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Published online 31 May 2007
Published in Crop Sci 47:1167-1176 (2007)
© 2007 Crop Science Society of America
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CROP ECOLOGY, MANAGEMENT & QUALITY

The Weed-Competitive Ability of Canada Western Red Spring Wheat Cultivars Grown under Organic Management

Heather E. Masona, Alireza Navabia, Brenda L. Frickb, John T. O'Donovanc and Dean M. Spanera,*

a Dep. of Agricultural, Food and Nutritional Science, Univ. of Alberta, Edmonton, AB, Canada T6G 2P5
b Dep. of Plant Sciences, Univ. of Saskatchewan, 51 Campus Dr., Saskatoon, SK, Canada S7N 5A8
c Agriculture and Agri-Food Canada, Lacombe Research Centre, 6000 C & E Trail, Lacombe, AB, Canada T4L 1W1

* Corresponding author (dean.spaner{at}ualberta.ca).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Competition from weeds can reduce grain yields in both conventional and organic systems. Plant height, tillering, and elevated photosynthetically active radiation interception are some of the traits thought to help confer competitive ability in cereal grains. Crop cultivars developed before the advent of modern, high-input agriculture may be better suited to lower soil nutrient levels and elevated weed competition. Twenty-seven spring bread wheat (Triticum aestivum L.) cultivars, representing 114 yr of Canadian wheat breeding, were grown at conventionally and organically managed sites in north central Alberta over a 3-yr period. Average conventional yields were 63% greater than organic yields, and average overall weed biomass was significantly greater under organic management. Earlier flowering and maturity were more important for achieving high grain yield in organic fields than in conventional fields. Greater numbers of spikes m–2 were associated with increased grain yield in organic fields but were not in conventional fields. In organic fields, increased plant height and early maturity were associated with reduced weed biomass, while strong early season vigor was related to increased yield, increased spikes m–2, and reduced weed biomass. A competitive crop ideotype for organically grown spring wheat in northern growing regions of the Canadian Prairies should include taller plants, with fast early season growth, early maturity, and elevated fertile tiller number.

Abbreviations: CWRS, Canada Western Red Spring • ERS, Edmonton Research Station • ZGS, Zadoks growth stage.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
ALTERNATIVE FARMING strategies are emerging because of increased concern over high herbicide and fertilizer use in agriculture systems worldwide. One such strategy is organic farming, a system of production that prohibits, among other things, the use of mineral fertilizers, synthetic pesticides, and genetically modified organisms (Bruinsma, 2003). In organic management systems, grain yields are commonly less than their conventionally managed counterparts (Walker and Smith, 1992; Entz et al., 2001; Kitchen et al., 2003; Ryan et al., 2004). In Canada, where research relating to organic grain production is limited to date, Entz et al. (2001) reported that wheat (Triticum aestivum L.), oat (Avena sativa L.), and barley (Hordeum vulgare L.) yields were 23 to 27% less on organic farms than on conventional farms.

Competition from weeds plays a role in reducing crop yields on organic farms. Studies in Canada and elsewhere have reported both greater numbers of weeds and greater diversity of weed species in organic cereal crops than in conventional ones (Samuel and Guest, 1990; Leeson et al., 2000; Entz et al., 2001). Nutrient limitation also serves to reduce organic crop yields (Barberi, 2002). On the Canadian Prairies, soil nutrient levels on organically managed soils were reported to be similar to or less than those of conventionally managed soils (Entz et al., 2001). In an attempt to overcome these production constraints, producers make use of farming practices such as crop rotations, changes to planting dates and density, intercropping, the use of animal and green manures, and varietal selection (Stopes and Millington, 1991; Barberi, 2002).

Breeding efforts in the Canada Western Red Spring (CWRS) class of wheat have focused mainly on developing high-protein, disease-resistant cultivars with broad adaptation. Other agronomic characteristics, such as the ability to compete against weeds (measured as weed suppression and/or weed tolerance) have been largely unaddressed in CWRS wheat breeding, likely in part due to the availability of effective herbicides over the past 50 yr.

