Agronomy Library > Forages & Forage Seed

Residual Weed Populations in No-till Annual Broadleaf Crop Rotations
Author: F. C. Stevenson and A. M. Hohnston
Date Created: February 10, 1999
Last Reviewed: February 03, 2009
Source: Weed Science. 47 (2): 208 - 214 Summary The development of problematic weed populations is a concern in western Canadian fields where canola (rape) and peas are grown in a 4-year sequence with spring cereal grains. Weed densities were examined at a site near Melfort, Saskatchewan, Canada, from 1994 to1997 in seven no-tillage managed crop rotations. Four rotations that included rape, peas, or flax in at least 3 of 4 years (HBF:high broadleaf-crop frequency) were compared with three rotations that included broadleaf crops grown in 2 of 4 years (LBF: low broadleaf-crop frequency). Spring wheat and barley were the cereal crops in rotation. Residual (post herbicide application) weed density for each weed species in a given year was recorded for all phases of each rotation to reflect the overall weed infestation. - Four annual broadleaf weed species were most abundant in 1996 and a second group of three species, with a variety of reproductive stages, became progressively less abundant as the study progressed. - The weed density difference between HBF and LBF rotations varied and was most prominent in years when environmental conditions were conducive for weed growth. - More frequent applications of the herbicide ethalfluralin (Edge), with its residual weed control, best explained why wild oat (Avena fatua) and cleavers (Galium aparine) generally were less abundant in the HBF rotations. - Of particular interest was the 8 plants/sq.m greater density of dandelion (Taraxacum officinale) and perennial sow thistle (Sonchus arvensis) in the HBF compared to the LBF rotations in the last year of the study. - Differences in herbicide use between the HBF and LBF rotations was considered the primary factor controlling the rotation effects of weed density. It is thought that limited herbicide options for the control of these species could present a future problem if they continue to develop in the HBF rotations.