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Production Papers

Determining the Feasibility of Early Seeding Canola in the Northern Great Plains

^a Central Agricultural Research Center, Montana State Univ., HC90 Box 20, Moccasin, MT 59462
^b Western Triangle Agric. Research Center, Montana State Univ., P.O. Box 974, Conrad, MT 59425
^c Dep. of Entomology, Montana State Univ., Bozeman, MT 59717
^d Northwestern Agric. Research Center, Montana State Univ., 4570 Montana 35, Kallispell, MT 59901

^* Corresponding author (cchen{at}montana.edu)

Received for publication January 3, 2005.

	ABSTRACT

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

 
Canola (Brassica napus L.) yield is often limited by heat and water stress. Early seeding may avoid the heat and water stress at critical growth stages but will encounter low soil temperatures and frequent frosts. Three experiments were performed at two locations in Montana from 2002 to 2004 to determine (i) early spring seeding effect on seed yield and oil content and optimum seeding rates for early seeding, (ii) base temperature (T_b) for germination and heat requirement for emergence, and (iii) suitable cultivars for early spring seeding. Late-March-seeded canola yielded 0 to 5% greater than mid-April seeding. Delaying seeding from mid-April to mid-May resulted in 43 to 63% yield reduction. Oil content was 12 to 22 g kg^–1 greater for mid-May seeding than mid-April seeding in 3 out of 5 site-year combinations. A seeding rate of 32 to 65 seeds m^–2 was found sufficient to produce optimum yields. Oil content tended to decrease 10 to 20 g kg^–1 when seeding rate increased from 11 to 97 seeds m^–2. The T_b for germination was less than 4°C, and the growing degree days for 50% emergence (GDD₅₀) were 42 to 81. Yield was negatively correlated (r = –0.46 to –0.65) to the days to 50% flowering, and biomass measured at 60 d after planting was negatively correlated to the chlorophyll fluorescence ratio (F_v/F_m) after cold stress (r = –0.58). The optimal seeding period for the region is between late March and mid-April. Several genotypes were found to have favorable characteristics for early seeding.

Abbreviations: ANOVA, analysis of variance • BM₆₀, biomass at 60 days after planting • CARC, Central Agricultural Research Center • DTF₅₀, days to 50% flowering • EL, electrolyte leakage • F_v/F_m, chlorophyll fluorescence ratio • GDD, growing degree days • GDD₅₀, growing degree days for 50% emergence • HTL₅₀, hours to 50% total leakage • T_b, base temperature for germination • T_max, maximum temperature • T_min, minimum temperature • WTARC, Western Triangle Agricultural Research Center

	INTRODUCTION

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

 
DRYLAND CANOLA PRODUCTION is often constrained by hot, dry summers and short growing seasons in the semiarid region of the northern Great Plains. Heat stress during flowering can cause flower abortion and greatly reduce seed yields (Angadi et al., 2000; Nuttall et al., 1992). The optimum daytime temperature was found to be 20°C for canola during flowering in the semiarid Canadian prairies (Angadi et al., 1999). A rise of maximum daily temperature from 21 to 24°C during flowering in July and August resulted in a substantial seed yield reduction (Nuttall et al., 1992). In a growth chamber study, Angadi et al. (2000) found that subjecting canola to 35/15°C of day/night temperatures for 7 d at the flowering stage injured canola reproductive organs and greatly reduced seed yield.

