Chapter 7 - Soil Preparation

Crop Rotations and Seedbed Preparation

Field selection and seedbed quality are two important aspects for productive canola crops. Good field selection mainly involves proper crop rotation. Seedbed quality is influenced by the tillage method and timeliness, and includes conventional and direct seeding methods.

Rotations and Crop Sequence

Crop rotation is an ancient agronomic practice compared to technology such as herbicides. Since rotation significantly increases yield, it is a cornerstone of farm management. The benefits to rotation are numerous, complex and still not fully understood. Studying rotation effects is difficult because long-term experiments are needed to exclude shortterm influences of weather, soil productivity and management changes.

Good crop rotation has a beneficial impact on weeds, diseases and insects. When a crop is grown continuously, pest populations adapted to that crop will increase. Rotating crops tends to reduce pest build-up, especially immobile types. For example, rotations with high canola frequency have more cruciferous weeds such as stinkweed, but less grassy weeds such as green foxtail or wild millet (Table 1).

Table 1. Effect of Crop Rotation on Stinkweed at Scott, SK (based on 1981, 1985 and 1990 plant counts)

Rotation	Stinkweed Plants/m2 in Canola Phase
Fallow-canola	190
Fallow-canola-wheat	129
Fallow-canola-barley	27
Fallow-canola-barley-hay	23
Fallow-canola-wheat-barley-hay-hay	50

The impact of crop rotation on weeds is partly due to different herbicides available in canola compared to cereals. Alternate crops in the rotation allow different herbicides to be used and thus can provide better overall weed control in the long term. Rotating crops and herbicides also helps to rotate herbicide groups and thus reduce the build-up of herbicideresistant weeds. The recent introduction of herbicide-tolerant canola systems (Roundup Ready, Liberty Link, Clearfield and Navigator) has improved weed control in canola and provided more options for rotating herbicide group use.

Similarly, crop rotation can lower the risk of diseases. Short rotation canola tends to have more problems with blackleg and seedling root rot (Table 2).

Table 2. Effect of Rotation on Blackleg in Canola at Scott, SK (1990)

Rotation	Blackleg (% of plants with basal lesions)
Fallow-canola	43.5
Fallow-canola-wheat	9.5
Fallow-canola-barley	9.0
Fallow-canola-barley-hay	8.0
Fallow-canola-wheat-barley-hay-hay	4.0

Proper rotation with cereals will reduce canola disease, provided that volunteer canola and cruciferous weeds are controlled in the cereal break crops. Also, rotation with canola can reduce many cereal diseases such as common root rot, take-all and tan spot. Australian research has documented a significant reduction in take-all disease of wheat following canola, indicating that canola has a "biofumigation" effect.

Rotation also affects water use. Oilseed crops like canola and flax may use less water than cereals, and thus improve subsequent crop yields in dry areas. Water use is more efficient when crops with different rooting systems are rotated.

In a diverse rotation of cereals, oilseeds, pulses and forages, the different kinds of residue returned to the soil can improve soil fertility over the long term. This fertility increase is due to differences in nutrient cycling, soil aggregation and complex interactions between plants, soil and microbes. Chemicals naturally present in plants and released during growth or decomposition can negatively or positively affect subsequent crops (this phenomenon is termed allelopathy).

Carefully plan crop rotation to avoid a serious build-up of disease, insects or hard-to-control weeds. Another factor to consider is volunteer growth from previous crops that may cause serious competition and seeding problems. Herbicide carryover from previous crops may also adversely affect canola. No single crop rotation will suit all circumstances. The choice of which crops to grow, and in what sequence, depends to a large extent on the soil and climatic conditions of a particular farm and also on the grower's management skills. In areas with adequate moisture, rotations that include cereals and broadleaf crops maintain pests at lower levels and produce consistently higher yield. An added benefit is that yields are often more stable because crops differ in their response to stress at various stages. Therefore, stress may damage one crop seriously, but others in the rotation may be less affected. Use the information in Table 3 as a guide in selecting a field for canola.

Table 3. Guide to Field Selection for Canola Crops

Crop Before Canola	Wait Period	Remarks
Barley, canary seed, spring rye, fall rye, oats, wheat, triticale, winter wheat	None	No diseases in common with canola. Can be grown the year before or after canola. Control stinkweed, cleavers, mustard, volunteer canola and other problem weeds in these crops. Consider potential herbicide residue carryover problems. A low probability of a volunteer problem for one year for canola on spring rye and triticale.
Buckwheat	One year	A high probability of a volunteer buckwheat problem exists for one year. A low probability of a damping-off and root-rot problem exists for one year. Where wild mustard was a problem in the buckwheat, a medium probability of a contamination problem for one year.
Canola	Three years	A high probability of a disease problem for sclerotinia for four years, blackleg for three years, white rust in B. rapa varieties for three years, alternaria for two years; and a low probability for root rot for two years and damping off for one year. If wild mustard was present, a medium probability of a contamination problem for several years. Consider potential herbicide carryover problems. A high probability of a root maggot problem for one year following a B. rapa crop.
Corn	Two years	Consider potential herbicide carryover problems. Soil test to a depth of 60 cm (24") to monitor nutrient levels and avoid over fertilization as corn is usually heavily fertilized.
Flax	One year	A high probability of a volunteer problem for one year. A low probability of a disease problem for root rot for two years and damping off and sclerotinia for one year.
Forage legume (alfalfa, clover)	One year	Consider potential herbicide carryover problems. A low probability of a disease problem for three years for sclerotinia, two years for root rot and one year for damping off. Available nitrogen may approximate summerfallow nitrogen levels if legume breaking is done before June 30. Perennial legumes have a relatively high sulphur requirement and may deplete soil supplies. Soil test to determine sulphur status.
Mustard	Two to three years	A high probability of a volunteer problem for one year, followed by a medium to low probability for two years. Mustard is a contaminant of canola, therefore, control volunteer mustard in the wait period. If wild mustard was present, a medium probability of a contamination problem for one year. A high probability of a disease problem for sclerotinia for four years, white rust for three years, and a low probability for root rot for two years and damping off for one year.
Potato	One year	Consider potential herbicide carryover problems. A low probability of a disease problem for three years for sclerotinia, two years for root rot and one year for damping off. If wild mustard was a problem in the potatoes, a medium probability of a problem for one year. Soil test to a depth of 60 cm (24") to monitor nutrient levels and avoid over fertilization as potatoes are usually heavily fertilized.
Pulse (pea, bean, lentil)	One year	Consider potential herbicide carryover problems. A low probability of a disease problem for three years for sclerotinia and one year for root rot and damping off.
Sugar beet	Three years	Cyst nematodes infest sugar beets causing root deformity, reducing production and sugar content. Nematodes may lie dormant in soil for several years as cysts (also attacks canola/rapeseed and mustard). Three- to four-year rotations among sugar beets, canola/rapeseed and mustard reduce nematode populations in the soil. A low probability of a disease problem for one year for root rot and damping off.
Sunflower	Three years	A high probability of a volunteer problem for one year. Consider potential herbicide carryover problems. A high probability of a disease problem for sclerotinia for four years, and a low probability for root rot and damping off for one year.

