Winter Rapeseed Recropping Considerations

Chapter 8 – Crops and Varieties, No. 14, Summer 1990

Roger Veseth

Winter rapeseed production in the Inland Northwest has largely been restricted to plantings on summer fallow. In the low-precipitation crop-fallow region, this is the main option for planting winter rapeseed. Even in the intermediate and higher precipitation areas (at least 16 to 18 inches of annual precipitation), where annual cropping is possible most years, winter rapeseed has usually been planted on fallow ground used as set aside acreage in the USDA Acreage Reduction Program (ARP). In these annual cropping areas, substantial expansion of winter rapeseed acreage in the future will require the development of agronomic practices that allow successful establishment of winter rapeseed on recrop ground.

The potential for increased flexibility in maintaining program crop base in the 1990 Farm Bill, expanding foreign and domestic markets, particularly for edible rapeseed (canola), and the possibility of new seed crushing facilities in the Northwest could expand winter rapeseed as a viable alternate crop. Low wheat prices are also stimulating interest in alternate crops. Rapeseed acreage in the Northwest is expected to increase in the 1990-91 crop year.

Although recrop winter rapeseed has generally had lower yields than rapeseed planted on summer fallow, growers are not losing a year of income with fallow and are avoiding fallow costs. Flexible recropping with winter rapeseed in years of adequate late summer or early fall rains may also be of interest to producers in the region. Recrop winter rapeseed could help lengthen crop rotations or possibly replace some less economical crop options.

Glen Murray, University of Idaho agronomist and crop physiologist, is researching management aspects of winter rapeseed production under annual cropping. Other cooperating UI scientists include agricultural engineer Charles Peterson and soil physicist John Hammel.

Murray points out that planting winter rapeseed on recrop land in northern Idaho usually occurs later than the recommended mid-August planting time because of dry soil conditions after harvest. Typically, later-planted rapeseed under conventional production practices sustains more winter injury and yields substantially less than early-planted rapeseed. The goal of the research effort is to develop an integrated crop management system, including: tillage practices; cultivar selection; planting method, date, rate and depth; seed treatment and pest control for economical winter rapeseed production under recropping situations. The development of a predictive model is also underway to help integrate and predict the impacts of different management options and environmental conditions on recrop winter rapeseed establishment and yield potential.

Murray points out that winter rapeseed, planted in August on fallow ground, provides excellent erosion control, rotational benefits for disease and weed control in the following cereal crop, and economical yields.

Later plantings on fallow or under recropping can result in smaller plants in the fall, thus reducing erosion protection and winter survival. Murray points out that planting winter rapeseed under conservation tillage in an annual cropping rotation may improve stand establishment and winter survival, as well as provide effective soil erosion protection.

Preliminary research by Murray and other scientists, and limited grower experiences, indicate that recropping with winter rapeseed can present additional production challenges compared to plantings on summer fallow. These include: lower soil water content at desired planting dates; potential increased competition from weeds and volunteer cereals; and problems associated with later planting dates. Later planting can result in a decrease in fall growth, due to lower soil and air temperatures, an associated decrease in winter hardiness, and an increase in damage from Pythium fungi in the soil. Murray’s research is focusing on developing methods to enhance recrop seedling establishment under dry soils with early planting after harvest, and under cool, wet conditions with later fall planting.

This research is being conducted near Moscow in a 22-inch annual precipitation zone. It is part of the STEEP (Solutions To Environmental and Economic Problems) conservation farming research effort in Idaho, Oregon and Washington. The High Erucic Acid Rapeseed (HEAR) project also provides funding for the research. Although Murray’s research is still in the early stages, some preliminary results are presented here, .& along with some general consideration for enhancing the success of recropping with winter rapeseed.

Preliminary Results and Management Considerations

Seedzone Water Requirement

Laboratory research by Murray has shown that a minimum soil water content of 12 percent (by weight) is needed in a Palouse silt loam soil to achieve at least 80 percent establishment (the soil has approximately 23 percent water content at field capacity). However, since the laboratory studies were done in a closed system that prevented evaporation, and evaporative demand was not measured in the preliminary field studies, he cannot accurately predict a soil water requirement for establishment from the current data. Although preliminary field data tends to confirm the 12 percent soil water requirement, Murray stresses that further field research will be needed to verify a minimum water content for germination and establishment. Wheat will germinate at lower soil water content than will rapeseed (down to 8 to 10 percent).

