Harvesting, Grain Drying and Storage
The thought of harvest usually brings to mind the image portrayed by traditional farm magazine cover pictures that feature a calm sunny day with a few fluffy clouds wandering across an otherwise blue sky. Farmers are hard at work sitting inside their air-conditioned combines that devour uniformly golden fields of wheat. The setting continues with an occasional truck wandering out to the field to collect the harvest bounty and transfer it to the elevator where it is loaded on railway hopper cars to be shipped to the four corners of the world to feed the starving masses.
In the real world, harvest is the most stressful, dangerous period of the entire year for a farmer. Harvest means long hours, weather that is often uncooperative and an increased risk of accidents. There is no pay day for the grain farmer as long as the crop is in the field.
Winter cereal production on the Canadian prairies adds another dimension to the harvest season - seeding. For this reason, forward planning and an efficient harvest operation are especially important to winter cereal growers. Many of the difficulties experienced by farmers attempting to insert winter cereals into their rotation have been due to a lack of emphasis on the harvesting, grain storage and drying segments of the crop production equation. Attention must be focused on the early harvest of crops to allow winter cereals to be seeded at the optimum time. This usually means a reassessment of the entire crop rotation and harvesting operation. Rather than reacting to the weather from the combine seat, winter cereal growers must plan to take advantage of the opportunities provided by the weather and modify their operations so that harvest and seeding can proceed under less than optimum conditions.
Wheat Kernel Development
[ Windrowing | Preharvest Glyphosate | Combining ]
[ Grain Moisture Content and Temperature | Aeration | Insects and Mites ]
[ Molds | Rodents | Prevention of Grain Spoilage ]
[ Natural-Air Drying | Heated-Air Drying ]
B. WHEAT KERNEL DEVELOPMENT
Wheat reaches physiological maturity long before the harvest operation normally starts. There are often long delays that result in many potential harvest days being lost while we wait for the moisture content of mature kernels to drop to levels that are safe for storage.
Kernel development starts once fertilization
of the flower has taken place (see
Figure 1. Relative kernel water content
(wet basis) between the end of flowering (Zadoks stage 69) and
overripe (Zadoks stage 94) growth stages. See Chapter
10 for a general description of Zadoks growth stages.
Figure 2. Relative kernel weight
(14.6 percent moisture, wet basis) for the development period
between the end of flowering (Zadoks stage 69) and the safe
grain storage stage (Zadoks stage 93). The weight loss after
Zadoks growth stage 93 is due to over-drying. See Chapter
10 for a general description of Zadoks growth stages.
Figure 2 suggests that there
is a uniform pattern of increase in dry matter which is accompanied
by a steady decline in water content (
Given favorable weather, the water content of the wheat kernel decreases rapidly after physiological maturity (33 to 43 percent water) has been reached (Figure 1). Sunny, warm, windy days with low humidity will quickly dry wheat to moisture levels that are safe for storage (less than 14.6% moisture). Cool, humid weather slows the drying process and rainfall will produce an increase in kernel moisture content. Heavy overnight dew can also cause an increase in the moisture content of unharvested dry grain.
The head and kernels of a healthy wheat plant are green before physiological maturity. Once physiological maturity has been reached, the chaff and upper part of the stem lose most of their green color. The kernel color changes from green to a light yellow at physiological maturity and the kernel then gradually takes on its mature color (red in the case of hard red winter wheat). Disease and insect damage, and nutrient, heat and water stress can influence plant color; therefore, kernel color is usually the best indicator of physiological maturity.
Harvest begins with the cutting of a standing crop. Harvest may be accomplished in a single step by straight (direct) combining or in a series of steps that may include windrowing (swathing), combining, and drying the grain to a safe moisture level for storage. Straight combining is the preferred harvest method in most wheat production areas of the world. Warm summer weather during the harvest season means that windrowing offers few advantages and often increases the risk of crop loss in most of these regions. In contrast, harvest in western Canada takes place in the fall when temperatures are cool, days are short, and winter is often just around the corner. There is usually a mad rush to get the crop under cover on the Canadian prairies and windrowing, which offers the opportunity to speed up harvest, has been widely adopted by wheat growers.