Effects of Preceding Crop on Canola

Growing canola on canola stubble usually results in reduced yield compared to canola on cereal, pulse or flax stubble.

Studies by Agriculture and Agri-Food Canada at Melfort and Aylsham, SK in the Moist Black soil zone have shown a large yield reduction in growing canola on canola (Table 4).

Table 4. Relative Yield of Crops Grown on Various Stubble Types at Melfort and Aylsham, SK (Moist Black soil zone) 1990-92

Stubble	Peas	Flax	Canola	Wheat	Barley
% Yield of check (crop on own stubble = 100%)
Cereal	125	111	152	98	109
Other oilseed	114	109	177	131	138
Pea	100	142	196	147	152

Source: Adapted from Townley-Smith Report to Saskatchewan Pulse Crop Development Board, 1994.

Alberta Management Insights summarized the effect of stubble type on canola yields using crop insurance records from 1992-1998 (Table 5). Three million acres of data are included in this study.

Table 5. Effect of Stubble Type on Canola Yield in Alberta

Soil Zone	Stubble Type	1992-98 Avg. Yield % Canola on Canola	Range of Annual Avg.
Black (south-central)	Wheat Barley Canola Summerfallow	107 115 100 118	96-130 99-127 - 99-139
Black - Dark Grey (north-central)	Wheat Barley Canola Summerfallow	106 125 100 121	87-133 109-158 - 97-155
Dark Grey - Grey (north-central)	Wheat Barley Canola Summerfallow	138 129 100 134	94-172 113-150 - 112-157
Thin Black	Wheat Barley Canola Summerfallow	109 115 100 125	96-120 103-131 - 107-136
Dark Brown	Wheat Barley Canola Summerfallow	116 119 100 152	96-152 97-164 - 123-219
Dark Grey - Black (Peace region)	Wheat Barley Canola Summerfallow	103 101 100 110	94-109 91-122 - 105-119

In addition, similar data from Manitoba is summarized in Table 6.

Table 6. Relative Yield of Major Crops Sown on Selected Stubble Types in Rotation in Manitoba during 1994-1998 (from Manitoba Crop Insurance Corporation)

	Stubble Type
	Wheat	Barley	Oats	Brassica napus Canola	Flax	Peas
Relative % yield (crop on own stubble=100%)
Wheat	100	109	110	118	114	120
Barley	115	100	110	119	122	122
Oats	114	103	100	124	123	115
Brassica napus canola	114	115	117	100	118	128
Flax	148	148	146	133	100	**

** Insufficient Data

In general:

• Canola yield is generally better on cereal stubble than canola. The exception appears to be in the Peace River region, perhaps due to the longevity of established canola diseases there (blackleg and brown girdling root rot) and the prevalence of canola.
• In Alberta, canola on barley appears to be slightly better than on wheat except in the Peace and Dark-Grey zones. Although the increased canola yield on cereal stubble is less than found in scientific experiments, this may be due to limiting factors of weeds, diseases and fertility in some commercial fields.
• In Manitoba, canola on flax and pea stubble had the highest yields.
• Yearly yield variation in the Alberta study is very large, indicating a strong weather influence on rotation response.

Rotational Impacts

Growing canola on canola stubble promotes disease and insect build-up. Light disease or insect levels will usually do little damage in the first crop of canola. However, their build-up in that crop can result in significant damage and yield losses in following canola crops. Yield reductions may occur when canola is grown on other crops susceptible to the same diseases and insects, such as flax, mustard, sweet clover, soybeans, field beans, lentils and sunflowers. As shown in the above tables, canola does well on cereal stubble if volunteer plants are controlled. Also, if cruciferous weeds or volunteer canola were a problem in the preceding crop, disease and insect problems could persist into the canola crop. If the preceding crop is a hay or pasture crop, use an early partial fallow period to facilitate seedbed preparations in the next spring. This will allow some weed control and soil moisture build-up. Late summer timing of sod-breaking may limit subsequent canola production because of depleted soil stored moisture, poor fertility and weed control, and the difficulty in preparing a good seedbed with sod clumps.

Another rotational impact of other crops on canola is the potential for toxins that inhibit the growth and yield of subsequent crops (allelopathy). Research around the world has documented that leachates from wheat, alfalfa and some forage grasses can be toxic to canola seedlings. Recent Australian research has found that wheat varieties can differ in the toxicity of their residues to canola. Wheat residue phytotoxicity may partly explain occasional poor emergence and vigour of canola direct seeded into heavy wheat straw. To reduce residue toxicity, spread wheat straw and chaff evenly behind the combine, and in cases of extreme straw production, bale wheat straw. Growing semidwarf cereal varieties may also reduce straw load.

Effects of Canola on Subsequent Crops

Positive Impact

Canola is an excellent break crop for cereals. However, when pre-emergent herbicides are used in the canola crop, sow oats and small seeded grasses next. Some studies have found higher soil moisture and nitrogen reserves on canola stubble than cereal stubble. Cereal root and foliar diseases are usually reduced after canola. The reduction in cereal diseases arises because canola is not a host of cereal diseases and thus their field populations tend to decline.

Also, recent Australian research indicates that canola actually kills certain cereal pathogens such as take-all root rot due to release of inhibitory compounds such as isothiocyanates during decomposition. This direct disease reduction has been termed biofumigation.

