Key points when considering harvesting surplus cereal straw:
- Maintain sufficient crop residue on the land to protect the soil from erosion by keeping the stubble standing particularly after seeding;
- Harvest surplus crop residue with an appropriate frequency so as not to lower soil organic matter, soil fertility and crop productivity; and
- Fertilize crops according to soil test recommendations.
Determining the inherent value of wheat straw
The decision regarding the amount of straw to be removed should be based on the inherent value of the straw for maintaining the viability of the cropping system and protecting the soil resource versus the value of the straw for other uses (e.g., feed and bedding for livestock, industrial straw board and feedstock for alternative energy forms).
The value of retaining the straw on the land is difficult to determine but should be based on a number of factors, such as:
- value of straw for soil erosion control;
- equivalent fertilizer value of the nutrients contained within the straw;
- value of the straw for building soil organic matter, soil quality and soil tilth; and
- value of the straw for soil moisture conservation.
If the value of the straw for the above factors exceeds its market value for other uses, then some or all of the straw should be retained on the land.
Stubble height and residue amount required for erosion control
Tall stubble provides many benefits which include:
- greater protection against wind and water erosion;
- improved soil moisture conservation through trapping snow and reducing evaporation losses; and
- keeping the surface soil in a moist state to improve seedbed conditions for shallow seeded crops.
Stubble should be cut as tall as possible without causing problems with plugging of seeding equipment.
Generally, the stubble height can be similar to the row spacing of the seeder. Most air-seeders have row spacing of eight to 12 inches, so stubble height can generally be from eight to 12 inches.
With direct seeding equipment that has four ranks of knife openers, some growers successfully manage stubble heights up to 1.5 times the row spacing. Newer direct seeding equipment with coulter or disc openers is able to handle taller stubble with few plugging problems.
In most years, shallower seeding results in less soil disturbance and better, more uniform crop emergence.
Things to consider:
- Slower speeds during seeding disturb less soil and bury less residue;
- Anchored, standing residue is much more effective for erosion control than loose, un-anchored residue (a 40 bu./ac. wheat crop leaves about 750 lb./ac. of six-inch tall standing stubble); and
- The amount of crop residue required for control of wind and water erosion varies with:
- field slope;
- soil texture;
- residue type;
- weather conditions;
- soil aggregation (clotting); and
- tillage practices.
Where surplus straw is harvested, direct seeding practices provide the best opportunity to maintain sufficient residue to protect the soil. To consistently maintain adequate residues, zero-till management practices are required.
Avoid harvesting straw in the following situations:
- Land with significant slope is vulnerable to soil loss by water erosion;
- Sandy textured soils are at high risk to erosion, especially in the Brown and Dark Brown soil zones;
- Eroded knolls need straw to protect the soil and to build soil organic matter; and
- Straw from canola, mustard or pulse crops decompose more rapidly and are less effective for controlling erosion.
Use of tree shelterbelts and barrier strips (perennial grass or annual barriers) is encouraged in drier areas and on vulnerable soils.
If straw is harvested from land that requires added erosion protection, consider planting winter cereals such as fall rye or winter wheat. This will provide continuous cover, especially during spring months when the land is most vulnerable to erosion.
Straw should not be harvested from fields that are to be summer-fallowed. Chemfallow is encouraged to minimize the risk of soil erosion.
Straw to grain ratio, and amount of straw that can be baled (harvested)
The ratio of total residue (straw plus stubble plus chaff) to grain for a wheat crop is typically 1.66 (1.66 lb. of residue per lb. of wheat (grain) harvested).
This ratio varies widely depending on:
- growing conditions;
- wheat class; and
- crop variety.
The ratio can be as low as one lb. of residue per lb. of grain, to as high as four lb. of residue per lb. of grain.
Generally, the quantity of straw that can be removed by baling ranges from 0.6 to 0.8 lb. for every lb. of grain. Besides the standing stubble that is left behind, balers often do not collect chaff and small pieces of straw that fall to the ground. Rotary combines may produce less baleable straw than conventional combines, especially under dry conditions. Chaff spreading during the combining operation is required.
Selecting a variety
Select varieties of wheat with tall straw to allow for the desired stubble height and for some surplus straw for baling. Semi-dwarf varieties produce shorter and less baleable straw and may be less desirable for straw harvesting.
Frequency for harvesting straw
Crop residue is essential to build and maintain soil organic matter. If straw is removed too frequently, soil organic matter levels will decline, and productivity and tilth of the soil will be reduced.
The harvesting of surplus straw must be planned for within a rotation cropping system. It is generally recommended that straw be harvested at a frequency of no more than once in four years.
