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Chapter 8 - Crops and Varieties, No. 5, Spring 1987

Winter Rape Management Research

Roger Veseth

Because of the current wheat and barley surpluses and low market prices, Northwest producers are seriously evaluating alternate crop options. Winter rape has received considerable interest over the past few years. After a relatively stable annual production on about 5,000 to 7,000 acres in the Northwest, winter rape acreage rapidly increased to 36,000 acres in 1986 because of increased prices, new varieties and expanding markets. Although lower prices have reduced the 1987 crop to about 25,000 "acres, strong producer interest suggests that they are ready to increase production when markets improve.

Besides providing potential economic diversity in crop options, winter rape offers additional advantages. It has been shown to be an excellent rotational crop, reducing the levels of some important soilborne diseases of cereals and perhaps peas. Producers commonly report that they achieve their highest winter wheat yields after winter rape in a 3-year rotation. Winter rape provides excellent soil erosion control if planted early. No-till and minimum tillage seeding of the following cereal crop after winter rape is becoming increasingly popular. Winter rape can be used as livestock feed as well.

New Management Options

Traditionally, winter rape has been seeded in August on conventionally tilled summer fallow in the higher precipitation areas receiving in excess of 22 inches annually. In the past few years, there has been increasing interest in seeding winter rape in an annual cropping rotation, particularly under no-till, as well as expanding the production area to lower precipitation areas of the Northwest where crop-fallow rotations are normally used. Little is known about winter rape establishment and management under these new conditions, however.

Research by University of Idaho crop physiologist and STEEP researcher Glen Murray is focusing on management considerations for winter rape under these new production options. His research is conducted in cooperation with Dick Auld, UI plant breeder; Charles Peterson, UI agricultural engineer; and other STEEP researchers involved in winter rape research,

Soil Water Content

As winter rape production expands to annual cropping systems and to drier Northwest regions, soil water content at planting can become more critical. From preliminary results of Murray's research over the past few years, it appears that a minimum soil water content of 12 percent (weight/weight basis) in the surface 6 inches is needed for adequate germination and emergence. This percent age was determined on a silt loam soil. For comparison, water content of the soil at field capacity is about21 percent.

Murray has also conducted laboratory studies on the relationships of soil water content to germination and emergence of winter rape using closed growth chambers for precise soil water monitoring. Results indicate that emergence can be delayed if soil water content is less than 10 percent. However, Murray estimates that the minimum soil water content in the field would again be about 12 percent after evaporation loss in the field is taken into account. Tillage and residue management practices such as minimum tillage and no-till are needed to focus on minimizing evaporative water loss and maximizing the storage of available precipitation to help ensure adequate soil water for winter rape establishment under annual cropping and in drier regions.

Annual Cropping and Conservation Tillage

Murray has conducted 2 years of research near Moscow on winter rape seeded in an annual crop rotation under conventional tillage compared to seeding on conventional summer fallow. The results indicate that winter rape seeded in early August, after green processing peas were harvested, offered the greatest potential for successful establishment under dryland recropping. However, yields were about 20 percent lower than yields of winter rape planted in fallow. Winter rape seedings after harvest of dry peas and spring barley were not successful in stand establishment.

Murray feels that no-till (one-pass direct-drill systems) may offer the best potential for successful winter rape establishment under annual cropping and in the drier areas of the Northwest. This is because no-till seeding results in lower rates of water loss by evaporation than with seedbed preparation methods used under conventional tillage. Also, if standing stubble remains after seeding winter rape, soils would remain warmer into the fall compared to a bare soil surface, typical of conventional summer fallow. This could allow development of a larger plant which would increase winter survival. The stubble would also moderate winter temperatures and potentially reduce winterkill.

To evaluate plant response to no-till seeding of winter rape in annual cropping, Murray no-till seeded Dwarf Essex winter rape directly into standing spring barley stubble on Sept. 15, 1986. He used a research no-till drill (Modified Haybuster 1000 Series) to plant the rapeseed perpendicular to the row direction in a wheat nursery at the UI Plant Science Farm near Moscow. This provided alternating plots of standing stubble and fallowed alleys, each 3 feet wide, The study initially focused on three factors: stand establishment, the ability of the plants to' 'harden off" under stubble for winter dormancy and overwinter survival.

Field survival of seedlings in the stubble and on bare fallow was not statistically different averaging 65 percent, Murray attributed the lack of difference between survival in stubble and on bare fallow to the mild winter and possible micro-climatic protection the adjacent stubble plots provided for the rape seedings on bare fallow. Pythiwn and winter kill, common problems on late-seeded winter rape on fallow, were not observed in the study.

No differences in field-hardening were noted between the stubble and fallow plots. Field-hardened winter rape seedlings were collected on November 4, frozen in freezing chambers and then grown in the greenhouse to estimate cold tolerance. Winter pea plants from an adjacent experiment were sampled the same day as a comparison of relative cold tolerance. At 17F in the freezing chambers, one-half of the winter pea seedlings were killed. Winter rape seedlings were not injured at that temperature. Cold tolerance of winter rape was previously thought to be similar to that of winter peas.

Future Research Plans

Murray plans to continue research on no-till winter rape under annual cropping. Larger, more extensive plots are planned for this fall. Murray also plans additional research on other factors involved in winter rape production under annual cropping. These include control of diseases associated with later seeding, weed control options and fertility management.

Pythium ControlVarious species of Pythium fungi can cause significant yield losses in late seeded winter cereals and early seeded spring cereal and spring peas in the intermediate and higher precipitation areas of the Northwest. Murray has documented Pythium damage to late seeded winter rape plants under annual cropping. He points out that Pythium damage maybe one of the important reasons why winter survival of winter rape is reduced with later seeding. Larger, earlier-seeded plants may be better able to resist Pythium infection and root damage. The effects of seed treatments and seeding date on Pythium damage will be evaluated.

Weed ControlWhere winter rape is seeded after cereals, volunteer grain the following year can become an important weed and no effective herbicide is available. Currently Treflan is the only herbicide labeled for weed control in winter rape. Cooperative weed control research is planned to evaluate future herbicide options.

Fertility ManagementWinter rape fertility management would also be different under annual cropping than after summer fallow. A Northern Idaho Fertilizer Guide for Winter Rape (UI Current Information Series No. 785) was completed in July 1986 by Robert Mahler, UI soil scientist, and Murray. It covers fertilizer considerations under several crop rotation options. Because of the high level of soil nitrogen which is often present in summer fallow, Murray points out that producers may think that winter rape is a low input crop. Nitrogen fertilizer requirements will be higher under annual cropping than under fallow. As a general rule, a 3,000 pounds/acre winter rapeseed crop will require 240 pounds/acre of nitrogen, about the same as a 90 bushels/acre winter wheat crop. For decomposition of the previous cereal crop residue, the application of an additional 15 pounds/acre nitrogen is suggested per ton of straw, up to 50 pounds/acre of nitrogen.


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