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a Potash and Phosphate Inst. of Canada, 12-425 Pinehouse Dr., Saskatoon, SK, Canada S7K 5K2
b USDA-ARS, Northern Great Plains Res. Lab., Box 459, Mandan, ND 58554
c Montana State Univ., Dep. of Land Resour. and Environ. Sci., P.O. Box 173120, Bozeman, MT 59717-3120
d Agric. and Agri-Food Can., Box 10, Scott, SK, Canada S0K 4A0
e USDA-ARS, Cent. Great Plains Res. Stn., 40335 Country Rd. GG, Akron, CO 80720
f Agric. and Agri-Food Can., Box 760, Indian Head, SK, Canada S0G 2K0
g North Dakota State Univ., Williston Res. Ext. Cent., 14120 Hwy. 2, Williston, ND 58101-8629
* Corresponding author (ajohnston{at}ppi-ppic.org)
Received for publication December 1, 2000.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONCANOLAMUSTARDFLAXSUNFLOWERSAFFLOWEROILSEED CROP RESEARCH NEEDS...REFERENCES |
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONCANOLAMUSTARDFLAXSUNFLOWERSAFFLOWEROILSEED CROP RESEARCH NEEDS...REFERENCES |
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The use of minimum and no-till seeding systems has been found to provide an effective means of controlling soil erosion in various regions of the Great Plains (Black and Power, 1965; Lindwall and Anderson, 1981). Improvements in seed yield with conservation tillage have been reported as a result of increased levels of plant-available water throughout the soil profile in the spring (Aase and Reitz, 1989; Brandt, 1992; Lafond et al., 1992) and increased water use efficiency due to favorable microclimate conditions created by standing stubble (Cutforth and McConkey, 1997). Some oilseed crops are small seeded, requiring good surface soil moisture for seed germination and crop establishment, as is effectively provided in direct-seeding systems in the northern Great Plains. As a result, adoption of conservation tillage management not only reduces soil loss by erosion, but also can facilitate extending the crop rotation and allowing for diversification of the crops grown. Economic success with a diversified crop rotation has been reported to be improved with the implementation of conservation tillage practices, such as minimum and zerotillage (Lafond et al., 1993; Rossetti et al., 1999; Zentner et al., 2002).
The objective of this review is to summarize information on the adaptation and production potential of some oilseed crops in dryland rotation with cereals in the northern Great Plains. Data has also been included from Akron, CO. Although located in the central Great Plains, it has similar climatic conditions to the northern Great Plains because of its high elevation. The oilseed crops of interest include canola, flax, mustard, sunflower and safflower. Soybean, while an oilseed, has been addressed in an associated review of grain legumes by Miller et al. (2002). Information will be presented from a variety of locations, representing a wide range of environmental conditions (Table 2).
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CANOLA |
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TOPNOTESABSTRACTINTRODUCTIONCANOLAMUSTARDFLAXSUNFLOWERSAFFLOWEROILSEED CROP RESEARCH NEEDS...REFERENCES |
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Canola is cultivated widely on the northern Great Plains as a spring crop because sufficient winter hardiness has not yet been developed. The spring growth habit also fits well for canola as a summer crop in rotation before winter wheat in the coolest parts of the central Great Plains (Nielsen, 1997, 1998a).
Crop Water Use
Field assessment of B. napus indicates that it is capable of extracting water from depths of 114 to 165 cm (Table 3). The shallow measurement of 114 cm was an actual root observation and more accurately represents rooting depth than the deeper estimates, which were based on water extraction. Nielsen (1997) reported that while neutron scatter measurements found that canola extracted water to depths of 165 cm, 92 to 95% of the growing season water use of the crop came from the surface 119 cm of soil profile. Brassica spp. have a taproot system giving the crop access to deep water and nutrients (Downey et al., 1974). When grown in semiarid regions, the rooting characteristics of canola require adequate subsoil moisture to sustain the crop during flowering and seed filling. In those areas of the northern Great Plains where growing season water deficit is high, canola should be grown on summer fallow rather than continuous crop. Root growth rates for canola were greater than several other spring crops in a multiyear study at Swift Current, SK, more quickly depleting soil moisture reserves, and thus sooner becoming dependent on rainfall to sustain growth (Angadi et al., 1999b).