Windrowing can start once the wheat has reached physiological maturity (33 to 43 percent kernel moisture, wet basis). The least mature kernels may still have a tinge of green, but most kernels should be a light yellow color and in the stiff dough stage (Zadoks stage 86). At this stage the kernel can still be crushed when squeezed between the thumb and forefingers, but beads of moisture should not appear when pressure is applied. Most of the grain yield and all of the grain protein will have been accumulated by the stiff dough stage. Consequently, windrowing at this stage should not cause a change in protein concentration or a reduction in test weight or seed weight unless the weather is extremely hot and dry and drying takes place at a very rapid rate. However, as a word of caution, the top wheat grades have low tolerance levels for green and immature kernels (see Chapter 24) and even the latest developing kernels must either be fully mature, or immature enough to be easily removed from the mature grain, before harvest can proceed without the risk of grade and price loss.
Grain filling may continue for a short time when the crop is windrowed at a kernel moisture content above 30 percent. The slower the drying rate, the longer grain filling can be expected to continue after cutting and the earlier the crop can be windrowed. Weather conditions, straw length, and windrow size all influence the windrow drying rate and, hence, the length of the grain filling period after the crop has been windrowed.
1. Drying Rate of Windrowed Wheat
Windrowing soon after the crop reaches physiological maturity can advance the harvest date if the weather cooperates. Studies conducted in western Canada suggest that, under cool reasonably dry conditions, windrowing can advance the harvest of spring wheat by up to six days. In regions where there is usually a rush to complete harvest before winter arrives, extra harvest days can be extremely valuable. In addition to reducing stress by spreading the work load, extra harvest days mean that more acres can be covered by each combine thereby lowering machinery investment costs. With late-maturing crops, such as spring wheat, any operation that speeds up the kernel drying process also reduces the risk of damage by early fall frost.
Standing wheat will usually dry as quickly as wheat in a windrow when the weather is hot and dry. Properly managed winter wheat matures 10 days to three weeks earlier than spring wheat in western Canada. The earlier maturity of winter wheat compared to spring wheat normally means that temperatures are warmer during the winter wheat harvest period. Consequently, when crop maturity is uniform, windrowing should not be expected to speed up the harvest of winter wheat to the same extent as it does later maturing crops like spring wheat.
Windrowing may not provide western Canadian farmers with a means of significantly speeding up winter wheat grain drying when crop maturity is uniform. However, it does have a place in dealing with uneven maturity, green weeds, and second growth.
2. Nonuniform Maturity
Winter wheat stands with populations of approximately 150 plants under dry conditions and 200 plants per metre square under favorable moisture conditions usually have two to three heads per plant and mature uniformly. However, winter wheat maturity is often delayed when plant populations are reduced due to low seeding rates, poor seedling establishment, winterkill, or flood damage. Winter wheat has the ability to compensate for low plant populations by producing a larger number of tillers. Because late developing tillers often mature later than early tillers, the larger the number of tiller heads per plant the greater the difference that can be expected in tiller maturity. Where large numbers of late tillers are produced, crop maturity may be delayed by several days to a week. Under these circumstances, windrowing serves to speed up the drying of late-maturing tillers and reduce the risk of grain loss from early-maturing tillers.
Crop lodging, unspread chaff rows (see Chapter 5), nutrient imbalances, and cool wet soils in low areas will often cause uneven crop maturity and harvest problems. Windrowing can speed up the drying of plants in the late-maturing patches and reduce grain losses from plants in areas that mature early. The effects of these variables on maturity can also be minimized by proper fertilization and crop residue management and the production of lodging-resistant, semidwarf cultivars in high moisture regions.
Plant parts and seeds from late-maturing weeds can delay harvest and cause grain storage problems. Farmers have a much more effective arsenal of herbicides available for weed control today than when swathing first became popular. However, the cost of herbicides, changes in management systems such as the recent shift to more continuous cropping and reduced tillage systems, herbicide resistant weeds, and new weed problems occasionally leave farmers with uncontrolled weeds to deal with at harvest. Until the pre-harvest use of glyphosate (Roundup, Laredo, Wrangler, and Victor) became a widely accepted method of perennial weed control, windrowing was the only effective method of dealing with late-maturing weeds so that harvest could proceed as soon as moisture levels were low enough for safe grain storage.
4. Second Growth
Second growth is a much more common harvest problem with winter wheat than spring wheat. The number of tillers produced by winter wheat seedlings is usually much larger than the plant can carry to maturity. The plants normally adjust their tiller number to reflect the environmental conditions encountered during the stem elongation stage by sluffing off late forming, poorly developed tillers. However, if a drought is broken or a nitrogen fertilizer deficiency is corrected when the winter wheat plants are in the stem elongation or early booting stages, late tillers may recover and produce heads.
