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Nitrogen Management

Brassica juncea Canola in the Northern Great Plains

Responses to Diverse Environments and Nitrogen Fertilization

^a Agriculture and Agri-Food Canada, Semiarid Prairie Agricultural Research Centre, P.O. Box 1030, Airport Rd. East, Swift Current, SK S9H 3X2, Canada
^b Agriculture and Agri-Food Canada, Research Farm, P.O. Box 1240, Melfort, SK S0E 1A0, Canada
^c Agriculture and Agri-Food Canada, Research Farm, Box 10, Scott, SK S0K 4A0, Canada
^d Agriculture and Agri-Food Canada, Research Centre, 107 Science Pl., Saskatoon, SK S7N 0X2, Canada

^* Corresponding author (gan{at}agr.gc.ca)

Received for publication October 24, 2006.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Brassica juncea var. juncea canola is a new oilseed species that is developed from B. juncea (L.) Czern. mustard with its oil and meal quality equivalent to conventional canola species. Understanding of the phenological characteristics and yield responses to diverse environments will allow the crop to be better adapted to target production areas. This study determined the responses of the juncea canola to various soil-climatic conditions and was compared with B. napus L. canola, B. rapa L. canola, juncea mustard, and Sinapis alba L. mustard. The five oilseed species/cultivars were grown under various N fertilizer rates (0, 25, 50, 100, 150, 200, and 250 kg N ha^–1), at four Saskatchewan locations from 2003 to 2005. On average, flowering began 40 d after seeding (DAS) for alba mustard and rapa canola (earliest), 49 DAS for napus canola (latest), and 44 DAS for juncea canola (intermediate). Flowering duration was longest for juncea canola (30 d) and shortest for napus canola (22 d). The napus canola and juncea mustard produced higher (1684 kg ha^–1) seed yields than the three other oilseeds (1303 kg ha^–1 on average). For all oilseed species, the seed yield was highly responsive to N fertilizer rates from zero to about 100 kg N ha^–1, and thereafter, the rate of yield responses declined. The amount of N fertilizer required to achieve the maximum seed yield was 106 kg N ha^–1 for rapa canola, 135 kg N ha^–1 for alba mustard and napus canola, and 162 kg N ha^–1 for the two juncea spp. Overall, juncea canola had lower seed yield than more popular hybrid napus canola, and the yield stability of juncea canola was lowest among the five oilseed species when examined across diverse environments. Earlier flowering, longer flowering duration, and greater tolerance to drought stress exhibited by juncea canola make the crop best adapted to the drier areas of the northern Great Plains. The improvement of seed yield and yield stability is the key to potentially adapt this new oilseed species to a wider range of environmental conditions.

Abbreviations: BLUP, best linear unbiased predictor • DAS, days after seeding

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
CANOLA QUALITY BRASSICA JUNCEA is a relatively new oilseed crop that has been developed from B. juncea mustard (Woods et al., 1991). This crop differs from conventional condiment (high glucosinolates) mustard because the quality of the seed oil and meal is equivalent to canola, and thus, the term "juncea canola" is used to distinguish these differences. The juncea canola seed has low levels of erucic acid and glucosinolates, a moderate level of oleic acid, and produces a product equivalent to that of more commonly-grown B. napus and B. rapa canola species (Burton et al., 2003).

The advantages of juncea canola compared with napus and rapa canola include more vigorous seedling growth, quicker ground covering, and enhanced resistance to blackleg, a serious disease of canola, caused by Leptosphaeria maculans (Desm.) Ces. Et de Not. (Woods et al., 1991; Burton et al., 2003). Other advantages of juncea canola over napus or rapa canola are less seed shattering at maturity, which facilitates direct combine of the crop. In addition, there is potential for higher yields of oil and protein because the seed of juncea canola usually has a thinner coat than other canola species. The benefits of growing juncea canola have been recognized in the northern Great Plains, where this nongenetically modified canola can provide growers with an opportunity to diversify their oilseed production systems (Potts et al., 2003).

