Agronomic Information - Residue Management

Introduction

It is hard to over-emphasis the importance of residue management in a direct seeding system. For producers looking to begin direct seeding, it is imperative to take the first step in their new cropping system at harvest the year prior. How crop residue, both the straw and the chaff, is managed the preceding year will dictate whether the crop residue is an asset or an Achilles heel for the following year of crop production.

A lack of residue management can create many problems. During seeding, plugging the seeding unit with straw, or hair-pinning if using a disc drill, can be an initial difficulty. Poor or uneven emergence, cold soils, nutrient tie up, delayed and/or uneven maturity, or thin crop stands may all result because of the toxic effect of chaff rows. All of these situations can be avoided through the proper management of crop residue.

Crop residues can be a major benefit to the cropping system when managed properly. The most notable benefit is the reduction in both wind and water erosion. Standing stubble also aids in the retention of snow, protects winter crops like fall rye or winter wheat from low soil temperatures, provides a beneficial microclimate for emerging spring seedlings, and enhances the infiltration of water. Crop residues provide weed growth suppression between rows and, over time, improve the overall health of the soil.

Factors that influence Residue Management

Crop Type

The types of crops included in crop rotation impact the amount of chaff and straw that must be managed. For instance, cereal crops such as wheat, barley, oats or some pea crops produce little chaff but have the potential to produce large quantities of straw. It is therefore vital to use proper straw management. It is usually considered adequate if straw is spread 80 % or more of the width of cut.

Oilseeds pose the opposite problem. Crops like canola and mustard produce much less straw in comparison to cereals, but oilseeds tend to produce large quantities of chaff. The chaff if not properly spread can cause problems to developing seedlings due to the toxins produced from these residues. The allelopathic nature of these residues can be overcome with proper chaff management. It is generally considered adequate if chaff is spread greater than 50% of the width of cut.

While there is potential to have too much residue, most problems can be eliminated with forward planning. Planned crop rotations (see Rotation Section) play a vital role in reducing the chance of heavy residue problems. The type of seeding unit and row spacing will dictate the amount of residue management required (see Seeding Section).

Flax presents its own special problems. Because of the “wire-y” nature of the straw it is extremely difficult to do a sufficient job of straw chopping. However some new straw choppers are capable of doing an adequate job, particularly when the flax has been pre-harvested with glyphosate and allowed to dry down completely. Removal or burning of flax straw is the only other alternative. Presently there is renewed interest in using the flax fiber for products such as fine linens and high quality paper products. Burning not only wastes nutrients particularly nitrogen, but also continues to fall out of favor with society as a whole because the smoke is extremely harmful to those suffering from asthma.

Width of cut

The width of cut, whether by a combine header or a swather, has a direct impact on the type of residue management required. Additional management must be used as the width of cut increases. As the size of combines continues to increase, producers continue to increase the width of cut to meet the capacity of their combine. It is important to remember the goal is to spread the straw greater than 80% the width of cut and to spread the chaff greater than 50% the width of cut. Ideally we want 100% spread of both straw and chaff.

For instance, a swather width of 45 ft (13.7 m) put through a combine with a standard straw chopper which only spreads the straw 15 ft (4.5 m), results in a straw concentration three times as heavy as it would be if compared to a full width of spread. An 80 bu/ac (5 t/ha) crop of barley quickly becomes a 240 bu/ac (15 t/ha) crop of barley where the straw is spread. This not only causes plugging problems even with the best clearance seeding machines, but also inhibits the proper development of young seedlings. Similarly the chaff row behind the combine will be much heavier than compared to a smaller header.

Stubble Height

Post-harvest stubble height can be a valuable tool or a hindrance in a direct seeding system. Tall stubble can be an asset in snow trapping, reducing evaporation, and providing protection to young seedlings. But stubble height must not hinder the capability of seeding equipment to pass through standing stubble.

In Saskatchewan snow represents as much as 1/3 of the annual precipitation. Snow trapping can enhance yields by more than 10% (deJong et al., 1986). Snow trapping can be improved with tall stubble, alternate height stubble, and trap strips, as well as field shelterbelts (see PFRA tree centre). Tall stubble is considered to be stubble in the range of 12 to 24 inches (30 to 60 cm). Alternate height stubble is the practice of leaving both short [6 to 12 in. (15 to 30 cm)] and tall 12 to 24 in. (30 to 60 cm) stubble in alternate swather passes. Trap strips are narrow strips of tall stubble surrounded by short stubble.

