheat harvest is unfavorable for fall sowing. In addition, a low number of growing degree days after sowing limits winter canola from reaching sufficient size to safely overwinter. In the intermediate rainfall zone (transitioning between crop-fallow and annual cropping zones), both spring canola and winter canola are grown. However, winter survival of winter canola in this area is one of the major factors limiting widespread adoption of winter canola. In the low-rainfall (grain-fallow) zone, winter canola is more common and has double the yield potential compared to spring canola. In this zone, spring canola is only grown when winter canola fails to establish or overwinter successfully.
Many studies have found that canola is a good rotational crop in wheat-dominated cropping systems. Rotational benefits include disrupting weed and disease cycles and increasing water and nutrient use efficiency because canola’s large taproot system can penetrate compacted soil layers and, thus, increase soil porosity. In turn, wheat following canola has the potential to develop more rigorous root systems, allowing more water and nutrient uptake and increasing resilience to extreme weather conditions. Many studies in the Pacific Northwest have reported positive yield benefits from wheat following canola. For example, a case study led by K. Sower (previous WOCS Extension Specialist) found wheat yield increased up to 27% following canola compared to following wheat. However, one study located in Davenport led by Dr. Schillinger and Dr. Paulitz found that spring wheat yield decreased by 17% following winter canola compared to following winter wheat.
Similar to any other crop, maximizing canola yield is challenging. The most common issues include: (1) crop establishment and uneven stand development, (2) overwinter survival, (3) pest and disease management, (4) harvest, and (5) fertility management. Nitrogen (N), phosphorus (P), sulfur, and micronutrients such as zinc and boron are the most commonly used fertilizers in canola fertility management. More research is needed to evaluate micronutrient fertility management strategies under WA soil and climate conditions. Dr. Tao (WSU Soil Fertility Specialist) is currently leading a study to evaluate critical levels of soil P and to determine the best P rate, timing, and application method for both winter and spring canola.
Managing Nitrogen for Canola
Currently, the yield-goal method is used to estimate N rates for canola. In another words, N rate is determined based on unit N requirements (UNR). A 7 site-year research study conducted across rainfall zones of WA found that approximately 5 to 7 lb N per 100 lb seed yield is sufficient for winter canola. A 12 site-year research study conducted across rainfall zones in WA found that approximately 7 to 17 lbs N per 100 lb seed yield is required for spring canola. Notably, however, when soil test N is higher than 100 to 150 ppm in the 6-foot depth, yield response to additional N fertilizer application is unlikely in both winter and spring canola. The N rate should be determined based on yield goal, UNR, and a soil test.
In general, the higher the yield potential, the lower the UNR. When canola is grown in higher yield potential areas, it develops more vigorous root systems that allow greater access to soil nitrogen and water. For winter canola, UNR is highly correlated with spring-available water, which includes both soil water and spring rainfall.
Timing of N application affects N use efficiency and N availability to canola. Spring application is a better practice than fall application in areas with high leaching potential, such as fields with sandy soils located in the high rainfall and intermediate rainfall zones. In the high rainfall zone, if soil test N is high in fall, no fertilizer N application is needed; if soil test N is low, 30 lbs/acre N as starter is recommended. Fall or splitting N applications between fall and spring in the low rainfall zone are good practices. Split application results in better yield in irrigated systems.
Canola seed quality is significantly affected by N management. Higher N availability leads to higher seed protein concentration. Typically, the higher the seed protein concentration, the lower the seed oil concentration. Timing of N application also affects seed oil concentration, mainly as a result of the timing effect on N availability. For example, in our 7 site-year research, we found that winter canola seed oil concentration was lowest with spring N application in a field located in the high rainfall zone. For that same field, the next lowest oil concentration was the split applications between fall and spring, followed by fall application.