Water stress at flowering and seed-filling stages also had a negative impact on canola seed yield (Brandt and McGregor, 1997; Nielsen, 1997). Using multiyear data from Scott, SK, Canada, Brandt and McGregor (1997) found a regression relationship between canola seed yield and precipitation from 21 June to 20 August and mean daily temperature from 15 June to 15 August. The regression equations indicated that for each millimeter increase in precipitation, the yield of canola increased by 5.9 kg ha^–1. Also, for each degree rise in mean daily temperature, there was a corresponding yield reduction of 188 kg ha^–1. In dryland cropping systems, water is the most limiting factor for crop production. In a review paper, Johnston et al. (2002) suggested that a minimum of 127 mm of water is required for canola seed production in the northern Great Plains. After the minimum water requirement is met, canola produces 6.9 to 7.2 kg ha^–1 of seed for every millimeter of precipitation consumed. Canola has a tap root system that can extract water from a soil depth of 1.1 to 1.7 m (Nielsen, 1997). In shallow soils that have a limited water-holding capacity, such as the Judith clay loam (fine-loamy, carbonatic Typic Calciborolls) in central Montana, canola may have to rely on frequent rainfall to sustain growth and produce seed during the latter part of the growing season. Precipitation timing and amount vary greatly year to year and location to location in Montana. Consequently, growers in the region encounter highly unstable canola yields.

A change in seeding date can alter the timing of plant growth and development and avoid the negative impacts of heat and drought stress at critical growth stages (Angadi et al., 2004; Chen et al., 2003; Ludlow and Muchow, 1990). Delaying the seeding date was reported to have resulted in seed yield reduction in the United States and Canada (Johnson et al., 1995; Kirkland and Johnson, 2000). Previous studies in North Dakota indicate that seed yields declined when canola was planted after 15 May. Lower seed yields were attributed to fewer pods per plant and a lower harvest index (Johnson et al., 1995). In the Canadian prairies, Kirkland and Johnson (2000) found that relative to the traditional mid-May seeding date, seeding in late April increased canola yields by 44%. Clayton et al. (2004) analyzed experiments conducted at 10 site-year combinations in Canada between 1999 and 2001 and found no significant difference in oil content between early (late April to early May) and normal (mid-May) spring seeding dates. In another study, Kirkland and Johnson (2000) did not find consistent oil content improvement for the April seeding date over mid-May seeding date. Most growers normally plant canola in early to mid-May in Montana, but this seeding date needs to be re-evaluated.

Early seeded canola may encounter suboptimal soil temperatures for seed germination and seedling establishment in the northern Great Plains. The optimum germination temperature for canola was 10 to 30°C (Thomas, 1984) and 15 to 20°C (Kondra et al., 1983) although the T_b for germination was reported to be 5°C (Morrison et al., 1989) and 0.4 to 1.2°C (Vigil et al., 1997). Soil temperature in April in most canola production areas of the northern Great Plains is usually lower than the previously given optimum temperatures (Zheng et al., 1994). Spring canola seeded into suboptimal soil temperatures had lower emergence and stand establishment rates due to the seed rotting in the cold soils (Blackshaw, 1991; Kondra et al., 1983; Livingston and de Jong, 1990). A significant reduction in canola germination was found at temperatures less than 10°C (Nykiforuk and Johnson-Flanagan, 1994), and it took as long as 18 d for 50% emergence at 5°C (Blackshaw, 1991). Vigil et al. (1997) reported that 65 to 81 growing degree days (GDD) are required for spring canola seedlings to emerge. Differences in GDD have also been found among the species and seed lots (Nykiforuk and Johnson-Flanagan, 1994; Thomas, 1984).

Plant densities of 80 to 180 plants m^–2 have been recommended for canola on the Canadian prairies (Thomas, 1984). Canola has been found to have great yield plasticity, but the plasticity of a plant to compensate for suboptimal plant population depends on the availability of resources such as light, water, and nutrients (Sultan, 2000). Under favorable growth conditions, Angadi et al. (2003) found no yield reduction when a uniformly distributed plant population was reduced from 80 to 40 plants m^–2 at Swift Current, SK. Morrison et al. (1990a)(1990b) found 1.5 to 3.0 kg ha^–1 (35 to 70 plants m^–2) was enough to produce maximum grain yield under generally good growing season moisture in southern Manitoba. With well below normal precipitation in 2001, Angadi et al. (2003) found seed yield decreased as plant density dropped below 40 plants m^–2. The optimal plant population has not been determined for early spring-seeded canola under Montana's climate and soil conditions.