In the semiarid regions of the northern Great Plains, such as northern Montana, southwestern Saskatchewan, and southeastern Alberta, napus or rapa canola often suffers from heat and drought stresses during flowering (Miller et al., 2001). These stresses can cause flower abortion (Morrison and Stewart, 2002) and failure to fill developing pods (Gan et al., 2004), which results in decreased seed yield (Angadi et al., 2000). Under stressful environmental conditions, juncea canola produces more flowers, resulting in a greater number of seeds per plant than napus or rapa canola species (Gan et al., 2004). However, there is little information available regarding the relative performance of juncea canola in comparison with napus canola, rapa canola, or condiment mustard species over a wide range of soil-climatic conditions.

Oilseed crops require adequate N supply for maximum productivity (Miller et al., 2001). In the subhumid environments of western Canada, for example, canola crops responded positively to N fertilizer up to application rates of 180 kg N ha^–1 (Brandt et al., 2002). Some hybrid cultivars of napus canola have a greater response to soil N supply than open-pollinated cultivars under more favorable environments (Brandt et al., 2002; Malhi and Gill, 2004). However, it is unknown whether a similar response can be expected between canola and mustard species in areas with low- and high-yielding potential. Little information is available regarding the yield response of juncea canola to N fertilization. We hypothesized that juncea canola would perform better than conventional canola and mustard species under more stressful conditions and that under more favorable growing conditions different canola and mustard species may perform similarly. The objectives of this study were to (i) understand how juncea canola interacts with varying soil and climatic conditions in comparison with commonly grown canola and mustard species, and (ii) determine the effect of N fertilization on plant establishment, start and duration of flowering, seed yield and biomass production among various canola and mustard species under conditions with different yield potentials.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Site Description and Experimental Design
Field experiments were conducted at four locations: Melfort, Saskatoon, Scott, and Swift Current, SK, Canada, from 2003 to 2005. Crops at Scott in 2005 were destroyed by hail during late flowering, thus no results from this location-year were obtained. The characteristics of the trial locations, soil types, and field conditions are summarized (Table 1). Before seeding, residual soil available N, P, and S were determined for each of three depths (0–15, 15–30, and 30–60 cm) at each of the 11 sites (the term site refers to location x year combinations and is used throughout the entire text of the paper). Soil samples of 8 to 12 cores were taken from plot areas, and were analyzed for NO₃–N, bicarbonate extractable P, and sulfate-S (Hamm et al., 1970). Soil nutrient concentrations were determined using bulk densities that were previously determined at each site (Table 2).

Fertilizer N was applied following recommended practices for placement of fertilizer and seed to minimize seedling damage (Malhi and Gill, 2004). At Melfort, ammonium nitrate was incorporated into soil 38 to 40 mm deep using a shallow rotary tillage before seeding. The tillage operation was oriented the length of the plots to minimize possible interplot movement of fertilizer. Immediately after tillage, plots were seeded 15 to 20 mm deep using a disc press drill with 17.8 cm row spacing. The seeding rows were between the fertilized rows. At the three other locations, no-till management practices were used to seed directly into wheat stubble 15 to 20 mm deep using a hoe press drill with 25.4-cm row spacing. Urea N fertilizer was mid-row banded to a depth of 38 to 40 mm. Blends of monoammonium phosphate (11–51–0, N–P–K) or triple superphosphate (0–45–0, N–P–K) and potassium sulfate (0–0–50–17, N–P–K–S) were applied at the same time as N fertilizer (Table 2). The amount of N from the blend fertilizer application was accounted in the N rate treatments. Weed control was achieved with a preseeding and a preemergent burn-off treatment of glyphosate, along with recommended postemergent sprays of grassy and broadleaf weed herbicides applied following label recommendations.

Plant population was determined by counting seedlings 10 to 14 d after initial seedling emergence in two to four 1-m rows (or 0.5–1.0 m²) per plot. Plant stand was assessed a second time at physiological maturity to determine the rate of plant survival during the period from emergence to maturity. Calendar dates were recorded for the start of flowering (the first flower was visible in a plot), completion of flowering (>95% of flowers had pods in a plot), and physiological maturity (seed moisture content of 20%). Days to first flower, duration of flowering, and duration of maturity were calculated based on the records of calendar dates. Aboveground plant biomass was determined by harvesting one 0.5- to 1.0-m² area of each plot at maturity. The plant samples were oven dried at 50 to 70°C for 7 to 10 d, and weighed. Entire plots were swathed or desiccated at physiological maturity with an application of glyphosate at label rates. After 7 to 10 d of drying in the field, the swathed windrows were combined with plot-scale equipment, and seed yields were adjusted to 11% moisture content.

