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Published online 27 May 2005
Published in Crop Sci 45:1410-1416 (2005)
© 2005 Crop Science Society of America
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CROP ECOLOGY, MANAGEMENT & QUALITY

Oat Caryopsis Size and Genotype Effects on Wild Oat–Oat Competition

Christian J. Willenborga, Brian G. Rossnagelb and Steven J. Shirtliffea,*

a Dep. of Plant Sciences, Univ. of Saskatchewan, 51 Campus Drive, Saskatoon, SK, Canada S7N 5A8
b Crop Development Center, Univ. of Saskatchewan, 51 Campus Drive, Saskatoon, SK, Canada S7N 5A8

* Corresponding author (shirtliffe{at}usask.ca)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
The inherent similarity between wild oat (Avena fatua L.) and oat (A. sativa L.) precludes selective herbicide control of wild oat. Previous studies have reported improved crop yield and competitiveness with the use of large seed. Therefore, a greenhouse experiment was conducted in 2002 and 2003 to examine the effect of caryopsis (dehulled kernel) size and genotype on wild oat–oat competition. Seeds of three oat cultivars (AC Assiniboia, CDC Boyer, CDC Orrin) were classified into three size classes based on caryopsis size (large, {approx}35 mg; medium, {approx}25 mg; and small, {approx}15 mg). Whole oat seeds of each genotype and caryopsis size treatment were grown either in monoculture or in mixture with wild oat (250 plants m–2). Neither genotype nor caryopsis size significantly influenced (P < 0.05) oat emergence. Nonetheless, oat plants established from large caryopses produced 17% more biomass (P < 0.001) and 15% more panicles m–2 (P < 0.001) than plants established from small caryopses. Wild oat produced 23% less biomass (P = 0.004) and fewer panicles m–2 when grown with oat plants established from large caryopses. Genotype only influenced oat panicle production (P = 0.05) and did not affect oat (P = 0.09) or wild oat (P = 0.34) aboveground shoot biomass production. These results suggest that oat plants derived from large-seeded caryopses may be better able to tolerate wild oat interference. However, further investigation is needed to examine the response of oat–wild oat competition to oat caryopsis size and genotype under field conditions.

Abbreviations: GDD, growing degree days • TGW, thousand-groat weight • TKW, thousand-kernel weight


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
DESPITE MANY EFFORTS to reduce its growth and fecundity, wild oat remains ubiquitously distributed across western Canada, occurring on approximately 57, 46, and 51% of the fields in Manitoba, Alberta, and Saskatchewan, respectively (Leeson et al., 2002a, 2002b, 2003). Wild oat competition causes extensive yield and quality losses in wheat (Triticum aestivum L.) (Martin et al., 1987; Kirkland, 1993) and oat (Willenborg et al., 2005). Because oat is grown primarily for consumption by humans and horses, high yield and quality are imperative to market acceptance (May et al., 2004). Therefore, it is essential to minimize the adverse effects of wild oat competition on oat yield and quality.

In most cereal crops, wild oat is controlled by herbicides. However, no herbicide exists to selectively remove wild oat from oat. Therefore, reliance on cultural weed control is central to successful wild oat management in oat. Traditionally, wild oat was controlled by delaying planting so that emerged wild oat could be controlled by tillage (May et al., 2004). However, delayed planting of oat causes substantial declines in grain yield, test weight, plump seed, and groat percentage, with a corresponding increase in thin seed percentage (Nass et al., 1975; Ciha, 1983; May et al., 2004). Consequently, alternative methods are needed to manage wild oat in oat.

Within a cereal crop, seed size variation may result from the position of the seed on the inflorescence, genetic differences, disease, nutritional, and environmental effects. Seed size affects ecological processes including seed dispersal and establishment (Leishman et al., 2000), as well as plant attributes such as growth form (Maranon and Grubb, 1993), plant height (Leishman and Westoby, 1994), and leaf area (Peterson et al., 1989). Thus, initial seed size may be an important determinant of crop competitive ability. Although large seeds generally appear to produce larger, more vigorous seedlings than small seeds (Kaufmann and McFadden, 1963; Kaufmann and Guitard, 1967; Lafond and Baker, 1986b), the effects of seed size on germination and emergence characteristics appear inconsistent. Kawade et al. (1987) and Larsen and Andreasen (2004) observed increased germination and emergence from large seeds of pearl millet (Pennisetum glaucum L.) and perennial ryegrass (Lolium perenne L.), respectively. In contrast, large wheat and rice (Oryza sativa L.) seeds were found to have a slower germination rate than small seeds at several temperatures and osmotic potentials (Lafond and Baker, 1986a; Roy et al., 1996).

