The development of integrated weed management strategies requires knowledge of mechanisms that influence compositional changes in weed flora. A 9‐year study was initiated in 1988 at Delhi, Canada, on a loamy sand soil to evaluate the effect of tillage systems [conventional (CT) and no‐till (NT)] and cover crops (only in NT) on weed density, species composition and associations, and crop yield in a winter wheat (Triticum aestivum L.)/bean/winter wheat rotation. Three bean types: soyabean (Glycine max L. Merr.), white bean (Phaseolus vulgaris L.) and kidney bean (P. vulgaris L.) were included. The NT system included variations: rye (Secale cereale L.) or maize (Zea mays L.) cover crop, volunteer wheat disked after harvest and wheat stubble. Data were collected in 1994, 1995 and 1996. Tillage systems, cover crops and crop type had differential effects on weed densities, species composition and associations. Weed densities were not affected by tillage or cover crops in wheat but, in the beans, densities were greater in the CT than in the NT systems. Various associations of weed species with tillage system, cover crop and crop type were observed. Crop yields were not affected by tillage type or cover crop, except that soyabean yields were highest in plots with cover crops.

The adoption of conservation tillage systems has been challenged by concerns potential weed species shifts. A 9-yr study from 1988 to 1996 was conducted Delhi, Ontario, on a loamy sand soil to evaluate the effect of tillage systems (conventional [CT] and no-till [NT]), cover cropSecale cereale, and nitrogen (N) rate (0, 50, 100, 125, 150, and 200 kg N ha−1) on monocropZea maysL. (corn) yield and changes in the composition of the weed flora. CT consisted of spring moldboard plowing followed by cultivation with a tooth cultivator. Weed counts were taken in the last 3 yr of the study (1994, 1995, and 1996) prior to postemergence herbicide application and then again 2 to 3 wk after herbicide treatment. Composition or the weed flora was analyzed by canonical discriminant analysis (CDA). The relationship between weed density and tillage system was not consistent. Weed species composition differed between CT and NT systems.Chenopodium albumandAmaranthus retroflexuswere associated with CT andDigitaria sanguinaliswith NT. N rate and cover crop did not affect weed density or species composition. Proper management of weeds with herbicides appeared to minimize any long-term effect on the weed flora resulting from varying N rates.Zea maysyields did not differ between CT and NT systems but were greater in both systems with a cover crop at the higher N rates. Disturbance caused by tillage was more important than N rate and cover crop as a mechanism influencing composition of the weed flora.

Influence of tillage type on vertical weed seedbank distribution in a sandy soil. Can. J. Plant Sci. 80: [455][456][457]. The vertical distribution of weed seeds in the seedbank of a sandy soil under three tillage systems (moldboard plow, chisel plow, and no-till) was estimated by a seedling-emergence method. The vertical distribution of the weed seedbank differed with tillage type and depth of tillage. The no-till system had the largest portion (90%) of the seedbank in the 0-to 5-cm layer. Chisel plowing distributed most of the seeds (66%) in the 5-to 10-cm layer. Moldboard plowing concentrated 71% of the seeds at the 10-to 15-cm depth. Our results suggest that the vertical distribution of the weed seedbank will be influenced by tillage type, depth of tillage, and soil type.Key words: Soil structure, moldboard plow, chisel plow, no-till Swanton, C. J., Shrestha, A., Knezevic, S. Z., Roy, R. C. et Ball-Coelho, B. R. 2000. Influence du régime de travail du sol sur la répartition verticale du stock semencier dans un sol sableux. Can. J. Plant Sci. 80: 455-457. Nous avons évalué, d'après le taux de levée observé dans des carottes de sol prélevées de diverses tranches de profondeur, la répartition verticale du stock de semences de mauvaises herbes dans un sol sableux. La répartition verticale de ce stock différait selon le régime de travail du sol et selon la profondeur de la tranche travaillée. La plus grosse portion du stock semencier adventice se retrouvait dans les 5 cm supérieurs (90 %) dans le régime de semis direct, dans la tranche de 5 à 10 cm (66 %) avec le labour au chisel et dans celle de 10 à 15 cm (71 %) avec le labour à la charrue à socs. Il découle de ces observations que la répartition du stock semencier de mauvaises herbes varie selon le mode de travail du sol, la profondeur de la tranche travaillée et aussi le type de sol.

The root system of a sugar cane crop on an Ultisol in northeastern Brazil was examined throughout the plant and first ratoon crop cycles, using both coring and minirhizotron methods. Total root masses (living plus dead, 0.9-1.1 kg m -2) and live root lengths (14.0-17.5 km m -2) were greater during the ratoon cycle than at the end of the plant cane cycle (0.75 kg m 2 and 13.8 km m -2, respectively ). Root die-back during the two weeks following ratoon harvest was estimated to be 0.15 kg m -, about 17% of the total root mass. Root die-back after the plant cane harvest was lower because fire was not used at this harvest and soil humidity was higher under the accumulated litter. A small amount of fine roots proliferated in the litter layer, amounting to 1% of the total mass and 3% of the total length. Root turnover could not be accurately assessed from minirhizotron observations due to variation in the relationship between coring data and the minirhizotron data with both time and soil depth.

Sugarcane (Saccharum spp.) production systems commonly include preharvest or postharvest burning to dispose of residue. In northeastern Brazil, no‐burn mulching practices are being introduced as a labor‐intensive but environmentally more friendly alternative. We examined the effect of burning crop residues on sugarcane yield and C, N, and P cycles. Carbon, N, and P losses during preharvest burning were measured for a sugarcane first ratoon crop. Postharvest burn and mulch treatments were compared for a plant crop grown on an Oxic Haplustult soil. During the preharvest burn of the first ratoon crop, 2600 kg C ha−1, 17 kg N ha−1 and 1 kg P ha−1 were lost by convection. The postharvest burn of the plant crop residue resulted in losses of 4800 kg C ha−1 and 42 kg N ha−1; P losses were undetectable in the burn. The postharvest burn of the plant crop residue had no significant effect on total C or NH4‐N plus organic N in soil during the first ratoon crop cycle. Soil pH in the top 7.5 cm layer of soil did not change after the postharvest burn, but increased 1.1 units in the top 1‐cm layer. In the unburned mulch treatment, the litter layer did not reduce tillering or N and P uptake. Harvestable cane yield of the first ratoon crop was 17 Mg ha−1 (wet wt.) greater in the mulch than the burn treatment (54 vs. 37 Mg ha−1). This yield response was attributed to increased soil water retention and reduced weed growth under the mulch. In this short‐term trial, mulching proved to be an alternative to the traditional burn system.