Seed size and seeding rate effects on canola emergence, development, yield and seed weight

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1 Seed size and seeding rate effects on canola emergence, development, yield and seed weight K. N. Harker 1, J. T. O Donovan 1, E. G. Smith 2, E. N. Johnson 3, G. Peng 4, C. J. Willenborg 5, R. H. Gulden 6, R. Mohr 7, K. S. Gill 8, and L. A. Grenkow 9 1 Agriculture and Agri-Food Canada, Lacombe Research Centre, 6000 C & E Trail, Lacombe, Alberta, Canada T4L 1W1 ( neil.harker@agr.gc.ca); 2 Agriculture and Agri-Food Canada, Lethbridge Research Centre, Lethbridge, Alberta, Canada T1J 4B1; 3 Agriculture and Agri-Food Canada, Scott Research Farm, Scott, Saskatchewan, Canada S0K 4A0; 4 Agriculture and Agri-Food Canada, Melfort Research Farm, Melfort, Saskatchewan, Canada S0E 1A0; 5 University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5A8; 6 University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2; 7 AAFC, Brandon Research Centre, Brandon, Manitoba, Canada R7A 5Y3; 8 Smoky Applied Research & Demonstration Association (SARDA), Falher, Alberta, Canada T0H 1M0; and 9 Western Applied Research Corporation (WARC), Scott, Saskatchewan, Canada S0K 4A0. Received 13 June 2014, accepted 4 September Published on the web 12 September Harker, K. N., O Donovan, J. T., Smith, E. G., Johnson, E. N., Peng, G., Willenborg, C. J., Gulden, R. H., Mohr, R., Gill, K. S. and Grenkow, L. A Seed size and seeding rate effects on canola emergence, development, yield and seed weight. Can. J. Plant Sci. 95: 18. Canola (Brassica napus L.) is the most common dicotyledonous crop in Canada. Here we determine the effect of canola seed size and seeding rate on canola emergence, development, yield and seed weight. In 2013, direct-seeded experiments were conducted at nine western Canada locations. Four canola seed sizes (1000-seed weights ranging from 3.96 to 5.7 g) and one un-sized treatment (4.4 g average) were seeded at two rates (75 and 150 seeds m 2 ). Higher seeding rates led to higher canola emergence and stubble density at harvest. Higher seeding rates also increased early crop biomass, 1000-seed weights and seed oil content and reduced days to start of flowering and days to crop maturity. Seed size effects on canola emergence, yield or seed quality were not significant. Increasing seed size had a positive linear association with early canola biomass and 1000-seed weights, whereas, both days to flowering and days to the end of flowering had a negative linear association with seed size. Greater biomass from large seeds increases crop competition with weeds and also hastens flowering, shortens the flowering period and reduces the risk that canola will be exposed to high temperatures that can negatively impact flowering and pod development. Key words: Crop density, flowering period, input costs, integrated weed management, maturity, oilseed rape, seed costs Harker, K. N., O Donovan, J. T., Smith, E. G., Johnson, E. N., Peng, G., Willenborg, C. J., Gulden, R. H., Mohr, R., Gill, K. S. et Grenkow, L. A Conse quences du calibre des semences et de la densité des semis sur la leve e, le de veloppement et le rendement du canola ainsi que le poids des grains. Can. J. Plant Sci. 95: 18. Le canola (Brassica napus L.) est la dicotyle done la plus cultive e au Canada. Les chercheurs ont pre cisé l effet du calibre des semences et de la densité des semis sur la leve e de cette culture, son de veloppement, son rendement et le poids de ses graines. En 2013, ils ont entrepris des expériences en proce dant a` des semis directs a` neuf endroits dans l Ouest canadien. A` cette fin, ils ont seme des graines de canola de quatre calibres (poids de mille graines variant de 3,96 a` 5,7 grammes) et d un me lange (moyenne de 4,4 grammes) a` deux densite s (75 et 150 graines par m 2 ). Une densite de semis plus e leve e entraıˆne une plus forte leve e et un chaume plus dense a` la récolte. Elle augmente aussi la biomasse de la culture en de but de saison, le poids de graines et la teneur en huile des graines, tout en diminuant le nombre de jours avant la floraison et jusqu a` maturite de la culture. Le calibre des semences n exerce pas une influence significative sur la leve e, le rendement ni la qualite des graines. Les graines de plus gros calibre pre sentent une corre