A number of studies have found differential competitive ability of genotypes or cultivars of wheat (Wicks et al., 1986; Huel and Hucl, 1996; Lemerle et al., 1996). Yield gains of 7 to 9% have been reported for "competitive" wheat cultivars when compared to "noncompetitive" cultivars (Hucl, 1998). Morphological, physiological, and biochemical traits are thought to control plant competitiveness (Baghestani et al., 1999; Iqbal and Wright, 1999; Lemerle et al., 2001a). Many studies have been conducted to determine which characters confer competitive ability in wheat. Studies examining aboveground morphology and physiology are most common, likely due to the ease associated with the selection for competitiveness based on visual characteristics. Greater tiller numbers, taller plants, elevated photosynthetically active radiation interception, and greater early season biomass accumulation were all found in the most competitive genotypes in a study of wheat genotypes (mainly Australian) from around the world (Lemerle et al., 1996). Crop height, crop biomass, ground cover, and flag leaf length of wheat were found to be negatively correlated with wheat yield reduction in Canadian wheat cultivars (Huel and Hucl, 1996). Hucl (1998) found that competitive wheat genotypes were taller and had greater tiller numbers compared with noncompetitive cultivars. For producers, knowledge about the competitive ability of cultivars would be useful for choosing cultivars suited to their environment (Lemerle et al., 2001b).

Some researchers question the value of using crop cultivars developed for low-stress, high-input production in higher-stress, low-input environments, such as organic systems (Laing and Fischer, 1977; Ceccarelli, 1996). It has been hypothesized that wheat cultivars developed before the advent of modern, high-input agriculture may be better suited to lower soil nutrient levels and elevated weed competition (Poutala et al., 1993). Researchers have reported that modern crop cultivars are better yielding under optimal conditions, yet suffer greater yield losses than ancestral cultivars when grown under stress conditions (Laing and Fischer, 1977; Ceccarelli, 1996; Guarda et al., 2004). In contrast, modern wheats of the UK out-yielded older cultivars in both weedy and weed-free environments (Vandeleur and Gill, 2004). In terms of the relative performance of historical and modern wheats subjected to stress, very few studies have been conducted in Canada. Hucl and Baker (1987) found that drought conditions had a greater negative impact on yield of the older Canadian wheat cultivars Red Fife and Marquis than on newer wheat cultivars, possibly due to the timing of drought stress in relation to the rate of plant development.

Much of the Canadian wheat competition research has involved the use of cultivated crop plants like A. sativa L., Brassica juncea L., and B. napus L. (Huel and Hucl, 1996; Weiner et al., 2001) or sown densities of wild oat (A. fatua L.) (Kirkland and Hunter, 1991) as weed analogs. While stressing the need for repeatable trials, Huel and Hucl (1996) suggested that more research was needed to investigate the effect of natural weed populations on the ranking of cultivars found to be competitive when tested in controlled environments. In addition, there has been virtually no attempt by Canadian researchers to investigate wheat competition with weeds on organic farms or to compare the varietal performance of wheat under conventional and organic management systems.

The objectives of the present study were to determine whether spring bread wheat cultivars exhibit different capabilities when grown under organic and conventional management, and to establish which, if any, agronomic traits affect the competitive ability of Canadian spring wheat cultivars in the two systems. Through this research, we sought to describe a competitive spring bread wheat ideotype for northern organic wheat production systems on the Canadian Prairies.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Twenty-seven Canadian spring wheat cultivars (Table 1) representing 114 yr of Canadian wheat breeding were grown under both conventional and organic management systems. Field trials were conducted at two locations in 2002, and at four locations in each of 2003 and 2004. In all 3 yr, the trial was conducted at the Edmonton Research Station (ERS), Edmonton, Alberta (53° 34' N, 113° 31' W), on paired sites, one organically managed and one conventionally managed, located approximately 1 km apart. In 2003 and 2004, the trial was also conducted in conventionally managed fields at the Alberta Agriculture, Food, and Rural Development Field Crop Development Centre Research Farm in Lacombe, Alberta (52° 28' N, 113° 44'W), as well as at a certified organic farm near New Norway, Alberta (52° 52' N, 112° 56' W). Soils at New Norway sites were Eluviated Black Chernozems (Albic Argicryolls), while soils at Edmonton and Lacombe sites were classified as Orthic Black Chernozems (Typic Haplustolls), typical of central Alberta (Alberta Agriculture, Food, and Rural Development, 2004).