If left to mature, these late developing tillers can delay winter wheat harvest by two or more weeks (see Figure 6, Chapter 10). When winter wheat fields have a significant amount of second growth, farmers have usually opted to wait and windrow the crop once the late-maturing tillers reach physiological maturity, even if the delay does cause an increased risk of grain loss from early-maturing tillers.
5. Losses Due to Insects, Weather, and the Harvesting Operations
The longer a crop stands in the field, the greater the risk of weather damage. Some cultivars, such as Selkirk spring wheat, are especially susceptible to shattering when they are mature and are less likely to experience yield loss from adverse weather if they are windrowed early. Cultivars that are susceptible to shattering are also likely to suffer greater grain loss from the action of reels and cutter bars when windrowing is delayed or they are straight combined.
The wheat stem sawfly has caused serious harvest losses to spring wheat in the prairie region. It attacks the base of stems causing tillers of mature plants to break off. Early swathing can reduce spring wheat harvest losses, but the most effective means of managing this insect pest has been the production of resistant cultivars. Winter wheat growers in North America have had greater problems with the Hessian fly than the wheat stem sawfly. Fortunately, only isolated incidents of Hessian fly damage have been reported on the Canadian prairies and this damage has been restricted to a minor breakdown of tillers in mature winter wheat crops.
6. Weathering and Sprouting
Kernel soundness and sprouting are wheat grading factors (see Chapter 24). Sound grain is mature and reasonably free from kernels damaged by frost, mildew, bleaching, or weather staining. Sprouting increases the kernel alpha-amylase level and reduces the baking quality of the wheat. Winter wheat has little or no seed dormancy and warm, wet weather can cause sprouting and rapid deterioration in quality. Therefore, it is important that every effort be made to minimize the opportunity for weathering once winter wheat is mature.
Kernel moisture may be as high or higher in standing wheat crops following a rain, but standing wheat crops usually dry more quickly than wheat in a windrow. Consequently, the quality of windrowed wheat often deteriorates more quickly than standing wheat during long periods of warm, wet harvest weather.
The windrows of short and/or thin wheat crops often fall through the standing stubble to the soil surface, especially if the crop was planted using a drill with wide row spacing. Windrows that have fallen through the stubble are more difficult to pick up with a combine. They also dry more slowly and are more likely to suffer loss in quality due to sprouting following periods of wet weather.
A preharvest glyphosate (Roundup, Laredo, Wrangler, Victor) treatment when wheat is in the hard dough stage (approximately 30 percent kernel moisture, Zadoks stage 88 to 90) can be used to speed up plant drydown and provide control of perennial weeds like Canada thistle and couch grass (quackgrass). Crop drydown following glyphosate treatment is usually slower than that provided by swathing. However, because it is translocated throughout actively growing plants, glyphosate is very effective in controlling perennial weeds that are building up nutrient reserves at this stage. The developing wheat kernel still contains approximately 30 percent water at the hard dough stage (a thumbnail impression will remain on the kernel), but the transport of nutrients from the leaves, stems, and spike to the kernel is complete. If glyphosate is applied earlier, while the plant is still actively translocating nutrients to the kernel, it may end up in the kernel as a herbicide residue that can affect germination and seedling vigor if the wheat is used for seed.
In western Canada, wheat is either straight combined (direct cut) or picked up from a windrow once it has matured.
In recent years, there has been a trend to increased straight combining. This trend has been especially evident in the harvesting of winter wheat where low seed dormancy and a normally warm harvest period increase the risk of grade loss due to germination when windrowed crops are exposed to extended periods of wet weather.
Unless maturity is very uneven, winter wheat dries down almost as quickly standing as it does in a windrow. Consequently, many farmers who do not have straight-cut headers (especially farmers with pull-type combines) windrow immediately ahead of the combine to avoid the potential weathering losses associated with early windrowing. Windrowing dry grain is an unnecessary step that adds to the time, labor, equipment, fuel, and repair costs of the harvest operation. However, these costs are usually offset by the cost of a direct cut header unless a large number of acres are straight combined each year.
Thin and/or short crops that have been windrowed are especially difficult to pick up with a combine. Therefore, harvest losses are usually greatly reduced when short to medium height (less than 60 cm) and thin crops are straight combined. For similar reasons, straight combining should facilitate a shift to semidwarf cultivars thereby reducing crop residue problems after high production years.