The relationship between seed size and yield, like that of germination, also appears to be somewhat ambiguous within and among cultivars. Emergence, head number, and grain yield among soft red winter wheat cultivars in Illinois were not affected by seed size (Mian and Nafziger, 1992). In fact, crops grown from small seed yielded more than large seed in some years. Dhillon and Kler (1976) also noted that plants established from small-seeded soybean varieties (Glycine max L. Merr.) yielded more compared with those established from large-seeded varieties. In these studies, seed size was confounded with genotype. In contrast, several studies have investigated the effect of seed size by comparing large and small seeds selected from within common cultivars. For example, small seed of winter wheat, spring wheat, and oat yielded 81, 82, and 83% as much as large, respectively (Kiesselbach, 1924). Likewise, yield from large barley (Hordeum vulgare L.) seed was 11% greater than the yield produced from small seed averaged among cultivars (Demirlicakmak et al., 1963). Among oat genotypes, plants established from primary kernels yielded 15% more than those established from secondary kernels (Brinkman, 1979).

The contribution of large seed size to crop growth and development may be enhanced under competition with weeds. Xue and Stougaard (2002) found that spring wheat competitiveness with wild oat increased as seed size and seeding rate increased. Sowing large seed reduced wild oat tillering by 15%, and biomass and seed production by 25%. Moreover, wheat yield increased 18% with the use of larger seed in the presence of wild oat competition (Stougaard and Xue, 2004). Thus, it appears that in cereal crops, planting large seed may reduce wild oat growth and fecundity and, as such, minimize the adverse effects of wild oat competition on crop yield and quality. However, their studies included various seed sizes of only one genotype, limiting the scope of inference because the effects of seed size may vary among genotypes.

A key component of integrated weed management systems is to grow competitive varieties (Lemerle et al., 2001). Genotypic differences in competitive ability have been previously reported in a number of crops (Challaiah et al., 1986; O'Donovan et al., 2000; Lemerle et al., 2001; Didon, 2002). Among cereal crops, genotypes with early emergence and tall stature appear to be most competitive with weeds. Didon (2002) demonstrated that the barley cultivar that emerged earlier had the fastest rate of stem extension, greatest leaf number, tallest stature, and was consequently more competitive with wild mustard (Sinapis alba L.). Tall barley cultivars also had a greater competitive effect (ability to suppress weed growth) on wild oat competition than shorter cultivars (O'Donovan et al., 2000).

Unfortunately, little effort has been directed toward improving the ability of oat to compete with wild oat. Furthermore, studies comparing seed size effects among various oat genotypes in response to weed competition are lacking and thus recommendations are limited. Our main objective was to assess the relative importance of oat caryopsis size and genotype in affecting wild oat–oat competition in the greenhouse. Genotypes were included in the study to ascertain whether caryopsis size effects were consistent or variable among genotypes. We conducted this study in a greenhouse because it provided us with a high degree of experimental control, repeatability, and precision, allowing us to isolate the effects of seed size on oat competitive ability.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Experimental Location and Design
A greenhouse experiment was conducted at the University of Saskatchewan at Saskatoon, SK, Canada, from September to December 2002 and October to December 2003. A three by three by two factorial randomized complete block design with treatments replicated four times was used. Our treatment factors were oat genotype (three), caryopsis size (three), and wild oat density (two). Oat genotypes evaluated were AC Assiniboia, CDC Boyer, and CDC Orrin. Genotypes were chosen on the basis of diversity in pedigrees and growth habit as well as heterogeneity of seed sizes. Caryopses were categorized into three size classes (small, medium, and large) of equal groat weight among genotypes as described in the following section. Wild oat density treatments consisted of oat grown without wild oat (0 plants m–2) or with wild oat present (225 plants m–2).

