No abstract

The occurrence of volunteer canola has been increasing in western Canada. The objective of this study was to determine the canola seedbank additions incurred during crop harvest on commercial farms. Over 2 yr, 35 fields of 15 different producers were sampled after harvest using a vacuum cleaner. The canola seeds were separated from the crop residue and soil, and yield loss, 1,000-seed weight, and seedbank additions were determined for each field. Further information for each field was obtained through a producer survey questionnaire. Average yield losses of 107 kg ha−1 or 5.9% of the crop seed yield were observed. This amounted to seedbank additions of approximately 3,000 viable seeds m−2. The yield losses among producers ranged from 3.3 to 9.9% or 9 to 56 times the normal seeding rate of canola. Even with relatively low persistence rates, seedbanks of this magnitude could result in significant volunteer populations for several years, without further seedbank additions from escaped volunteers.

Summary Secondary seed dormancy has been linked to seedbank persistence of volunteer oilseed rape (Brassica napus) in western Canada. It has been suggested that there is a genetic component to secondary seed dormancy expression in oilseed rape, but little is known of its importance in relation to non‐genetic factors. In a series of experiments we investigated the relative importance of genotype, seed size, time of windrowing and pre‐ and post‐harvest environment on the expression of secondary seed dormancy. We found that genotype contributed between 44 and 82% to the total variation in secondary seed dormancy. A broad range in secondary seed dormancy expression was observed among 16 genotypes examined. Nevertheless, three‐quarters of the genotypes investigated exhibited relatively high potential for the expression of secondary seed dormancy (back‐transformed mean 71% dormant seeds). Seed size contributed 21% to the total variation, while the influence of seed maturity (harvest regime) on secondary seed dormancy expression was negligible. Despite diverging environmental conditions during the four growing seasons spanning these experiments, the influence of pre‐harvest environment on seed dormancy expression was relatively small and ranged from 0.1% to 4.5%. Secondary seed dormancy potential decreased over time during seed storage. This decrease was greatest when seeds were stored at ambient temperatures and least when seeds were stored at −70°C.

Cropping records from 13 organic farms in the eastern Canadian prairies and one in North Dakota (1991 to 1996) were surveyed to determine crop rotation pattern, yields and soil nutrient status. Major crops included cereal grains, forages, and green manure legumes. Organic grain and forage yields averaged from one-half to almost double conventional yields. Soil N, K and S levels on organic farms were generally sufficient; however, levels of available soil P were deficient in several instances. Key words: Crop rotation, weeds, forages, green manure crops

The fundamental role of integrated weed management (IWM) is to provide a source of scientifically based knowledge from which growers can make informed weed-management decisions. The objectives of this article include (1) highlighting the essential knowledge base required for the success of an IWM cropping system, (2) identifying the barriers to acceptance of IWM, and (3) discussing the future research opportunities for IWM. The minimum knowledge base consists of four key components: the effect of tillage on weed population dynamics, the time of weed emergence relative to the crop, the critical period for weed control, and the concept of a harvest window. There are substantial barriers, however, that reduce the willingness of growers to adopt the components of an IWM cropping system. IWM systems can be perceived as unreliable resulting in increased risk to management. No direct economic benefit can be defined clearly nor has there been sustained support for the adoption of IWM. In the future, IWM must change from a descriptive to a predictive science. As new markets evolve for agricultural products, new quality issues will arise that may influence weed management. Environmental auditing of IWM systems in terms of ISO 14000 accreditation, total carbon credits, or energy use will provide an important template from which comparisons of alternative weed-control strategies can be assessed. IWM strategies must be developed to reduce the risk to management and to gain broader support from the crop-protection industry, growers, and government.