The amount of snow trapped is directly proportional to the stubble height. More snow increases the water supply to the crop. Tall standing stubble also aides in reducing both wind speed and solar radiation, which in turn helps reduce evaporation and keep the soil cooler (Cutforth and McConkey, 1997). However the tall stubble does not appear to reduce plant photosynthesis. Research at Swift Current has shown yield increases in response to tall stubble over cultivated plots in wheat, pulses and canola (Table 1).

Table 1: The effect of stubble height and seeding management on the mean seed yield of Argentine canola (cv. Arrow). The results are mean averages from two years of research at Swift Current (adapted from Cutforth et al., 2000).

Treatments

Seeding

Late Fall

Early Spring

Late Spring

Mean Yield

Tall Stubble

39.2

32.5

27.6

33.1 (b)

Tall Stubble + 34 lbs extra N

42.3

39.8

29.2

37.1 (a)

Short (Fall)

34.8

29.9

26.3

30.3 (b,c)

Short (Spring)

36.3

31.6

25.5

31.2 (b)

Cultivated (Fall)

31.2

27.3

24.7

27.7 (c,d)

Cultivated (Spring)

N/A

29.1

24.9

27.0 (d)

Mean Yield

36.7 (a)

31.7 (b)

26.4 (c)

 

Numbers followed by the same letter are not significantly different.

Straight cut vs swathed

The cutting method does not influence the importance of proper residue management. Straight cutting often results in taller stubble height than does swathing, but it should not be so tall as to hinder the ability of seeding directly into that stubble. Taller stubble means less straw is going through the combine requiring less straw management to provide good seeding conditions. Contrarily, there is often more straw to manage when the crop is swathed (see stubble height discussion above).

Stripper Header

resimage1The use of stripper headers provides unique situations for producers wanting to direct seed into standing stubble (Figure 1). The stripper takes only the grain and some chaff into the combine, therefore increasing the harvesting capacity of your combine. The small amount of chaff is easily spread. The greatest difficulty is dealing with the very tall stubble. The use of disc-type seeders are well suited to this tall residue, but hoe-type seeders are not an option without mowing or cutting the tall stubble ahead of seeding.

Figure 1: Harvesting using a stripper header.

The very tall stubble left from stripper headers can be beneficial in trapping snow. This reduces wind speed and evaporation, and protects young seedlings. It important that a well managed crop plan is in place to take advantage of the tall stubble to prevent a residue management headache (see stubble height discussion above).

Conventional vs rotary combine

Whether you are a conventional or rotary combine owner you must implement good residue management practices. Rotary combines typically discharge shorter straw than conventional combines that in some cases may not require additional chopping. But it is important that the straw be spread back evenly across the width of cut. However in tough conditions the length of straw will increase. This can cause problems the following spring by hindering material from flowing properly through the seeding unit.

Both types of combines will require additional straw spreading capacity as the width of cut increases. As well both systems require chaff spreaders when they are part of a direct seeding system.

Straw choppers and spreaders

The cheapest and most effective straw management occurs at the back of the combine. Until the late 90’s most original choppers and spreaders did an inadequate job of straw spreading particularly with high yields and increasingly greater widths of cut (Figure 2).

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Figure 2: Poor Straw management.

Today most combine manufacturers offer optional straw choppers that provide satisfactory straw spread up to 30 or 35 ft (Figure 3). As well several after-market straw chopper options are available. The key is to match the straw spreading capability to the width of cut. Note that the power requirements of some systems can be quite high which can affect the performance of the combine in certain conditions. There are also reports from some producers of high maintenance requirements for some systems. There are several PAMI reports for straw and chaff spreaders.

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Figure 3: Two examples of good straw and chaff management.

Chaff spreaders

The importance of spreading chaff should not be overlooked. Most factory-equipped combines do not have chaff spreaders and the chaff is usually not spread beyond the width of the combine body (Figure 4). This can result in heavy chaff rows left behind the combine, particularly for crops like canola or mustard. Chaff from these crops contains toxic chemical substances that can effect crop establishment and growth.

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Figure 4: Poor chaff management.

Most experienced direct seeders suggest that good chaff management is a critical component to successful direct seeding. There are numerous after-market options for managing chaff at the back of the combine. Research by PAMI shows that chaff should be spread back over 50% the width of cut.