Commercial field data from provincial crop insurance departments confirm cereal yield advantages after canola. The benefit of canola stubble on subsequent wheat yield has also been confirmed in Manitoba from crop insurance records (Table 6).

Cereal yield response to canola stubble from these crop insurance records is lower than found in research experiments. This may be due to limiting factors of weeds, disease and fertility in some commercial fields. Rotations with canola have been shown to reduce year-to-year yield variability. In areas with adequate moisture for continuous cropping, a good rotation is cereal-broadleaf-cerealbroadleaf- for example, wheat-canola-barley-peas.

Negative Impact

The phytotoxic effect of canola residues has been documented in Canada by several researchers (Figure 1 and Figure 2) and is attributed to toxins leached from residues as well as toxins produced during microbial decomposition.

Figure 1. Effect of Incorporated Rapeseed Mature Residue or Fresh Herbage on Various Crop Plant Densities at Saskatoon, SK in 1977

Figure 2. Effect of Incorporated Rapeseed Mature Residue or Fresh Herbage on Various Crop Yields at Saskatoon, SK in 1977

Table 7. Effect of Stubble Type on Barley and Wheat Yield in Various Alberta Soil Zones (from Agriculture Financial Services Corporation)

Soil Zone	Stubble Type	1992-98 Avg. Yield % of Barley on Barley	1992-98 Avg. Yield % of Wheat on Wheat
Black (south-central)	Wheat Barley Canola	105 100 111	100 120 119
Black - Dark Grey (north-central)	Wheat Barley Canola	98 100 104	100 127 120
Dark Grey - Grey (north-central)	Wheat Barley Canola	115 100 111	100 ** **
Thin Black	Wheat Barley Canola	105 100 112	100 106 105
Dark Brown	Wheat Barley Canola Summerfallow	94 100 104 118	100 107 105 117
Dark Grey - Black (Peace region)	Wheat Barley Canola	93 100 122	100 107 128

** Insufficient Data

Phenolic compounds are considered to be the main class of phytotoxic compounds that reduce the growth rate of roots and shoots. Isothiocyanate, a breakdown product of glucosinolate, has been shown to reduce weed seed germination, especially with small weed seeds such as spiny sow thistle and flixweed. The phytotoxicity is greatest with fresh residues and young tissue (herbage). Mature canola residue does not normally cause subsequent crop germination and growth reductions except for nitrogen deficiency due to immobilization by the decomposing residue and colder soil temperature. This is likely due to leaching of the phytotoxins out of the residues by rain and snow before the next crop is seeded.

Therefore, problems may occur in situations where thick volunteer canola is worked under in the spring before seeding flax or a cereal crop. Also, poor straw and chaff spreading with the combine may result in an N deficiency and toxicity problems in the chaff row for the next crop, especially if heavy volunteer canola growth occurs in the chaff row. Stress conditions such as an N deficiency and high temperatures can increase the allelopathic effect. The allelopathic nature of canola and other Brassica species have potential to be used in green manure systems for weed control.

Canola may reduce beneficial organism populations such as rhizobia and vesicular-arbuscular mycorrhizal fungi (VAM) in following crops. These microorganisms do not colonize canola and thus their populations will decline after a canola crop. However, the breakdown products of glucosinolates may also reduce the populations with a result similar to the effect on take-all disease. A recent Australian study found that VAM colonization in wheat and flax was generally lower following Brassica crops, but this did not negatively affect crop uptake of P or Zn, or affect crop yield. Further research is needed.

An average canola crop provides less trash cover than an average cereal crop because it produces slightly less residue and breaks down more quickly. Recent research at the Agriculture and Agri-Food Canada (AAFC) Lethbridge, AB Research Centre found that crop residue losses during fallow were lentil>canola>rye>barley>wheat>flax. Lentil and canola residues often break down twice as fast as wheat. Thus, wind or water erosion risks increase with canola especially when it is followed by summerfallow. When land is frequently cropped to canola, there is a slight risk that soil structure may deteriorate because of the short residue life. However, crop rotations with canola are much less damaging to soil quality than rotations with summerfallow. Such cropping practices lead to poor soil tilth and an increased tendency to crust after heavy rains, especially on soils with high clay and low organic matter contents (such as Grey-Wooded soils). Studies at AAFC Melfort, SK and Beaverlodge, AB Research Centres have shown an advantage for a grain-forage rotation on degraded and Grey-Wooded soils. Several years of other crops like cereals and forages will build up or maintain soil organic matter due to crop residues with slower decomposition rates. This will promote better soil structure and tilth, and reduce soil erosion when a trash cover is maintained. Researchers at Beaverlodge have also reported that canola is a suitable companion crop for grass establishment.

Frequent canola production can also increase hard-tocontrol volunteers and weeds like stinkweed, cleavers, wild mustard and Canada thistle. Ensure fields considered for conventional (non-herbicide-tolerant) canola production are as free as possible of wild mustard, stinkweed, cleavers and cow cockle. B. napus varieties have little or no seed dormancy and usually pose little problem for long-term volunteering. However, seeds of B. rapa varieties, buried by deep tillage and exposed to certain temperature and moisture conditions, may develop dormancy similar to wild oats. The seed can stay dormant for many years then germinate, causing volunteer and contamination problems. One way to reduce this problem is to keep the seed on or near the soil surface to promote germination and mortality and prevent induced dormancy.

Seedbed Requirements and Preparation

Canola emergence is greatly influenced by seedbed conditions. Good seedbed conditions are more important for canola than cereals due to the shallow planting depth necessary for this small-seeded crop. A good seedbed will:

• supply enough moisture for germination and seedling establishment
• provide adequate warmth and aeration
• have minimal physical resistance for the seedling to emerge
• be relatively free of weeds and disease
• offer some resistance to erosion

The wide range of soil characteristics, residue levels and weather across the prairies has caused a corresponding evolution of a wide range of tillage implements, seeding systems and ground openers.