To make early spring seeding feasible, suitable canola cultivars must be selected. The suitable cultivars must have quick germination, emergence, and establishment at low temperatures, and seedlings must be tolerant to early spring freezing and thawing events. Freezing on the functionality of the photosynthetic apparatus can be used to assess the cold tolerance of plant genotypes. The photosynthetic apparatus function can be evaluated by measuring the F_v/F_m, which indicates the efficiency of the excitation capture by open Photosystem II reaction centers (Fracheboud et al., 1999; Rizza et al., 2001). A significant reversible decrease in F_v/F_m was found in all genotypes of oat (Avena sativa L.) during acclimation to low, nonfreezing temperatures, and F_v/F_m measurement was found to be highly correlated with field-evaluated frost damage (Rizza et al., 2001). Measurement of F_v/F_m is rapid and noninvasive.

Freezing tolerance of plant tissue is evaluated by measuring whether the tissues are alive or dead after subjecting the tissue to a range of freezing temperatures. The extent of damage caused by the freezing can be evaluated by increased rate of electrolyte loss, determined by placing plant tissue in distilled water and measuring the electrical conductivity of the resultant solution (Levitt, 1980; Madakadze et al., 2003; Murray et al., 1989). The electrolyte leakage (EL) method is based on objective measurements, utilizes small quantities of tissue, and is relatively cheap. However, it takes more time than the chlorophyll fluorescence method. Researchers in the Canadian prairies observed tolerance of young canola seedlings to freezing temperatures in early spring (Kirkland and Johnson, 2000), but no study has been conducted to assess cold-tolerant canola cultivars in Montana.

Central Montana is characterized as having a high altitude (1200 m), a short growing season (<115 d frost-free period), shallow soils (<1.0 m), and warm summers with infrequent rainfall. Suitable cultivars must be selected for this environment, and an optimal seeding date and rate must be determined for superior and stable yields. The objectives of this study were to determine (i) effect of early spring seeding date on canola seed yield and oil content and optimum seeding rate for early seeding, (ii) T_b for germination and heat unit requirement for emergence, and (iii) suitable cultivars for early spring seeding in Montana. Results obtained in this study are applicable to other regions of the United States and the world with similar geological and climatic conditions.

	MATERIALS AND METHODS

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

 
This paper reports the results from one field seeding date and rate study (Exp. 1), one field cultivar evaluation study (Exp. 2), and one growth chamber seed germination and emergence study (Exp. 3). Experiment 1 was performed for 3 yr from 2002 to 2004 at the Central Agricultural Research Center (CARC; 47°03'30'' N, 109°57'30'' W, 1400 m elev.) and for 2 yr from 2003 to 2004 at the Western Triangle Agricultural Research Center (WTARC; 48°18'24'' N, 111°55'30'' W, 1125 m elev.) of Montana State University. Experiment 2 was performed for 1 yr in 2004 at CARC and WTARC. Soils are classified as fine-loamy, carbonatic Typic Calciborolls (Judith clay loam series) at CARC and fine montmorillonitic Ardic Argiborolls (Scobey clay loam series) at WTARC. The 98-yr average annual precipitation at CARC is 392 mm, with an average annual air temperature of 5.9°C and an average frost-free period of 110 d. At WTARC, the 19-yr average annual precipitation is 293 mm, with an annual average temperature of 6.8°C and a frost-free period of 128 d. Experiment 3 was conducted in a growth chamber at CARC during the period between 2002 and 2003.