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  		Table 1. Description of Canada Western Red Spring wheat (Triticum aestivum L.) cultivars included in trials conducted in 2002, 2003, and 2004 at four sites in north-central Alberta, Canada.

 
Each of the seven trials was arranged in a randomized complete block design with four replications. All plots were seeded at a rate of 300 viable seeds m–2. At the ERS in 2002 and at the ERS and the certified organic farm in 2003, plot dimensions were 6 m x 0.9 m and consisted of four rows spaced approximately 23 cm apart. Plots were seeded using a four-row, double-disk drill (Fabro Enterprises Ltd., Swift Current, SK). In 2004, plot dimensions at ERS and the certified organic farm were 4 m x 1.38 m, consisting of six rows spaced approximately 23 cm apart. Plots were seeded using a six-row, no-till double-disk drill (Fabro Enterprises Ltd., Swift Current, SK). At Lacombe in 2003 and 2004, plot dimensions were 4.5 m x 1.12 m, consisting of eight rows spaced approximately 14 cm apart. Seed used each year was increased the previous year at the ERS. The trials were not irrigated. All trials were planted in mid- to late May and harvested in early to mid-September. Precipitation and temperature data for each year and location are presented in Table 2. Climate data for the certified organic farm at New Norway was taken from the nearest provincial weather station, located approximately 20 km away, at Camrose, Alberta.


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  		Table 2. Mean monthly precipitation and temperature data for the 2002–2004 growing seasons at Edmonton, Camrose, and Lacombe, AB, Canada.

 
Conventional sites were managed according to local recommendations (Alberta Agriculture, Food, and Rural Development, 2003, 2006). Mineral fertilizers were applied following soil fertility testing in early spring. At the 2003 ERS-conventional site, fertilizer (90 kg ha–1 N as urea and 28 kg ha–1 P as 8-24-24) was broadcast after seeding. At the 2004 ERS-conventional site, fertilizer (39 kg ha–1 N as urea) was banded at a depth of 8.5 cm into the soil in the fall of 2003 and again in spring 2004 (11 kg ha–1 N as urea and 6 kg ha–1 P as 8-24-24) before seeding. Soil tests were conducted after seeding each year at each site (Table 3). Soil samples were taken from random points in and among experimental plots and were taken after seeding to obtain an estimate of available soil nutrients at the start of the growing season. All ERS-conventional fields were cultivated and harrowed before planting in spring and received late-spring applications of MCPA Amine 500 (present as dimethyl amine) (Nufarm Agricultural, Inc., Calgary, AB) at a rate of 1.5 L ha–1 to control broadleaf weeds (Alberta Agriculture, Food, and Rural Development, 2006). In both years, the Lacombe sites received applications of seed banded 6-25-30 at 112 kg ha–1 and late-spring applications of Refine Extra (thifensulfuron-methyl/tribenuron)/CurtailM (clopyralid + MCPA ester [present as 2-ethyl hexyl ester]) (DuPont Canada Agricultural Products, Mississauge, ON) at 19 g ha–1 & 1.5 L ha–1 to control broadleaf weeds. Soil testing was not performed at Lacombe in either year.


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  		Table 3. Soil properties of 0- to 15-cm depth soil samples taken at experimental sites at the Edmonton Research Station, Edmonton, AB, and the certified organic farm at New Norway, AB, in the 2003 and 2004 growing seasons directly after seeding.

