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Determining the inherent
value of wheat straw | Stubble
height and residue amount required for erosion control | When
to avoid harvesting straw | Shelterbelts/barrier
strips and strip cropping | Crop
rotation | Straw
to grain ratio | Selecting
a variety | Frequency
for harvesting straw | Fertilizer
use and application | Nutrient
content of wheat straw | Greenhouse
Gases and Carbon Sequestration
Last Updated: July 2006
Key points when considering harvesting surplus cereal straw:
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 strawboard 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:
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. The following pages provide a discussion of factors to consider when determining the inherent value of straw.
Stubble height and residue amount required for erosion control
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Tall stubble provides greater protection against wind and water erosion and improves soil moisture conservation through trapping snow and reducing evaporation losses. Tall stubble also helps maintain the surface soil in a moist state, which improves seedbed conditions for shallow seeded crops. Therefore, 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.
- Slower speeds during seeding disturb less soil and bury less residue.
- Anchored, standing residue is much more effective for erosion control than loose, unanchored residue. A 40 bu./ac. wheat crop leaves about 750 lb./ac. of six-inch tall standing stubble.
- 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 (clodiness), and tillage practices. Saskatchewan Agriculture and Food recommends a general guideline of 750 lb./ac. of standing stubble plus 250 lb./ac. of loose residue be retained on the soil surface after seeding operations are completed.
This applies to well-aggregated loam soils. Increasing amounts of residues must be retained to prevent erosion on more sensitive soils.
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.
- A survey of 4,500 fields across Saskatchewan in the spring of 1998, found that 62 per cent of the fields had less than 500 lb./ac. of residue cover after seeding. Only six per cent of the fields had standing stubble after seeding which offers the best protection against erosion.
- A wheat crop yielding 20 to 23 bu./ac. under average fertility and growing conditions will provide enough residue for adequate protection of the soil from wind erosion if low disturbance direct seeding (zero-till) is practiced. This assumes about 15 per cent of the residue (leaves and chaff) is decomposed between harvest and spring seeding, and 10 per cent of the residue is buried during the zero-till seeding operation. However, the straw to grain ratio of wheat is quite variable, so in some years a producer may bale some straw from his/her 20 to 23 bu./ac. wheat crop, while in other years he/she cannot. Careful consideration of all the factors is required before straw is removed. (See section “Determining the Inherent Value of Wheat Straw” above.)
When to avoid harvesting straw
Shelterbelts/barrier strips and strip cropping
- Use of tree shelterbelts and barrier strips (perennial grass or annual barriers) is encouraged in drier areas and on vulnerable soils.
- Strip cropping is also an effective means of controlling soil erosion in drought-prone areas.
Crop rotation
- 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)
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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). However, this ratio varies widely depending on growing conditions, fertility, 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.
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.
Fertilizer use and application
Nutrient content of wheat straw
The generally accepted levels of nutrients removed by crops and residues under good growing conditions are published in the Canadian Fertilizer Institute fact sheet “Nutrient Uptake and Removal by Field Crops, Western Canada ”, Table 1.
A Saskatchewan Agriculture and Food initiated study of the nutrient content of Saskatchewan wheat straw under farm conditions for the 1998 and 1999 crops found average nutrient levels in agreement with the industry fact sheet. The values in the Canadian Fertilizer Institute fact sheet represent the total nutrients at maturity and are intended as a general estimate of the nutrients used by crops. However, there is considerable variability in nutrient uptake among wheat classes and varieties.
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.
Grain |
|
N (lb./ac.) |
P2O5 (lb./ac.) |
K2O (lb./ac.) |
S (lb./ac.) |
Spring wheat |
Total uptake |
76 – 93 |
29 - 35 |
65 - 80 |
8 -10 |
40 bu./ac. producing |
grain |
54 – 66 |
21 - 26 |
16 -19 |
4 - 5 |
3984 lb./ac. of straw |
straw |
22 – 27 |
8 - 9 |
49 - 61 |
4 – 5 |
* Source. Nutrient Uptake and Removal by Field Crops, Western Canada 1998. Canadian Fertilizer Institute.
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.
Calculating the fertilizer equivalent value of the nutrients in the straw (Table 2)
Table 2. Nutrient content and value of nutrients in one tonne of wheat straw*
Nutrient |
Price of the fertilizer |
Value of nutrient |
(lb./tonne) |
nutrient ($/lb.) |
($/tonne) |
N (12 – 15) |
0.40 |
4.80 – 6.00 |
P2O5 (4.5 – 5) |
0.35 |
1.58 – 1.75 |
K2O (27 – 34) |
0.20 |
5.40 – 6.80 |
S (2 – 3) |
0.28 |
0.56 – 0.84 |
Total |
|
12.34 – 15.39 |
* Values have been rounded. Nutrients contained in agriculture residues may be exposed to increased losses as compared to chemical fertilizer nutrients.
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. For more information on potassium and other nutrients visit Saskatchewan Agriculture under Crops/Soil Fertility/Fertilizer documents.
To determine the present market value for each fertilizer nutrient use the fertilizer prices in your area. In this example, assume:
- Nitrogen-N sells for $0.40/lb. N,
- Phosphate (P2O5) sells for $0.35/lb. P2O5,
- Potassium (K2O) sells for $0.15/lb. K2O, and
- Sulphur (sulphate form) sells for $0.28/lb. S.
Greenhouse Gases and Carbon Sequestration
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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 a small amount 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.
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Removing carbon dioxide from the air by growing plants and field crops, and storing carbon in the soil as soil organic matter is called a "Soil Carbon Sink". The United Nations Framework Convention on Climate Change requires Annex 1 countries to report greenhouse gases by sources and removals by sinks.
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Straw can be used for manufacturing strawboard. The carbon in the straw that makes up the strawboard 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 strawboard 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.
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Direct Seeding Manual – A Farming System for the New Millennium, 1999. Prairie Agricultural Machinery Institute, Humboldt, Saskatchewan. Call 1-800-567-7264.
- Securing Acceptable Erosion Risks after Pulses and Oilseeds in the Brown and Dark Brown Soil Zones. Semiarid Prairie Agricultural Research Centre, Saskatchewan Agriculture and Food.
- Saskatchewan Agriculture and Food website: www.agr.gov.sk.ca
- Nutrient Uptake and Removal by Field Crops – Western Canada 1998. Visit Canadian Fertilizer Institute at www.cfi.ca
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