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.

In western Canada, little is known about the seedbank ecology of volunteer canola. Therefore, integrated recommendations for the management of this weed are limited. In this study, we investigated the seedbank persistence and seedling recruitment of two spring canola genotype groups with different secondary seed dormancy potentials under contrasting tillage systems. The study was conducted at two locations with different soils in the Mixed Moist Grassland ecoregion of Saskatchewan. A single cohort seedbank was established in 1999 and was followed for 3 yr in successive wheat crops. In a separate laboratory study, the six canola genotypes examined were classified as those with high and those with medium potentials for the development of secondary seed dormancy (HD and MD, respectively). After one, two, and three winters, maximum persistence of 44, 1.4, and 0.2% of the original seedbank was observed among the treatments, respectively. In 2001, HD canola genotypes tended to exhibit 6- to 12-fold greater persistence than MD canola genotypes, indicating lower seedbank mortality in HD canola. Seedling recruitment of HD canola also was higher than MD canola when differences were observed between these genotype groups. Therefore, long-term seedbank persistence of canola can be reduced by growing genotypes with low inherent potential for the development of secondary seed dormancy. The proportion of persisting seeds tended to be higher under conventional tillage than under zero tillage because of lower seedbank mortality, but no clear distinction in seedbank persistence in terms of absolute time could be made between these two tillage systems. Volunteer canola seedling recruitment followed the pattern of a typical summer-annual weed, where seedling emergence was observed only during May and June.

Avena fatua seeds remaining on the plant at harvest and taken into the combine harvester may be dispersed over large areas. The objective of this study was to characterize the development of A. fatua in comparison to spring Triticum aestivum. As part of this objective, the rate of seed shed in A. fatua relative to development of T. aestivum was determined. Avena fatua and T. aestivum had similar phyllochron intervals within locations but differed between locations. Plant development as measured by the Zadoks plant development scale was consistent within plant species between locations. Seed shed in A. fatua was also consistent between locations. Most of the seed shed occurred within 2 wk, and the cumulative seed shed followed a sigmoidal pattern. The seed shed occurred as T. aestivum was ripening, and the percentage of seed shed appears to be related to the water content of the T. aestivum spike. Because of this relationship, the proportion of seed remaining on A. fatua at harvest could be managed by changing the timing of crop harvest.

Structural equation modeling in the plant sciences: An example using yield components in oat. Can. J. Plant Sci. 91: 603Á619. Structural equation modeling (SEM) is a powerful statistical approach for the analysis of complex intercorrelated data with a wide range of potential applications in the plant sciences. In this paper we introduce plant scientists to the principles and practice of SEM using as an example an agronomic field trial. We briefly review the history of SEM and path analysis and introduce the statistical concepts underlying SEM. We demonstrate the use of observed and latent variable structural equation models using a multi-site multi-year field trial examining the effects of seed size and seeding density on the plant density and yield of oat in Saskatchewan. Using SEM allowed for insights that a standard univariate analysis would not have revealed. We show that seeding density has strong effects on plant and panicle density, but has very limited effects on final yield. Plant density has a consistent non-linear effect on panicle density across location that was not affected by precipitation. In contrast, the implicit effect of precipitation on seed number appears to be the main driver for final yield. Incorporating precipitation data into the model demonstrates how mechanistic models can be developed by including in the path diagram variables that would normally treated as random factors in a mixed model analysis. Finally, we provide a guideline to assist plant scientists in determining whether SEM is an appropriate method to be used for the analysis of their data.Can. J. Plant Sci. Downloaded from pubs.aic.ca by University of P.E.I. on 08/11/15For personal use only.Can. J. Plant Sci. Downloaded from pubs.aic.ca by University of P.E.I. on 08/11/15For personal use only.

Combine harvesters have the potential to disperse weed seeds great distances. Reducing this dispersal may be important in an integrated weed management system. The objectives of this study were to determine the distance that wild oat seeds are dispersed by a combine harvester and the effect of chaff collection on combine harvester seed dispersal. This was measured by sampling wild oat seeds at varying distances behind a combine equipped with a removable chaff collection system after it passed through a wild oat patch. Chaff collection consistently reduced the amount and distance that wild oat seeds were dispersed. This occurred because more than 74% of the total wild oat seed that were ejected from the combine were in the chaff. Because most of the chaff falls in a row directly behind the combine, chaff collection only affected dispersal in this area. In 1996, chaff collection reduced wild oat seed dispersal past the wild oat patch to less than 10 seeds m−2at 45 m, whereas without chaff collection, there was greater than 10 seeds m−2up to 145 m. At distances beyond 145 m, chaff collection had no significant effect on seed dispersal. Chaff collection may be an important tool in an integrated weed management program because it may slow weed invasions and reduce the expansion of weed patches.