 
Organically managed sites did not receive any applications of chemical fertilizers and herbicides and were managed according to Organic Crop Improvement Association International Certification Standards (Organic Crop Improvement Association, 2000). Soil tests were conducted after seeding each year at each organic site (Table 3). The ERS-organic sites were designated to be organically managed in the spring of 2001 but were not certified. The 2003 ERS-organic site was planted to fall rye (Secale cereale L.) in the fall of 2001, which was mowed throughout the summer of 2002. The vegetative fall rye was disked under in the fall of 2002, just before an application of composted dairy manure at a rate of 60 t ha–1. In the spring of 2003, the land was cultivated and harrowed just before seeding of the 2003 trial. The 2004 site was left to triticale stubble in the fall of 2001 and was seeded with berseem clover (Trifolium alexandrinum L.) in the spring of 2002. Extreme drought across the Canadian Prairies in the 2002 growing season caused the clover crop to fail, and the land was seeded to fall rye in late summer of 2002. In the summer of 2003, the fall rye was harvested, and the soil was disked and treated with an application of composted dairy manure at a rate of 60 t ha–1. In the spring of 2004, the land was cultivated and harrowed before planting. Composted dairy manure was estimated to contain approximately 50% dry matter and 13 g kg–1 total N.

At the certified organic farm, experimental trials followed cereal-legume plowdowns without crop removal in the year before planting. The 2003 trial was planted at a site that received a green manure plowdown in 2002 and 2001 and was seeded to barley in 2000 and oats in 1999. The 2004 site received a green manure plowdown in 2003 and was seeded to a pea/barley intercrop in 2002. The 2003 certified organic farm trial was lost to cow grazing in late June, allowing only a modest amount of data to be collected; the 2003 certified organic farm data is therefore not included in the subsequent analyses and discussion.

Disease assessments were conducted at all site-years, and very mild and sporadic incidences of powdery mildew (Erysiphe graminis) and stripe rust (Puccinia striiformis) were observed, with no apparent differences between sites or years. Because of the low and infrequent disease incidence, it was determined that no control measures were necessary.

Data Collection
Emerged seedlings were counted in one 1-m row per plot at the one- to three-leaf stage (Zadoks growth stage [ZGS] 11–13) (Zadoks et al., 1974). Early season vigor was rated at the 3- to 4-leaf stage (ZGS 13–14). Early season vigor was based on plant leaf size, number, and overall form on a scale of 1 to 5, with 1 being the least vigorous and 5 the most (Revilla et al., 1999).

Spike emergence (heading) was recorded as the day when 75% of the emerged spikes in the plot had visible peduncles. After stem elongation was complete, plant height (representing the distance from the soil surface to the tip of the spike, excluding awns in awned cultivars) was recorded on a per plot basis. At early dough development (ZGS 80), incidence of both powdery mildew (Erysiphe graminis; rating from 1 to 5) and leaf spot diseases (mainly Septoria tritici; 1 = no disease incidence to 9 = highest incidence of disease) was recorded. Maturity was recorded as the day when 75% of the spikes and peduncles in the plot were brown, which was estimated to be approximately 30% seed moisture content. At maturity, all spikes in a 1-m row section of each plot were counted and used to calculate spikes m–2.

In 2002, weed presence at the time of grain harvest was negligible, due to existing drought conditions in that year. In 2003 at the ERS, weed biomass m–2 in each plot was determined by separating and weighing the aboveground portion of the weeds from a harvested 1-m x 0.23-m row of wheat at maturity. In 2004 at the ERS and the certified organic farm, weed biomass m–2 in each plot was determined by collecting the aboveground portion of weeds from within a randomly placed 0.0625-m2 quadrat at harvest maturity.