Compared to windrowing, straight combining provides the farmer with greater control over the amount of straw put through the combine and the height at which the crop is cut. Proper manipulation of these two variables is critical to effective residue management and efficient snow trapping, two of the primary factors that determine the success of direct seeding and continuous cropping systems (see Chapter 5). The Prairie Agricultural Machinery Institute (PAMI) has also estimated that combine capacity may be up to 50 percent greater because less straw is usually handled during straight combining compared to threshing windrowed wheat.
- Combine Adjustment
Proper combine adjustment insures that the grain is harvested with minimum loss. Grain loss during harvest means lost income and volunteer plants that must be dealt with later. With the current shift to direct seeding in western Canada, volunteer plants have become one of the primary weed problems and an extra effort to minimize grain losses during harvest usually pays worthwhile dividends later in the rotation.
Combine pickups and headers should be adjusted to minimize wheat shattering and head losses. The grain should be fed into the combine uniformly to minimize equipment wear and optimize the threshing operation. Holes and other leaks in the combine should be patched to eliminate losses as the wheat travels through the combine. Cylinder speed and concave clearance should be adjusted to thresh but not crack the grain (see Chapter 24). Fan speed, sieves, and strawwalker or rotary cylinder load must be set to efficiently separate the grain from the straw and chaff. Finally, the straw and chaff should be uniformly spread over the field to facilitate subsequent tillage and seeding operations (see Chapter 5).
Effective grain storage systems maintain grain quality and prevent grain losses. In order to prevent spoilage, storage structures must be designed and maintained to protect the grain from weather, bird, animal, insect, mite, and mold damage. Grain that is to be stored for any length of time must be kept under conditions that are uniformly cool and dry.
Moisture content and temperature are the main factors that determine the length of time that grain can be safely stored (Figure 3). Enzyme activity and micro-organism growth can be expected to increase dramatically when the moisture content of the air (relative humidity) in a storage bin rises above 70 percent, i.e., the relative humidity levels found in tough and damp grain. The rate of enzyme activity and micro-organism growth and development is also a function of temperature. As a consequence, tough and damp grain that is stored at warm temperatures will rapidly spoil while dry grain stored at cool temperatures can be safely stored for a long period of time.
The quality of wheat will deteriorate if it is harvested and stored at a high moisture content (Figure 3). Because of its major influence on safe storage time, grain moisture content is a grading and pricing factor in most wheat markets (see Chapter 24). In western Canada, wheat with less than 14.6 percent moisture (wet basis) is considered dry enough for safe storage and can be moved directly into the marketplace without a price discount. Wheat with 14.6 to 17.0 percent moisture is graded tough and must be dried to less than 14.6 percent moisture for long- term storage. Damp wheat, which contains over 17.0 percent moisture, deteriorates very quickly and can only be stored with extreme care and attention.
The safe storage time of grain decreases as temperature increases (Figure 3). The uniformity of grain temperatures within a storage bin can also affect the safe storage time of grain. Condensation occurs when moist, warm air comes in contact with cold objects. Consequently, temperature differences within a grain bin cause moisture migration from warmer to colder areas and, if air movement is insufficient to remove the moist warm air from the storage bin, conditions may become favorable for rapid grain spoilage. These conditions are most likely to develop when grain is stored in large (greater than 3,000 bushels), poorly ventilated bins. Differences in grain temperature and the outside air can also create high moisture areas within a bin. Moisture usually migrates to the top center of the bin when the grain is warmer than the outside air and to the center bottom of the bin when the grain is colder than the outside air.
Tough and damp grain will quickly heat and spoil unless grain temperature is controlled (Figure 3). Therefore, cooling must be given first priority when dealing with high moisture grain that is in storage. Aeration has been used by many farmers to cool tough and damp grain and to maintain uniform temperatures throughout the bin so that the formation of high moisture areas and hot spots can be delayed or prevented.
The airflow for cooling grain in storage may be as low as 10 percent (0.1 cfm/bu or 1.3 L/s/m*m*m) of that used for natural-air drying. When cooling high moisture grain, the aeration fan should be run continuously as long as the outside air temperature is at least 5° cooler (5° warmer if high moisture grain has been stored over winter) than the grain temperature, even during rain or other periods of high humidity.
Some drying will take place at low airflow rates, but even under ideal conditions no more than a two to three percent reduction in the moisture content of the grain should be expected. Consequently, low airflow aeration systems are not considered suitable for grain drying.