lation line aire positive avec la biomasse du canola en de but de saison et le poids de graines. En revanche, le nombre de jours avant la floraison et celui jusqu a` la fin de la floraison sont ne gativement corre le s au calibre des semences. La biomasse plus importante re sultant de l utilisation de grosses semences rend la culture plus compétitive avec les mauvaises herbes, mais elle acce le` re aussi la floraison, raccourcit celle-ci et atte nue le risque que le canola soit exposé a` des tempe ratures e leve es susceptibles d avoir une incidence ne gative sur la floraison et le de veloppement des gousses. Mots clés: Densité des peuplements, floraison, couˆt des intrants, lutte inte gre e contre les mauvaises herbes, maturite, colza ole agineux, couˆt des semences Canola is the most common dicotyledonous crop in Canada. Since 2003, the canola harvested area increased from 5 to 8 million ha [Canola Council of Canada (CCC) 2014a]; canola production increased from 7 to 18 million tonnes (CCC 2014b). It is anticipated that demand for canola will continue to increase (CCC 2014c). Canola growers are concerned about the input costs required to grow and protect crop yield potential. Canola Can. J. Plant Sci. (2015) 95: 18 doi: /cjps

2 2 CANADIAN JOURNAL OF PLANT SCIENCE seed costs are second only to fertilizer inputs required for canola production (Alberta Agriculture and Rural Development 2014). Relatively high seed costs have prompted growers to seed canola at suboptimal rates. Recent surveys indicate that approximately half of all western Canada canola growers have crop densities B40 plants m 2 ; optimal yields require a minimum of 40 plants m 2 (CCC 2013). Furthermore, over a wide range of western Canada sites and conditions, only about 50% of planted canola seeds emerge above the soil surface (Harker et al. 2003, 2012a). McWilliam (1998) found that an average of only 30% of winter oilseed rape seeds emerged in Great Britain. In recent years, canola seed size has increased substantially; seed weights greater than 6 g 1000 seeds 1 are not uncommon. In 1995 to 1997 field experiments, Elliott et al. (2008) classed seed ranging from 1.9 to 4.0 g 1000 seeds 1 as small to large ; very large seed ranged from 4.2 to 4.7 g 1000 seeds 1. Canola seed size effects on crop emergence and yield have been inconsistent. Under greenhouse conditions, Hwang et al. (2014) showed that midsize seed (0.7 to 2.0 mm) had a higher emergence rate than smaller or larger seed, but that larger seed led to greater shoot weight and plant height. Under field conditions, the same authors found no seed size effect on canola emergence, but at one of two very low-yielding sites inoculated with Rhizoctonia solani, midsize seed (0.7 to 2.0 mm) yielded greater than smaller or larger seed. Using relatively small seed, Elliott et al. (2008) showed consistent increases in canola emergence and yield as seed weight increased. In winter oilseed rape, seed greater than 2 mm led to greater emergence than smaller seed (Lunn et al. 2003). In other studies, seed size did not influence canola emergence or yield (Lamb and Johnson 2004; Clayton et al. 2009). However, with continual canola improvement and cultivar changes, we felt it necessary to re-examine seed size effects on growth and yield. Our hypothesis was that several canola growth, productivity and quality traits would improve as seed size increased. Specifically, we hypothesized that larger seeds would increase canola biomass and reduce compensatory branching within the crop canopy; as a result, flowering, maturity, seed size and yield would be impacted. Our objective was to determine the effect of two canola seeding rates and a range of currently relevant Table 1. Site, plot, and seeding details canola seed sizes on crop emergence, growth, yield and quality at nine western Canadian sites. The four different seed sizes were only made available for a single year. MATERIALS AND METHODS Direct-seeded (no-till) experiments were conducted at nine western Canadian sites in Alberta, Saskatchewan and Manitoba in 2013 (Table 1). All plots were established in fields previously sown to wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), or oats (Avena sativa L.). Prior to seeding, an application of glyphosate (900 g a.e. ha 1 ) was applied to the entire plot area to