Statistical Analysis
Data were analyzed using the PROC MIXED model of SAS (SAS Institute, 1999) where N fertilizer rate and crop type (i.e., various canola and mustard species/cultivars) were designated as fixed effects, and blocks and sites were considered random effects (Littel et al., 1996). In the analysis, N fertilizer rate was considered a continuous variable rather than a class variable. Therefore, all interactive responses of crop types and sites to the various N rates were determined by incorporating the intercept and slope coefficients of linear regressions in the ANOVA. Variance estimates and P values were used to determine the relative importance of these interactions. The quadratic (N x N) regression coefficients were excluded in variance estimates because exploratory analysis indicated that the quadratic coefficients were too small to be important. With sites designated as random effects, inferences on optimum oilseed crop management could be extended to the other canola and mustard growing regions with similar environmental conditions as those in the northern Great Plains. Treatment effects were declared significant at P < 0.05.

Significant responses of the various oilseeds to N fertilizer rates were investigated by regressing seed and straw yield against N fertilizer rates using a segmented quadratic-plateau model as follows:

[1]

where W is the yield (kg ha^–1), N is fertilizer rates (kg N ha^–1), N_join is the join point or the N fertilizer rate at which the plateau begins, and a, b, c, and plateau are model coefficients. The plateau of the regression estimated the maximum yield, and the join point of the regression estimated the N fertilizer rate at which the maximum seed yield was achieved. These nonlinear regression coefficients were estimated using the PROC NLMIXED model of SAS (SAS Institute, 1999). Similar to the linear model described in the previous paragraph, the nonlinear regression model variance estimates were determined for site x intercept coefficient and site x linear slope coefficient interactions.

Preliminary analysis revealed that the five oilseed crops did not respond to N fertilizer rates in a similar manner when examined across diverse environments. To further categorize their responses for a given treatment (i.e., crop type) or treatment combination (i.e., crop type x N rate combination), a grouping methodology, as described by Francis and Kannenberg (1978), was used to explore random variability or stability. Means and coefficients of variation (CV) for each crop x N rate combination were estimated across the various levels of yields and N rates. Each yield-related variable was plotted against its corresponding CV, producing a biplot. The biplot, together with the scatter of data points, was used to identify four response categories: Group I, high yield, low variability (optimal); Group II, high yield, high variability; Group III, low yield, high variability (poor); and Group IV, low yield, low variability. These biplots give an indication of the relative variability or stability of a crop type across the various environments.

Because of the complex nature of the interactive responses for crop type x various N rate combinations across the 11 diverse environments, more complex analysis was employed to further determine the intensity or magnitude of the variability associated with contrasting environmental sites for each of the five oilseed crops. This was achieved by determining variance estimates for the intercept and linear slope coefficients separately for each crop type or group of crop types. A trial-and-error/elimination process was used to obtain a stable model fit (corrected Akaike's information criterion) that was close to that achieved through linear ANOVA. Best linear unbiased predictor (BLUP) estimates for the intercept and linear slope coefficients were considered as deviations relative to the overall means of coefficient estimates across sites (Littel et al., 2002). The BLUP estimates were outputted for each crop type when there were significant differences between crop types, or the BLUP estimates were outputted for group of crop types when the estimates were similar for more than two crop species/cultivars.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
The overall performance of the five oilseed crops was assessed by comparing their means of the 11 sites for yield-related variables at 80 kg N ha^–1, an N rate typically recommended for oilseed production in the northern Great Plains. Responses to various N fertilizer rates were determined using linear and nonlinear regressions for each crop or group of crop types. Yield variability (or stability) across diverse soil-climatic environmental conditions was assessed using biplot methodology, while the interactive effects of crop type by N rate combinations across environments were examined using variance estimates for the intercept and linear slope regression coefficients and their deviations from overall means.

Plant Stand and Development
Seedling emergence and plant survival were greatest for alba mustard, followed by napus canola, while the three other oilseeds had significantly lower seedling emergence and plant survival (Table 3). Nitrogen fertilizer rate did not affect seedling emergence or the percentage of plants that survived after emergence for any of the oilseed crops.

