Investigating red clover (Trifolium pratense L.) stand survival and uniformity when under-seeded to winter wheat (Triticum aestivum L. em. Thell) under low soil moisture
Red clover (Trifolium pratense L.), when included in corn (Zea mays)-soybean (Glycine max)-winter wheat (Triticum aestivum L. em. Thell) rotations in relay cropping with winter wheat, has been shown to increase organic matter, soil tilth, yield and productivity of the rotation and to decrease soil erosion, nitrate leaching and synthetic nitrogen fertilizer requirements. In spite of these benefits, red clover inclusion into crop rotation has declined, largely due to non-uniformity of red clover stands. In Ontario, causes of heterogeneous red clover stands have not been conclusively determined although the effects of tillage and shifts in competition for light and nutrients between wheat and clover during relay cropping have been investigated. As suggested by the high year effect across those studies, variation in soil moisture during relay cropping likely has a large effect on red clover stand uniformity. We conducted a field experiment repeated over three years to test the impact of different soil moisture deficits on the establishment of clover varieties with different growth habits (single-cut and double-cut). Low soil moisture, beginning either late May or late June, under winter wheat decreased red clover stand densities and above-ground biomass. A second field experiment more specifically examined the effect of water exclusion on red clover stand uniformity over a large area under irrigated and non-irrigated conditions. In 2015, low soil moisture in spring increased stand non-uniformity in non-irrigated plots. Differences between varieties in response to drought and drought tolerance mechanisms were investigated in growth room studies. The double-cut variety Belle had greater leaf area and shoot water content during mild to moderate drought conditions compared to the single-cut variety Altaswede. In contrast, Altaswede had significantly higher survival rates than Belle after moderate to severe drought stress. Differences in drought response and recovery were also observed between other single-cut and double-cut varieties. By demonstrating that red clover growth and stand uniformity can be impacted by drought mechanisms, effective strategies for the development of improved varieties through breeding or screening efforts can be identified.