control weeds. Soil samples were collected at each site before seeding and analyzed for available soil nutrients. On the basis of the soil analyses, fertilizer additions were made to achieve 100% of the soil test recommendations for each site. Yield targets varied from 2800 to 3900 kg ha 1 at different sites. Most fertilizer was side-banded 2 cm beside and 3 to 4 cm below the seed row with small amounts of nitrogen and phosphorus also placed with crop seeds. At Scott, SK, and Brandon, MB, most of the nitrogen fertilizer was applied in a mid-row band; at Saskatoon, SK, liquid 2800 was applied when canola reached the two-leaf stage. Sulphur was broadcast at Brandon, MB. Canola was seeded at a target depth of 1 to 2 cm in 19- to 30-cm rows and in plots ranging from 2 by 6 m to 4 by 15 m on the dates and with the equipment specified in Table 1. The experimental design was a randomized complete block design with four replications. A factorial arrangement of treatments (52) was imposed with four canola seed size treatments (average 1000-seed weights: 3.96, 4.6, 4.8, 5.7 g) and one un-sized treatment (4.4 g) seeded at 75 or 150 seeds m 2. Screens employed for 3.96, 4.6, 4.8, and 5.7 g 1000 seed 1 were 4.2/64ƒ, 4.9/64ƒ, 5.4/64ƒ,and 5.9/64ƒ, respectively. The seed was treated prior to seed size segregation. Preliminary germination tests indicated that germination exceeded 97% for all seed size classes. The canola cultivar planted was glyphosate-resistant 7375 (Monsanto Canada Inc., Winnipeg, MB, R3T 6E3) at all sites. One or two in-crop applications of glyphosate at 450 g a.e. ha 1 were applied as required to control local weed populations. Fungicides and insecticides were applied Site GPS Coordinates Plot size (m) Seeding date Seeder Opener Row spacing (cm) Beaverlodge, AB 55.28N, W 415 May 10 Conserva Pak air 1 cm knife 23 Brandon, MB 50.08N, 99.98W 28 May 15 Versatile zero till 1 cm knife 20 Carman, MB 49.58N, 98.08W 58 May 16 Double disc Notched disc 19 Falher, AB 55.78N, W 26 May 17 Fabro cone 8 cm hoe 23 Lacombe, AB 52.58N, W 415 May 9 Conserva Pak air 1 cm knife 30 Melfort, SK 52.88N, W 410 May 21 Edwards hoe air drill 1 cm knife 20 Saskatoon, SK (1) 52.18N, W 26 May 23 Fabro cone disc 20 Saskatoon, SK (2) 52.18N, W 26 May 27 Fabro cone disc 20 Scott, SK 52.48N, W 410 May 22 R-Tech cone 1 cm knife 25

3 HARKER ET AL. * SEED SIZE AND SEEDING RATE EFFECTS ON CANOLA 3 as needed according to local disease and pest insect infestations. Data collection included crop emergence density (2 crop rows1 meter2 places), crop biomass at the six-leaf stage (2 rows 1 meter2 places), days to start and end of flowering, days to maturity, canola yield, and productive plant stubble density. Plots were either swathed at the appropriate time ( pushed at Brandon) or harvested with combines or straight-combined after desiccation. Plots at Brandon experienced hail damage. Canola seed was cleaned, weighed and corrected for percent moisture (8.5%). Thousand-seed weights, percent green seed and oil and protein content are presented based on an 8.5% moisture adjustment. Statistical Analyses Data were initially subjected to the UNIVARIATE procedure in SAS software (SAS Institute Inc., Cary, NC) to detect and remove outliers. All data were then subjected to the MIXED procedure in SAS software to determine the significance of seeding rate and seed size main effects and their interaction. Sites and replications were treated as random effects and seed rate and seed size were treated as fixed effects. In agronomic experiments with limited replication, Type II errors (acceptance of the null hypothesis when it is false) are more common than Type I errors (rejection of the null hypothesis when it is true) (Carmer and Walker 1988). By treating sites as random effects we increased the likelihood of making appropriate inferences at untested locations (Yang et al. 2010), but also increased replications to enable us to detect differences that were not apparent when analyzing individual sites. We also examined seed rate and seed size effects at individual sites (i.e., we also treated sites as fixed effects) to determine how those effects varied among locations and compared to overall means across sites. Linear, quadratic, and cubic contrasts for unequally spaced seed size treatments were determined and included