Nitrogen fertilizer rate affected flowering responses and time to maturity, and interacted with the effect of crop types (Table 3). However, the preceding interactions were marginal because additional N fertilizer (i.e., 0 vs. 80 kg N ha^–1) delayed the start of flowering and time to maturity by about 1 d for napus canola but it had no effect on the other crop types (data not shown). Increasing rates of N fertilizer shortened the duration of flowering by 1 d for alba mustard, lengthened it 1 d for napus canola, and had no effect on the other crop types. Obviously, the effect of N fertilizer rate on flowering and maturity of oilseed crops was not of practical importance, despite statistical significance in most cases.

Crop Yield
Seed yield differed significantly among the five oilseed crop types: napus canola and juncea mustard achieved the highest seed yields (Table 3), both producing about 35% greater yields than the lowest-yielding oilseeds, alba mustard and rapa canola. The seed yield of juncea canola was intermediate among the other crops. The differences in straw yield among the five oilseeds followed a similar trend to seed yield, with the exception that straw yield was intermediate for the two juncea species (Table 3). The difference between the lowest- and highest-yielding crop types was 490 kg ha^–1 for seed yield and 1153 kg ha^–1 for straw yield. The preceding differences among crop types for seed and straw yields meant that harvest indices were greatest for juncea mustard and rapa canola, lowest for alba mustard and juncea canola, and intermediate for napus canola (Table 3).

Both seed and straw yield responded to N fertilizer rates in a curvilinear manner, and the responses were consistent among the five oilseed crops (Table 3). Similarly, harvest indices also responded to N fertilizer rates in a curvilinear manner, but only a small change (about 0.01 harvest index units) occurred between N fertilizer rates. The segmented quadratic-plateau model revealed that seed yield increased sharply with increasing N fertilizer rates up to 100 kg ha^–1 (Fig. 1A ). Beyond 100 kg N ha^–1, the yield response to fertilizer N rates was generally leveled off or the rate of increase in yield declined. The rate of N fertilizer at 100 kg N ha^–1 was greater than the current recommendation of 80 kg N ha^–1 (the vertical dashed line in Fig. 1). The responses of straw yield to N fertilizer rates followed a similar trend as that in seed yield, although sharp increases in straw yield occurred between 0 and 50 kg N ha^–1 (Fig. 1B). Thereafter, the rate of increase in straw yield was either slowed or declined before the straw yield curves leveled off at about 100 kg N ha^–1.

View larger version (24K): [in this window] [in a new window]   Fig. 1. Nonlinear regression output for (A) seed and (B) straw yields produced by alba mustard (cv. AC Base), juncea canola (cv. Amulet), juncea mustard (cv. Cutlass), rapa canola (cv. Hysyn 110), and napus canola (cv. InVigor 2663). The data were collected from 11 sites (location x year combinations) in Saskatchewan from 2003 to 2005. The trend lines were derived from predictions associated with the nonlinear regression coefficients, and the vertical dash line indicates the recommended rate of N fertilizer (80 kg N ha–1) for oilseed production under normal growing conditions.

View larger version (31K): [in this window] [in a new window]   Fig. 2. Summary of the nonlinear regression coefficients (a, intercept; b, linear slope coefficient; c, quadratic slope coefficient; join point, N fertilizer rate at which the plateau begins; plateau, upper asymptote) for seed and straw yield produced by alba mustard (cv. AC Base), juncea canola (cv. Amulet), juncea mustard (cv. Cutlass), rapa canola (cv. Hysyn 110), and napus canola (cv. InVigor 2663). The data were collected from 11 sites (location x year combinations) in Saskatchewan from 2003 to 2005. The horizontal error bars represent confidence intervals for coefficient estimates; error bars that do not overlap are significantly different at (P < 0.05).

View larger version (44K): [in this window] [in a new window]   Fig. 3. Biplot (means vs. CV) of crop type x N fertilizer rate combinations for data collected at 11 sites (location x year combinations) in Saskatchewan from 2003 to 2005. The first letter of the data point labels indicates crop type (am, alba mustard; jc, juncea canola; jm, juncea mustard; rc, rapa canola; nc, napus canola) and the following number indicates the N fertilizer rate. A number closely clustered or clustered near the origin was not labeled. The large open triangle symbols in each graph represent the mean values for each crop averaged across N rates. Group I, high yield, low variability (optimal); Group II, high yield, high variability; Group III, low yield, high variability (poor); Group IV, low yield, low variability. DAS, days after seeding.