Caryopsis Size Classification
Seeds of each genotype were obtained from common seed increases at the time of harvest in 2002 from the Crop Development Center at Saskatoon, SK, Canada. All seed used was obtained from the same location and year and was thus exposed to the same environmental conditions. Because of the hulled seed structure of A. sativa, a series of fractionation tests were conducted where seeds of each genotype were fractionated into several proportions using no. 7 through 5 sieves (Can-Seed Equipment Ltd., Saskatoon, SK, Canada), ranging in size from 1.95 by 8.33 to 2.75 by 8.33 mm. Subsequent to fractionation, the variability in seed size distributions, thousand-kernel weight (TKW), test weight, and thousand-groat weight (TGW) between and among these seedlots was determined.

To eliminate differences in hull content between genotypes, caryopsis size classifications (small, medium, and large) were derived based on groat weight. The required TKWs needed to obtain the desired caryopsis size classes were determined and chosen based on regressions of TKW vs. TGW for each genotype independently. Seedlots of each genotype were then separated into size classes, and from those fractions containing the seeds that were the size of the predetermined groat size classes (small, 14–16 mg; medium, 24–26 mg; and large, 34–36 mg), seedlots of 200 caryopses each were counted and removed. Each 200 caryopses seedlot was then placed on a scale and the largest or smallest seeds were removed and replaced by medium-sized seeds of the respective size fraction, one at a time, until the final weight was equal to 200 times the target seed weight of the caryopsis size class of the genotype. Caryopsis weights, corresponding groat weights, and regressions are shown in Table 1. Wild oat seed used in the study consisted of those retained on a 1.95- by 8.33-mm sieve.


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  		Table 1. Thousand kernel weight (TKW) and thousand groat weight (TGW) of the various caryopsis size classes and genotypes used in the study.

 
Experimental Procedures
Ready Earth (W.R. Grace and Co., Ltd., Ajax, ON, Canada), a vermiculite, soilless mix, was added to each of 100, 20-cm-diam. (9 L) pots. All pots were watered to field capacity and lined with a coffee filter to aid in growth media retention. A circular pot-planter, 18 cm in diam., was constructed to simultaneously create holes for planting each oat or wild oat seed, ensuring that an identical seeding depth and spacing was used between all pots. Seeds were planted on 8 Oct. 2002 and 29 Sept. 2003 in a square arrangement at 3.8-cm intervals and at a depth of 3 cm (Fig. 1). Temperature in both experimental runs was maintained at 24/18°C (day/night) with heating or evaporative cooling as needed. Relative humidity among the two experimental runs averaged 25 ± 3%. Natural irradiance was supplemented 24 h d–1 with artificial lighting provided by 1000-W high pressure sodium lamps at a photosynthetically active radiation level of 250 µmol m–2 s–1.



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  		Fig. 1. Example of the planting pattern for evaluating the effects of oat genotype and caryopsis size on oat–wild oat competition. Filled squares represent individual oat plants, while open triangles represent individual wild oat plants. Dimensions are given demonstrating that spacing was equidistant and equiangular.

 
Oat and wild oat densities were established to simulate field oat densities (generally 200 to 250 plants m–2) and the effects of a wild oat population of equal density. Therefore, eight oat caryopses per wild oat-free treatment (caryopsis size within genotype) were sown in eight holes per pot to achieve a target density of 225 plants m–2. An additional eight wild oat seeds were placed in pots that contained the wild oat treatment (equal to an additional 225 plants m–2). In addition, 16 pots were sown to wild oat alone (450 plants m–2) and placed around the periphery of the experiment to minimize edge effects. All pots were covered with 2 cm of dry potting mixture and slightly compacted. Where final emergence was less than the target density, seedlings from spare pots were transplanted 8 d after emergence to compensate for germination and emergence that had failed. All experimental pots were rerandomized every 4 d to minimize environmental variability and possible border effects.