in the MIXED procedure to determine significant regressions of all dependent variables against the four seed size treatments. Additional single degree of freedom contrasts determined if there were differences between the four distinct seed size treatments and the un-sized seed. RESULTS AND DISCUSSION Weather patterns were similar to long-term averages at most experimental sites (Table 2). Moisture conditions in May were generally favourable for canola emergence and early seedling growth. In June, most sites had abundant rainfall; precipitation was at least 75% higher than normal at three sites (Falher, AB; Saskatoon, SK; Scott, SK). In August, Melfort and Saskatoon received only 11 mm of precipitation. Higher-than-normal May temperatures at five of eight locations also favoured canola emergence and early growth. Average May temperatures in Beaverlodge and Lacombe, AB, were at least 28C higher than normal. Seeding Rate Effects Seeding rate did not interact with seed size for any dependent variable (P]0.2031) (Table 3). Canola seeding rate influenced most dependent variables (Table 4). Similar to the results of Kutcher et al. (2013), a higher seeding rate did not increase canola yield (P ). For individual sites, Melfort was the only site where yield increased with seeding rate; the higher seeding rate Table 2. May through August monthly precipitation and mean temperatures and long-term averages at all experimental sites May June July August Actual Avg. Actual Avg. Actual Avg. Actual Avg. Precipitation (mm) z Beaverlodge, AB Brandon, MB Carman, MB Falher, AB Lacombe, AB Melfort, SK Saskatoon, SK Scott, SK Temperature (8C) z Beaverlodge, AB Brandon, MB Carman, MB Falher, AB Lacombe, AB Melfort, SK Saskatoon, SK Scott, SK z Actual monthly precipitation 75% higher or lower than long-term averages and actual monthly temperatures 28C higher or lower than long term averages are underlined.

4 4 CANADIAN JOURNAL OF PLANT SCIENCE decreased yield at Beaverlodge (Fig. 1; Table 4). However, in other research, higher seeding rates did increase canola yield (Brandt et al. 2007; Hanson et al. 2008; Harker et al. 2012b). As expected, higher seeding rates increased canola emergence and productive plant stubble density (Table 4). Despite significant site seeding rate interactions for both variables, all individual site data were in complete compliance with the all-site means. Emergence percentage was 55% for both seeding rates. In a previous study, canola seeded at a depth of 1 cm had 45% emergence (Harker et al. 2012a). In the current study, productive plant stubble density was 47 and 45% for the seeding rates of 75 and 150 seeds m 2, respectively. Slightly higher canola emergence in the current versus the previous study could be related to more favourable precipitation and moisture conditions at the time of seeding (Table 2). Canola biomass at the six-leaf stage increased with the higher seeding rate (150 seeds m 2 ) (Table 4). Results from seven of nine sites supported the seeding rate effect on biomass; the two remaining sites were not statistically significant. Higher levels of crop biomass are important for crop competition with weeds (Johnson et al. 1998). Seeding rates leading to higher crop biomass help to rapidly close the crop canopy and facilitate integrated weed management strategies (Blackshaw et al. 2008) that delay weed resistance to herbicides (O Donovan et al. 2007). The higher canola seeding rate (150 seeds m 2 ) also increased 1000-seed weight and seed oil content (Table 4). The latter seeding rate effects were not significant at most individual sites, but no sites demonstrated a contrary effect. Seed weight is an important yield component for canola productivity. Greater crop biomass from the higher seeding rate probably reduced compensatory growth (McGregor 1987; Angadi et al. 2003) and the number of smaller, less-mature (see below) seeds. Similarly, All Sites Scott Saskatoon 2 Saskatoon 1 Melfort Lacombe Fahler Carmen Brandon Beaverlodge * kg ha 1 * Seed rate 150 Seeds 75 Seeds Fig. 1. Seeding rate (no. m 2 ) effects on canola yield for individual sites and the means of all sites. Horizontal lines on each black bar are standard errors for the difference among means for each site and for the all sites means. Asterisks (*) adjacent to bar ends indicate statistically