Granary weevil (Sitophilus granarius). The granary weevil is a shiny dark -brown color. They are 4 to 5 mm long and have a distinctive long, slender snout. They deposit their eggs inside kernels through a small hole that the female drills and seals. The larvae feeds inside the kernel leaving only the hull when the weevil emerges.
Meal moth (Pyralis farinalis). The meal moth prefers damp, mouldy environments. The adult meal moth has a wingspan of approximately 2.5 cm. Its forewings are grey-brown with dark brown patches at the tip and base. The larvae are cream colored with a dark brown head. They web wheat kernels together in clusters around cocoons.
Red flour beetle (Tribolium castaneum) and confused flour beetle (Tribolium confusum). These two beetles are very similar in appearance. The confused flour beetle is more likely to be found in areas where flour is stored while the red flour beetle is commonly found in grain storage areas. The adults are reddish-brown and about 4 mm long. The larvae are worm-like and white in color with light brown bands. They feed on the germ of the kernel, ground grain, flour, and grain dust.
Rusty grain beetle (Cryptolestes ferrugineus). A shiny, flat, reddish-brown beetle that is about 2 mm long. The larvae are about 3 mm long, white, and worm-like. The larvae and adults feed on the germ of the kernel, damaged kernels, and wheat dust. The rusty grain beetle can live in dry grain.
Saw-toothed grain beetle (Oryzaephilus surinamensis). A dark-brown, slender beetle that is 3 to 4 mm long. It gets its name from saw-like projections that can be seen on each side of the thorax when magnified. The larvae and adults feed on the germ of the kernel and wheat dust.
Mites. There are a large number of species of mites. They are distinctive from insects in that they have eight rather than six legs. Mites are very small in size and secrete an oily substance that has a distinctive, minty smell. They feed on the germ of the kernel, the kernel itself, and foreign material in the grain.
There are many different species of molds that can damage stored grain. These species thrive under a wide range of conditions, but warm, damp, well ventilated storage areas generally provide them with the most favorable environments for growth and development. Grain with damaged, broken, nonviable kernels and high levels of foreign material molds more readily than sound, clean grain. Improper drying methods and long periods of storage can also increase the susceptibility of grain to mold damage.
Molds may cause considerable damage before they become obvious to the naked eye. They generally attack the germ of the kernel first and can reduce the level of seed germination before any visible signs of damage are evident. Heating of grain and a musty odor indicates that serious mold infestations have occurred. If the environmental conditions are favorable, several species of molds can produce toxins that may cause serious illness when ingested by animals, including humans.
Rats and mice eat and damage large amounts of grain if they are not controlled. Rodents also carry diseases that are transmitted through direct contact or the fleas and mites that they carry and the food they contaminate.
Control measures include keeping storage facilities and surrounding areas clean and free of garbage and debris in which rodents can hide and nest. Waste grain that provides an easily accessible food source should be removed from the grain storage area. Storage facilities should be made rodent proof and an active eradication program should be followed.
The Hantavirus Respiratory Syndrome virus has made headlines in North America since 1993. This virus causes an illness with "flu-like" symptoms that include a cough, muscle aches, headaches, fever, nausea, and vomiting. Death from heart and lung failure and shock has resulted in more than 60 percent of the cases reported.
Hantavirus is spread by deer mice (Peromyscus maniculatus) and probably other rodents. Anyone who comes in direct contact with an infected animal or inhales dust that has been contaminated with excreta containing the virus is at risk. Early diagnosis of this disease increases the likelihood that treatment will be successful.
Fortunately, the Hantavirus Respiratory Syndrome has affected very few people and it can be prevented by a common-sense approach to rodent control. Standard rodent control practices should be followed. Special precautions should be taken to avoid direct contact with rodents and dusty air that may have been contaminated with their saliva or excretions. For further information, contact your local Public or Occupational Health Office.
There are a number of grain storage management practices that should be followed in order to minimize grain losses due to spoilage.
1. Bins, air ducts, perforated floors, etc. should be swept clean or vacuumed and spoiled grain in and around storage and harvest facilities should be cleaned-up and buried or burned. Holes and leaks in bins should be patched to prevent grain loss, keep out the rain, and limit the access of rodents.
2. Cleaned bins should be sprayed for insects well before harvest starts, especially if there is a history of insect problems in the storage facility. Always read and follow the instructions on the label when applying insecticides, fungicides, fumigants, and rodent poisons.
3. Properly sample the grain as it goes into the bin. This will provide a clear picture of the grain condition at the start of storage and help to identify potential problems that should be monitored and/or corrected.
