INTRODUCTION

In the last twenty years, much of the research effort on winter wheat production in western Canada has been conducted in cooperation with farmers who were convinced that no-till cropping was the way of the future. One of the most difficult early lessons novice no-till farmers had to learn was that the development of an effective system for handling trash (crop residues) must be placed at the top of their revised list of management priorities.

Simple changes to systems are often the most difficult to accept, especially when they are in conflict with established traditions. Most farmers grew up with the attitude that crop residues were nothing more than a trash nuisance. If crop residues were so heavy they couldn't be ignored or buried, then you burned them. In contrast, in no-till production systems, properly managed crop residues are valued as an ally that will provide opportunities to reduce production risks, increase crop productivity, and ensure agricultural sustainability.

Crop residue management programs need not be complex. However, there is no single set of rules that can be applied to cover all situations. In other words, to be effective crop residue management systems should be custom-designed to individual needs.

There are four basic principles that should be emphasized when custom designing crop residue management systems for successful no-till winter wheat production.

1) The snow trapping ability of the standing stubble must be adequate to ensure successful overwintering of the winter wheat.

2) Water is normally the most limiting factor for crop production in western Canada. Therefore, snow that is trapped in standing stubble, and retained in the soil profile as melt water the following spring, is potential money in the farmers' pocket.

3) Crop residues must be managed so that they do not interfere with the seeding operation.

4) Crop residues must be distributed uniformly when they are returned to the field so that they do not interfere with winter wheat stand establishment and uniformity of development.

Snowcover and Risk of WinterKill
Snow Trapping, Moisture Conservation and Crop Water Use
Seeding into Crop Residues
Influence of Crop Residues on Plant Establishment and Development

SNOWCOVER AND RISK OF WINTERKILL

The importance of snowcover insulation

The importance of snow management in the overwintering of crops has been recognized by researchers and farmers for many years. In North Dakota, research dating as far back as 1923 established that winter wheat sown into standing stubble had the best chance of winter survival. Subsequent work on the Canadian prairies has confirmed the importance of snow cover in overwintering wheat in regions with harsh winter climates.

Deeper snow usually means warmer winter soil temperatures and less cold stress on winter wheat plants. However, the insulative value of snow is also dependent upon properties such as composition of the solid phase or matrix, liquid water content, impurities, reflectiveness, and density. Snow that is loosely packed has the best insulative value. Consequently, when checking on winter wheat fields in January, remember that snow you can walk on top of does not usually have as good insulative value as the same quantity of snow that you sink into with each step.

In Saskatchewan, differences in soil temperature at a depth of two inches (5 cm) can be as large as 10°C for bare summerfallow fields compared to adjacent snow covered stubble fields (-21°C for summerfallow versus -11°C for stubble). Field studies have also shown that minor variation in snow cover over a distance of just a few feet can cause large differences in soil temperatures. The large effect that small differences in snowcover have on soil temperature, and hence winter stress level, have made it difficult to place numerical measures on the importance of snowcover insulation for winter wheat.

The field survival index (FSI) was developed to provide a numerical rating that represents a cultivar's winter survival potential (Table 1). Differences in FSI reflect average percent differences expected in field survival between cultivars. For example, a 38% (514 - 476 = 38) advantage is expected in winter survival potential for Norstar compared to Norwin. The FSI has also been used to establish the winter hardiness requirements for crop survival under different depths of snowcover (Table 2).

Estimates of minimum winter survival potential required to achieve undamaged winter cereal stands in Saskatchewan were obtained from field trials that included cultivars with a wide range of FSI. These estimates demonstrated that a cultivar FSI greater than 650 is required to ensure an undamaged winter cereal stand on bare summerfallow (Table 2). This means that only the hardiest winter ryes, grown under optimum management, have a chance of surviving without snowcover. Two inches (five cm) of unpacked snow greatly reduces the winter hardiness requirement but the risk for winter wheat is still high. With four inches (10 cm) of snowcover, cultivars with an FSI greater than 430 may be considered.