Plots were harvested using a Wintersteiger plot combine following maturity. Grain yield was recorded on a dry weight basis. Harvested grain samples were dried at 60°C for ~24 h, and weed seeds were removed using a 2-mm mesh sieve (Canadian Standard Sieve Series No. 10). At the certified organic farm in 2004, there was a substantial wild oat presence, requiring that plot harvests be cleaned using a Vac-A-Way Seed Cleaner with a No. 12 screen (Hance Corp., Westerville, OH). Plot grain yield at that site was then determined by weighing the cleaned sample. Hectolitre weight was determined by weighing a 1-pint (473 mL) subsample of plot yield, except in 2002 when drought conditions reduced yields, allowing only a 60-mL subsample to be used.

Data Analysis
All data were subjected to analysis of variance (Steel et al., 1996). For each location-year (environment), analysis of variance was performed using the PROC GLM procedure of SAS (SAS Institute, 1999). For each management system, an analysis of variance using PROC GLM (fixed effects) was used to obtain the percent sums of squares breakdown, providing information about relative sources of variation.

A combined analysis of all data was performed using the PROC MIXED procedure of SAS to determine differences between management systems (SAS Institute, 1999). Management system, cultivar, and the management system x cultivar interaction were considered fixed effects, while environment within management system, replication within environment, and associated interactions were considered random. Subsequently, analyses of variance within each management system (i.e., conventional, organic) were performed using the PROC MIXED procedures of SAS, where location-years (environments), replications within environment, and the environment x cultivar interaction were considered random. Cultivar was considered as a fixed effect because cultivars were selected so as to adequately represent 114 yr of CWRS wheat breeding. In addition to including some of the most important CWRS wheats released in Canadian history (e.g., Red Fife, Marquis, ‘Thatcher’), the selected collection of cultivars contain representatives from each decade from the 1880s through 1990s, with the exception of the 1950s. Pearson's coefficients of correlation were computed within each management system using the least-squares means from each of the environments (location-years) with the PROC CORR procedure of SAS.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
In 2002, a provincewide drought reduced grain yield, with average yields of 0.8 t ha–1 at ERS-conventional and 1.2 t ha–1 at ERS-organic. At the conventional site, grain yield ranged from 0.4 to 1.2 t ha–1, and at the organic site, grain yield ranged from 0.8 to 1.5 t ha–1. Overall yields in that year were reduced by a factor of ~4 in conventional plots and by a factor of ~2 in organic plots compared to the overall average (averaged over years) for each system (data not shown). Cultivars did not differ for yield (p {approx} 0.30) under conventional management; however, under organic management, cultivars did differ for grain yield (p < 0.01). Due to the high variability of the 2002 season, we have removed the 2002 data from the overall analyses, and they are not included in the subsequent analyses and discussion. The grain yields achieved in 2002 were well below the long-term average and would be considered crop failures.

In 2003, grain yield averaged 3.7 t ha–1 at Lacombe, 4.3 t ha–1 at ERS-conventional, and 3.7 t ha–1 at ERS-organic. In 2004, grain yield averaged 5.6 t ha–1 at Lacombe, 3.5 t ha–1 at ERS-conventional, 1.2 t ha–1 at the certified organic farm, and 2.9 t ha–1 at ERS-organic. Percent sum of squares breakdown indicated that environment was the largest source of variation for most traits, particularly grain yield and days to maturity (data not shown). When analyzed in fixed effects models within each management system, environment x cultivar effects were significant for most traits; however, the percentage of variation attributed to that interaction was small (≤16%) for all traits (data not shown). Thus environmental effects were a large source of variation but environment x cultivar interactions were not. Disease incidence was low, and preliminary analyses showed no significant differences in disease incidence among environments or cultivars; thus disease data is not included in the subsequent results and discussion.

Differences were observed in the performance of CWRS wheat in conventional and organic management systems (data not shown). Grain yield (p = 0.07) and weed biomass (p = 0.06) differed significantly between management systems, with conventional yields 63% greater than organic yields, and an average weed biomass of 134 g m–2 under organic management and 1.4 g m–2 under conventional management (Tables 4 and 5). Weeds present in both 2003 and 2004 at ERS included stinkweed (Thlaspi arvense L.), common lambsquarters (Chenopodium album L.), wild buckwheat (Polygonum convolvulus L.), shepherd's purse (Capsella bursa-pastoris L.), and Canada thistle (Cirsium arvense L.), while weeds in both years at the New Norway site were mainly wild oats (A. fatua L.) and common lambsquarters. Plant emergence, early season vigor, time to heading and maturity, plant height, and spikes m–2 were not significantly different between the two systems. A significant (p < 0.01) management x cultivar interaction for weed biomass was detected. To further investigate each management system more thoroughly, separate analyses for each management system were performed. Data are presented by management system (i.e., organic, conventional).