4. Keep the amount of foreign material in the grain to a minimum. Use a grain spreader to distribute broken seeds and grain fines evenly throughout the bin and to maintain a level grain surface. The formation of dockage piles within the bin will affect air movement and aggravate insect and mold problems.
5. Avoid mixing newly harvested grain with grain that has been in storage. Grain that has been in storage may contain insects. Freshly harvested grain may also heat if it is placed on top of grain that has been in storage.
6. Regularly monitor grain temperature and moisture. The frequency that bins should be checked depends upon the moisture and temperature of the grain when it goes into storage. When potential problems exist, inspection every two weeks during the winter and once a week during the rest of the year is recommended to allow the opportunity for corrective action to be taken before significant grain spoilage occurs. Surface inspection is not sufficient for the early detection of high moisture areas, hot spots, or the presence of mold and insects.
7. Use natural or heated-air to artificially dry high moisture grain. Use aeration to keep high moisture grain cool if it cannot be dried immediately.
Canadian prairie winters provide an excellent environment for grain storage. High moisture grain can be stored for a long period of time if its storage temperature is maintained below 10°C. Low temperatures also limit insects and rodent population size and the growth of molds.
8. Move grain that has started to heat. This will temporarily cool the grain. It can also be used as a temporary measure for the control of insects and mites, especially if the grain is moved during the winter when outside air temperature is cold. The grain should be cleaned and sold, treated with an insecticide or fumigated if insects are detected.
9. Keep detailed records on the condition of the grain in storage. Accurate records can help in identifying potential storage problems and in planning preventative action.
Moisture will move from the grain into the surrounding air until an equilibrium is reached. The equilibrium moisture content of grain is dependent upon the relative humidity and temperature of the air (Figure 4). The equilibrium moisture content of grain decreases when the relative humidity decreases and temperature increases. However, the speed at which the moisture content of stored grain can be reduced by the introduction of warm, dry air is dependent upon the airflow rate. Therefore, air temperature, relative humidity, and airflow must all be considered in discussions that deal with the drying of tough and damp wheat.
The systems for drying high moisture grain can be divided into natural-air drying and heated-air drying.
Mature grain dries in the field when moisture is transferred from the kernel to the surrounding air. Air temperature, wind speed, and humidity affect the rate of drying. Similarly, the rate of natural-air drying is dependent upon the temperature and humidity of the air and the rate of airflow through the grain. The advantage of natural-air drying over field drying is that the grain is protected from wet weather by the roof of the storage bin. The challenge with natural-air drying is to successfully dry the grain before spoilage takes place.
content (wet basis) of cereal grains.
humidity during August, September, and October in Saskatchewan.
during August, September, and October in Saskatchewan.
The temperature and relative humidity of the air determine the equilibrium moisture content of grain (Figure 4). There are normally large fluctuations in relative humidity during harvest in western Canada. However, the average relative humidity is in the 60 to 68 percent range during the August to October harvest period (Figure 5). When wheat is exposed to air temperatures above 12° C and relative humidities less than 70 percent, its equilibrium moisture content drops below 14.6 percent (Figure 4). Consequently, there is a good potential for natural-air drying during August and early September in most regions of the Canadian prairies. Warm average daily temperatures in August (Figure 6) provide an especially favorable environment for natural-air drying during the normal winter wheat harvest period provided air flow rates can be maintained at high enough levels to remove excessive moisture before the grain spoils.
Aeration units that deliver airflow rates as high as 1.5 cfm/bu (19.5 L/s/m*m*m) are normally used for natural-air drying. The aeration fans inject air at the bottom of the grain bin (Figure 7). The air picks up moisture as it moves up through the grain to be exhausted out the top of the bin. Moisture is removed from the grain at the bottom of the bin until there is an equilibrium established between the grain moisture content and the relative humidity of the air passing through the grain. Consequently, airflow rate has only a minor effect as long as the grain moisture content is below the equilibrium level determined by air temperature and relative humidity (Figure 4). For this reason, airflow in natural-air drying is most important on warm, dry days.
During natural-air drying, the moisture content of the grain at the bottom of the bin is determined by the incoming air temperature and relative humidity while the grain at the top of the bin remains at the same moisture content as when the bin was filled. The area between these two layers is known as the drying front (Figure 7). The speed at which the drying front moves up the bin is dependent upon the airflow rate and the moisture content of the grain.