Table 1. Winter hardiness of selected winter wheat cereal cultivars.

The above observations emphasize the importance of snowcover in determining the winterkill risk for wheat. In the zero to four inch (0 to 10 cm) range, each additional 0.4 inch (1.0 cm) of snowcover reduces the cultivar winter hardiness requirement for successful overwintering by at least 22 FSI units (Table 2). In other words, an additional 0.3 inch (0.8 cm) snowcover is all that separates the winterkill risk for CDC Kestrel from that of Norstar [(514 - 497)/22 = 0.8 cm].

Cultivars with poor winterhardiness potential have survived Saskatchewan winters when the snowcover is greater than six inches (15 cm) (Table 2). In spite of this, cultivars with FSI less than 420 are considered to have an unacceptably high risk of winterkill when grown in most of western Canada.

Generally, the deeper the snowcover, the less winter stress on plants. However with very deep persistent snowcover, there is the danger of damage from diseases such as snow mould. It is also important that snow be kept in its place in the normal sequence of seasonal changes. A deep early snowcover, when soil temperatures are warm and winter wheat has not yet fully acclimated, can reduce the plant's overwintering ability.

Methods of snow trapping

There are many methods for trapping snow. However, for winter cereal production, it is important that the required snowcover is uniformly in place over the entire field before soil temperatures approach critical levels for the crop. With this restriction in mind, several methods of snow trapping have been considered for winter wheat production under Saskatchewan conditions. Most methods have shown some promise in providing a snow trap for overwintering cereals. However, they all have limitations.

a) Thin stands of rapeseed/canola, flax or other summer annual grown as a companion crop with winter wheat. To be effective, this method of snow trapping usually requires that the field be seeded twice. The trap crop is seeded in late July or early August to allow for sufficient growth to ensure adequate snow trapping. Summerfallow seedbeds are often loose and dry at this time of the year making the establishment of small seeded crops difficult. Seeding a cover crop this early can also result in weed problems that have to be controlled through the use of chemicals. The winter wheat is seeded at the recommended date, in late August or early September.

b) Trap strips of summer annual crops or perennial grasses sown at intervals across the wheat field, perpendicular to the prevailing winter winds. This method also requires two seeding operations. In addition, where perennial grasses such as tall wheat grass are utilized, the strips interfere with normal tillage operations.

c) Tree shelterbelts. Shelterbelts have been utilized to reduce soil erosion by wind. Similarly, they can be effective in trapping and reducing the drifting of snow. However, where there are only a few shelterbelts, most of the snow will collect in or adjacent to the trees leaving the area between the shelterbelts free of snow. A further problem arises in that snow banking of this nature often produces an ideal environment for snow mould.

d) Underseeding winter wheat with a spring crop, such as barley, in the spring. The spring crop is harvested, leaving stubble for snow trapping and the winter wheat remains to produce the next crop. This system has received considerable attention recently. It is not a new idea, but it has never been adequately researched. Several disadvantages argue against its use. The underseeded winter wheat competes with the spring crop for moisture and nutrients during the first growing season. Early sown winter wheat is more subject to winter damage. Winter wheat is also more productive when sown at the optimum date. In addition, the risk of a build-up of diseases, such as wheat streak mosaic virus, is a genuine concern with this system.

e) "Zero-till" summerfallow. The main advantage with this system is that it provides standing stubble for a snow trap and, at the same time, solves the problem of limited moisture for winter crop establishment in the fall. In the past, the main limiting factor for zero-till summerfallow has been high chemical costs. In addition, when zero-till summerfallow is practised in regions with favorable moisture, there have been difficulties in maintaining standing stubble for a snow trap during the second winter, which is the winter wheat crop year.