Experiment 1
Two glyphosate [N-(phosphonomethyl)glycine, in the form of its isopropylamine salt]-tolerant canola cultivars, Hyola357 (Interstate Seed Co., West Fargo, ND)¹ and DK223 (Monsanto Co., St. Louis, MO), were planted into standing spring cereal stubble at two field test sites (CARC and WTARC) on three seeding dates using a no-till plot drill with a 0.3-m row spacing. The seeding dates for each year and location are presented in Table 1. Canola was seeded at 133 seeds m^–2; then the seedlings were thinned to four plant densities of 11, 32, 65, and 97 plants m^–2 at the three- to four-leaf stage in 2002. The thinning was done rapidly and by each replicate to minimize the confounding influence of plant growth with plant densities. In 2003 and 2004, the canola plots were planted at seeding rates of 11, 32, 65, and 97 seeds m^–2. Canola seed was mixed with a teaspoon of peat moss powder to ensure a uniform distribution, particularly at low rates. Plant stands were counted after emergence. The experiment was a split-split plot design with four replications. Planting dates were assigned to whole plots, cultivars were assigned to subplots, and plant densities were assigned to sub-subplots. Plot dimensions were 1.5 by 8.5 m at CARC and 1.8 by 7.6 m at WTARC. Blended fertilizer consisting of N, P, K, and S was applied at rates of 67 kg N ha^–1, 22 kg P₂O₅ ha^–1, and 17 kg S ha^–1 at CARC and 163 kg N ha^–1, 34kg P₂O₅ ha^–1, 34 kg K₂O ha^–1, and 22 kg S ha^–1 at WTARC. Fertilizer was applied using a broadcast method, except for P at WTARC, which was placed with the seed at planting. Glyphosate was applied for weed control as needed. Canola seed was harvested by either direct combining (CARC) or by swathing and then thrashing (WTARC) with a plot harvester. Oil content was measured at the chemical laboratory at the Eastern Agricultural Research Center, Montana State University, using the American Oil Chemists Society method AK 4-95 (AOCS, 1994) with a Maran Ultra 10 MHz Nuclear Magnetic Resonance (NMR, Resonance Instruments Ltd., Witney, UK).

Field Plant Emergence and Flowering Date
Plant seedlings were counted in five rows 1 m long everyday in the middle of each plot, from the first day of seedling emergence to the day of complete emergence (no more plants emerging). Flowering dates were also recorded for each cultivar, and the days to 50% flowering (DTF₅₀) were calculated.

Plant Biomass
Canola plants were dug from a 0.3-m length within the middle row 60 d after planting. Roots were cut from plants at the soil surface level and kept separate. Both roots and shoots were dried at 60°C for 72 h. Dry weights were measured and converted to biomass at 60 days after planting (BM₆₀).

Chlorophyll Fluorescence Measurement
Four completely developed leaves from the upper part of four representative plants were measured for chlorophyll fluorescence using an OS1-FL portable pulse-modulated fluorometer (Opti-Sciences, Tynggsboro, MA) on the second and fourth day after a snowstorm on 12 May. The measurements were done using a saturated light pulse (15000 µmol m^–2 s^–1) for 0.8 s after a dark treatment using dark clips. The post-snow readings, F_v/F_m, represent the functionality of the photosynthetic apparatus, and the differences between the measurements, (F_v/F_m), were considered as the recovery of the photosynthetic functions.

Electrolyte Leakage Measurement
Six completely developed leaves from each plot were cut a day after the snowstorm on 12 May. The center of each leaf was punched with a 13-mm-diam. hole punch. The leaf discs were then placed into a vial filled with 15 mL of distilled water. An Oakton CON 510TDS electrical conductivity meter (Eutech Instruments, Singapore) was used to measure the electrical conductivity of the solutions. The first measurement of EL was done at 40 min after the leaf discs were placed into the water. Samples were kept at 4°C in a refrigerator, and the following measurements were conducted at 24, 48, 72, and 144 h after leaf discs were placed in water. Before each measurement, samples were equilibrated in a water bath at 23°C for 2 h. Finally, leaves were cooked at 105°C for 4 min in an autoclave to destroy the cells, and the electrical conductivity was measured after cooling to 23°C. Murray et al. (1989) used a first-order rate constant with units per time to measure the rate of leakage. In this paper, the rate of EL was expressed as the hours to 50% of total EL after autoclaving (HTL₅₀).