Moisture zones develop within the bin when grain is aerated
from the bottom. Drying starts at the bottom of the bin while
grain at the top of the bin remains damp. The success of bin
drying depends on moving the drying front through the bin as
quickly as possible without overdrying grain at the bottom of
the bin or causing spoilage at the top of the bin.
An uniform airflow throughout the bin is necessary to prevent the grain from spoiling before it dries. Regular temperature monitoring and/or grain sampling is important so that potential problems can be identified when drying does not proceed as expected. The temperature of the grain should be warmest below the drying front and coolest at the drying front. Unless the grain is mixed, the drying front must move through the grain to the top of the bin.
A farmer has essentially no control over air temperature and relative humidity when natural air is used to dry grain. This leaves airflow as the primary management tool that can be adjusted to accommodate variable storage situations. Airflow is influenced by fan specifications, duct design and layout, kind of grain, amount and type of dockage, depth of grain, method of filling, and ventilation.
At the time of installation, the fan system should be matched to the size of bin and the kind and moisture content of the grain to be stored. Duct design and layout must also be given due consideration as they determine the airflow distribution through the grain. The most uniform airflow is usually achieved when a perforated drying floor is used for air distribution.
Moist air must be allowed to escape from the bin without hinderance if high airflow rates are to be maintained. Ventilation openings that are equivalent to at least three percent of the floor area should allow moist air to be exhausted from the bin without significantly restricting airflow rates.
The air pressure produced by the fan forces the air through the grain in a drying bin. The airflow resistance the fan works against is known as static pressure. A manometer installed in the fan air stream provides the most practical method of determining static pressure. Airflow rates through the grain and the operating efficiency of the drying system can be quickly estimated once manometer readings are available.
Seed size must be taken into consideration when estimating the safe harvest moisture content and the depth of grain or oilseed that can be safely dried with natural air. Small seeded grain and oilseeds usually offer more resistance to airflow and, as a result, dry more slowly than large seeded crops.
It is generally recommended that the harvest moisture content of wheat should not exceed 18 to 20 percent when natural air drying. At higher moisture levels there is a danger that the drying time may be longer than the allowable storage time (Figure 3) and the grain will spoil unless special precautions are taken.
The drying time of damp grain may be reduced by decreasing the depth of grain in the bin. When bins are partially filled the distance the drying front has to travel to reach the top of the grain is shorter and airflow is increased because the static pressure is reduced.
Aeration fans should be turned on as soon as the floor and air ducts have been covered with grain. This allows grain cooling to start immediately and prevents chaff and other foreign material from collecting in piles and reducing the drying efficiency of the system. A properly installed grain spreader in the centre fill collar will assist in uniform drying by levelling or creating a small depression in the centre of the bin and ensuring that the grain is uniformly distributed around the outside of the bin.
A properly designed, well managed natural-air drying system provides a number of advantages to farmers who have to contend with poor harvest weather and tight production schedules.
- It provides the opportunity for greater control over the harvesting operation. This in turn allows for more efficient planning and greater flexibility of farming operations during the normally rushed harvest season.
It increases combining time by allowing for harvest under less than ideal conditions without increasing combine operating costs. Studies conducted by the Prairie Agricultural Machinery Institute (PAMI) have shown that differences in capacity and fuel consumption of a properly adjusted combine are minimal when grain is harvested at moisture contents between 14 and 23 percent.
It allows for an earlier start to the harvest season thereby reducing the risk of shattering loses and loss of grain quality.
An early start to harvest means that there are more standing stubble fields ready for seeding near the optimum seeding date for winter wheat. This reduces the inconvenience and stress of seeding during the harvest period and allows for a smoother integration of direct seeded winter wheat into a rotation.
An extended harvest period allows more acres to be harvested by each combine thereby reducing machinery investment and operating costs.
It makes straight combining a more attractive option because grain from stands that vary in maturity can be safely dried to a uniform moisture content.
It provides the farmer with greater control over grain moisture content. Damp and tough grain may spoil in storage without drying and grain harvested at moisture levels less than the dry cut-off (14.6% for wheat) weighs less and returns less when marketed.
Natural-air drying can result in increased grain handling and labor costs. However, a little time spent planning the layout and organization of grain storage, handling and drying systems can create efficiencies and savings in this area.
The drying capability of natural air is dependent upon air temperature and relative humidity. A heavy reliance on ambient (natural) air conditions for grain drying can create problems during extended periods of humid, warm weather.
Periods of high humidity and low air temperature occur most frequently during harvest in the northern and eastern part of the Canadian prairies (Figures 5 and 6). In years with wet harvest weather, supplemental heat is often required for effective grain drying in these regions.