f) "Stubbling-in". Winter wheat is no-till seeded into standing stubble in the fall immediately after harvest of the previous crop. The first three to four inches (8 to 10 cm) of snow have the greatest effect in buffering soil temperatures and this depth is the easiest to maintain with the stubbling-in production system. Where stubble land is seeded to winter wheat in the same year that it was cropped, there must be sufficient moisture for germination, the recommended seeding date must be adhered to, weeds must be controlled, and adequate fertilizer must be provided. Where these requirements have been met, this method of snow trapping has shown the most promise. The primary advantages of stubbling-in are that seedbed preparation costs are minimal and each year there are approximately 57 million acres (23 million hectares) of standing stubble available in western Canada. The main limitation of stubbling-in is the occurrence of cool, wet fall weather that delays harvest and restricts access to stubble fields until after the optimum date for seeding winter wheat.

Snow trapping potential of stubble

The amount of snow available determines the upper limit of snowcover that can be maintained on a field. Standing stubble will only assist in maintaining a snowcover when there is snow available to be trapped.

Height and density (number of stems per square foot or metre) of standing stubble determine the snow trapping potential of a stubble field. Within the limits imposed by crop height and snow availability, snow depth on a field can be increased by simply leaving taller stubble. More snow will blow out of thin than thick stubble stands. Consequently, compared to thick stands, stubble height must be taller to maintain a similar depth of snow when stubble stands are thin. Snow also often packs more solidly into thin stands of stubble. Greater packing of snow may mean more water from spring snowmelt, but the insulative value is usually higher for a similar quantity of loosely packed snow.

Type of crop has an influence on how stubble should be managed for effective snow trapping. The stubble of rapeseed/canola and mustard is often thinner than stubble from cereals and, therefore, may have to be cut higher to compensate for poorer snow trapping capabilities. Cereal stubble is also usually more resilient than canola stubble, especially when the fall weather has been hot and dry or the canola was infected with blackleg. Stubble that is brittle and breaks down easily must be carefully managed to maintain a good snow trapping potential.

Drill type and opener design both have a direct influence on the snow trapping potential of stubble fields. Most hoe drills breakdown more stubble than disc drills during the seeding operation. More tillage and greater stubble breakdown can be expected with wide compared to narrow openers on hoe drills.

Special attention should be given to the maintenance of standing stubble in high traffic areas, especially during harvest and winter wheat seeding. Stubble breakdown at entrances to fields and on headlands often reduces snow trapping potential in these areas resulting in winterkill and weed patches. Stubble height should be maximized on dry years when crops are short and in areas where crop stands are poor, such as on eroded knolls or saline patches. When stands are especially short or thin, extra snow may be trapped by leaving narrow strips of stubble where only heads have been removed. In extreme cases, the crop may be left standing to maximize stubble height and snow trapping potential.

SNOW TRAPPING, MOISTURE CONSERVATION, AND CROP WATER USE

Moisture availability during the growing season is a major factor limiting crop productivity on the prairies. One third of the prairie annual precipitation falls as snow. This snowfall represents an average of three to 5.5 inches (75 to 140 mm) of water equivalent.

When properly managed, crop residues offer the farmer an opportunity to conserve and more efficiently utilize total annual precipitation for crop production. Crop resides can be used to:

- trap the snow uniformly,
- retain snowmelt on the field,
- optimize water infiltration into the soil profile, and
- maximize water use for crop production.

No-till seeding into standing stubble from a previous crop (stubbling-in) has been the most widely accepted method of snow trapping for winter wheat production in western Canada. In this system, height and density of the standing stubble determines the snow trapping potential of a stubble field. Any additional gain in snow melt can only be accomplished by increasing the packing of the snow trapped in the stubble.

Assuming there is snow available to trap, methods to increase the snow trapping potential of a stubble field include uniformly increasing the stubble height, leaving narrow trap strips where only the head of cereal crops has been removed or the crop is not harvested, and alternating strips of short and tall stubble. Use of a snowplow to ridge fields may also increase the snow trapping potential of a field. However, the increased risk of winterkill where the snow has been removed to form ridges makes this method of snow trapping impractical for winter wheat production systems.