Conventional Tillage Seedbed Preparation

Prior to the widespread adoption of conservation tillage or direct seeding, canola seedbed preparation involved numerous tillage operations for the following reasons:

• to bury previous crop residues that interfere with herbicide/fertilizer application or seed placement
• to control weeds which have germinated
• to place soil-applied herbicides or fertilizers
• to create a fine soil structure in the zone of seed placement that balances water infiltration and storage, and for adequate air movement

Although tillage can help prepare a good seedbed and control weeds, there are disadvantages. Excessive or untimely tillage can:

• deplete seedbed moisture
• degrade soil structure that encourages crusting
• contribute to soil compaction
• create large lumps
• increase soil organic matter losses and erosion

Unnecessary tillage adds unnecessary fuel, machinery and labour cost. Use just enough tillage with conventional tillage seeding systems to achieve a reasonably level, uniform, wellpacked, granular surface structure with a mix of granules in the 1 to 5 mm size. The small granules will provide good seed to soil contact for water absorption, while the larger granules will provide some wind erosion protection. Achieving this mix of soil granules without increasing the soil erodability is a challenge since granules smaller than 0.84 mm are susceptible to wind erosion. Leave enough residue on the soil surface to reduce wind and water erosion without interfering with seeding operations.

Several factors must be considered to achieve a good seedbed in conventional tillage systems:

• the tillage implement is suitable for the soil and residue conditions
• the soil moisture content is suitable ("timeliness")
• the implement is properly adjusted for depth and speed

Considerable experience is needed to select the most suitable implement, to properly adjust it for the correct uniform depth and to begin tilling at the right soil moisture content in order to achieve a granular seedbed rather than a powdery or lumpy one. Sandy soils are easily worked into a fine seedbed with minimal tillage. However, the workability of sandy soils leaves little margin for error-overworked sandy soils quickly become very fine structured and susceptible to water and wind erosion.

In contrast, clay-textured ("heavy") soils cannot be worked under wet conditions because large lumps or clods develop which prevent good seed to soil contact. Subsequent tillage to break up the lumps can pulverize the remaining soil and thus make the soil prone to crusting. Crusting of low organic matter clay soils (Grey-Wooded) is a major challenge for canola germination and establishment. Work clay soils at moisture contents slightly drier than field capacity-at this stage, the moist clay can be squeezed by hand into a pliable ball but no free water appears on the soil or hand.

Medium textured ("loam") soils are more forgiving than clay or sandy soils and are best worked when moist. Loam soils worked wet can still create clods, while excessive tillage can reduce them to a very fine structure that can crust or be vulnerable to erosion.

Ensure the spring tillage depth is shallow-2.5 to 5 cm (1 to 2")-since soils tend to dry out quickly to the depth of tillage. Canola is different than cereals. You can't seed to moisture if the top 5 to 7.6 cm (2 to 3") of soil have dried out. A common mistake is to cultivate deeply or an excessive number of times in the spring, then try to firm up the seedbed by several harrow/packing operations. This dries out the seedbed and often pulverizes the surface structure, creating a significant crusting and erosion risk. Grower surveys have reported that minimum shallow tillage (one to two operations) before seeding results in the highest yields on stubble.

A firm, well-packed seedbed will:

• provide good seed to soil contact for moisture absorption during germination
• retain moisture in the seed zone
• provide adequate aeration
• facilitate uniform shallow seeding depth

Excessive tillage can create loose, dry seedbeds that are difficult to firm up and susceptible to erosion. Ensure the seedbed is firm enough that footprints are not deeper than the thickness of the sole of a boot. Packing operations mainly reduce the granule and pore sizes in the surface soil, which will reduce moisture loss. However, do not pack the soil too much because the granules can be pulverized, restricting water infiltration, aeration and predisposing the soil to crusting and erosion. On-row packing during seeding is beneficial, especially for canola. The decision about how much extra packing should occur when seeding canola is a difficult one. Too little packing could result in poor emergence if dry conditions prevail after seeding while too much could result in erosion or crusting if wind or heavy rain follows seeding. This variable effect of extra packing on canola germination, establishment and yield has been shown in research trials. The Canola Council of Canada Crop Production Centres have conducted many experiments comparing post- and pre-packing prior to 1997 and found varying results. Generally, pre-packing is more desirable than post-seeding packing in conventional tillage systems because it will enable more uniform, shallow seeding. Post-seeding packing by rolling is occasionally used to firm up the seedbed and push down rocks. However, the extremely smooth surface left after rolling makes these fields very prone to wind erosion.

Fall tillage has declined dramatically over the past decade as producers strive to increase water capture from snow, retain surface residues for erosion control, and to lower fuel, labour and machinery costs. One fall tillage operation to place fertilizer, control weeds or work in heavy residue can still be beneficial if it avoids extra spring tillage that can dry out the seedbed. Fall fertilization also can take advantage of lower fertilizer prices and reduce the workload in the busy spring seeding period.

If canola is planted on summerfallow, enough residue must be left on the soil surface to reduce erosion and crusting potential. The reasons to summerfallow include:

• weed control
• soil moisture conservation
• increased short-term nutrient availability
• reduced residue-borne plant diseases
• reduced risk of crop failure due to drought

Conservation summerfallow maintains sufficient plant residues on the soil surface to prevent soil erosion while controlling weeds and increasing stored soil moisture. Tillage operations are reduced in number or intensity, and are replaced with herbicides. By using residue-conserving practices, adequate cover can be maintained through the fallow period until the next crop is sufficiently established to protect the soil from erosion. Use a minimum residue cover of 1,513 kg/ha (1,350 lb/ac) to protect most soils from serious wind or water erosion. This is roughly equivalent to the residue left after harvesting a wheat crop yielding 785 kg/ha (14 bu/ac) of grain. In practice, most cereal fields will have crop residues that exceed this level. Residue levels are reduced through natural decomposition (sunlight, oxidation, and microbial activity). Canola residue breaks down about twice as fast as wheat, and this is why summerfallow after canola is not a wise practice. Residue cover declines after each tillage operation (Table 8).