Chaff collection systems

Chaff collection offers an alternative to spreading the chaff uniformly back across the field. The collection of chaff not only eliminates the problems associated with chaff rows, but it is also beneficial in removing weed and other grain seeds which typically are spread back on the land.

Crop management is often not considered to be part of harvest. But the removal of weed seeds from the field helps reduce the spread of weed patches and ultimately reduces the overall herbicide usage.

The removal of chaff, weed seeds, cracked grain and a percentage of the parent grain provides an excellent feed source for cattle producers. This could be used in the producer’s own operation or it could be sold to local livestock producers. This adds value to a normally non-economic product and it adds to the bottom line of the crop.

Presently there are two main chaff collection systems. The first system involves a wagon pulled behind a combine. Everything that does not fall through the grain sieves is blown into the chaff wagon (Figure 5). These wagons are dumped periodically across the field. The chaff piles can then be used in the field or brought back to a central livestock feeding location.

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Figure 5: Chaff collection system.

The second system is completely unique in that the grain and chaff together are mechanically separated from the straw in the field (Figure 6). The combined grain and chaff, called graff, are brought to a central location where further threshing occurs. The grain is separated from the graff and the chaff and other material are blown into a pile that can then be used for livestock feed.

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Figure 6: Grain/chaff collection system whereby grain, weed seeds, and chaff are separated from the straw in the field and taken to a central collection area for further processing.

Post-harvest residue management

It is generally considered most effective and cost efficient if residue management occurs at the back of the combine. However when the residue can not be adequately spread back on the land or collected from the field, other management techniques must used. The goal is to ensure that residue is managed sufficiently so the seeding tool does not plug and crop growth is not hindered. This provides beneficial growing conditions for the new crop.

Harrowing is an effective tool for spreading straw, but it should be noted that harrowing does not spread chaff. Therefore chaff spreading or collection must occur at the back of the combine. For best results harrowing should occur as close to harvest as possible in dry, windy conditions. Delaying harrowing until late in the fall or the spring allows the straw to settle into the stubble, making the harrowing operation less effective. Harrowing in the fall has the added drawback of knocking down standing stubble thereby reducing the potential to trap snow.

Anytime we add additional operations to the farm there are some inherent costs. Costs such as labour, fuel, and the cost of the equipment must be taken into account. They all add to the overall costs of the farming system. But none-the-less residue must be managed for the upcoming crop.

There are a number of harrow designs and manufacturers in the market place, but there are some common features among many of them. Oscillating harrows may be the best implement for spreading straw, particularly if the straw has settled into the stubble. Due to their aggressive action, oscillating harrows may also be used to provide limited surface incorporation of granular herbicides.

Stiff tooth or diamond harrows also do a good job of spreading straw (Figure 7). Replacing the 5 in. (12 cm) tines with 9 in. (22 cm) tines can improve the clearance, but it does make them susceptible to damage particularly at higher operating speeds. The 3-foot (0.9 m) sections do a superior job of spreading straw compared to the 5-foot (1.5 m) sections, probably because the reduced weight and width of the narrower harrow make it more mobile and flexible.

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Figure 7: Stiff tooth and light tine harrows.

Heavy harrows have been the tool of choice in the last number of years (Figure 8). Their aggressive action can provide excellent straw spread as well as a good mixing of the surface layer of the soil. Bent designs do not clear residue as well straight tines. If conditions are not ideal (dry and windy) heavy harrows may cause straw bunching. Aggressive tine angles do a better job of spreading straw, but they are also more likely to bunch straw. It is recommended that the tine angle be set at the most aggressive angle so that there is no straw bunching and maximum straw spreading.

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Figure 8: Heavy harrows.

Rotary harrows are typically associated with conservation farming (Figure 9). This is because the rotary harrow conserves crop residues and leaves the residue on the soil surface. However their design limits the movement of straw to approximately 3 feet (0.9 m) from where it lays. This may not be suitable for straw spreading where the straw must be moved and spread evenly across large areas. Bunching and plugging are not usually a problem with this harrow design.

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Figure 9: Rotary harrow.

On some soils, particularly grey wooded or other soils very low in organic matter, harrowing in the spring may be necessary to break-up or prevent crusting. This can be a particular problem as soils low in organic matter. Crusting can develop as these soils go from being saturated with water (flooded) to a much lower moisture content. The harrowing operation acts to maintain a loose granular surface.