There was a clear effect of crop type by N fertilizer rate on straw yield variability (Fig. 3). In general, napus canola had the highest straw yields with lowest variability (Group I), along with juncea mustard and juncea canola under higher N rates. Crops receiving the lowest (0–25 kg N ha^–1) rates of N fertilizer had the highest variability in straw yield. In general, straw yield variability due to N supply was greater than the variability among crop types. There was a weak association between the values of harvest indices and their corresponding variability. The most prominent trend was that napus canola had the lowest variability in harvest index and alba mustard the greatest.

Interactive Responses to Environmental Conditions (Sites)
Analysis of variance revealed significant site x crop type and site x intercept coefficient (a value) interactions for all the yield-related variables (Table 3). Also, there were significant site x linear slope coefficient (b value) interactions for seed and straw yields. Variance estimates indicated that site x intercept interactions for plant stand responses were lowest for juncea and napus canola and greatest for alba mustard (Table 4). For the start of flowering, variance estimates for the site x intercept interactions were similar among crop types. However, for the duration of flowering and time to maturity, the variability for the site x intercept interaction was greatest for juncea canola, followed by juncea mustard and napus canola. For seed yield, variance estimates for site x intercept were greatest for juncea and napus canola and lowest for juncea mustard. Similar differences were apparent for straw yield, with a few exceptions. Variance estimates for the site x slope interaction for seed and straw yields indicated that the responses to N fertilizer rates 150 kg N ha^–1 (i.e., within the linear portion of N fertilizer effect) were the most variable for the highest-yielding crops, juncea mustard and napus canola.

View this table: [in this window] [in a new window]   Table 4. Variance estimates for yield-related variables for five oilseed crops tested at 11 sites (location x year combinations) in Saskatchewan, Canada, from 2003 to 2005.

View larger version (39K): [in this window] [in a new window]   Fig. 4. Best linear unbiased predictor estimates for seed and straw yield deviations of the intercept and linear slope coefficient estimates for each crop type (or crop type group) relative to the overall estimate across sites (location x year combinations). Statistical significance of deviations for each crop type/group at a particular site is indicated as follows: * = 0.05 P 0.01, and ** = P < 0.01.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
In the present study, only one cultivar was selected for each of the five oilseed species, mainly because of the consideration of the size of the experiment. Although the cultivar selected was the best representative for each species, the results from the specific cultivars may not be applicable to other cultivars if the traits are substantially different. However, previous studies have shown that the differences between cultivars within a species are always smaller than differences between species (Angadi et al., 2000; Gan et al., 2004). Therefore, in the discussion below, we tended to discuss differences among species.

Plant Establishment
Among the five oilseed species/cultivars tested, alba mustard and napus canola had significantly greater seedling establishment and plant survival than the three other species/cultivars. The alba mustard had largest seed size (5.8 mg seed^–1), while the napus canola was a hybrid with high seed vigor, both of which have strong influence on seedling establishment (Steppuhn and Raney, 2005). Nitrogen fertilizer rates did not have any effect on seedling emergence or plant survival for any of the oilseed crops in this study. Nitrogen fertilizer was applied using either middle-row banding (at 8 sites) or preseeding soil incorporation (at three sites); these fertilization practices have been proven to be safe for canola and mustard crops (Malhi and Gill, 2004).

In the present study, the average plant density was 65 plants m^–2, which was lower than planned target, but it was within the range recommended for optimum plant population of oilseeds in the northern Great Plains (Angadi et al., 2000). Seed and straw yields were poorly associated with either percentage seedling emergence, plant survival, or the variability of these factors. For example, the napus canola and the alba mustard had greatest percentage emergence with lowest variability in plant stand, but the napus canola produced the highest seed yield while the alba mustard the lowest. Canola and mustard species have a strong ability to compensate for low plant density (Degenhardt and Kondra, 1981). The compensatory effect is achieved mainly through production of additional primary and secondary branches (Angadi et al., 2003) and more pods per plant (Kirkland and Johnson, 2000). Due to the strong compensatory effect of canola and mustard, a crop with a plant population of 50 plants m^–2 can produce similar seed yield per unit area as a crop with a population of 80 plants m^–2 (Angadi et al., 2003).