Table 8. Residue Left on the Soil Surface after Various Tillage Operations

Tillage Implement	% Residue Left After One Pass	% Residue Left After Four Passes
Wide-blade cultivator	90	60-65
Chisel plow with low-crown shovel	85	40-45
Chisel plow with normal shovels	80	35-40
Chisel plow with normal shovels plus mounted harrows	60	10-15
Heavy tandem or offset disc	35-65	5-15
Moldboard plow	0-10	0

Tillage operations can be managed to maintain surface residues while preventing serious erosion. Residue left standing will help trap snow and increase spring soil moisture. Usually 45% of soil moisture conserved in an 18- month fallow period is received over the first fall and winter. By trapping snow more effectively, more soil moisture can be conserved. This increased moisture conservation can reduce the need for summerfallow and allow more stubble cropping.

Conservation Tillage Seeding Systems

Conservation tillage seeding systems (direct seeding, no-till or zero-till, minimum or reduced tillage) aim to improve or maintain soil quality and conserve soil moisture. The development of these tillage-seeding systems has been one of the major changes in agriculture during the past two decades. The major advantages of these conservation tillage systems are:

• less soil erosion by wind and water due to retention of surface residue
• maintained or increased soil organic matter contents
• increased soil microbial and faunal populations
• increased soil moisture storage and infiltration rate
• improved soil tilth
• reduced N and S leaching losses
• reduced root diseases
• reduced salinization
• reduced overall machinery investment
• reduced labour needs
• reduced energy requirements
• comparable to better yields and net returns

In contrast, the following disadvantages of conservation tillage have been raised, although many have been resolved through new technology or management practices:

• inadequate and expensive seeding equipment for direct drilling into heavy residue conditions
• poor weed control
• lower spring soil temperatures that reduce and delay seedling emergence
• increased foliar disease from residue borne inoculum
• poor fertilizer efficiency due to placement difficulties and increased N denitrification with higher soil moisture
• delayed seeding in spring due to high soil moisture
• increased surface soil compaction
• greater management skills needed since fewer alternatives are available for weed control and fertilizer application
• increased herbicide usage
• poorer yields and net returns, particularly in wet years

Direct seeding is more flexible than no-till since some tillage can solve immediate weed problems and deal with high moisture and heavy clay soil conditions. In direct seeding, soil is not tilled in the spring before seeding to conserve seedbed moisture. Any fall tillage performed must leave the soil surface compact and level to preserve soil moisture. Most of the crop residue is retained on the surface with at least half the stubble remaining upright and anchored to trap as much snow as possible. Typical operations are fall fertilizer banding with knives, and redistributing crop residue and incorporating herbicides with heavy or rotary harrows.

The amount of soil disturbance during direct seeding varies with the type of opener. With low soil disturbance direct seeders, less than 40% of the soil surface is physically worked by the openers to form the seedbed furrow. Some soil from the opener's action may be deposited between furrows, giving the appearance of more soil disturbance. Low soil disturbance can be expected from 75 cm (3") wide openers spaced at 22.5 cm to 30.0 cm (9 to 12"). Soil firmness, moisture conditions and planter speed affect the amount of soil disturbance. Low disturbance direct seeding systems are very much like no-till systems except that some tillage options remain available in direct seeding.

High soil disturbance direct seeders disturb more than 40% of the soil surface. If fall tillage was done, then spring seeding occurs in loosened soil and most of the surface is disturbed. Wide ground openers that overlap will disturb the entire soil surface to some degree. Sweep openers produce high disturbance. They give varying degrees of weed control, so a pre-seeding herbicide application may not be needed. However, they may stimulate weed growth since weed seeds and volunteer seeds from the previous crop will be incorporated into moist soil. High disturbance openers may require additional seedbed finishing to cover the seed and to improve weed control.

In no-till or zero-till systems, seeding is the only operation that disturbs the soil. Only 25 to 35% of the soil surface is disturbed, just enough to place the seed and fertilizer into a seedbed. No-till is similar to low disturbance direct seeding except that direct seeding systems allow some tillage to deal with unusual conditions. No-till aims to minimize soil disturbance and maintain as much crop residue cover as possible because:

• low disturbance reduces soil moisture loss
• weed seeds are less likely to survive and grow on the undisturbed soil surface
• crop residue cover protects soil from wind and water erosion
• standing stubble traps snow

Residue management is a crucial aspect for successful conservation tillage seeding. Successful residue management needs to consider various factors, including:

• crop residue amounts and condition, particularly green, lodged or damp straw
• capability of the seeding machine and other implements to clear through the crop residue without plugging or "hairpinning"
• combine or swather cut width compared to the spread width of straw and chaff behind the combine
• alternative uses for straw when residue is plentiful
• weed control methods

In direct seeding systems, begin residue management at harvest with a wide and even spread of straw and chaff behind the combine. Extra operations after harvest to manage heavy residue are time consuming and costly. Most new combines are equipped with good straw and chaff spreaders or are easily adapted with after-market units. Converting older combines to a better spreading system is usually more difficult. The cost, horsepower needs, type of drive and spread width varies between the different after-market units.

Residue clearance is the ability of seeding equipment to allow crop residue to pass through without bunching. Ground openers and shanks are shaped to prevent dragging and subsequent bunching of straw residue. The opener must prevent chaff residue from falling into the seed furrow and causing poor seed cover and poor furrow closing. Critical dimension is the distance between two points in a machine where plugging with straw is likely to occur. The most common locations for plugging in the seeder are:

• between the underside of the shank (may be the spring trip supporting mechanism) and the soil surface
• between one ground opener and the next
• between a ground opener and a wheel or some other adjacent structural member

The critical dimension may vary with the amount of crop residue on the surface, the moisture content of the residue or even the air humidity. Damp straw plugs the seeder quite easily. Fluffed up straw plugs more easily than straw lying on the soil surface. Dry straw on a warm, breezy day will pass through a seeder while damp straw may not. Changing speed and direction of travel may help a seeder to clear crop residue.

The straw handling performance of many seeders may be improved by modifying the location where plugging most often occurs. The space between the opener and "plug point" may need to be increased, sometimes greatly, to prevent plugging. Four-rank cultivator units on air-drills will plug less than three-rank units.