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This relationship indicates that for each degree rise in mean daily temperature, the yield of B. napus declines by 188 kg ha-1 and B. rapa by 217 kg ha-1. This indication of greater heat sensitivity for B. rapa contrasts with the observations in controlled environment conditions by Angadi et al. (1999a). Also, for each millimeter increase in precipitation, there is a corresponding 5.9 kg ha-1 yield increase with B. napus and 3.3 kg ha-1 yield increase with B. rapa, indicating a marked difference in water use sensitivity, contrasting with the equations presented in Table 4. Given that Brandt and McGregor (1997) used a common measurement period for their water use assessment and that B. rapa starts and finishes flowering 2 wk before B. napus, they may have introduced some bias into their evaluation of these crops.
Canola Management in Rotation
Including canola in rotation with wheat can have a positive effect on wheat yield. Brandt and Zentner (1995) reported that spring wheat yields were 24% higher on canola stubble than when grown on wheat stubble in a continuous wheat rotation at Scott, SK. At Melfort, SK, wheat yields were 30% higher when grown following canola than following wheat (Zentner et al., 1986). The influence of the canola crop in breaking plant disease [Septoria spp. and tan spot (Pyrenophora tritici-repentis)] buildup in the rotation was suggested as the main benefit derived from the preceding canola crop. In southern Alberta, winter wheat yields were increased by 25% when grown on canola vs. winter wheat stubble (Larney and Lindwall, 1994).
Tillage studies with canola have found that the crop is well suited to no-till seeding (Lindwall et al., 1994). Canola seed yield on no-till ranged from 1 to 14% higher than on conventional tillage. In drier regions, the maintenance of surface soil moisture with no-till has improved establishment of small-seeded crops like canola, attested by numerous observations by agronomists. In addition, the high water use requirements of canola can benefit from any additional water stored in the profile of no-till fields and may have increased water use efficiency from the microclimate of standing cereal stubble (B. McConkey, personal communication, 1999).
Ludlow and Muchow (1990) pointed out in a review that the ability to match key phenological growth period of a crop to a less stressful growth period in the growing season can be an effective means of avoiding the negative impact of heat and drought stress. Seeding date has been found to be a very important management tool in minimizing the negative impact of high temperature and moisture stress during the critical flowering and seed-filling periods in canola. Delaying canola planting past mid-April to early May has been shown to result in final seed yields that were 22 to 38% lower than late fall or early spring (Johnson et al., 1995; Kirkland and Johnson, 2000; Brandt et al., 1999). In North Dakota, the reduction in canola yield with delayed seeding was attributed to fewer pods per plant, leading to a lower harvest index (Johnson et al., 1995). Research on the Canadian prairies reported that when freezing temperatures were recorded with early planting dates, they did not negatively affect crop survival as the young seedlings appear tolerant to cold stress if acclimated (Kirkland and Johnson, 2000). A combination of factors, including cultivars, stage of development, pattern and duration of freezing temperatures, surface residue cover, and proximity of the seedlings to the soil surface, all play a role in the reaction of the seedlings to freezing temperatures.
In Saskatchewan, where winter soil temperatures generally never rise above freezing, Kirkland and Johnson (2000) have taken the early seeding of canola one step further and evaluated dormant seeding of canola in the fall just before soil freeze-up. This dormant-seeded canola is ready to begin growth as soon as temperature and moisture conditions permit in the following spring. The researchers report that relative to the traditional mid-May seeding date, late-April and fall dormant seeding increased canola yields by 44 and 34%, respectively. While crop establishment with dormant seeding is usually never as good as that achieved with spring seeding, the adaptable nature of the canola crop allows it to compensate for a thin stand through increased branching. Additional advantages of dormant seeding included earlier crop flowering (25 d), windrowing (18 d), and harvesting (19 d) (Kirkland and Johnson, 2000).