All pots were watered to field capacity twice a week (biweekly) until the flag leaf stage after which watering occurred three times weekly. Water soluble fertilizer (20–20–20, N–P–K) was applied evenly to pots at a rate of 1.2 g L–1 weekly until the flag leaf stage, after which it was applied biweekly. From planting to 8 d after emergence, oat seedling emergence was recorded three times daily (0700, 1400, and 2000 h). In oat, the Haun (1973) growth stage was recorded biweekly on three plants per pot until the six-leaf stage. Panicle production (panicles m–2) was determined just before harvest by counting the number of panicles on five plants of each species per pot. Plants were harvested at the late milk to early dough stage (17 Dec. 2002 and 9 Dec. 2003). Final plant height was determined after wild oat and oat plants were cut at the soil surface and separated. Plants of both species were subsequently dried for 96 h at 40°C to determine shoot dry weight.

Statistical Analysis
Haun (1973) growth stage data were regressed against cumulative growing degree days (GDD) from emergence using PROC REG (SAS Institute, 1996), with the inverse of the slope taken as the phyllochron interval in GDD (Cao and Moss, 1994). Growing degree days were calculated using the following equation:

[1]
where Tmaxi is the daily maximum air temperature on the ith day, Tmini is the daily minimum temperature on the ith day, and Tbase is the base temperature (0°C) for growth (Sonego et al., 2000). Oat median emergence time, or the time to 50% emergence, was described by fitting the following logistic function to each experimental unit (pot):

[2]
where Pt is the proportion of caryopses emerged at time t, t is thermal time in GDD (base temperature = 0°C) accumulated since the initiation of the experiment, a is the estimated rate of emergence (number of emerged caryopses per GDD), and b is the estimated median emergence time (GDD) in each experimental unit.

Residuals for all data were initially tested for normality with the UNIVARIATE procedure in SAS (SAS Institute, 1996). Data for oat panicles and wild oat biomass were log (base 10) transformed, to normalize the residuals and in these cases, ANOVA was preformed on transformed data. With the exception of oat emergence data, all data for each species were subjected to a three-way (genotype x seed size x wild oat density) factorial ANOVA, using the mixed model procedure of SAS (PROC MIXED; Littell et al., 1996), with degrees of freedom calculated by Satterthwaite's approximation method. Genotype, caryopsis size, wild oat density, and their interactions were considered fixed effects, whereas blocks, runs, and their interactions with fixed effects were considered random effects. Within the mixed procedure, the log likelihood ratio was employed to test the significance of the random effect of run and its interactions with genotype, caryopsis size, and wild oat competition (Littell et al., 1996). This test indicated whether data could be combined over runs. Fixed effects and random variance components in the mixed model were estimated by restricted maximum likelihood, which iteratively estimates fixed treatment effects by least squares and then estimates random effects by maximum likelihood based on the residuals of the fixed effects (Steele et al., 1997). All data were analyzed within wild oat density and genotype as significant interactions demanded. Means were separated using Fisher's protected LSD with treatment effects declared significant at P < 0.05.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
The main effect of caryopsis size was significant for shoot biomass and oat panicle counts (Table 2). Oat derived from large caryopses produced 17% more biomass (P < 0.001) and 15% more panicles m–2 (P < 0.001) than plants derived from small caryopses, irrespective of genotype or wild oat competition (Table 3). Similarly, plants established from medium-seeded caryopses produced 11% more biomass (P < 0.001) and 9% (P < 0.001) more panicles than plants established from small caryopses. However, no significant differences in biomass or panicle production were observed between large and medium caryopses (Table 3).


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  		Table 2. P values for factors in mixed model analysis of oat median emergence time, final emergence, height, biomass, panicle and seed production as affected by oat genotype, initial caryopsis size, and wild oat competition.

 

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  		Table 3. Oat and wild oat dry matter production, oat panicle production, and oat phyllochron of three oat genotypes and caryopsis sizes with or without wild oat competition.

 
Likewise, the main effect for caryopsis size also was significant (P = 0.004) for wild oat shoot biomass (Table 4). Wild oat shoot biomass was approximately 23 and 14% lower when competing with oat established from large and medium caryopses, respectively, compared with small (Table 3). Thus, all oat genotypes examined in this study that were established from large caryopses not only produced more dry matter both in the presence and absence of wild oat competition, they suppressed wild oat dry matter production. However, caryopsis size did not affect (P = 0.30) wild oat panicle production (Table 4). Nevertheless, competition with wild oat had a large effect on oat shoot biomass. Among genotype and caryopsis size, oat grown in mixture with wild oat produced 65% of the biomass (P = 0.05) of oat grown in monoculture (Table 3).