Seeding Date

Preliminary research results on planting date by Murray, Dick Auld, a UI plant geneticist, and other UI researchers indicate that seedling establishment and winter survival both decline sharply with delays in planting past a September 1 planting date in Moscow. However,the importance of planting date can be strongly influenced by soil moisture and temperature at time of planting.

One example of the influence of soil water content is from a 1987 study near Moscow. Winter rapeseed was planted on September 3, September 18 and October 1 on conventional fallow and direct-seeded after spring barley. A 12-foot, double-disk no-till drill was used to seed all the plots. Successful establishment after spring barley was not achieved at any of the planting dates because soil water content at the 2- to 4-inch soil depth was below 8 percent on all planting dates. However, rapeseed did establish at all planting dates on fallow where soil water content at the 2- to 4-inch depth ranged from 8 to 14 percent.

Another example shows the interaction of fall precipitation and planting date. Winter rapeseed was seeded on fallow ground on August 10, August 30 and September 13, 1988 (Table 1). Because of rains between the first and last planting dates, stand establishment increased from 4 plants per foot of row (on a 7-inch row spacing) for August 10 planting to 6 plants per foot of row for the September 13 planting. Soil water contents for the two planting dates were 11 percent and 14 percent, respectively. Due to mild winter conditions, winter survival in this study was not influenced by planting date and averaged 88 percent across the three planting dates.

Table 1. Interaction of fall precipitation and planting date on stand establishment, winter hardiness and yield of winter rapeseed planted on summer fallow, 1988-89, near Moscow, ID.

Laboratory research with rapeseed plants from the 1988-89 study indicated that cold tolerance of field acclimated winter rapeseed was influenced by planting date even though field survival was not affected because of the mild winter. Plants collected in December and frozen at various temperatures had LT50 cold tolerances (temperature required to kill 50 percent of the plants) of 11°, 16° and 25°F for the August 10, August 30 and September 13 planting dates, respectively (Table 1). Murray found that LT50 was inversely correlated to taproot diameter and rosette diameter (Correlations of –0.78, –0.76, respectively, significant at the 99 percent probability level). In other words, the larger the plant going into the winter the better the cold tolerance. Plants dug in March had LT50 values and correlations to plant size similar to those from the December sampling. Murray concludes that rosette and taproot diameters are good indicators of cold tolerance.

In a separate seed treatment-planting date study on fallow in 1988, winter rapeseed was planted on August 15, September 1, September 16 and October 1. Seed yields declined from 4,560 to 3,540 pounds per acre as planting date was delayed from August 15 to September 16. Winter survival of plants seeded on October 1 was 12 percent, too low for economic production.

In the fall of 1989, Murray planted winter rapeseed directly into spring barley stubble on August 30, September 13 and September 27. Soil water content was more than adequate for all planting dates because of above-average August precipitation. On September 27, stand establishment for the August 30 and September 13 planting dates was 7.2 and 3.4 plants per foot of row, respectively (Table 2). By October 25, some stand loss was evident in the August 30 planting. Heavy concentrations of barley residue in combine straw and chaff rows contributed to some of the seedling loss. Plant counts to determine winter survival were made in March 1990. Plant survival was 45, 31 and 1 percent from the August 30, September 13 and September 27 plantings, respectively.

Table 2. Effect of planting date on stand establishment, winter survival and yield of winter rapeseed direct-drilled after spring barley, 1989-90, near Moscow, ID.

Based on these preliminary studies, Murray concludes that a planting date in late August or early September appears essential to obtain economic yields in the Moscow area. Also, crop residue should be uniformly distributed by the combine at harvest of the previous crop or spread out after harvest to reduce planting and stand establishment problems.

Murray stresses that further research is needed on planting dates and other planting considerations under different precipitation-and climatic conditions. August or early September rains in the Moscow area the past 2 years, precipitation conditions not “normal” for the area, have significantly influenced the results of the studies.

Seeding Rate

With winter wheat, slightly higher seeding rates are sometimes used at later planting dates to help offset the possibility of increased soilborne disease and weed pressure, and decreased emergence, winter survival and tillering. However, Murray cautions growers that the effect of increased seeding rate (and narrower row spacings) for winter rapeseed, with delayed planting dates, is still largely unknown. He explains that higher seeding rates (and narrower row spacings) may increase intra-crop competition, causing smaller plants, higher crown sets and consequently, lower winter survival. Conversely, a denser plant canopy with higher plant populations may provide some additional winter protection. Murray points out that rapeseed plants have an exceptional ability to “fill in” the field space available. For example, he has found that rapeseed populations from 2 to 5 plants per foot of row on a 7-inch row spacing often produce about the same yield.

