and three tillage treatments (moldboard plowing (MP), chisel plowing (CP) and no-tillage (NT)) were compared in a split-plot design. Total organic C was affected by the tillage treatments but not by the rotations. In the topsoil layer (0-7.5 cm), NT and CP treatments had C,contents 20% higher than the MP treatment. In the same soil layer, MBC averaged 300 mg C kg-' in the MP treatrnent and up to 600 mg C kg-l in the NT soil. Hot-water-extractable and acid-hydrolyzable carbohydrates were on average 4O% greater under reduced tillage than under MP. Both carbohydrate fractions were also slightly larger in the rotation than in the soil under continuous barley. The ratios of MBC and carbohydrate C to total organic C suggested that there was a significant enrichment of the OM in labile forms as tillage intensitywas reduced. Alkaline phosphatase activity was 507o higher under NT and 20% higher under CP treatments than under MP treatment and, on average, 15% larger in the rotation than in the continuous barley treatment. Overall, the management-induced differences were slightly greater in the top layer (0-7.5 cm) than in the lower layer of the Ap horizon (7.5-15 cm). All the properties measured were highly correlated with one another. They also showed significant temporal variations that were, in most cases, independent of the treaftnents. Four years of conservation tillage and, to a lesser extent, rotation with red clover resulted in greater OM in the top soil layer compared with the more intensive systems. This organic matter was enriched in labile forms.

No abstract

Generalizations concerning the effects of management practices on weed community dynamics often lack robustness, most likely because of the concomitant effects of agronomic and environmental factors. However, such generalizations, when valid, provide useful grounds for predictions and are thus desirable. This study attempted to evaluate the relative importance of crop rotation, tillage, and weed management as factors affecting weed communities and tested the hypothesis of an association between management practices and weeds from certain life cycle groups. Principal component analysis (PCA) of weed density data from a 4-yr field study conducted on a Kamouraska clay and a Saint-André gravelly sandy loam at La Pocatière QC, Canada, identified groups of weed species, while an analysis of variance (ANOVA) of PCA scores associated these groups with management factors. A multivariate analysis of variance (MANOVA) of regression coefficients describing time courses of density for each species confirmed treatment effects. Species segregated roughly according to life cycles. Interactions among weed management intensity, tillage, and crop rotation mostly explained species dominance in the various cropping systems. A first group of species, mostly annual dicots, largely dominated in minimum weed management treatments; their relative importance in each rotation varied with their level of susceptibility to postemergence herbicides. A second group included annuals and perennials, whose commonality seemed to be their tolerance to herbicides; these species also had a particular affinity for chisel and no-till treatments. A third group was formed by perennial species, each with a different response to tillage. The tenuous correspondence between commonly used classification schemes and management factors suggests that other aspects of weed biology (e.g., seed size, dispersal, production, germination requirements, and seedbank longevity) should be considered when trying to explain and predict the presence and dominance of certain weed species with regard to management practices.

Constraints to the adoption of conservation tillage in central and eastern Quebec potentially include cool wet springs, short growing seasons, and variable precipitation patterns. This study was conducted to determine the suitability of conservation tillage practices to cereal cropping systems in this area of Quebec. The effects of crop rotation [spring barley (Hordeum vulgare L.) monoculture; spring barley‐red clover (Trifolium pratense L.) rotation], tillage [fall moldboard plow (MP); fall chisel plow (CP), and direct‐seeded no‐till (NT)], and weed management intensity (intensive, moderate, minimum) on populations and dry weights of crop and weeds at midseason, and on grain yields and yield components were examined on a Kamouraska clay and a Saint‐André gravelly‐sandy loam. On the clay site, moderate weed management in NT treatments resulted in crop establishment, growth, and yields comparable to those in MP treatments. Over five years, grain yields in NT treatments averaged 2877 kg ha−1, compared with 2870 and 2260 kg ha−1 in MP and CP, respectively. Grain yields were also consistently but not statistically higher in the rotation (3014 kg ha−1) than in the monoculture (2322 kg ha−1). On the loam site, crop establishment, growth, and yields responded negatively to reductions in tillage and weed management intensity. Barley stand establishment and weed control in CP and NT treatments on the loam site were less successful in the monoculture than in the rotation. No‐till grain yields (1494 kg ha−1) were on average 7% lower than CP yields (1608 kg ha−1), and 29% lower than MP yields (2076 kg ha−1). Rotation had no effect on grain yields on the loam site. Findings confirm the potential of conservation tillage to generate sustained yield returns in spring barley cropping systems, provided that proper attention is given to critical aspects of the cropping system, including crop establishment and weed management.

The main study objective was to measure the effects of tillage (moldboard plow, chisel plow, and no-till) and weed management (intensive, moderate, and minimum) on weeds and crops in a spring barley monoculture compared with a spring barley–red clover rotation. The study was initiated in 1987 and conducted at two sites. Residual effects of treatments were measured in a wheat test crop at the loam site in 1994 and at the clay site in 1995–1996. Weed seed bank densities ranged from less than 300 to nearly 30,000 seeds m−2 and plant densities from 30 to 6,000 plants m−2. Seven species were recorded on average per plot aboveground and 10 species per plot in the seed bank. Species number in the seed bank varied little with treatments compared with species numbers aboveground. Crop rotation and tillage had little effect on weed species diversity but affected relative species dominance. The presence and abundance of species was also influenced by their degree of tolerance to the herbicides used in each system. Annual dicots largely dominated in minimum weed management treatments. Their relative importance in each rotation varied with their level of susceptibility to the different postemergence herbicides. Perennials were not exclusively found in reduced tillage systems. The relationship between perennials and tillage was dependent on the response of perennating structures to the type and frequency of soil disturbance. For example, quackgrass dominated in chisel and moldboard plow systems where rhizomes would be frequently fragmented. Field horsetail, also a rhizomatous species, dominated in the monoculture/direct-seeded no-till treatment under minimum weed management. Its absence from the rotation was explained by the regular removal of aboveground biomass during the forage production year. Overall, weed response was regulated by agronomic factors but was largely determined by specific biological attributes and environmental conditions.