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  		Table 4. P values for factors in mixed model analysis of wild oat height, biomass production, and panicle production (panicles m–2) as affected by oat genotype and initial caryopsis size.

 
As expected, genotype significantly (P = 0.05) affected oat panicle production (Table 2). CDC Boyer and AC Assiniboia produced 18 and 17% more panicles m–2 than CDC Orrin, respectively (Table 3). Although genotype affected oat panicle production, it did not significantly affect oat (P = 0.09) and wild oat shoot biomass production (P = 0.34) or wild oat panicle production (P = 0.63) (Tables 2 and 4). Nevertheless, a trend was observed among genotypes with regard to oat biomass production, whereby CDC Boyer produced 23% greater biomass than AC Assiniboia and 12% greater biomass than CDC Orrin (Table 3).

Interestingly, the differences we observed in oat biomass and panicle production could not be attributed to variation in oat emergence. Median emergence time and final emergence percentage were not affected by genotype, caryopsis size, or wild oat competition, although wild oat presence x caryopsis size interactions were significant for final emergence (Table 2). When interactions were analyzed within wild oat treatments, however, caryopsis size did not significantly affect final emergence of oat (data not shown). This was because of large seed having greater germination than small when wild oat was absent compared with when wild oat was present. The lack of differences may be due to the hulled structure of Avena spp. seeds, but given that this is the first study in a hulled cereal to account for initial differences in hull content between seed sizes and genotypes, no evidence exists to support this hypothesis. Nevertheless, these results are congruent with those obtained in Rumex species, where no differences in emergence between large and small seeds appear to exist (Cideciyan and Malloch, 1982; Martinková et al., 1999).

Similarly, variation in oat biomass and panicle production could not be attributed to differences in height. Neither genotype nor caryopsis size influenced oat height in monoculture (Table 2). However, with wild oat competition present, AC Assiniboia and CDC Boyer derived from large caryopses grew substantially taller than plants established from small caryopses (Table 5). Caryopsis size did not affect plant height of CDC Orrin when competing with wild oat. Neither genotype (P = 0.25) nor caryopsis size (P = 0.25) affected wild oat height (Table 4). Likewise, oat phyllochron intervals did not differ among oat genotypes (P = 0.28) or caryopsis sizes (P = 0.50) (Table 2). Therefore, the differences observed in oat height, biomass, and panicle production were not due to differences in rate of leaf appearance. The rate of leaf appearance rate was 4 GDD longer (P = 0.02) when oat was grown in mixture with wild oat compared with oat in monoculture (Table 2). This may have resulted because oat plants grown in mixture were shaded by taller wild oat plants. Therefore, they may have been less able to compete for light, likely resulting in a reduced assimilate production and a lower rate of leaf appearance. Gautier and Varlet-Grancher (1996) reported that lowering blue light, as would occur when a plant is shaded, increased the phyllochron in fescue (Festuca arundinacea L.). Likewise, Gautier et al. (1999) noted an increase in the phyllochron of perennial ryegrass (Lolium perenne L.) under reduced photosynthetic photon flux densities experienced under shaded conditions.


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  		Table 5. Effects of genotype and caryopsis size on oat height with wild oat competition present.