Seeding rates for winter rapeseed are usually between 4 to 12 pounds per acre on a pure live seed basis. In dryer areas, particularly where late summer water stress is a possibility, lower seeding rates of 4 to 7 pounds per acre are suggested. Murray points out that the higher seeding rates (8 to 12 pounds per acre) might be considered in higher production areas if planting date is delayed significantly, when weed competition is expected to be higher, or where seed-soil contact might be restricted by surface residue or in dry, cloddy seedbeds.

Seeding Depth

Murray suggests that shallower planting depths be used as the planting date is delayed. A 2-inch seeding depth has been fairly standard, although if surface water content is good, 1/2 to l-inch seeding depths are preferred, especially when seeding date is delayed. He stresses that seed-to-soil contact is very important for germination and establishment, and is particularly important under limited soil water conditions. Firming the seedbed, at planting with the drill or after planting with implements such as a roller harrow, can improve seed-soil contact and stand establishment under low moisture conditions. Because of soil and topographic variability in fields, seeding depth may have to be adjusted for different parts of the field to adequately reach moist soil.

Murray recommends that rapeseed be seeded into moisture if possible. However, if dry soil conditions persist and seeding rapeseed into moisture is not possible, even at 3 to 4 inches depth, Murray suggests that it maybe better to seed at a shallow depth (about 1 inch) in dry soil and allow the rapeseed to germinate and emerge after later rains, instead of delaying the planting date much beyond September 15 (in the Moscow area) to wait for rain. This approach may permit just enough additional fall growth, once the rain occurs, to ensure winter survival, in contrast to planting after the rain. He points out that even a few days delay in emergence in September can make a substantial difference in growth potential as soil and air temperatures begin to drop sharply.

The use of granular applicators with air delivery systems is becoming more popular for planting rapeseed in reduced tillage systems in some areas when there is adequate surface soil moisture for establishment. For example, fertilizer can be direct-shanked into cereal stubble with a heavy-duty applicator or modified tillage implement, the seed broadcast with an air applicator and incorporated with a standard harrow, and the field packed with a roller harrow to firm the seedbed and improve seed-soil contact. A second standard harrow operation, with the tines turned back to minimize soil disturbance, can sometimes substitute for a roller harrow operation.

Seed Germination Enhancement

In 1989, Murray and Peterson conducted an evaluation of seed treatments to enhance germination and establishment of winter rapeseed seeded directly into spring barley stubble on August 30, September 13 and September 27 planting dates. The treatments included in-furrow application of water (150 to 300 gallons per acre), premoisturized seed (20 to 40 percent by weight), planting sprouted seed in a gel, seed priming (treatment with polyethylene glycol — a commercial treatment used on some vegetable crops to accelerate germination) and normal dry seed. They did not find any improvement in plant establishment with any of the treatments over normal dry seed at the three planting dates. However, they point out that the unusually high precipitation in August 1989 prevented potential interactions between effects of seed treatments and planting dates. In earlier research with the same seed treatments with winter rapeseed on summer fallow, they observed some benefits of seed priming and in-furrow water application on seedling establishment. Planting date also influenced results from seed treatments. Further research with tests to enhance germination is planned under recrop conditions.

Disease Control

Seed rot, seedling blight and root rot caused by Pythium fungi could be more damaging with late-seeded winter rapeseed under recropping compared to earlier plantings on fallow. This is because the Pythiwn fungi is more active under cold, moist conditions which are common with late seeding. Murray has isolated Pythium from late-seeded rapeseed plants in the fall and from dead plants in the spring.

He conducted a field experiment in 1988 to evaluate the effect of three chemical seed protestants for winter rapeseed direct-seeded into spring barley stubble on August 15, September 1, September 16 and October 1 planting dates. None of the treatments are currently labeled for use on rapeseed. One of the chemical treatments with activity on Pythium did improve yield on both early and late plantings, but the other treatments had no effect. Additional research is needed to determine conditions required for beneficial responses with seed treatments.

If the frequency of rapeseed increased in the crop rotation, particularly if there are other broadleaf crops in the rotation, diseases common to rapeseed and other crops could increase. Sclerotinia white mold is one example of a diseases which affects rapeseed, peas, beans, potatoes and other broadleaf crops. Cereals are not susceptible. Murray speculates that recrop rapeseed may have a slightly lower potential for damage from Sclerotinia compared to rapeseed on fallow because of lower soil water and nitrogen levels, and consequently, smaller plants and lower associated microenvironment humidity which is less conducive to the disease.