different (PB0.05) seeding rate effects on yield at the respective sites. oil content is increased in larger, more-mature seeds. Oil content can be important for canola producers where canola prices reflect oil content levels. Higher seed oil content after higher seeding rates is consistent with previous research (Brandt et al. 2007; Harker et al. 2012b). Canola seed protein content is usually inversely related to seed oil content; such was the case in the current project. The higher canola seeding rate also reduced days to the start of flowering and days to maturity, with a trend (P ) to reduce days to the end of flowering (Table 4). Individual site data supported those effects. Greater plant biomass and intraspecific competition from the high seeding rate probably reduced compensatory growth (McGregor 1987; Angadi et al. 2003) and hastened flowering and maturity. Although the differences in flowering and maturity were small, those differences could have large impacts on canola yield and quality in Canadian prairie environments. Earlier flowering and maturity increase the probability that canola will avoid relatively high temperatures that reduce canola yield potential (Polowick and Sawhney 1988; Nuttall et al. 1992; Young et al. 2004; Kutcher et al. 2010; Harker et al. 2011, 2012b). In a similar manner, earlier maturity can reduce the likelihood of frost damage and subsequent yield and green seed-related quality reductions. Seed Size Effects Yield is the usually the most important response variable in agronomic experiments. Therefore, even though there were no significant regression contrasts for canola yield versus seed size with sites treated as a random effect (P]0.2249, Table 3), it remains useful to observe individual site effects and variability. Site-to-site yield variability was dramatic; yields at the highestyielding site were approximately threefold greater than the lowest-yielding site. The site-by-seed size interaction for yield was highly significant (P ). At Scott, yield increased with each positive increment in seed size (Fig. 2). However, at all of the remaining sites, there were no significant regressions for seed size versus yield. Given these data, we reject our hypothesis that canola seed size would influence yield. Four dependent variables (shoot biomass, days to flowering, days to end of flowering and 1000-seed weight) had a significant linear association with canola seed size (Table 3). Early canola biomass increased linearly as canola seed size increased (Fig. 3A). Only one individual site had a similar significant contrast, contrasts for the remaining eight sights were not significant. A single degree of freedom contrast confirmed significantly greater crop biomass when the largest seed size (5.7 g 1000 seeds 1 ) was compared with the un-sized seed (P ). Using smaller seed (2 to 4 g 1000 seed 1 ) than in the current study, Elliott et al. (2008) also demonstrated greater early shoot biomass with larger seed. Higher canola biomass early in the growing season is probably

5 HARKER ET AL. * SEED SIZE AND SEEDING RATE EFFECTS ON CANOLA 5 Table 3. ANOVA showing the statistical significance of treatment and site sources of variation, including regression contrasts for dependent variables Dependent variables Protein content (%) Oil content (%) Green Seed (%) 1000 seed weight (g) Stubble density (no. m 2 ) Yield (kg ha 1 ) Crop maturity (d) End of flowering (d) Start of flowering (d) Crop biomass (kg ha 1 ) Emergence density (no. m 2 ) Source Seed rate (R) B B Seed size (S) RS Site B B B B B B B B B B B SiteR B B B B B B SiteS B SiteRS Regression contrasts for seed size (S) Linear Quadratic Cubic All Sites Scott Saskatoon 2 Saskatoon 1 Melfort Lacombe Fahler Carmen Brandon Beaverlodge kg ha 1 Seed size Fig. 2. Seed size (g 1000 seed 1 ) effects on canola yield for individual sites and the means of all sites. Horizontal lines on each black bar are standard errors for the difference among means for each site and for the all sites means. The asterisk (*) adjacent to bar ends at Scott indicates a statistically different linear contrast (P0.0236) for yield versus seed size. All other linear, quadratic, and cubic contrasts for yield versus seed size at any site or for the all site means were not significant (P] 0.05). the best indication of plant