Straight combining permits the tallest stubble because long straw is not required to keep a swath from falling to the ground. A wind or air reel on the combine header allows a higher straight cut than the conventional bat reel. When double swathing, stubble height can be increased in alternate swaths by laying the swath with the longest straw, and therefore shortest stubble, on the bottom of the windrow. It has also been suggested that alternate height cutting will increase the snow trapping potential of fields that are straight combined.

The snow trapping efficiency of a rough stubble, where narrow deflector or unharvested trap strips are left, is greater than for a stubble of uniform height. For best results the strips should be less than 50 feet (15 m) apart. More snow will collect near the strips. Therefore, the closer the strips are together, the more uniform the snowcover will be on the field.

Tall, dense stubble stands provide the opportunity for greater snowcover in the winter and extra melt water. They may also result in a slower snow melt and cooler soil temperatures in the spring. These factors, combined with increased shading, can result in slower spring plant growth. However, there is little evidence to indicate that either shading of seedlings or cooler soil temperatures in the spring reduce the final yield of winter wheat. The possibility of excessive melt water causing flood damage to winter crops in low lying areas (sloughs) in some regions in some years has been the only problem associated with deep snowcovers from stubble traps.

A word of caution. It is recommended that nitrogen fertilizer be broadcast on winter wheat as early as possible after the soil has thawed in the spring. As early as possible does not mean go out and get the fertilizer spreader stuck in the mud. Check your winter wheat fields, not adjacent fields where the stubble was worked down the previous fall, to determine when they are dry enough to allow for spreader traffic. Results of the pickup test (driving a half-ton across the field to determine when spring field operations can start) can be very different for no-till stubble fields which have trapped snow compared to fall tilled fields that the snow has blown off.

The amount of soil moisture derived from snow trapping will be dependent upon the amount of snow held in the stubble and the degree of melt water infiltration. The mulch layer formed by crop residues is very absorbent and slows surface runoff allowing more time for water to enter the soil. Infiltration rate is the highest when the surface layers of the soil are dry and lowest when they are saturated at freeze-up. When snow melt is greater than infiltration, the excess water runs off into sloughs and other drainage areas.

Once melt water has entered the soil profile, it must be retained for use by the crop during the growing season. On the Canadian prairies, the average amount of water that can be obtained from snow trapping has been estimated at 1.2 inches (30 mm). It has also been estimated that each tillage operation in conventional cropping systems wastes as much as a half inch (13 mm) of water through increased evaporation. Therefore, with conventional cropping systems all the water gained from trapped snow can be lost during the first six weeks after snow melt. In no-till production systems, crop residues reduce evaporation by providing a protective mulch, decreasing wind velocities, and shading the soil surface. This means that more water is available for the crop. Early spring growth allows winter wheat to efficiently utilize this additional water, which is part of the reason why properly managed Norstar winter wheat has had a 36 percent yield advantage over hard red spring wheat in Saskatchewan.

SEEDING INTO CROP RESIDUES

Most of the problems encountered in no-till seeding are directly related to the amount and type of crop residue present on the field. Both amount and type of crop residue vary from region to region, farm to farm, and even field to field on the same farm. Consequently, successful no-till seeding requires a flexible adaptive approach to the management of crop residues. This approach must minimize the opportunity for drill plugging and emphasize shallow seed placement with good seed-to-soil contact.

Residues from low yielding crops are easy to manage. However, serious winter wheat producers seed a crop every year. Therefore, they plan their crop residue management systems as if every year was going to be a high residue year.

Special equipment is required for no-till seeding of winter wheat. There are a large number of makes and models of minimum tillage drills available on the market and experience with this equipment often determines whether or not farmers continue with no-till production systems.