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  		Table 4. Conventional management overall least-squares means for grain yield, days to heading, days to maturity, plant height, spikes per m–2, and weed biomass of cultivars grown in 2003 and 2004, at four site-years in north-central Alberta, Canada, and arranged in descending order of grain yield.{dagger}

 

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  		Table 5. Organic management overall least-squares means for grain yield, days to heading, days to maturity, plant height, spikes m–2, and weed biomass of wheat (Triticum aestivum L.) cultivars grown in 2003 and 2004, at three site-years in north-central Alberta, Canada, and arranged in descending order of grain yield.{dagger}

 
For conventional management, the best-yielding 8 cultivars out of 27 were not found to be significantly different for grain yield based on the Fisher-protected LSD test, while 21 of the 27 cultivars under organic management were found in the first LSD grouping for that trait (Tables 4 and 5). Cultivars found to be among the top 5 yielding cultivars in their respective management systems were ranked among the top 10 yielding cultivars in the opposing management system, with the exception of Garnet. Garnet was the fifth highest yielding cultivar under organic management and the second lowest yielding cultivar under conventional management.

Correlations of Competitive Traits in Conventional and Organic Management Systems
Early season vigor and time to heading and maturity were similarly associated with yield in the two management systems, though the association between yield and time to maturity was somewhat stronger under organic management than under conventional management (Table 6). Spikes m–2 and yield were strongly positively correlated at organic and negatively correlated at conventionally managed sites. Height and yield were found to be correlated at conventional and organic sites. Yield and weed biomass were strongly negatively correlated in organic fields.


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  		Table 6. Least-squares mean (based on environment within management system) genotypic correlations of eight agronomic traits for 27 wheat (Triticum aestivum L.) cultivars grown at four conventionally (n = 108) and three organically (n = 81) managed sites during 2003 and 2004 in north-central Alberta, Canada.{dagger}

 
Because of herbicide use in the conventional system, weed biomass under conventional management was not found to be significantly correlated with any of the competitive plant traits. However, under organic management, weed biomass was positively correlated with maturity and negatively associated with height and spikes m–2 (Table 6).

Time to heading and number of spikes m–2 were found to be negatively correlated in organic, but not in conventional systems (Table 6). Maturity and spikes m–2 were positively correlated under conventional and were negatively correlated under organic management. Spikes m–2 and height were strongly negatively correlated under conventional management and were not correlated under organic management.

Early season vigor and spikes m–2 were positively correlated in organic fields but were not correlated in conventional fields. Early season vigor and maturity were more strongly negatively correlated in organic fields than in conventional fields. Time to heading and maturity were strongly positively correlated under conventional management but were not associated under organic management (Table 6).


   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Following the 2002 drought and resulting crop failure, precipitation levels in the north-central Alberta region increased in the subsequent two growing seasons, almost reaching the 30-yr average in Camrose and Lacombe in 2004 and surpassing it in Edmonton in the same year. Regardless, average grain yields were notably low at the certified organic farm and at ERS-organic site in 2004, likely a result of a combination of intense weed competition and lower nutrient levels (Table 3), particularly at the certified organic farm.

Similar to other reports (Walker and Smith, 1992; Entz et al., 2001; Kitchen et al., 2003; Ryan et al., 2004), the mean yield of cultivars grown under conventional management was greater than that of cultivars grown under organic management. This may be due to increased stress under organic management caused by nutrient limitation and, more importantly, weed competition. While the soil nutrient and moisture status at the various organic sites was variable (Tables 2 and 3), overall mean weed biomass at organic sites (134 g m–2) was much greater than at conventionally managed sites (1.4 g m–2).