vigour. The vigour index developed by Elliot et al. (2005) combined seed weight and germination; that index was strongly correlated with shoot biomass. Early plant vigour is important relative to establishing a plant canopy that can optimize photosynthetic radiation capture and carbon fixation. In addition, Pavlychenko and Harrington (1934) showed that the final competitive outcome between crops and weeds is disproportionately influenced by competition at early growth stages (i.e., early plant vigour). Furthermore, greater biomass increases intraspecific competition and thereby reduces compensatory branching (McGregor 1987; Angadi et al. 2003), which could hasten flowering and reduce the overall flowering period. Days to flowering and days to end of flowering decreased linearly with increased seed size (Fig. 3B and C). For both variables, three individual sites demonstrated a similar significant effect and no sites revealed a contrary pattern. A single degree of freedom contrast revealed significantly earlier flowering (P ) and end of flowering (P ) when the largest seed size (5.7 g 1000 seeds 1 ) was compared with the un-sized seed. The flowering time difference from the smallest to the largest seed size category was only about 1 d, but every day that canola growers can avoid high temperatures during the flowering period is important. These effects on flowering are probably associated with greater canola biomass with larger canola seeds. Lamb and Johnson (2004) did not detect canola seed size impacts on canola flowering or maturity timing. In accordance with the current study, Clayton et al. (2009) also failed to detect seed size effects on canola maturity. *

6 6 CANADIAN JOURNAL OF PLANT SCIENCE (a) 900 Site by seed size: P = Compliance (1, 8, 0) (b) 47 Site by seed size: P = Compliance (3, 6, 0) Kg ha y = x R 2 = Days y = x R 2 = (c) Days Site by seed size: P = Compliance (3, 6, 0) y = 0.474x R 2 = Seed size (g 1000 seeds 1 ) Harvested canola seed size increased linearly as planted canola seed size increased (Fig. 3D). A similar significant effect was detected at only one individual (d) Grams Site by seed size: P = Compliance (1, 8, 0) y = x R 2 = Seed size (g 1000 seeds 1 ) Fig. 3. Canola seed size (weight) effects on shoot biomass at the six-leaf stage (A), days to flowering (B), days to end of flowering (C) and 1000-seed weight (D). Stars indicate the unsized (4.4 g) treatment response (not included in regression analyses). P values for linear regression contrasts for A to D were , , , and , respectively. The F test P value for the siteseed size interaction is shown at the top of each dependent variable graph. Compliance values (in parentheses): first value is the number of sites (nine total) with a statistically significant regression in the same direction as the respective fitted line on the graph; second value is the number of sites with a nonsignificant (P]0.05) regression response; third value is the number of sites with a statistically different regression in the opposite direction to the respective fitted line. Vertical bars are standard errors for the difference between means. site. Here, as well as for the canola biomass data, it is apparent that the additional statistical power gained by treating sites as a random effect prevented a Type II Table 4. Dependent variable means across sites and the site by seeding rate interaction for two canola seeding rates Seeding rate (seeds m 2 ) Site by seeding rate interaction Variable z F test P value Compliance y Emergence density (no. m 2 ) , 0, 0 Crop biomass (kg ha 1 ) B , 2, 0 Start of flowering (d) B , 4, 0 End of flowering (d) B , 4, 0 Crop maturity (d) B , 2, 0 Yield (kg ha 1 ) , 7, 1 Stubble density (no. m 2 ) B , 0, seed weight (g) , 6, 0 Green seed (%) B , 8, 0 Oil content (%) , 5, 0 Protein content (%) , 7, 0 z Within each dependent variable, significantly different (PB0.05) means for the 150 versus 75 seeds m 2 rate are underlined. y The first value is the number of sites (nine total) with statistically different means in the same pattern as the across-site mean difference. The second value is the number of sites where seeding rate was not statistically significant (P]0.05). The third value is the number of sites with statistically different means in a pattern opposite to the across-site mean difference.