In western Canada, seasonal seedling recruitment has been reported in weedy canola populations, and seed persistence has been linked to the secondary seed dormancy potential of a genotype. Temperature influences secondary seed dormancy induction in this species. In these experiments we (1) investigated the influence of temperature and osmotic potential on secondary seed dormancy induction in canola, (2) related these to seedbank dynamics and seedling recruitment of two canola genotypes with different seed dormancy potentials in the field, and (3) investigated the influence of residue, burial depth, and soil type on seedbank dynamics and seedling recruitment in the field. In the laboratory, rates of seed dormancy induction were positively correlated to increasing temperatures and water stress. The role of temperature was approximately threefold more important to seed dormancy development than was osmotic potential within the tested ranges of these variables. In the field, seasonal seedbank dynamics of canola buried at 10 cm were strongly influenced by a genotype's inherent potential for secondary dormancy. An increase in the ungerminable portion of the seedbank was observed in the high-dormancy genotype as soil temperatures increased during spring. This did not occur in the low-dormancy genotype, resulting in sixfold less seed persistence in this genotype by midsummer, by which time, the total remaining seedbank was ungerminable in both genotypes. At the 1-cm burial depth, most of the seedbank was depleted by midsummer of the year after seedbank establishment because of high seedbank mortality in all treatments. Thus, the seasonal recruitment behavior in canola was primarily a function of seed death in the shallow seedbank and a shift to an ungerminable state in the deep seedbank.

Ashraf and Abu-Shakra, 1978; Hao and de Jong, 1988). Several studies have indicated that germination is de-Oat (Avena sativa L.) yield and quality on the northern Great layed and reduced by low water potential (Rao and Plains are consistently reduced by frequent drought and wild oat Dao, 1987; Hao and de Jong, 1988), low temperature (Avena fatua L.) competition. Wild oat cannot be selectively removed (Livingston and de Jong, 1990; Willenborg et al., 2004), from oat with herbicides. Identifying genotypes or seed size(s) with high germination potential under moisture stress may facilitate im-high salinity (Romo and Haferkamp, 1987), and combiproved seedling vigor, stand establishment, and crop competitiveness.nations of these factors (Willenborg et al., 2004; Living-Therefore, a germination study was conducted to determine the effects ston and de Jong, 1990; Hao and de Jong, 1988). The of genotype and seed size on the germination of oat seed subjected resulting late and inadequate germination and subseto moisture stress. Large, medium, and small seeds of six common quent seedling establishment critically affect crop prowestern Canadian oat genotypes were germinated in polyethylene duction this region (Livingston and de Jong, 1990). glycol (PEG 8000) solutions with initial osmotic potentials rangingOat is one of the major economic grain crops grown from 0 to Ϫ0.4 MPa at 5؇C. Generalized linear mixed models fit to in the northern Great Plains region, planted annually the data provided a statistically valid, appropriate, and convenient on 1.7 million hectares of land in western Canada alone method to analyze germination data. In all genotypes examined, de-(Statistics Canada, 2004). Oat is a crop that normally creasing seed size and osmotic potential increased median germination time (MGT) and lowered final germination percentage (FGP). Among produces seed of varying size as a result of the multigenotypes, CDC Bell had the fastest MGT while AC Mustang had floret habit of the oat spikelet (Doehlert et al., 2002, the highest FGP. Delays in MGT and reductions in FGP resulting 2004). Oat seed size is inherently nonuniform because oat from increased moisture stress were similar to those observed in other may produce one, two, or three seeds per spikelet. The cereals, suggesting that oat may be as capable of germinating under innermost seed, called the primary seed, is the largest low spring soil moisture conditions as wheat and barley. The results and seed size and mass decrease with increasing seed of this study also indicate that large seed of genotypes such as AC order (Doehlert et al., 2002). This relationship results Mustang and CDC Bell appear better suited to germinate under the from reduced sink strength and photoassimilate acquirange of osmotic potentials in this study. Although differences in sition of higher order seeds (Palagyi, 1983; Doehlert et MGT and FGP between seed sizes in this study were statistically al., 2002).significant, they were generally small and thus, oat germination characteristics may not be...