According to LSD groupings, cultivars yielded more similarly under organic management than under conventional management. This may be due to the increase in crop stress associated with organic crop management, likely resulting from reduced nutrient availability (Table 3) and elevated weed competition. Another study similarly reported no significant varietal differences in grain yield of spring wheat in an ecological cropping system, whereas significant differences were detected between the same cultivars grown in a conventional system (Poutala et al., 1993). The lower yield potential of organic systems highlights the need for the development of cultivars with increased performance within such systems.

With most of the cultivars, there were no clear indications that some were more suitable for organic management systems than others (Tables 4 and 5). Some cultivars (e.g., AC Intrepid, Sinton) performed well in both systems, while others (e.g., Red Fife, Chester) performed relatively poorly in both systems. The cultivar Garnet was an exception, yielding comparatively greater in the organic system and less in the conventional system. Similarly, an eastern Canadian study reported that while AC Walton typically out-yielded AC Barrie in conventional cultivar trials, AC Barrie out-yielded AC Walton under organic management (Nass et al., 2003). Collectively, these results indicate that there may be some cultivars more suited for production in organic compared to conventional management systems.

The negative relationship between yield and time to maturity observed in both systems indicates that early maturing cultivars are appropriate for use in northern wheat cropping systems, both conventional and organic. Because delayed seeding is a common practice among organic wheat producers as a means of weed control, early maturing cultivars would be particularly desirable in northern regions. Poutala et al. (1993) reported that an early maturing cultivar (Satu) performed well in an ecological cropping system, contrary to suggestions that early maturing cultivars are not well suited to northern ecological systems. Early maturing cultivars, which develop more quickly than later maturing cultivars (Karimi and Siddique, 1991), may have greater nutrient demands early in the season than their late maturing counterparts. Cooler spring temperatures associated with northern climates may reduce and/or delay the release of plant available nitrogen (Agehara and Warncke, 2005).

Number of fertile tillers may be an indicator of competitive ability in organic systems, as spikes m–2 and yield were positively correlated in organic management and were negatively correlated in conventional management. Tillering capacity and spikes m–2 have been previously reported to be associated with competitive ability, although in experiments conducted in conventionally managed fields (Hucl, 1998).

The stronger association between height and yield at the conventional sites compared to the organic sites suggests that height alone may not be a good indicator of competitive ability in organic systems. Since overall average plant height remained similar between the two management systems, the negative correlation between weed biomass and plant height in organic fields implies that weed biomass decreased as height increased, suggesting that height does help to suppress weeds. In previous studies, plant height was associated with competitive ability in both conventional (Huel and Hucl, 1996; Lemerle et al., 1996; Hucl, 1998) and organic systems (Gooding et al., 1993).

The strong negative correlation between yield and weed biomass observed here in organic fields has been reported in many studies in both organic (Ryan et al., 2004) and conventional fields (Lemerle et al., 1996; Hucl, 1998). The often-studied association between increased weed biomass and reduced yield can be explained by competition for growth-limiting resources (i.e., light, water, nutrients) between weeds and crop plants.

The positive correlation between weed biomass and time to maturity of cultivars in organic fields indicates that weed growth was higher in cultivars with increased time to maturity. Thus, it may be desirable for organic wheat producers to use early maturing cultivars to reduce weed biomass in the field. A competition experiment in Sweden using wheat breeding lines from an organic breeding program reported no significant correlation between weed biomass and wheat maturity (Bertholdsson, 2005).

The negative correlation between weed biomass and spikes m–2 under organic management may indicate that weed growth was suppressed by cultivars with high fertile tiller number. That the overall average number of spikes m–2 in organic fields was 8% less than in conventional fields suggests that competition with weeds reduced the number of spikes m–2, which is again supported by the findings of various studies (Kirkland and Hunter, 1991; Lemerle et al., 1996; Hucl, 1998).

