Canola rotation effects on soil water and subsequent wheat in the Pacific Northwest USA

Farmers in Mediterranean climate regions are increasingly growing canola (Brassica napus L.) in wheat (Triticum aestivum L.)‐based systems to break soil‐borne pathogen disease cycles, control weeds, and enhance crop marketing opportunities. A 6‐year rainfed cropping systems experiment was conducted near Ritzville, Washington USA from 2015 to 2021. The objective was to compare performance of winter and spring canola, winter triticale (× Triticosecale Wittmack) (WT), and winter wheat (WW) and measure their effects on soil water dynamics and subsequent performance of spring wheat (SW). Overwinter soil water gain in canola stubble was significantly reduced compared to water gain in WT and WW stubble. The more winter precipitation, the greater the differences in soil water among treatments in early spring. There was a trend in all years for diminished overwinter soil water gain in canola stubble. Averaged over years, grain yield of SW was 1940, 2340, and 2210 kg/ha grown on canola, WT, and WW stubble, respectively (p = 0.045). Coefficients of determination from regression analysis conducted each year showed a high correlation between water content in the 180 cm profile in early spring and SW grain yield, except in one year of extreme drought. This paper provides a first report in a Mediterranean climate region of decline in wheat grain yield after canola versus after a cereal crops, primarily due to reduced overwinter soil water storage. Core Ideas Canola is an important rotation crop in rainfed wheat‐based rotations. We conducted a 6‐year study to determine performance of spring wheat grown after canola, triticale, and wheat. Overwinter soil water storage was significantly reduced in canola stubble versus the stubble of triticale and wheat. Soil water at time of planting was closely correlated with spring wheat grain yield.