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Cover cropping with fall rye? Careful!

By Rupert Jannasch

Cover crops are invaluable for controlling weeds, preventing soil erosion and preserving soil organic matter. Ideally, growers will vary cover crops from year to year to take advantage of different botanical characteristics such as rooting patterns, allelopathy and nitrogen fixation. Variety also helps thwart the establishment of certain weeds and pests. The hectic pace of the farming season, however, often means growers are scrambling to plant any crop at all by autumn. For the sake of easy establishment and reliability many will choose the old standby, fall rye.

By late August, cover crop choices are limited. Oats, brassicas (oilseed radish, rape/canola, mustard) and possibly buckwheat (very frost sensitive, little biomass) are a few options. Unlike fall rye, oats and brassicas winterkill and leave little surface trash to contend with in the spring. Hairy vetch will not over-winter in many regions of Canada, but in some areas, late summer can be an excellent time to establish this acid tolerant, leguminous crop for plough-down the following summer. It bears mentioning that in Atlantic Canada, at least, late summer is a good time to establish more long-term forage stands such as white clover, timothy and meadow fescue, partly because weed pressure is usually lower in the autumn than in spring.

For squeezing in a growing crop between fall and spring, however, fall rye is without equal. Rye can yield 2-6 tonnes dry matter per hectare depending on soil fertility and when it is ploughed under. The crop's expansive root system, autumn tillering, early spring growth and long straw provide excellent weed control.

Rye is also well known for its release of allelopathic chemicals - substances that retard the growth of certain weeds. These are found in a variety of crops and weeds and are being studied as possible alternatives to synthetic herbicides. The chemicals released by rye have inhibitory effects on weeds such as wild oats, red rooted pigweed, ragweed and common purslane.

Allelopathy is complex and requires the correct combination of crop, soil and weather conditions to work, so growers should not assume rye is a panacea for all weed problems. For instance, the allelopathic effect appears to be less effective when rye is either tilled or chisel plowed as opposed to being left on the soil surface. Small seeded vegetables such as carrots may be sensitive to the inhibitory chemicals. Larger seeded crops, legumes and transplanted crops like tomatoes are not.

Normally, fall rye is direct seeded (100-135 kg/ha) after the main crop, preferably by the middle of September. In some years, it may establish as late as November. Alternatively, rye can be overseeded into soybeans from an airplane, high bodied tractor, or with other types of broadcast seeders. Optimum timing is at leaf yellowing or early leaf drop. In Minnesota, fall rye has also been established in corn with aerial seeding.

Rye can follow potatoes - either by drilling or broadcasting - directly after harvest. The earlier the potato variety, the more chance of success. With late season varieties, post-harvest field work may be impossible and the days too short for establishing the crop. One short-cut method is to broadcast rye several days before harvest. It's a gamble, but worth some experimenting.

Despite rye's many advantages, the decision to plant should not be taken lightly. Establishing fall rye is relatively easy, but spring management is an enormous challenge if the following crop is to be planted in time for the main growing season. Rye is an extremely difficult crop to kill. It will regrow after ploughing and it is prone to secondary tillering after mowing. Consider also that rye can soak up a lot of water in early spring, but if left growing too long the soil can dry into concrete.

A relatively short mat of rye may be directly incorporated in the soil, but as the crop grows beyond about 12 cm, mowing or flail chopping first is advisable. Some organic farmers recommend that green manures should be cut and wilted for several days before incorporation to lessen the acidifying effects of decomposition on the soil. After incorporation the rule of thumb is to wait two weeks before planting - perhaps longer with mature stands.

The regrowth problem can be overcome by waiting until at least the early heading stage or even flowering before cutting or ploughing the crop. The problem, of course, is that planting may be delayed well into June - beyond the date still feasible for planting a main crop.

To use rye successfully as a winter cover, growers need to fully appreciate the crop they are dealing with. According to one vegetable production text from 1945, growers need "not only the necessary equipment, but the determination to get the crop plowed under before it becomes too large and strawy …"

See Minnesota Department of Agriculture's Greenbook 2005 for results of a 2003-2005 Minnesota trial investigating the effect of fall rye (varieties, seeding rates, and the method and timing of killing the rye) planted in either corn or small grain residues on two organic and two conventional farms prior to subsequent soybean production.

Rupert Jannasch, M.Sc., P.Ag. is a consultant for the Organic Agriculture Centre of Canada. Please send comments or questions by phone (902)893-7256 or by email at oacc@dal.ca.

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