The negative correlation between spikes m–2 and height at conventional sites that was not seen at organic sites may reflect the trend toward reduced height and greater kernels per unit area in Canadian CWRS breeding (McCaig and DePauw, 1995). High weed populations present at the organic sites may have altered this relationship. In the current experiment, negative associations were observed between spikes m–2 and both time to heading and maturity at organic sites. Spikes m–2 and time to heading were not associated under conventional management, while spikes m–2 and time to maturity were positively correlated under conventional management. The combination of high tiller numbers and weed competition in organic fields may have increased the rate of development of certain cultivars. Increasing plant densities have increased the rate of maturation in winter wheat (Gooding et al., 2002), and although Hameed et al. (2003) reported no significant effect of seeding rate on time to heading or maturity, they did observe significant decreases in time to heading and maturity in plots receiving no N fertilizer. We observed that time to heading and maturity were strongly correlated under conventional management and were not correlated under organic management. The dynamics of weed development, timing of competitive stress in terms of crop development, and resource limitation likely weaken the relationship between heading and maturity in organic fields.

Early season vigor and yield were positively correlated in both management systems; however, early season vigor was positively correlated with spikes m–2 and negatively correlated with weed biomass in organic fields, but not in conventional fields. Further, there was a stronger correlation between early season vigor and maturity in organic fields than in conventional fields. Early season vigor may be more important in organic fields than in conventional fields, allowing a plant to develop more tillers and reach maturity faster, thereby reducing the effects of weed competition and enabling a plant to maintain yield in weedy conditions. In support of these findings, another study reported that early vigor, as measured by early wheat biomass, was negatively associated with weed biomass (Bertholdsson, 2005). Regression models used in that study predict that a 20% increase in early biomass would reduce weed biomass in wheat by 15 to 39% (Bertholdsson, 2005).


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
Organic and conventional management systems differed greatly in terms of weed biomass, which in turn decreased overall wheat grain yield. From observations made at the experimental sites and from the results of soil testing, it is likely that moisture and nutrient availability in some organic location-years were somewhat limiting as well. Spring wheat cultivars performed differently in the two management systems. Differences among cultivars were more pronounced in the conventional system, probably because genetic differences were expressed to a greater extent in the absence of stresses associated with weeds and low soil nutrient status in the organic systems. Modern cultivars, typically selected in high-yielding environments, may be more responsive to inputs than older cultivars and may or may not perform poorly in low-yielding environments (Calderini and Slafer, 1999). When grown in low-yielding environments, barley cultivars selected at high-yielding sites yielded up to 49% less than barley cultivars selected at low-yielding sites (Ceccarelli et al., 1992).

This study identified traits that have the potential to improve wheat competition with weeds and result in better grain yields in organic production systems in northern regions. Earlier heading and maturity were found to be more important for achieving improved grain yield in organic fields than in conventional fields. Greater numbers of spikes m–2 were also found to be associated with increased grain yield in organic fields. Increased plant height and faster time to maturity were found to be associated with reduced weed biomass. Vigorous early season growth was related to increased yield, increased spikes m–2, and reduced weed biomass in organic fields. Based on these findings, a suitable spring wheat ideotype for organic management may be a taller cultivar with fast early season growth, early maturity, with a greater number of fertile tillers.


   ACKNOWLEDGMENTS
 
The authors wish to gratefully acknowledge Steven Snider of Little Red Hen Mills organic farm for his farmer cooperation. Thanks are also expressed to Cliff Therou for managing the fields at the Edmonton Research Station as well as to Dione Litun, Byron Cordero, Klaus Strenzke, Amy Kaut, Travis Eldstrom, Todd Reid, Susan Albers, and technical staff at the AAFRD Field Crop Development Centre in Lacombe, AB. Over the tenure of the preparation of this research article, H. Mason was supported by a Canadian Wheat Board Post-Graduate Fellowship and by an NSERC Discovery Grant.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
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Received for publication September 24, 2006.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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