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MUSTARD |
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TOPNOTESABSTRACTINTRODUCTIONCANOLAMUSTARDFLAXSUNFLOWERSAFFLOWEROILSEED CROP RESEARCH NEEDS...REFERENCES |
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Crop Water Use
Information on the water use characteristics of mustard was reported in recent research by Brandt et al. (1999), which included both B. juncea and S. alba. They report that B. juncea is very similar to canola in its conversion of water into seed yield (Table 4). However, while S. alba requires less water to establish its minimum seed yield, it also has much lower seed production per unit of water use relative to B. juncea. Angadi et al. (1999b) reported that B. napus and B. juncea were similar in their water use characteristics, and as a result, they concluded that despite its reputation as a drought-tolerant crop, mustard was no better adapted to semiarid regions than canola. In fact, the high water use characteristic and deep rooting pattern of the Brassica species of canola and mustard make them better suited than spring wheat or field pea (Pisum sativum L.) for production on summer fallow in semiarid regions (Brandt and McGregor, 1997). Given that summer fallow is no longer part of many cropping systems in semiarid regions, positioning canola in rotation following low water-use, shallow-rooted crops and optimizing water conservation with no-till seeding may be required to achieve successful production.
Optimum daytime air temperature for maximizing yield of B. juncea was similar to that of B. rapa, close to 28°C, and higher than the 20°C optimum for B. napus (Angadi et al., 1999a). Subjecting Brassica species to a high-temperature stress during the flowering stage of development using a temperature of 35 to 15°C (day and night, respectively) reduced the seed yield of B. juncea by 34% and B. rapa by as much as 93%. Brassica napus and S. alba both flowered with fewer accumulated heat units when seeded early rather than late (Miller et al., 1998a). Delayed seeding of S. alba resulted in the largest proportional (30%) reduction in seed yield among 12 cereal, oilseed, and pulse crops grown in a seeding date study at Scott and Swift Current, SK. For both B. juncea and S. alba, the reduction in seed size was equal in proportion to the reduction in pod density.
Mustard Management in Rotation
Little research has been conducted into the management of mustard in rotation with cereals in the northern Great Plains. In one cropping sequence study at Swift Current, SK, B. juncea did not increase seed yield of spring wheat significantly over that of continuous spring wheat though a small increase (0.5 protein units) in wheat grain protein was observed (Miller et al., 1998a). During one of eight site-years of that study, volunteer mustard seedlings competed strongly with spring wheat at the seedling stage, reducing wheat yield by 49% compared with continuous spring wheat, despite complete control with herbicide application within the recommended window. Given that mustard has a water use, rooting depth, and growth and development pattern similar to canola, it is likely to provide cropping system effects similar to canola (Angadi et al., 1999b). As discussed for canola above, optimizing the yield of Brassica sp. oilseeds in semiarid environments will depend on the timing of water and high-temperature stress relative to crop flowering and seed formation.
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FLAX |
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TOPNOTESABSTRACTINTRODUCTIONCANOLAMUSTARDFLAXSUNFLOWERSAFFLOWEROILSEED CROP RESEARCH NEEDS...REFERENCES |
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Crop Water Use
Relative to the other oilseed crops, flax is a shallow-rooted crop (Table 3). Research on flax water use and extraction from the soil has shown that flax will extract 90% or more of plant-available water from the 0- to 60-cm soil layer (Lafond, unpublished data, 1998). In agreement, Campbell and Zentner (1996) showed that flax grown on fallow often conserved more water in the 60- to 120-cm soil layer than did spring wheat. Seed yield response to water use from trials conducted at Swift Current and Scott, SK, indicates little difference between flax and B. napus at the same location (Table 4). Once the minimum water use of approximately 127 mm was achieved, seed yield increased at a rate of 7.6 kg ha-1 mm-1. However, at Indian Head, SK, 12 yr of water use (WU) data with flax gave a very poor relationship with seed yield [Yield (kg ha-1) = 1.9 (WU + 532 mm); r2 = 0.06]. This poor relationship at this higher-yielding location indicates that something other than water is influencing the seed yield response of flax (Lafond, unpublished data, 1998).
The shallow rooting behavior of the flax crop makes it well adapted to the improved moisture conditions of the surface soil found in no-till production systems (Lafond et al., 1992). The benefits of no-till production were demonstrated over a wide range of growing conditions, i.e., from hot and dry to cool and moist conditions (Lafond and Derksen, 1996). In fact, flax was shown to have greater water use efficiency under no-tillage (5.4 kg ha-1 mm-1) relative to conventional tillage (4.9 kg ha-1 mm-1).