It is currently unknown if other diseases would be more or less of a problem for recrop winter rapeseed under various tillage systems and other management practices compared to plantings on summer fallow.

Tillage

Murray points out that maintaining all or a portion of the previous crop’s residue on the surface through conservation tillage plantings of winter rapeseed offers two potential advantages for winter rapeseed establishment and survival compared to more intensive, low residue tillage systems. First, reduced tillage intensity and increased surface residue can increase conservation of existing soil water and fall precipitation before and after seeding. Direct seeding may offer the greatest water conservation potential. Under dry conditions, even a small increase in soil water content could mean the difference in whether stand establishment is successful and there is adequate fall growth for winter survival.

Second, surface residue, particularly standing stubble, may increase winter survival of small, late-seeded rapeseed plants. The stubble provides thermal insulation from wind and cold temperatures, as well as snow trapping for added protection and water storage.

An important additional benefit of surface residue is enhanced protection from soil erosion. Winter rapeseed, planted early on summer fallow, generally provides dense plant cover for erosion protection overwinter. However, late-seeded rapeseed plants under recropping do not provide adequate erosion protection. Surface residue, or surface roughness and residue, must also be utilized to control soil erosion.

If volunteer cereal and other weeds are expected to be a problem, Murray suggests that a minimum tillage system might be preferable to direct seeding. For example, a minimum tillage “shank fertilizer-cultivate-seed” approach may help improve germination of volunteer and other weed seeds to permit their removal before planting.

When winter rapeseed is planted directly into stubble, the plants tend to be somewhat taller and spindlier in the fall compared to plants growing under low-residue tillage systems because of the reduced light penetration through the stubble. This can make the plants more vulnerable to freeze injury even though the stubble helps provide thermal and wind protection for the plants. Murray plans to continue his research on direct no-till and minimum tillage planting of winter rapeseed under recrop conditions.

Fertility

Murray and Robert Mahler, UI soil scientist and STEEP researcher, have been researching fertilizer requirements of winter rapeseed. They point out some differences in fertility management between planting winter rapeseed after another crop compared to after summer fallow.

One of the biggest changes is in the nitrogen (N) fertilizer requirement. For example, approximately 80 to 90 pounds N per acre is commonly mineralized (converted from organic form to plant-available inorganic form) by microbial decomposition of soil organic matter and crop residue during a fallow year in northern Idaho. When winter rapeseed is recropped after spring barley, the amount of N that would have been mineralized in the fallow year needs to be applied as fertilizer. The actual amount needed would depend on the amount of soil test N, soil organic matter content, precipitation zone and other factors. Approximately 15 pounds of N per acre would also need to be added per ton of soil-incorporated cereal residue to replace soil N temporarily tied-up in microbial decomposition of the cereal residue. The potential for overwinter loss of N should be considered when selecting the fertilizer application timing and method.

Adequate phosphorus (P) is important for vigorous root growth and plant development, particularly when cool soil and air temperatures restrict root growth. Winter rapeseed has a moderate P requirement. The researchers point out that early root access to P fertilizer can become more important in recropping, particularly as the planting date is delayed and when soil test P levels are inadequate, Deep banding of P below seed depth and near the seed rows at or before planting are effective fertilizer placements under these conditions. Phosphorus fertilizer may also be placed with the seed. However, the total seed-placed N, sulfur (S), and potassium (as K₂O) should be less that 25 pounds per acre with good soil moisture, since rapeseed is particularly sensitive to salt damage from these fertilizers when placed with the seed. A smaller amount should be considered under dry conditions.

Winter rapeseed is also a fairly high user of S and boron (B). The availability of these are commonly overlooked in winter rapeseed production. The researchers stress the need to soil test for S and B along with the other nutrients under recropping.

More information on winter rapeseed fertility management is available in UI Cooperative Extension System publication Northern Idaho Winter Rapeseed Fertility Guide, CIS 785, by Mahler and Murray at the local County Extension Office. It includes some specific information on N fertilizer requirements under different recropping situations.

Weed Control

Murray stresses that growers need to keep in mind there may be higher weed competition for winter rapeseed under recrop than after fallow, since fallow does help to reduce weed populations for the next crop. Weed control is an important consideration because recrop winter rapeseed plants are often smaller and less competitive with weeds during the late fall, winter and early spring. Treflan currently is the only labeled herbicide in the Northwest for rapeseed and soil incorporation if required. This usually prevents its use in direct seeding systems, and may restrict its use in some reduced tillage systems.