Yield and Yield Stability
The napus canola and juncea mustard produced the highest seed yields with the greatest yield stability (i.e., lowest variability) among the five oilseed species, while the juncea canola produced moderate seed yield with the lowest yield stability (i.e., greatest variability) when were tested across various rates of N fertilizer and diverse environments. These results indicate that juncea canola can be adapted to some environments but may be challenged when grown under other environmental conditions. In contrast, the napus canola and juncea mustard can be adapted to more diverse environments. Furthermore, variance estimates revealed that the two highest-yielding oilseed species (napus canola and juncea mustard) had the most pronounced intercept deviations from the means (indicated by more negative intercept deviations at Melfort and more positive intercept deviations at Saskatoon) relative to other oilseed species. The wide range of intercept deviations indicates that the high-yielding napus canola and juncea mustard are more sensitive than the other oilseed species/cultivars to conditions where no N fertilizer is applied (note that the intercept of the linear regression to N fertilizer rates was obtained when N rate was zero). This sensitivity was probably influenced by growing season rainfall, heat units, and residual soil N, among other factors.

Coincidently, the napus canola and juncea mustard had large linear slope coefficients when seed yield was plotted against various rates of N fertilizer. Also, these two high-yielding oilseed species/cultivars had the greatest deviations in the linear slope coefficients when the seed yield responses were assessed across diverse environments. For example, at Melfort and Swift Current, the napus canola and juncea mustard had more positive slope coefficient deviations, while at Saskatoon and Scott, they had more negative slope coefficient deviations than the other species. These results indicate that the high-yielding napus canola and juncea mustard are the most sensitive of the oilseed species tested to the supply of N fertilizer, and that more pronounced responses to N fertilization can be expected under more favorable growing conditions.

Responses to Nitrogen Fertilizer
In the present study, the responses of the oilseed species/cultivars to N fertilization was assessed under two scenarios: (i) at the rate typically recommended (80 kg N ha^–1), and (ii) at rates increasing from 0 to 250 kg N ha^–1. At the typical rate of recommendation, the seed yield of juncea canola was similar to that of alba mustard; both 300 kg ha^–1 (18%) less than seed yield produced by napus canola. The seed yields of juncea canola and alba mustard were less responsive than other species/cultivars to various rates of N fertilizer, while the napus canola had the greatest response in seed yield to increased N fertilization. These results indicate that juncea canola is grown with a yield penalty under the typical N recommendation (i.e., 80 kg N ha^–1), and that this penalty is not alleviated with an adjustment of the N fertilizer rate. It is unlikely that an improvement of N fertilizer management schemes can help enhance yield potential for juncea canola more effectively than for the more popular napus canola. Furthermore, our results revealed that the extremes of fertilizer rates (0 vs. 250 kg N ha^–1) changed flowering duration and maturity by no more than a few days, suggesting that adjustment of N fertilizer rates is unlikely to be an effective tool to altering phenological differences among the oilseed species.

Overall, seed and straw yield responded to N fertilizer rates in a curvilinear manner for all the oilseed species/cultivars studied. Seed yield increased sharply with increasing N fertilizer rates up to 100 kg ha^–1. The rate of increase in seed yield peaked as fertilizer N rates approaching 100 kg ha^–1, which was higher than the current recommendation of 80 kg N ha^–1. The magnitude of the responses to N rates varied among the species/cultivars. Increasing the supply of N increased the straw yield more prominently than for seed yield in juncea canola, as reflected by the lower harvest index (0.244), compared with the high-yielding juncea mustard (0.269) or napus canola (0.251). More N fertilizer was required to maximize seed yield in juncea canola relative to napus canola under the same growing conditions. It is speculative that the juncea canola has a more structured physiological response to N fertilizer rates compared with a more flexible response in juncea mustard and napus canola. Structured physiological responses to growth resources limit the ability of crop plants to convert extra photosynthetic biomass associated with additional N fertilization into seed yield (Angadi et al., 2000). In practice, one may expect that the impact of N application on seed yield will be less for the new oilseed species, juncea canola, than for the conventional canola and mustard species, because of less sensitivity of juncea canola to addition of N fertilization.

