Cereal cover crops for weed suppression in a summer fallow-wheat cropping sequence

Cropping systems in western Canada that include summer fallow can leave the soil exposed to erosion and require frequent weed control treatments. Cove r crops have been used for soil conservation and to suppress weed growth. E xperiments were conducted under rain-fed conditions at Lethbridge, Alberta to determine the effect of short-term fall rye (Secale cereale L.), winter wheat (Triticum aestivum L.) and annual rye cover crops in the fallow year on weed growth and subsequent wheat yield. Under favorable weather conditio ns fall rye was as effective as post-harvest plus early spring tillage or h erbicides in spring weed control. Winter wheat and fall rye residues, after growth was terminated in June, reduced weed biomass in September by 50% co mpared to no cover crop in 1993 but had little effect on weeds in 1995. Fal l-seeded cover crops reduced the density of dandelion (Taraxacum officinale Weber in Wiggers) and Canada thistle [Cirsium arvense (L.) Scop.] but incr eased the density of downy brome (Bromus tectorum L.), wild buckwheat (Poly gonum convolvulus L.), and thyme-leaved spurge (Euphorbia serpyllifolia Per s.) in the following fall or spring. Wheat yields after fall rye and no cov er crop were similar but yields after spring-seeded annual rye were less th an after no cover crop. Spring-seeded annual rye did not adequately compete with weeds. Cover crops, unlike the no cover crop treatment, always left s ufficient plant residue to protect the soil from erosion until the followin g wheat crop was seeded.