Flax Management in Rotation
Although flax has been grown for a long time on the Canadian prairies and in the northern Great Plains of the USA, few rotational studies have included it (Lafond et al., 1992). Campbell and Zentner (1996) reported that flax conserved soil NO3 and water below the 60-cm soil depth compared with spring wheat though it inexplicably failed to provide a rotational benefit to a subsequent wheat crop compared with continuous spring wheat. Soil under flax stubble dries and warms sooner than that under wheat stubble, and it is possible that timeliness of water use was compromised due to a common date for spring tillage operations, which had to be delayed until the wheat stubble was sufficiently dry. It has also been recommended that flax should not be grown after legumes or potato (Solanum tuberosum L.) because of the potential for infection from Rhizoctonia spp. bacteria (Flax Council of Canada, 1998). While the recommendations have never been substantiated, it is believed that flax grown after alfalfa does exceptionally well in subhumid regions of the Canadian prairies. Other studies that have examined flax from a crop rotation basis have revolved around seeding flax into canola stubble. These studies have shown that when young volunteer canola seedlings are tilled just before the seeding of flax, yield reduction is experienced, supposedly as a result of indole glucosinolates in the vegetative material inhibiting germination (Vera et al., 1987). However, this effect was muted and even eliminated when flax was seeded directly into canola stubble as in a no-till production system (Gubbells and Kenaschuk, 1989). A survey of the Manitoba Crop Insurance Database has revealed that the yields of spring wheat were generally higher following flax than following canola or field pea (Bourgeois and Entz, 1996). Compared with canola, this can be explained in part by the fact that the shallow-rooting behavior of flax would favor the deeper-rooting crops like wheat because of extra moisture and nutrients conserved below the 60-cm soil layer.
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SUNFLOWER |
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TOPNOTESABSTRACTINTRODUCTIONCANOLAMUSTARDFLAXSUNFLOWERSAFFLOWEROILSEED CROP RESEARCH NEEDS...REFERENCES |
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Crop Water Use
Sunflower is a deep-rooted crop that requires 10 to 30 d more to mature than cereals, making it a high water-use crop. Plant rooting depths of 120 to 269 cm have been reported (Table 3). Jaafar et al. (1993) found that sunflower in Kansas was rooting to a depth of 269 cm, with 87 to 96% of the roots observed in the surface 165 cm of the soil profile. Nielsen (1998a) also found that sunflower in northeastern Colorado extracted water to a depth of 165 cm and that it showed very uniform water extraction to this depth. Unlike canola, which developed a deeper and more prolific root system in a dry vs. wet growing season, sunflower was very consistent in its rooting depth and water extraction regardless of the growing season conditions (Nielsen, 1998a). On loam and silt-loam soil types, sunflower has been shown to be capable of extracting soil water to a lower matric potential than corn (Zea mays L.), proso millet (Panicum miliaceum L.), and winter wheat (Unger, 1990; Nielsen, unpublished data, 1995).
Work in Colorado indicated that sunflower seed yield was less responsive to water than canola (Table 4) (Nielsen, 1998a). Similar results were reported in work on early maturing open-pollinated, short-stature sunflower in a comparative trial with B. juncea at Swift Current, SK (Miller et al., 1998a). Miller et al. (1998b) concluded that short-stature sunflower used water inefficiently relative to mustard and, as such, may not be as well suited as other oilseed crops in semiarid regions. However, subsequent research with a dwarf hybrid sunflower, slightly later in maturity than the short-stature sunflower, showed water use efficiency equal to B. juncea (Miller et al., 1998a). Direct comparisons of water use efficiency between sunflower, a warm-season plant, and cool-season oilseeds are difficult to make accurately in the highly variable climatic conditions of the northern Great Plains. The much longer growth cycle of sunflower causes a higher likelihood of anthesis and seed fill occurring during summer drought. However, midsummer rainfall received after cool-season oilseeds have ceased growth can be converted efficiently to seed yield by sunflower. Also, the seeding date of sunflower is often delayed weeks to months after Brassica spp., indicating that these crops utilize the environment in completely different patterns. This may be because sunflower is a warm-season broadleaf and most of the preanthesis growth occurs during periods of high evapotranspiration demand, whereas B. juncea is a cool-season broadleaf and most of its vegetative growth occurs during the low evapotranspiration part of the growing season.