Experiment 3
Fifty uniform seeds from each cultivar or breeding line used in Exp. 2 were placed on water-saturated germination paper and inserted into a Petri dish. The Petri dishes were incubated in a growth chamber at 4, 6, 8, 10, and 15°C. The experiment was repeated three times.

Three replicates, each consisting of 50 seeds from every cultivar or breeding line, were planted into a Petri dish filled with sieved Judith clay loam soil. The Petri dishes were incubated at 4, 6, 8, 10, and 15°C in a growth chamber. The planting depth was 13 mm, and the soil moisture was kept about 0.2 kg kg^–1.

The Petri dishes were checked everyday, and germinated and emerged seeds were counted. The seeds were considered germinated when the seed coat was broken and the radicle started to grow. Seeds were considered emerged when the cotyledons appeared on the soil surface. Germination and emergence rate were calculated by dividing germination and emergence counts by the total seeds planted.

The T_b was calculated by using a linear regression method (Gbur et al., 1979). Growing degree days required for 50% emergence were calculated by subtracting the T_b from the incubation temperature and then multiplying the result by the days of incubation.

Data Analysis
Analysis of variance (ANOVA) was conducted using a split-split-plot design model with combined years or locations for Exp. 1 (McIntosh, 1983). Year and location were considered as fixed effects. Three ANOVAs were performed: The first was for the combined data for the mid-April and mid-May seeding dates at CARC from 2002 to 2004, the second was for the combined data for the mid-April and mid-May seeding dates at WTARC from 2003 to 2004, and the third was for the combined data for the late-March, mid-April, and mid-May seeding dates at CARC and WTARC in 2004. Since there was no seed harvested at the early June seeding date in 2002 and 2003, no data were included in the ANOVA. For Exp. 2, seed yield and oil content were analyzed using an ANOVA model for randomized complete block experiments combined over locations (McIntosh, 1983). One-way ANOVAs were also used to analyze the differences of cultivars in T_b, GDD₅₀, F_v/F_m, (F_v/F_m), EL, HTL₅₀, DTF₅₀, and BM₆₀ in Exp. 2 and 3. Fisher's protected LSD (P < 0.05) was used to compare means. A statistical package, SYSTAT 10.2 (SYSTAT Software Inc., Richmond, CA) was used to perform the analyses.

	RESULTS AND DISCUSSION

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

 
Weather
The monthly average maximum and minimum temperature (T_max and T_min) distributions were similar at CARC in 2002, 2003, and 2004, except March and April in 2002 were colder than in 2003 and 2004 (Fig. 1a) . The 2003 summer months, particularly June and July, were hotter than in 2002 and 2004. The growing season precipitation and monthly distribution patterns were different from year to year. The total growing season precipitation was 182, 123, and 181 mm in 2002, 2003, and 2004, respectively. In 2002 and 2004, the monthly precipitation patterns were similar to the long-term average. However, after the harvest of the first-seeding-date canola in 2002, there were several storms that delivered a total of 114 mm of water. In 2003, April was wetter than average, receiving 94 mm of precipitation, but was followed by an extremely dry summer with a total precipitation of less than 25 mm in July and August.

View larger version (46K): [in this window] [in a new window]   Fig. 1. Precipitation (Precip) and minimum and maximum temperatures (Tmin and Tmax, respectively) for the Central Agricultural Research Center (CARC) and Western Triangle Agricultural Research Center (WTARC) in 2002, 2003, and 2004. The long-term average precipitation (LTA Precip) was calculated from 95- and 18-yr averages at CARC and WTARC, respectively.