Interactive Responses to Diverse Soil-Climatic Conditions
The five oilseed species/cultivars varied in their response to diverse environments encountered in this study. The sensitivity of these responses was partly reflected by large random variance estimates for flowering and maturity among other phenological traits. For example, juncea canola flowered 4 d earlier than napus canola and the duration of flowering lasted 7 d longer averaged across diverse environments. Earlier flowering increased the likelihood of longer flowering duration, which increased the capacity of juncea canola plants to buffer high temperature and drought stresses during the reproductive period. In the northern Great Plains, high temperature and drought stresses often occur during the latter part of the growing season (historical records, McCaig, 1997). Earlier flowering and the longer flowering duration exhibited by the juncea canola is probably the key mechanism responsible for high production in the dry areas of the northern Great Plains. Kirkland and Johnson (2000) recognized the promoting early flowering in oilseed crops was one of the key management strategies to avoid hot weather during the reproductive period. However, variance estimates for the duration of flowering and time to maturity were greater for juncea canola than for other oilseed species; this partly explained the larger variance in seed yield for the juncea canola across diverse environments. In contrast, napus canola exhibited a consistent time for the start of flowering and time to maturity across various environments; this also partly explained the consistently higher seed yield of the napus canola than other oilseed species/cultivars. Because of the close association among the start of flowering, time to maturity, and the stability of seed yield in oilseed species, one might expect that management of these factors in oilseed species, particularly in juncea canola, may have great potential of improving yield potential and stability.

Among the five oilseed species/cultivars tested, the high-yielding napus canola and juncea mustard were the most sensitive to variation in growing season rainfall, growing degree-days, and residual soil N availability. This was indicated by the larger variance estimates and the greater intercept deviation in seed yield when none to lower rates of N fertilizer (0–25 kg ha^–1) were applied. Overall, residual soil N had a more negative effect on straw yield than on seed yield, while growing degree-days had a greater impact on seed yield than on straw yield. The preceding effects were most notable in juncea canola. However, there did not appear to be any consistent pattern in the way that the three factors (i.e., growing season rainfall, growing degree-days, and residual soil N) affected the stability of straw or seed yield. Averaged across the 11 diverse environments, the low-yielding oilseed species/cultivars (namely alba mustard and rapa canola) appeared to have a high degree of yield stability. However, there was subjective evidence presented in the biplots (Fig. 3), which indicated that low N fertilizer rates resulted in the low yield stability (i.e., the greatest variability). Therefore, regardless of the oilseed species/cultivars, the application of adequate N fertilizers will help minimize yield variability and reduce production risks.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
The results of this study indicate that juncea canola can be considered as an alternate oilseed crop that is adapted to the semiarid areas of the northern Great Plains where high temperature and drought stresses often limit the productivity of conventional napus and rapa canola species. It appears that the juncea canola cultivars have improved some key phenological traits such as earlier flowering, longer duration of flowering and maturity, and improved drought tolerance during the reproductive growth period. These improved phenological characteristics help improve the adaptation of this new oilseed species to the drought-prone regions of the northern Great Plains. However, the current cultivars of juncea canola do not have the yield potential of the more popular hybrid cultivars of napus canola, and the yield stability juncea canola is lower than other oilseed species/cultivars when tested across diverse environments. Longer maturity of juncea canola crop may be a concern in cooler and short-season areas of the northern Great Plains. Compared with the high-yielding napus canola and juncea mustard, the seed yield of juncea canola had a weaker response to increased rates of N fertilizer, suggesting that N use efficiency of juncea canola is low. Further genetic enhancement in conjunction with improved management practices may be able to narrow the gaps in yield potential and N use efficiency between juncea canola and napus canola. There exist needs to determine if crop management practices such as altering seeding date or adjusting seeding rates would increase seed yield and improve yield stability and N fertilizer use efficiency for juncea canola. Further investigation of these factors on juncea canola is warranted.

	ACKNOWLEDGMENTS

 
The authors acknowledge the funding from the Saskatchewan Canola Development Commission, Agricultural Development Fund of Saskatchewan Agriculture and Food, and Agriculture and Agri-Food Canada Matching Investment Initiative. We also acknowledge the expert technical assistance of C. McDonald, G. Ford, L. Spensor, D. Leach, K. Strukoff, C. Nielsen, C. Kirkham, D. Cross, D. Rode, C. Powlowski, and D. Chen.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

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