Direct Seeding Equipment

Air seeders and air drills have become common machines on both conventional and conservation tillage farms. An air seeder uses a medium or heavy-duty cultivator, a central pneumatic seed and fertilizer delivery system and a ground opener for seed and/or fertilizer placement. This system offers many options and adaptations to meet a variety of conditions. The seeder's mainframe is carried and controlled by wheels inside the frame. Levelling (fore-aft) is controlled by caster wheels in front of the frame (floating hitch type). This method of depth control is superior on land with sharp hills or gullies. Install seed row finishing equipment on the rear of the air seeder. Separate soil finishing passes may be needed over the seeded field to ensure good seed placement depth.

An air drill is an adaptation of the air seeder. The main difference is that air drills do not have wheels inside the frame carrying the ground opener hardware. Machine support and depth control comes from dedicated packer wheels on the rear of the drill. The front is carried and controlled by forward caster wheels as with any floating hitch cultivator.

An air drill has all the advantages of an air seeder including:

• good seed depth control
• wing-up convenience for transport
• a central seed and fertilizer metering system
• excellent field efficiency and capacity

It also has two main advantages over an air seeder--the relatively constant packing force delivered by each on-row packer, and the increased residue clearance made possible by the absence of inside-the-frame wheels.

One key aspect of direct seeding units is ground opener selection. The ground opener is the part that penetrates the soil to place seed and fertilizer. It has a soil-breaking wear point, a soil dividing body, and delivery tubes to guide seed and fertilizer to the furrow bottom. Also, deflecting surfaces may be present to guide soil back around the fertilizer bands and seed rows.

A ground opener must:

• leave at least 15 mm (0.6") of soil between the fertilizer bands and seed rows
• not allow seeds to fall in a concentrated fertilizer zone
• create a good soil structure (fine aggregates) in the seed zone
• have low draft requirements and resist wear
• scour well in moist and high clay content soils-many openers tend to build up with soil, causing the furrow opening to be too large resulting in the seed not being covered with sufficient soil
• leave the soil surface smooth enough for subsequent operations like crop spraying and harvesting
• adequately "blacken" the soil surface over the seed row if there is a concern about soil temperature for seed germination

Opener performance is influenced by:

• soil moisture content
• soil texture
• soil density
• seeding depth
• forward speed

Choosing ground openers to suit the conditions on a farm will be difficult but manageable. Seed must be placed into moist soil and surrounded by soil particles small enough to reduce open spaces in the seedbed. Coarse soil lumps in the seedbed increase soil moisture loss and reduce seed germination. For good seed-to-soil contact, ensure the ground opener either causes very little soil movement so that the soil profile is fractured very little, or causes enough agitation to create soil particles small enough to fall in around the seed. The latter case may require harrowing to spread the soil over the seed row. Soil cover depth is measured after the last implement passes over the seeded field.

Every ground opener design creates its own particular flow of soil around it. Therefore, each opener design results in a different furrow opening, seed placement and soil cover. The furrow from a specific opener is affected primarily by the soil's clay and moisture content. Additional passes with equipment using on-row packing wheels, harrows, or other soil levelling and packing equipment, will change the soil cover depth above the seed.

Soil must be repacked around and above the seed to prolong seed contact with moist soil. Packing reduces moisture loss from the seedbed by creating a denser soil layer at the surface with fewer large air spaces. A fine balance often exists between packing enough to reduce moisture loss and packing too much, which in some soils may promote crusting that hinders seedling emergence. Adequate packing is achieved when all the soil lumps are crushed both around and above the seed.

Most ground openers require a packer to close and pack the soil in the furrow to create a good seedbed. The packer's shape and width must conform to the furrow and the location of the seed underneath. A direct, minimum disturbance seeder requires an on-row packer. A wide sweep opener that cuts the full width of the seedbed requires a harrow, rod-weeder, packer or a combination of several systems to finish the seedbed.

Fertilizer must be placed near enough to the seed to supply nutrients for good early growth but not too close for crop safety. Too much fertilizer placed too close to the seed can cause injury to the germinating seeds, resulting in reduced crop emergence (see N and S sections in Soil Fertility chapter).

In a double-shoot system (Figure 4), the soil buffer is a zone between the fertilizer band and seed row where there is neither seed nor fertilizer. The opener, since it is placing both the seed and the fertilizer, must leave a soil buffer of at least 1.3 to 2.0 cm (0.5 to 0.75"). To achieve this buffer width, the spacing between the centres of the fertilizer and seed outlets has to be at least 5.0 cm (2").

The soil buffer zone can be lost if the scatter of seed and fertilizer increases when planting in clay or wet soil, from travelling too fast and from too much fan speed on an air cart. Seed that lands in the fertilizer band may not germinate or the seedlings may emerge later.

The greater the number of tasks the opener must perform, the more complicated the opener and the seeding operation become. Complicated ground openers are usually more sensitive to varying soil conditions.

Single and Double-Shoot Systems

The terms "single shoot" and "double shoot" refer to how material (seed and fertilizer) is delivered by the planter to the ground opener. The differences are:

• A single-shoot system has only one delivery line going to the ground opener. The line carries seed and possibly some granular fertilizer (Figure 3).
• A double-shoot system has two lines going to the ground opener. These may be two airflow lines or an airflow line and a liquid fertilizer or anhydrous ammonia (NH₃) line (Figure 4).

Figure 3. Single Shoot Opener

Figure 4. Double Shoot, Paired-row Opener

Most often, a double-shoot system has a ground opener, which opens two separate furrows so the seed is placed in one furrow and the fertilizer in the other. This type of fertilizer placement is called double-shoot side banding. The fertilizer band is usually placed deeper than the seed, usually about 2.5 cm (1"). Seed is placed either in one row, above and to the side of the fertilizer band, called single side banding, or in two rows above and on both sides of the fertilizer band, called paired-row double shooting (Figure 5).

Figure 5. Double Shoot, Single Side-band Opener

A semi-dependent opener (Figure 6) is a variation on the double-shoot, single side-band opener. It forms distinct furrows and leaves a seed row slightly narrower than the opener. The seed opener follows behind and slightly to the side of the fertilizer point, ensuring that soil covers the fertilizer band before the seed is placed. There is little chance of fertilizer and seed mixing. The advantage of the semi-dependent opener is that the seeding depth more closely follows the land contours.