Experiment 1
The canola had very good emergence in all years and seeding dates, and plant densities were close to the target seeding rates (data not shown). Canola seed yield varied greatly from year to year. There were significant effects of seeding date, and seeding date x year interactions (P < 0.05, Table 2). Therefore, means of canola seed yield were presented by year for the seeding date effects at CARC and WTARC (Fig. 2) . Seed yield decreased greatly with delayed seeding dates. Canola yields for the mid-May seeding date at CARC were 59, 63, and 55% less than the mid-April seeding date in 2002, 2003, and 2004, respectively. Similar results were found at WTARC. Seed yield for the 13 May seeding date was 43% less than the 25 April seeding date in 2003, and seed yield for the 30 April seeding date was 24% less than the 13 April seeding date in 2004 (Fig. 2). However, differences of yield between the late March and mid-April seeding dates were small. The yield difference at WTARC was only 84 kg ha^–1 (or 5%) and was not statistically different at CARC (Table 2, Fig. 2). Canola yield response to seeding date was similar at both locations in 2004 (no seeding date x location interactions). Canola seed yields were extremely low in 2003 at both locations due to a severe summer drought.

View larger version (19K): [in this window] [in a new window]   Fig. 2. Response of canola seed yield to seeding date at the Central Agricultural Research Center (CARC), Moccasin, MT from 2002 to 2004 and at the Western Triangle Agricultural Research Center (WTARC), Conrad, MT from 2003 to 2004. Error bars in the graphs represent ± 1 SE.

Seeding rate had significant influences on canola yield at both locations, and the seeding rate influences differed by years and locations (i.e., seeding rate x year and seeding rate x location interactions were significant, P < 0.01). In years with average precipitation amount and distribution, such as 2002 and 2004, the canola yield increased greatly when the seeding rate increased from 11 to 32 seeds m^–2, and the yield curve flattened out when the seeding rate was greater than 32 seeds m^–2 (Fig. 3) . Canola yield tended to decrease when plant density was greater than 65 plants m^–2 at CARC in 2002. Therefore, a seeding rate of 32 to 65 seeds m^–2 is sufficient for an average year in central Montana and other regions in the Northern Plains with similar climate and soil conditions. In Saskatchewan, Canada, Angadi et al. (2003) found that, with slightly above-normal growing season precipitation, canola yield did not change with plant densities ranging from 40 to 80 plants m^–2, but under well-below-normal precipitation, the seed yield declined as populations dropped below 40 plants m^–2. In 2003, a severe drought year, canola seed yield was low and did not respond to seeding rate at CARC. Seed yield increased with planting rate at WTARC in 2003; however, the greatest yield was only 350 kg ha^–1 (Fig. 3).

View larger version (21K): [in this window] [in a new window]   Fig. 3. Response of canola seed yield to seeding rate at the Central Agricultural Research Center (CARC), Moccasin, MT from 2002 to 2004 and at the Western Triangle Agricultural Research Center (WTARC), Conrad, MT from 2003 to 2004. Error bars in the graphs represent ± 1 SE.

Canola oil content changed from year to year (Table 3 and Fig. 4) . Oil content responses to seeding date were inconsistent among years and locations (i.e., seeding date x year and seeding date x location interactions were significant, P < 0.01, Table 3). At CARC, the mid-May seeding date had 12 to 17 g kg^–1 greater oil content than the mid-April and late March seeding dates in 2002 and 2004. However, the oil content was 21 g kg^–1 less for the mid-May seeding date than for the mid-April seeding date in 2003. This was likely due to limited canola seed filling and oil formation caused by the severe summer drought in 2003. Similar results were obtained at WTARC. The canola oil content was 16 to 22 g kg^–1 greater for the 13 May and 13 April seeding dates than for the 30 March seeding date in 2004, and the oil content was 17 g kg^–1 greater for the 13 May seeding date than for the 25 April seeding date in 2003. Other researchers have also found inconsistent responses of canola oil to seeding dates in Canada (Clayton et al., 2004; Kirkland and Johnson, 2000). The drought during the summer of 2003 not only reduced canola seed yield but also decreased oil content at both locations (Fig. 4).

View larger version (19K): [in this window] [in a new window]   Fig. 4. Response of canola oil content to seeding date at the Central Agricultural Research Center (CARC), Moccasin, MT from 2002 to 2004 and at the Western Triangle Agricultural Research Center (WTARC), Conrad, MT from 2003 to 2004. Error bars in the graphs represent ± 1 SE.