7 HARKER ET AL. * SEED SIZE AND SEEDING RATE EFFECTS ON CANOLA 7 error. A single degree of freedom contrast confirmed significantly greater harvested seed weight when the largest seed size (5.7 g) was compared with the un-sized seed (P ). It would seem reasonable that plants growing from larger seeds would also produce larger seeds. Elliott et al. (2008) also demonstrated that harvested canola seed had higher seed weights after larger seeds were planted. It is also reasonable to assume that if greater crop biomass from larger seeds reduces compensatory growth, it could also reduce the number of smaller, less-mature seeds at harvest. In other studies, planted seed size did not influence harvested seed weights (Lamb and Johnson 2004; Clayton et al. 2009). Large canola seed has advantages and disadvantages. One advantage of larger seeds is that seedlings from larger seeds are less susceptible to flea beetle [Phyllotreta cruciferae (Goeze)] damage (Bodnaryk and Lamb 1991; Elliott et al. 2008). The advantage of more rapid early shoot growth and its impact on early crop competition with weeds was mentioned above. One disadvantage of large seeds is that, given the lack of an emergence benefit [as shown here (P0.4656, Table 3) and in other research], a higher weight and cost of large seeds is required to achieve the same plant population compared with when smaller seeds are planted. As seed costs increase, growers are pressured to reduced seeding rates; the combined effect of reduced seeding rates and the trend for larger seeds can threaten canola yield potential. Indeed, surveys of western Canada canola fields indicate that approximately half of all fields have crop densities that will not allow optimal yields (minimum of 40 plants m 2, CCC 2013). It is noteworthy that seed size did not influence canola emergence. One might assume enhanced recruitment with larger seed given greater resources to reach and penetrate the soil surface. However, the lack of a seed size effect on canola emergence is consistent with other studies (Lamb and Johnson 2004; Elliott et al. 2005; Clayton et al. 2009). Indeed, Lunn et al. (2003) noted that selection of the very largest seeds could reduce winter oilseed rape emergence. It is possible that there is a threshold canola seed size that optimizes canola emergence and yield. Significant canola emergence and yield benefits existed when seed weight increased from 1.9 to 4.0 g 1000 seeds 1 (Elliott et al. 2008). If such a threshold exists, canola emergence and yield benefits from seeds with weights (sizes) above the threshold would be unlikely. CONCLUSIONS Increasing canola seeding rate from 75 to 150 seed m 2 did not increase canola yield (P ). As expected, higher canola seeding rates increased early crop biomass. Higher seeding rates also increased 1000-seed weights and seed oil content, and reduced days to start of flowering and days to crop maturity. Neither canola emergence nor yields were influenced by canola seed size. Increasing seed size had a positive linear association with early canola biomass and 1000-seed weights. Days to flowering and days to the end of flowering had a negative linear association with seed size. Greater biomass from large seeds increases crop competition with weeds and also hastens flowering, shortens the flowering period and reduces the risk that canola will be exposed to high temperatures that can negatively impact flowering and pod development. The negative effect of current grower trends to reduce or even maintain the same weight per unit area seeding rates is exacerbated by the fact that canola seed size is generally increasing. A failure to consider seed numbers for a given weight of seed may threaten canola yield potential. ACKNOWLEDGMENTS We thank L. Michielsen, P. Reid, E. Sroka, J. Zuidhof, G. Semach, I. Murray, T. Coelho, J. P. Pettyjohn, G. Finlay, S. Neudorf, D. W. Lewis and C. Kirkham for technical support. We also acknowledge the funding support of Monsanto Canada Inc., the Agriculture and Agri-Food Canada Canola Cluster Initiative, the Alberta Canola Producers Commission, the Saskatchewan Canola Development Commission, the Manitoba Canola Growers Association, and the Canola Council of Canada. Alberta Agriculture and Rural Development Average farm input prices for Alberta. [Online] Available: [2014 May 12]. Angadi, S. V., Cutforth, H. W., McConkey, B. G. and Gan, Y Yield adjustment by canola grown at different plant populations under semiarid conditions. Crop Sci. 43: Blackshaw, R. E., Harker, K. N., O Donovan, J. T., Beckie, H. J. and Smith, E. 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