When selecting a no-till seed drill, winter wheat producers should take the following factors into consideration:

- simplicity of design,
- cost,
- durability,
- adaptability for seeding other crops,
- versatility,
- crop residue clearance,
- draft requirements,
- ease of transport,
- accuracy of depth control,
- precision of seed placement.

In the absence of excessive trash, most minimum tillage drills will give adequate soil penetration if properly adjusted. Drag and horsepower requirements are usually lower for disc-type drills. However, hoe drills (including most air seeders) give more positive depth control. There is usually less soil and stubble disturbance with disc drills than hoe drills. However, as long as sufficient stubble is left standing, and the seedbed is not dried out, a small amount of soil disturbance is not critical to winter wheat production. In fact, a small amount of soil disturbance may have a beneficial effect in ensuring good seed coverage.

Excessive crop residue can reduce penetration with disc drills. In addition, even where penetration is adequate, "hairpinning" (forcing of uncut straw or chaff into the furrow) may result in seed "pop-up" after the disc drill passes, thereby reducing seed-to-soil contact. Poor seed-to-soil contact interferes with germination and seedling establishment and is often responsible for poor stands in chaff rows.

Harrowing before seeding is not an effective method of spreading excess chaff. Equipping combines with properly adjusted chaff spreaders is the only practical means of managing excessive chaff when seeding is done with a disc drill.

Compared to earlier designs, the introduction of openers with a leading (off-set) disc has improved the residue cutting of double disc drills. However, this improvement has not eliminated the hairpinning problem associated with inadequately spread chaff.

Fields that have a long history of no-till often build up a soft undisturbed surface mulch. Hairpinning is a greater problem when the soil is soft and the residues are damp and tough. Consequently, continuous no-till cropping can aggravate seed placement problems with disc drills.

Hoe openers lift or push crop residues out of the seed row. Consequently, penetration and hairpinning are not usually problems with hoe type drills. However, excessive amounts of tall stubble and long, poorly chopped or unchopped straw can cause plugging. Problems with plugging are minimized when straw is finely chopped and hoe drills have been designed with adequate space between ranks and sufficient shank length to allow crop residues to pass through without piling up.

There is a rule of thumb that states stubble height should not be taller than the row spacing of a hoe drill. For example, this rule suggests that stubble should not be cut more than eight inches (20 cm) tall when hoe drill row spacing is eight inches. However, vertical clearance (length of shank), number of ranks of openers (2, 3, or 4), distance among ranks, and placement of wheels near shanks are also important factors that must be considered when assessing the flow of crop residues through a hoe drill.

In regions with short growing seasons, there is often an overlap between harvest of spring sown crops and the optimum seeding dates for winter wheat. When this happens, the most opportune time to seed winter wheat is when conditions are too wet for combining. Seeding through trash that is damp or wet provides the ultimate test of trash management systems and drill design. Wet, loose trash has a tendency to pile up in front of the discs, resulting in plugging of disc drills. The increased tendency of wet trash to pile up also aggravates plugging problems with hoe drills. Where straw is damp, hoe drills will sometimes pull out tall stubble, creating additional plugging problems. Once again, these problems are mainly a result of poor systems for trash management and/or drill design deficiencies that can be corrected by proper equipment selection.

INFLUENCE OF CROP RESIDUES ON PLANT ESTABLISHMENT AND DEVELOPMENT

Crop residues that are not uniformly spread over the field can have a significant, and often highly visible, effect on plant establishment, growth, and productivity.

Improperly managed straw slows and frustrates the seeding operation by causing drill plugging, especially when hoe type drills are used for seeding. Trash piles are often left in the field when drills are unplugged. These trash piles become obstacles to other field operations. On farms where all crops are no-till seeded, trash piles can become a long term nuisance that interferes with the seeding of subsequent crops. Poor or nonexistent crop growth in trash piles also reduces plant competition and increases the opportunities for crop volunteers and weed growth.