View larger version (24K): [in this window] [in a new window]   Fig. 5. Oil content of two canola cultivars, DK223 and Hyola357, at the Central Agricultural Research Center (CARC), Moccasin, MT from 2002 to 2004 and at the Western Triangle Agricultural Research Center (WTARC), Conrad, MT from 2003 to 2004. Error bars in the graphs represent ± 1 SE.

View larger version (19K): [in this window] [in a new window]   Fig. 6. Response of canola oil content to seeding rate at the Central Agricultural Research Center (CARC), Moccasin, MT from 2002 to 2004 and at the Western Triangle Agricultural Research Center (WTARC), Conrad, MT from 2003 to 2004. Error bars in the graphs represent ± 1 SE.

[1]

where Precip is the precipitation (mm) between 20 June and 20 August and T is the mean daily temperature between 15 June and 15 August.

Using pooled data from the seeding rate treatment of 65 seeds m^–2 in 2002, 2003, and 2004, a nonlinear regression function was established between seed yield and growing season precipitation (Precip), average daily maximum temperature (T_max) during the reproductive period (from flowering to harvest), and soil water at seeding (SW) for CARC:

[2]

It is clear that both Eq. [1] and [2] show negative effects of daily temperature at reproductive stages on canola seed yield, but the temperature impact was less in Eq. [2] than in Eq. [1], i.e., 1.6 versus 188 kg ha^–1 per degree of temperature change. Other researchers also found negative impacts of high temperature at reproductive stages (Angadi et al., 2000; Nuttall et al., 1992).

Furthermore, canola seed yield responded exponentially to growing season precipitation in Eq. [2]. The reasons canola yield was more sensitive in this study compared with the study by Brandt and McGregor (1997) in Saskatchewan, Canada (Eq. [1]), include that the canola in this study was planted into a recropped field condition where there was little moisture stored in the soil profile and the crops were solely relying on the growing season precipitation. Second, the precipitation distribution was highly variable from year to year. In 2003, for example, CARC received 123 mm of total precipitation, but only 25 mm of the precipitation was delivered in July and August, which caused severe water stress during the reproductive stages and greatly reduced the seed yield. In 2004, however, there was 181 mm total growing season precipitation, and the precipitation was reasonably distributed (107 mm was distributed in June, July, and August), producing very good canola yields. This precipitation distribution resulted in the exponential response of yield to precipitation from 2003 to 2004. More multiyear studies are needed to validate Eq. [2].

Experiment 2
Canola seed yield varied greatly among cultivars and locations; there were significant location x cultivar interactions (P < 0.05, Table 4), indicating cultivars performed differently at CARC and WTARC. The canola seed yields ranged from 880 to 1350 kg ha^–1 at CARC and from 1800 to 2610 kg ha^–1 at WTARC (Table 5). The reason for greater yields observed at WTARC compared with CARC was due to a deeper soil profile, timely rainfall, and the trial being planted into a summer-fallowed field. Several cultivars exhibited superior yield potentials at both locations, including UISC00135, UISH0031923, DK223, Hyola357, and UISH003197. Some cultivars yielded well at one location but not at the other location. Those cultivars include DK3455, Clearwater, Sunrise, and UISC0038DE, reflecting the environmental sensitivity of these cultivars. Oil content also varied among cultivars and locations, and there were significant location x cultivar interactions (P < 0.01, Table 4). Oil content appeared greater at WTARC than at CARC, ranging from 377 to 425 g kg^–1 at CARC and from 428 to 460 at WTARC. Cultivars that had greater oil content at both locations include 95SH2511017, DK3455, UISC02314, Garnet, and Impact. The commercial cultivars, DK223 and Hyola357, had average performances at WTARC in terms of oil content levels but had lower-than-average oil contents at CARC. Therefore, further genetic improvement is needed to develop cultivars that will produce both stable yield and oil content across Montana. Several breeding lines in this study, such as UISC02314 and UISH003197, had similar yields but superior oil contents over DK223 and Hyola357.