When winter wheat is the previous crop, volunteer wheat can be a major weed problem. Although a spring cereal is generally a preferred previous crop for recropping winter rapeseed, growers should try to avoid crops, such as Steptoe spring barley, which can cause volunteer problems due to winter hardiness. Weed competition in winter rapeseed after spring peas and lentils can also be particularly severe because these crops are less competitive against weeds and herbicide options are limited. Wild oat is often another potential problem in ~crop rapeseed because of the smaller, less-competitive rapeseed plants. Some grassy weed herbicides for winter rapeseed are being considered in the pesticide registration program for minor crops (IR-4).

Ideally, growers should make an effort to choose fields that have low weed populations for recropping with winter rapeseed. The potential for rapeseed damage from soil-residual herbicide applications in the previous years should also be considered.

Recrop Winter vs. Spring Rapeseed

Approximately 60 percent of the rapeseed in eastern Washington is now spring rapeseed and 40 percent is winter rapeseed. In contrast, winter rapeseed predominates in northern Idaho, where there is only a small spring rapeseed acreage.

Murray points out that, in some areas, growers may want to consider spring rapeseed instead of winter rapeseed in a recropping program. He explains that the choice will depend on a number of factors including: possible planting dates, length of growing season, soil water content, expected growing season precipitation, overwintering problems and the potential for flea beetle damage. Spring rapeseed may be attractive for recropping in areas where winter rapeseed often fails to emerge, due to low soil water content, or winterkills. If winter rapeseed fails to establish or is winterkilled, Murray suggests that spring rapeseed may still bean option in some areas the following spring, as well as possibly replanting with winter or spring cereals.

Flea Beetle Damage

Joe McCaffrey, UI entomologist and STEEP researcher, points out that flea beetle damage can be severe on spring rapeseed in northern Idaho and the surrounding region. The flea beetle is a small black beetle which can damage or kill small seedlings by defoliation. The flea beetle has been less of a concern in dryer production areas, possibly because earlier planting dates in these areas may help escape some of the flea beetle damage since the insect is less active at cooler temperatures. Currently, no insecticide is registered in the U.S. for flea beetle control in rapeseed. Winter rapeseed is generally not damaged by flea beetle, unless seeded extremely early on summer fallow.

Flea beetle damage is often most severe on the field margins. Double seeding of field margins of spring rapeseed is a practice commonly used by some Canadian growers to minimize stand losses from the flea beetle. However, it is unknown how effective that practice would be in this region.

McCaffrey cautions growers that the cabbage seed pod weevil can also be a problem in spring rapeseed as well as winter rapeseed, even though spring rapeseed is somewhat later maturing. Damage can be particularly significant when spring rapeseed blooms near the time of winter rapeseed bloom in the area.

Yield Comparison

Murray points out that spring rapeseed typically has a better overall success of establishment than winter rapeseed. But, when successfully established and overwintered, winter rapeseed usually out-yields spring rapeseed. At Moscow and other northern Idaho areas, spring rapeseed yields are generally about half of winter rapeseed yields. In eastern Washington, Spectrum Crop Development agronomist Andy Thostenson reports that, with good production management practices, spring rapeseed can yield 65 to 70 percent of winter rapeseed yields.

Murray stresses that early seeding is very important for spring rapeseed production. Consequently, the potential planting date for spring rapeseed can be an important factor in deciding whether to plant spring or winter rapeseed. For example, Murray points out that spring rapeseed often cannot be planted in the Moscow area until late April, due to wet soil conditions, while late February or early March plantings are often possible in somewhat dryer areas of eastern Washington. Higher yield potentials are possible with earlier seeding dates.

Future Implications

At the present time, winter rapeseed production is less risky when it is planted on fallow land compared to recrop land. However, Murray points out that growers can still take advantage of opportunities for recropping with winter rapeseed when they arise – when early rains or cropping systems provide adequate soil water content for seeding establishment in August or early September. Direct seeding and other conservation tillage systems may facilitate recropping with winter rapeseed by increasing retention of soil water and providing overwinter protection. Murray hopes that research will also identify seed treatments, to enhance establishment and disease control, and weed control options. Prospects for recropping with winter rapeseed continue to be evaluated in current research efforts at the University of Idaho.