Figure 6. Semi-dependent Opener

Selecting and Using Ground Openers

All commercially available openers work under some conditions, but few, if any, work well under all conditions. How does a grower know if a particular opener design will work well under the conditions on the farm? Begin by discussing ground opener performance with neighbours to learn more about options suited to the conditions in the area. Next, determine which openers will likely meet the requirements for seed and fertilizer placement, handle the amount of residue cover that usually exists on the farm, and meet other requirements specific to the operation (such as providing some weed control). Seedbed utilization (SBU) (see N section in Soil Fertility chapter 9) is an important consideration. Install one or more of these openers on the seeder, try them in several typical conditions on the farm and assess the results.

Ground opener design is often influenced by soil and crop residue conditions in the area where the opener was developed. If these conditions are similar to the grower's farm, chances are better that the opener will do a good job. Therefore, ask the manufacturer about the conditions in the area where the opener was developed.

Opener comparison tests have been conducted by government engineering research agencies. For example, the Alberta Farm Machinery Research Centre at Lethbridge, AB (now called the Ag-Tech Centre) has conducted performance tests on a wide variety of ground openers. "Testing of Double Shoot Openers" (AFMRC Report 721) provides comparative data on seed band depth, seed band width, fertilizer band width, spacing between the fertilizer and seed bands, opener wear, draft, power requirements and other characteristics for 15 ground openers. Check with provincial departments of agriculture for information applicable to your area.

Although it takes experience and perseverance to make ground openers work in direct seeding, many growers have developed successful direct seeding systems. Here are some tips for ground opener use:

• Check the planter's adjustments in each new field and in areas of the field where conditions are very different. Compromising here can be costly in terms of stand establishment and often in yield.
• No particular opener works well on all soil textures. If the farm has a variety of soil textures, openers may have to be changed to suit certain fields.
• Double-shoot openers may require frequent adjustment of depth and forward speed to ensure good placement of seed and fertilizer.
• When soil conditions are very moist, particularly in fine clay soils, ensure the seed row furrow is sealing adequately to preserve seedbed moisture.
• When assembling components from several sources, pay special attention to ensuring good mechanical arrangements of the planter. Poor assembly causes costly downtime and repairs.

Pre-Seeding Weed Control

The reduction in tillage with direct seeding results in a heavier reliance on herbicide weed control. Winter annual and early spring germinating weeds will compete strongly with canola and, therefore, pre-seeding herbicide application (burn-off) is a common practice. If pre-seeding herbicides are not applied, the initial weeds become large and often pass the growth stages needed for good control by in-crop herbicide applications. Each direct seeded field must be scouted for weed emergence-scouting must be done on foot, and sometimes on hands and knees, to find and identify weed seedlings. Although there are no published threshold numbers of emerged weeds for preseeding herbicide applications, experience has shown that growers tend to underestimate the tiny weed seedlings present before seeding in early spring.

The best time to control annual weeds with a non-residual herbicide is usually just before seeding. Spraying too early before seeding can allow new weed seeds to germinate before the crop emerges. In conditions of heavy weed infestations and low soil moisture, it may be necessary to spray early to stop soil moisture depletion. Spraying after seeding can be effective, but there is a risk that bad weather could prevent or delay the burn-off herbicide application, and lower crop yields. Also, soil particles on weed leaves from the seeding operation may reduce herbicide performance.

Several effective pre-seeding weed control herbicides are available. Glyphosate is the active ingredient in most preseeding burn-offs, and is sold under several different brands and formulations (for example, Roundup Original, Victor, Renegade, Roundup Transorb, Touchdown, Touchdown iQ and Glyfos). The reasons for glyphosate's popularity in burnoff sprays are:

• no herbicide residues to harm seeded canola emergence and growth
• a wide range of annual weeds is controlled and most perennial weeds are suppressed
• herbicide rotation options are increased with minimal resistance concerns
• cost is very affordable

Phenoxy herbicides such as 2,4-D and MCPA are also used occasionally in pre-seeding herbicide mixes. Research has shown that significant canola stand thinning and yield loss can result if dry conditions occur between burn-off and crop emergence.

Canola Response to Conservation Tillage Seeding

Several research reports have been published on the response of canola to different tillage seeding systems. However, firm conclusions are difficult to draw from the research for several reasons. First, some of the research was conducted on plots that were previously in conventional tillage systems and, therefore, results may not indicate the outcome on long-term direct-seeded fields. Secondly, direct seeding equipment has evolved significantly and early research will not reflect these advances. Finally, herbicidetolerant canola systems have been recently developed and widely adopted which improve weed control and thus may affect the performance under direct-seeded systems.

Weed Control

In addition to equipment concerns, weed control is often stated as a major obstacle that hinders adoption of directseeded systems. Initially, a change to direct seeding was expected to bring more weed problems, especially perennials and wind disseminated species. For example, at the Agriculture and Agri-Food Canada Scott, SK Research Centre a 12-year (1979-1990) study comparing zero tillage (ZT) with conventional tillage (CT) in two rotations found that ZT only increased yield by increasing soil moisture where weed control was adequate. Yield decreases with ZT were associated with poor control. However, during this period, suitable post-emergent herbicides to control annual broadleaf weeds such as stinkweed, wild buckwheat, lamb's quarters and red root pigweed were not available.

At the Agriculture and Agri-Food Canada Beaverlodge, AB Research Centre a study compared CT, ZT and reduced tillage (RT) on clay Grey-Wooded soils from 1989-1991. As this short study progressed, there was a trend of relatively greater weed density under ZT and a shift in species composition. The weeds that increased in density under ZT canola were stinkweed, wild oat, smartweed, field horsetail and dandelion while wild buckwheat decreased. However, canola yields were not significantly reduced under ZT compared to CT.

In contrast, other recent research on the prairies has found that clear changes in weed communities do not generally occur with adoption of conservation tillage seeding. Weed communities are more influenced by locations, rotations and years than by the tillage system. For example, an Agriculture and Agri-Food Canada study at Rycroft, AB from 1989-1993 compared the impact of three tillage systems (CT, RT and ZT) on the weed population during early crop growth. The study found that the relative contributions to the size and diversity of weed flora are likely to be greater by common species under CT and by rare species under RT and ZT. No consistent increase in the weed population occurred with time under all three systems. Similarly, a study by Agriculture and Agri-Food Canada at three Saskatchewan locations from 1986-1990 did not find any increase in perennial and annual grass weeds with ZT. Weed community changes were influenced more by location and year than by tillage system.