View this table: [in this window] [in a new window]   Table 6. Base temperature (Tb), growing degree days for 50% emergence (GDD50), Fv/Fm, Fv/Fm recovery [(Fv/Fm)], electrolyte leakage (EL) after snow event, and hours to 50% of total electrolyte leakage (HTL50) of 17 canola cultivars.

The heat units required for emergence also varied among cultivars. The GDD₅₀ ranged from 42 to 81, which is equivalent to reaching 50% emergence under 6°C in 7 to 14 d. Some cultivars required more heat units than others for emergence. The cultivars that required less heat units for emergence (quick emergence) included Hyola357, DK223, Garnet, 96SI510312, Impact, and Sterling (Table 6). This heat requirement has a great impact on seedling establishment in the early spring seeding dates. Field stand counts in Exp. 2 agreed with the growth chamber seedling emergence study (data not shown). Seed rot may occur due to slow emergence in cold soils (Blackshaw, 1991; Kondra et al., 1983; Livingston and de Jong, 1990; Nykiforuk and Johnson-Flanagan, 1994). Aside from the genetic differences, seed vigor and seed age could also affect the speed and percentage of emergence (Nykiforuk and Johnson-Flanagan, 1994; Thomas, 1984). The calculated T_b was reported between 0.4 to1.2°C, and emergence began between 65 and 81 GDD (Vigil et al., 1997).

The T_b and GDD₅₀ were not correlated with canola seed yield in this study. However, GDD₅₀ had a weak positive correlation with DTF₅₀. Days to 50% flowering was negatively correlated to seed yield at CARC (r = –0.65) and WTARC (r = –0.46). The negative correlations between DTF₅₀ and seed yield at both locations indicated cultivars that flower earlier produce greater seed yield. These results differed from Madakadze et al. (2003), who found positive correlations between T_b and rate of EL, chlorophyll fluorescence, and leaf damage of warm-season grasses. Several cultivars in this study possess favorable characteristics for northern regions of the Great Plains, such as low T_b, fast emergence, low (F_v/F_m) after cold stress, early flowering, and good seed yield (Table 5 and 6).

Using the information obtained in this study, one can determine an optimal seeding date in the northern Great Plains or other regions with similar climate and soil conditions as central Montana. First, days from seeding to emergence can be predicted from long-term weather data based on the T_b and GDD₅₀ information obtained from Exp. 3. Second, based on the cold tolerance information in Exp. 2, one can decide which cultivar to plant and the risk of frost damage for a given early seeding date. Third, using the DTF₅₀ information from Exp. 2 in combination with long-term weather data, the maximum daily temperatures at flowering stage can be forecasted; thus, the potential impact of T_max on seed yield can be estimated for a given seeding date.

	CONCLUSIONS

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

 
Canola production has the potential to expand in Montana. Early seeding between late March and mid-April is the key to achieve a good and stable canola seed yield. This study indicates that the optimal seeding rate for early spring seeded canola is 32 to 65 seeds m^–2. Although early spring seeded canola will encounter cold soil temperatures and frequent frosts, canola can germinate at less than 4°C and requires 42 to 81 GDD for emergence. Further studies are needed to test the threshold temperature and duration of canola genotypes to cold stress. Several genotypes were found to have favorable characteristics for the semiarid region in the northern Great Plains, such as low T_b, fast emergence, relatively cold tolerant, early flowering, and good seed yield and oil content.

	ACKNOWLEDGMENTS

 
The authors thank John Miller, Shanning Ji, and Leann Fox for their help in the field and laboratory experiments. The authors also thank Dr. Jack Brown at University of Idaho, Douglas Ryerson at Monsanto, and Jim Johnson at Interstate Seed Company for providing canola seed for this project. Funding for this study came from the Montana State Experiment Station, USDA Pacific Northwest Canola Program, and the Bio-Based Products and Food Safety Institute of Montana State University.

	NOTES

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

 
¹ Mention of trade name does not constitute an endorsement.

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

 

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