Sunflower Management in Rotation
Crop management trials in Colorado have found that sunflower fits well into cereal-based crop rotations but should not be grown more than once every 3 or 4 yr due to persistence of the damaging plant disease phoma (Phoma macdonaldii Boerma) (Anderson et al., 1999). The high water use of sunflower reduced the yield of subsequent crops in rotation, especially during dry years. Results from a crop rotation study at Akron, CO, showed reduced average yields of winter wheat when the crop was grown following sunflower in rotation compared with rotations without sunflower (Nielsen et al., 1999). Yield reductions were much larger in a 3-yr rotation (wheat–sunflower–fallow) than in 4-yr rotations (wheat–corn–sunflower–fallow or wheat–millet–sunflower–fallow). In wet years, the yield-reducing effect of the previous sunflower crop was not evident in the 4-yr rotations but still manifested itself in the 3-yr rotation.
To reduce the potential for crop failure after sunflower, good soil and water conservation practices need to be used, and a drought-tolerant crop should be planted that uses water efficiently. Nielson (1998b) showed that much of the detrimental effect of the high soil water extraction by sunflower could be offset when sunflower stalks are left standing after harvest. In years with normal to above-normal snowfall in the central Great Plains, 6 to 15 cm of soil water recharge can occur while concurrently providing adequate protection against wind erosion (Nielsen and Aiken, 1998).
Wind erosion potential and soil water evaporation are minimized when no-till or reduced-till weed control and sunflower-planting practices are employed to maintain previous crop residues on the soil surface during the sunflower growing season. In recent years, there has been increased interest in minimum- and no-till sunflower production practices (Blamey et al., 1997). Maintaining surface residue from the previous crop results in reduced soil erosion and evaporation and increased rainfall infiltration and soil water storage. Maintaining surface residue using no-till helped suppress evaporation and increased sunflower seed yield 16% compared with conventional tillage (Norwood, 1999).
The maintenance of surface crop residues in sunflower rotations helped to reduce or prevent the buildup of certain weed species. Weed populations often proliferate within the open canopy of sunflower, but combining cultural practices, such as delayed seeding, narrow rows, banded fertilizer, and increased plant population, has improved weed control (Tanaka and Anderson, 1999).
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SAFFLOWER |
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TOPNOTESABSTRACTINTRODUCTIONCANOLAMUSTARDFLAXSUNFLOWERSAFFLOWEROILSEED CROP RESEARCH NEEDS...REFERENCES |
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The crop was introduced into North America as early as 1899 from its origin in the Middle East and South Asia. Safflower was initially tested in the USA in 1925 as a new oilseed crop and was commercially grown in the Great Plains beginning in the late 1950s (Knowles, 1958). Oil content of safflower seed ranges from 35 to 40%. There are two types of safflower cultivars: those producing oil high in oleic or monounsaturated fatty acid and those high in linoleic polyunsaturated fatty acid (Berglund et al., 1998). After oil extraction, the seed meal has a protein content of about 24% (Hoag et al., 1969). Safflower seed that is uniformly bright white in color or lightly striped can be marketed as birdseed.
Crop Water Use
Safflower generally is considered a daylength-neutral, long-day plant that needs at least 1400 degree days (5°C basis) to reach maturity. Miller et al. (2001) used a very early maturing cultivar of safflower (Saffire) to calculate degree-day requirements of 1430 to 1480 at Swift Current, SK, concluding that maturity requirements for safflower generally were too great for that location. Plants have a deep taproot that can extend to a depth of 220 cm (Table 3) and xerophytic spine attributes that contribute to good drought and heat tolerance (Dajue and Mundel, 1996). Safflower will not survive in standing water for even a few hours during warm weather (Mundel et al., 1997). Therefore, good estimates of yield per unit of water are difficult to determine. Research at Mandan, ND, found that the water use efficiency of safflower ranged from 1.2 to 5.1 kg ha-1 mm-1 for crops seeded mid-May or early June (Alessi et al., 1981). Studies conducted by Hang and Evans (1985) using a line-source irrigation method determined that safflower's water use efficiency was 8.45 kg ha-1 mm-1 at 50 cm of irrigation. In north-central Montana, Brown and Carlson (1990) collected water use data in safflower cultivar trials over 10 yr, generating a rainfed yield–water use equation of:
Water use was estimated based on soil water extraction to an assumed 213-cm soil depth, with an estimated 254 mm of plant-available water. The Brown and Carlson (1990) equation appears similar to the high end of the range reported by Alessi et al. (1981). In general, safflower grown in the northern Great Plains of the USA yields best in areas where along with good soil moisture at seeding, growing season precipitation (May through August) is in the 15- to 25-cm range, with at least 75% of the precipitation occurring before the end of July. The flowering and pollination of safflower can be severely reduced by rainy days or days with excessive dew and high humidity. Plant diseases such as alternaria (Alternaria carthami) and bacterial blight also flourish under these conditions and can cause major yield reductions.