Studies on Frequency of Straw Harvest
A 40-year crop rotation experiment at the Indian Head Research Farm has shown that a properly fertilized fallow-wheat-wheat rotation can maintain crop yields and soil organic matter even when the straw is baled after each wheat crop. The removed straw represented about 40 per cent of the total residue produced, and in this case, the roots and residue that were left behind was sufficient to maintain soil organic matter. The only soil factor affected by straw harvesting was soil aggregate size, which was slightly reduced when the straw was harvested. These plots were converted to zero-till management in 1990, and between 1987 and 1997 there was an increase in soil organic matter, despite having harvested the straw.
A 13-year study conducted in the Thick Black and Gray soil zones of Alberta determined the effects of removing all of the above-ground crop residue. When fertilizer was added according to soil test recommendations, soil organic matter increased over time even in plots where all crop residue was removed.
Returning the straw, fertilizing to soil test recommendations, and eliminating tillage had the greatest effect on increasing soil organic matter.
It is difficult to make a general recommendation on how often a portion of the straw can safely be harvested from a particular field.
In soils with low initial soil organic matter (Brown and Gray soil zones):
- a frequency of once every five to seven years may be acceptable.
In a well fertilized continuous cropping system in the Black soil zone:
- surplus straw may be baled each year that a cereal crop is grown.
- chaff should be spread; and
- remaining stubble should be left anchored to protect the soil from erosion.
Fertilizer use and application
Soil testing and subsoil moisture sampling are valuable tools to ensure optimum grain and straw yields. A cereal crop fertilized to soil test recommendations will produce more straw and grain than an unfertilized crop.
Where possible, fertilizer should be banded during the one-pass seeding operation. Low disturbance sidebanding openers also allow for fertilizer banding at the time of seeding. Coulter, disc and knife openers can be used to sideband or mid-row band fertilizer during the seeding operation, or as a separate operation.
Nutrient content of wheat straw
The generally accepted levels of nutrients removed by crops and residues under good growing conditions are published in the Fertilizer Canada fact sheet, Nutrient Uptake and Removal by Field Crops, Western Canada, Table 1.
Other factors that influence nutrient levels of straw and lead to wide variation in the nutrient levels from field to field are: fertilizer practices (amount and balance of nutrients), weathering of the straw after maturity and before baling, and soil and climatic conditions.
Nutrient content of wheat straw from Table 1 expressed as a percent by weight is as follows: N (0.55-0.68%); P2O5 (0.20-0.23%); K2O (1.23-1.5%); S (0.10-0.13%).
Table 1. Average nutrient uptake and removal by a 40 bu./ac. wheat crop with a straw to grain ratio of 1.66 under Western Canada conditions.
|40 bu./ac. producing
|3984 lb./ac. of straw
* Source. Nutrient Uptake and Removal by Field Crops, Western Canada 1998. Fertilizer Canada
From the nutrient levels in Table 1, one tonne of wheat straw has the following levels of nutrients given a 1.66 straw to grain ratio: N (12-15 lb.); P2O5(4.5-5 lb.); K2O (27-34 lb.); S (2-3 lb.).
If the nutrient content of straw is in question, a representative sample should be collected and submitted for analysis to a soil testing laboratory. Also, pre-feasibility studies should include determining nutrient levels of baled straw by crop type and harvesting method from the procurement area for the proposed processing plant due to the wide variation of nutrient levels in straw.
Note: Most Saskatchewan soils are high in potassium. Fields should be soil tested on a regular basis to monitor the change in available potassium levels and other nutrients because of the extra nutrients being removed by harvesting the straw.
Greenhouse Gases and Carbon Sequestration
When crops grow, they remove carbon dioxide from the air. The carbon in carbon dioxide is used in all plant parts. A portion of the carbon taken up by the plants is returned to the soil as residues. When these crop residues decompose, they release carbon dioxide back to the air, but some of the carbon residue becomes tied up in soil organic matter. Carbon dioxide is a major greenhouse gas and so there is interest in determining to what extent this natural process can be used to reduce carbon dioxide concentrations in the air. Increasing soil organic matter represents atmospheric carbon that is sequestered (or fixed) in the soil.
Research has shown that soil organic matter can be built up by: fertilizing to soil test recommendations, increasing cropping intensity, reducing the amount of summerfallow and reducing the amount of tillage used.
Straw can be used for manufacturing. The carbon in the straw that makes up the straw board will not decompose, and thus will not be released back to the air as carbon dioxide during the life of the product (e.g., furniture or houses). Storing carbon in straw board is a form of carbon sequestration. This can be extended by further recycling the straw-based products.
There is also growing interest in using straw as a feedstock to make fuels, such as ethanol which could replace fossil fuels. When fuels made from straw are burned, the carbon in the fuel is released back to the atmosphere as carbon dioxide. However, since the carbon dioxide was initially removed from the air by the crop, there is no net addition of carbon dioxide into the atmosphere.