 
The lack of interactions between caryopsis size and genotype observed in this study indicates that oat genotype and caryopsis size are additive (or independent) in their effects on wild oat–oat competition (Tables 2 and 4). This was likely because of the fact that we accounted for differences in hull content and groat percentage among the various genotypes examined in this study. Oat genotypes vary considerably with respect to hull content and corresponding groat percentages (Doehlert et al., 1999). Although it appears logical, if not essential, to account for differences in hull content among genotypes, this standardization has not occurred in previous studies examining the response of various plant attributes to seed characteristics of a hulled species. In a series of experiments, Kaufmann and McFadden (1960)( 1963) determined that large barley seed produced plants that were more productive and higher yielding than small seed. However, in both experiments, seed size classes were determined simply by sieving. Similarly, Brinkman (1979) demonstrated that oat plants of various genotypes derived from primary kernels yielded 15% more than those established from secondary kernels, but again differences between hull content were not accounted for. Our study is the first to base seed size on caryopsis size in a hulled species and thereby account for differences in hull content among genotypes. We essentially eliminated genotypic effects by normalizing for groat/caryopsis size. Consequently, genotype did not affect the ability of oat to suppress weed growth after this normalization procedure (Table 3). However, because seed sizes and distributions vary among oat genotypes (Doehlert et al., 2002, 2004), genotypes may differ in competitive ability, mediated through their different seed sizes.

Our results clearly demonstrate the importance of initial caryopsis size to the outcome of wild oat–oat competition (Tables 2 and 4), and suggest that the ability of oat to exhibit both an increased competitive response and effect to wild oat competition may be a product of specific crop traits such as caryopsis size. When competing with wild oat, greater oat biomass and panicle production in conjunction with reduced wild oat biomass production were observed in oat plants established from larger caryopses, indicating that they were better able to tolerate (competitive response) and suppress (competitive effect) wild oat competition. This suggests that the ability of oat to tolerate and suppress wild oat competition is positively correlated to caryopsis size, and agrees well with previous findings from earlier field trials (Kiesselbach, 1924; Demirlicakmak et al., 1963; Xue and Stougaard, 2002). Thus, potential may exist to use crop competition, via the use of large caryopses, as a component of integrated weed management in oat.

Whereas caryopsis size influenced both oat and wild oat biomass and panicle production, genotype only affected oat panicle production (Tables 2 and 4). On this basis, we contend that the positive effects of caryopsis size on competition between wild oat and oat were greater than were observed for the three genotypes examined (Table 3). Stougaard and Xue (2004) reported that seed size had a greater effect on spring wheat yield than seeding rate when competing with wild oat. Furthermore, the increased biomass production associated with increased caryopsis size indicates more effective resource capture by plants derived from large caryopses. However, because differences in emergence and phyllochron intervals were not observed across caryopsis sizes, we speculate that the greater competitive ability of oat plants established from large-seeded caryopses was likely related to greater occupancy of space resulting from greater intrinsic growth rates rather than from a faster rate of leaf expansion. Increased growth rates and early seedling vigor in plants established from large seeds have also been observed in wheat (Lafond and Baker, 1986b) and switchgrass (Panicum virgatum L.) (Aiken and Springer, 1995).

Although the results reported herein come from greenhouse studies, the data suggest that planting large oat caryopses could increase oat dry matter production and panicle production both in the presence and absence of wild oat competition. This may translate into increased yield because yield increases with the use of large seed have generally been due to increased panicle production (Kaufmann and McFadden, 1963). Moreover, because of the linear relationship between biomass and seed production (Cousens and Mortimer, 1995), increased biomass production generally translates into greater seed production, resulting in increased yield. The lack of differences generally observed in this study between large and medium caryopses suggests that more competitive oat crops may be planted by simply ensuring that small caryopses are removed from seedlots. This should be easily achieved, considering that the small caryopses used in this study should not be present in Certified Seed in Canada. However, the results of this greenhouse study cannot completely describe the response of wild oat–oat competition to oat caryopsis size and genotype. Therefore, further investigation is needed to examine the response of oat–wild oat competition to oat caryopsis size and genotype under field conditions. However, we predict that the results observed in this study would be enhanced under field conditions due to environmental heterogeneity. It is likely that the ability of large-seeded materials to compete better for belowground resources would be very important under the moisture-limited conditions frequently experienced in the Northern Great Plains region.


   ACKNOWLEDGMENTS
 
The authors gratefully acknowledge funding from the Quaker Oats Company and Cargill Ag Horizons for this research, as well as Dr. Robert Gulden for providing insightful comments on the manuscript, as well as statistical advice. In addition, the technical expertise and diligent assistance of Aaron Miller and Rachelle German is greatly appreciated.

Received for publication November 1, 2004.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 


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