Safflower in Rotations
Including safflower in wheat-based rotations can improve use of deep soil water and N (Black, 1993). The long growing season required by the crop permits deep root growth, enabling safflower to use water and nutrients from a greater soil depth than crops with a shorter growing season. Because safflower uses more soil water and requires a longer growing season than small grain crops, it and other crops of similar rooting depth should be grown in 3-yr or longer rotations to reduce potential failure of other crops in a rotation. In areas that have dryland saline-seep problems, safflower can be used in rotation to use surplus water from recharge areas to prevent the expansion of saline seeps (Mundel et al., 1992). Control of grassy weeds in safflower can benefit subsequent small grain crops in a rotation. Conversely, broadleaf weed control in small grains can also benefit safflower production because few herbicides are registered for use in this minor crop.
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OILSEED CROP RESEARCH NEEDS IN THE NORTHERN GREAT PLAINS |
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TOPNOTESABSTRACTINTRODUCTIONCANOLAMUSTARDFLAXSUNFLOWERSAFFLOWEROILSEED CROP RESEARCH NEEDS...REFERENCES |
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Canola and mustard crops are capable of producing large amounts of crop biomass under conditions of good early season moisture and warm temperature. However, low-water and high-temperature stress conditions during flowering and seed filling can result in poor seed formation and a very low harvest index. There is a need to continue to evaluate canola and mustard cultivars under high-stress environments to determine if there are plant types better suited to these environments, with conservative growth habits that are better matched to soil water constraints. Further research is required across the semiarid regions of the northern and central Great Plains to evaluate the full impact of alternative seeding dates on the suitability of Brassica sp. oilseeds in cropping systems. This type of seeding-date manipulation may greatly expand the production area of these cool-season oilseed crops.
The authors' understanding of the water use characteristics and yield potential of flax is very poor. In the more humid areas of the northern Great Plains, there is a need to understand why the positive relationship between seed and water use becomes very weak as available water and water use increases. However, likely the greatest need for research in flax is to develop a better understanding of yield formation, with an emphasis on which management practices have the greatest influence on yield and how this knowledge can be used in the development of new flax cultivars.
Future research needs to continue to define where sunflower has the best fit in the overall crop production system. The role of a deep-rooted crop like sunflower in utilizing subsoil moisture and nutrients has not been fully exploited in cropping systems where both deep- and shallow-rooted crops are grown.
Safflower seedlings lack the vigor displayed by many other crops (Berglund et al., 1998). Spring stand establishment is a major problem in the northern Great Plains because of inadequate seed–soil contact, soil crusting, lack of adequate soil moisture in the seedbed, or seedling diseases. Early spring planting can also be a problem because of cool soil temperatures. Future research areas for safflower include improving seedling vigor, improving seed dormancy to minimize sprouting at harvest, increasing seedling cold tolerance, developing seeding techniques, and other management practices to improve spring stand establishment, including those improving resistance to seedling disease. Areas such as weed control, rotations, and value-added products are major research topics currently being pursued. But one research area that the safflower industry sorely needs is marketing research and creation of a national organization to promote safflower as a very healthy, edible oil; viable industrial oil, and food for birds.
The successful inclusion of oilseed crops in cereal-based cropping systems has been shown in this review to have positive agronomic and economic impacts. Genetic improvement in oilseed crop yields will continue to make them economically competitive with cereals. Evaluation of oilseed crops through agronomic management will ensure their role as a diversification option in Great Plains cropping systems.
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TOPNOTESABSTRACTINTRODUCTIONCANOLAMUSTARDFLAXSUNFLOWERSAFFLOWEROILSEED CROP RESEARCH NEEDS...REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONCANOLAMUSTARDFLAXSUNFLOWERSAFFLOWEROILSEED CROP RESEARCH NEEDS...REFERENCES |
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