Potentially, certain weeds can proliferate under direct seeding if management is not careful-examples are dandelion, narrow-leaved hawk's beard and foxtail barley. Given the different environment found in direct-seeded fields, it should not be surprising that different weed species with adaptation to residue-covered habitats will become dominant over species commonly found in conventional, cultivated fields. But research indicates that weed shifts are manageable by careful attention to rotations, herbicide selection and timing (especially pre-harvest glyphosate for perennials). Herbicide-tolerant canola systems have recently added effective options for improved weed control in directseeded systems.

Soil Moisture and Moisture Use

Many studies in western Canada and world-wide have reported higher soil moisture under conservation-tillage seeding compared to conventional tillage. The improved soil moisture under conservation tillage is due to reduced evaporation from residue-covered soil as well as increased infiltration rates. Higher soil moisture will often improve germination, emergence, early crop growth, moisture use efficiency and yield. For example, in the 12-year study by Agriculture and Agri-Food Canada at Scott, comparing ZT with CT in two rotations with fallow, canola and wheat, yield increased with ZT where spring soil moisture was increased. In the 36 comparisons over three rotation phases, ZT increased spring soil moisture in nine cases and there were no decreases. Yield was increased in nine cases but decreased in three, and moisture use efficiency increased in six cases with two decreases. The Beaverlodge, AB tillage study found that ZT and RT increased soil moisture in the top 10 cm (4") in dry periods. In excessively wet years, canola growth, moisture use efficiency and yield can suffer under direct seeding.

Soil Temperature

Spring soil temperatures are often cooler under conservation-tillage seeding systems than conventional tillage. Most of the western Canadian research studies have found that residue covered soil is 0 to 2°C cooler (daily average temperature) than cultivated soil. In some cases (sunny days), temperatures during midday can vary by 5°C. The colder soil is due to increased soil moisture (water is slower to warm up than air) and more heat reflectance by the residue. Although emergence is sometimes several days longer with direct seeding compared to conventional tillage, the delay usually disappears after canopy closure. Increased soil moisture usually improves yield in spite of the initial cooler temperatures.

In most cases, the cooler soils will not hamper final crop stands or yield. Direct seeders often can seed shallower (which is warmer) due to better moisture, and this largely compensates for temperature differences. However, experience has shown that excessively wet springs can negatively affect canola growth and yield, partly due to temperature effects. Also, frost damage to canola seedlings has been more severe on residue-covered fields when frost followed a warm sunny day. The greater injury was likely due to lower heat radiation in the critical early morning period under residue-covered soil compared to bare soil. Pay very close attention to achieving uniform residue spreading, preferably with the combine, to reduce cold temperature problems and enable good ground opener performance and seed placement. Research in the Peace River, AB region by Agriculture and Agri-Food Canada found that a narrow strip of bare soil over the seedbed can overcome most of the cold temperature and excessive moisture disadvantage of direct seeding in unfavourable situations.

Canola Yield Under Conservation Tillage Seeding Systems

Research conducted on the prairies has reported variable success with conservation tillage seeded canola-ZT or direct-seeded canola has yielded less, the same or more than conventionally seeded canola. The many changes in direct-seeding technology and variable weather effects makes it difficult to apply some of the past research findings to the farm level. Perhaps the best indication of canola performance under direct seeding compared to conventional and reduced tillage is from hail and crop agency records. Three years of recent records (1999-2001) from the Agriculture Financial Services Corporation in Alberta shows that direct seeding and reduced tillage produced 109% and 108% yield of conventionally seeded canola on average in the major canola growing areas. Based on 4.6 million insured canola acres seeded on stubble over these three years, 29%, 43% and 28% were conventional tillage, reduced tillage and direct-seeded, respectively. This shows that conservation-tillage seeded systems are more popular now than conventional-tillage systems. Figures 7-9 illustrate the yield comparisons between tillage systems reported in the major canola growing soil zones in Alberta.

Figure 7. Canola Yield Comparisons Between Tillage and Direct-Seeded Systems in the Black Soil Zone of Alberta (AFSC 1999-2001 Data)

Figure 8. Canola Yield Comparisons Between Tillage Systems in the Dark Brown Soil Zone of Alberta (AFSC 1999-2001 Data)

Figure 9. Canola Yield Comparisons Between Tillage Systems in the Peace River Region - Dark Grey and Grey Soil Zones of Alberta (AFSC 1999-2001 Data)

Net Returns of Different Tillage Systems

Compared to yield, there have been fewer studies that investigated canola net returns seeded with the various tillage systems. Such studies are difficult since there are numerous combinations of machinery and, therefore, production costs can vary dramatically. Many other confounding variables, other than the tillage seeding system, can also significantly affect the net returns. While direct seeding systems can reduce labour, fuel and some equipment costs, herbicide and other equipment costs may increase. With the widespread adoption of air-drills, the equipment cost has become less of an issue.

Alberta Agriculture Food and Rural Development conducted a survey of 185 growers in 1994 and 1995 to assess the short-term economics of conservation-tillage practices. Based on growers' costs and returns, partial budgets were compared between the systems. Machinery fixed costs were estimated through mathematical formulae. The main report findings were:

• Zero-and reduced-tillage systems, on average, may have slight economic advantages over conventional-tillage systems.
• Zero-and reduced-tillage systems had marginally better contribution margins, and returns to land, labour and management on average, compared to conventional-tillage systems.
• Contrary to expectations, herbicide costs did not vary consistently between the tillage systems.
• Fuel, repair and depreciation costs increased as the tillage intensity increased. While this is an economic advantage for zero tillers, the more expensive machinery needed for direct seeding tends to offset this.

Overall, conservation tillage systems are suitable for canola production. The higher seedbed moisture can encourage better canola emergence. However, heavy residue fields can create significant problems for good canola seed placement, and increases the risk of frost mortality. Canola seedlings are sensitive to seed placed fertilizer, therefore, ground openers need to be chosen carefully. Further information on conservation tillage practices can be obtained from provincial and federal soil conservation agencies.

References