Nonuniform stands and highly visible striped patterns in winter wheat fields are the most dramatic effects of improperly managed chaff. There are a number of possible causes for the variation in crop stand caused by chaff rows.

Heavy chaff rows, which are left behind combines that are not equipped with chaff spreaders, often result in poor seed placement, especially with disc type drills. Poor seed-to-soil contact interferes with germination and produces seedlings that are spindly. Weak, spindly plants do not compete well with weeds and crop volunteers. They may also be more susceptible to diseases and are often later maturing and lower yielding.

A high concentration of crop residues in chaff rows slows the soil warming process in the spring. In fact, ice can sometimes be found under heavy crop residues in chaff rows after the winter wheat between the chaff rows has begun spring "green-up". Delayed spring development, due to cool soil temperatures in the chaff rows, often has a season long effect that results in uneven crop ripening and distinctive green strips in the field as the crop approaches maturity.

High microbial activity, due to large amounts of crop residues, can temporarily immobilize nitrogen and increase the probability of early season crop nitrogen deficiencies in chaff rows. Early season nitrogen availability is critical to the development of healthy winter wheat stands. Consequently, prior to crop heading, nitrogen deficiencies associated with unspread chaff rows often show up as yellow strips of nonvigorous, unproductive plants. When the crop residue has been consumed by the microorganisms and their populations die-off, the nitrogen once again becomes available for plant use. However, nitrogen release from the decayed chaff rows usually occurs too late to benefit the winter wheat crop.

Seeds of weeds and volunteer crops that are thrown over the back of the combine are concentrated in a narrow strip when chaff is not spread as part of the harvesting operation. Weak, uncompetitive winter wheat plants and a concentration of seed provides an opportunity for weeds and volunteer crops to thrive in chaff rows. This results in weed control problems, increased dockage and, in some cases, grade loss when the crop is sold.

Plants and their residues often exude inhibitors that suppress the growth of other plant species. A species' ability to inhibit the growth of another species through biochemical interactions is known as an allelopathic or phytotoxic effect. The probability of allelopathic interactions that suppress crop growth is increased when residues, weeds, and volunteer plants are concentrated in chaff rows.

An effective straw and chaff management system is a must for successful no-till farming. There is a wide selection of straw and chaff spreading options available. Straw that is finely chopped presents fewer problems for hoe drills. Therefore, it is important that cutting edges be kept sharp on straw choppers when seeding is to be done with a hoe drill. Deflector design will influence spread width and minor modifications to choppers may be necessary to obtain uniform straw distribution on the field. One of the most effective spreader designs has two large and two small optional deflectors that deflect the straw at a 45° angle. Chaff spreading options range from simple inexpensive homemade systems, which are basically spinning deflectors mounted behind and below the combine sieves, to expensive factory designed units that use air blasts across deflectors or modified straw spreaders and choppers to spread chaff. The important feature to look for is a durable spreader combination that will return the chaff and straw uniformly to the field. A spread width of 25 feet (7.5 m) for chaff and 40 feet (12 m) for straw should be a minimum goal when a wide swather or combine header is used for the harvest operation.

	A well planned crop residue management system and proper seeding equipment are necessary for successful no-till winter cereal production.
	Snow that is loosely packed has the best insulative value. Note winterkill in area where snow was packed by a heavy float used to flatten roadside grass.
	Poor winter wheat stand establishment in unspread canola chaff rows. The winter wheat crop was seeded with a hoe drill.
	Winter wheat stand suppression by volunteer spring wheat in unspread chaff rows. Straw was baled off this field and it was seeded with a hoe drill.
	Trash piles left in the field when drills are unplugged become long-term nuissances that interfere with other field operations, especially no-till seeding of subsequent crops.
	Water from snow melt can have a dramatic efect on crop growth and productivity. Note improved crop growth where snow was banked out from the caragana hedge.