When temperatures are below 10°C, the air will not carry much water and the transfer of water from the grain to the air is slow. Consequently, drying efficiency is greatly increased when heat is added to outside air that is colder than 10°C. The colder the outside air temperature, the more heat that must be added for effective drying and the more expensive the drying operation becomes. Consequently, it is much cheaper to complete the grain drying operation early in the harvest season when air temperatures are warm (Figure 6 ).
1. Supplemental Heat for Natural-Air Drying
The drying capacity of air can be greatly improved by increasing the air temperature of natural-air drying systems. As a general rule, the relative humidity can be expected to decrease by one-half for each 10°C increase in air temperature. For example, when the outside air temperature is 15°C and the relative humidity is 80 percent, a 10°C increase in air temperature to 25°C will reduce the relative humidity of heated air to approximately 40 percent. A further 10°C increase in air temperature to 35°C will reduce the relative humidity of the air to approximately 20 percent. Consequently, small increases in air temperature can be expected to produce large increases in the effectiveness of natural-air drying systems when the relative humidity of the outside air is high.
The lower the relative humidity of the air, the lower the equilibrium moisture content of the grain (Figure 4). When bin drying, the grain below the drying front (Figure 7) dries to an equilibrium moisture determined by the incoming air while the grain above the drying front remains at the same moisture content as when it entered the bin. Consequently, as long as the relative humidity of the unheated air is low enough to produce a grain equilibrium moisture content less than 14.6 percent (Figure 4), there is little to be gained by heating the air unless the overdried grain below the front can be mixed with the tough or damp grain above the drying front.
Overdrying means increased drying costs and market losses due to lower grain weight. Therefore caution must be exercised to ensure that overdrying does not occur when heated air is used in a system that is designed for natural-air drying. As long as the outside air temperature remains above 10°C, warming the air by approximately 5°C is usually sufficient to bring natural-air systems into the optimum range for drying wheat during periods of high humidity.
The weight loss (shrinkage) from drying can be calculated by using the equation:
where Mi and Mf are the initial and final grain moisture contents, respectively. For example, when wheat at 19 percent moisture is dried to 14.5 percent moisture, the shrinkage is 5.26 percent. If the same wheat were dried from 19 to 12 percent moisture, the shrinkage is 7.95 percent.
The safe storage time for grain is reduced when temperatures are increased (Figure 3). Therefore, airflow rates must be adequate to ensure that the moisture content is reduced to a safe storage level before the grain has had time to spoil when heat is added to natural-air drying systems.
2. Types of Heated-Air Dryers
Heated-air dryers use heat, high airflow rates and grain mixing to speed up the grain drying process. There are three main types of heated-air drying systems that have been incorporated into stationary bins or portable units.
1. Non-recirculating batch type dryers. The grain is loaded into these types of dryers as a batch and there is no mixing until after the heated-air drying phase has been completed.
2. Recirculating batch type dryers. The grain is loaded in a batch and there is mixing during the drying process.
3. Continuous flow type dryers. Grain is loaded and unloaded during the drying process. Grain movement through continuous flow dryers is usually adjusted to accommodate different levels of grain moisture content.
Airflow rate and grain depth have a large influence on the drying speed of heated-air dryers. Bin dryers usually employ lower airflow rates but, because of their larger size and grain depth, the daily output of dry grain may be as great for bin dryers as for "high speed dryers".
Dryers with high airflow rates can quickly and efficiently reduce the moisture levels of damp grain. However, dryers with lower airflow rates are usually more energy efficient and, because drying proceeds more slowly, they allow more time for moisture to move from the inside to the outside of the kernel. Consequently, lower airflow drying at cooler temperatures is often less expensive and more effective for removing the final one or two percent moisture from grain that is being dried.
Drying air that is too hot denatures proteins that determine the baking properties of wheat. Heat damage of this type does not cause visible changes to the kernels, but it can reduce high-grade wheat to feed wheat. Consequently, when a heated-air dryer is first used, it is a worthwhile precaution to have samples evaluated to ensure that the dried grain meets market standards.
The Canadian Grain Commission has provided the following guidelines for the safe drying of wheat.
The baking quality of wheat is damaged if the temperature of the grain reaches 60°C for any significant length of time. Accordingly, the first principle of safe drying is that the grain temperature in any part of the dryer should never exceed 60°C.
Wet grain exposed to a hot, dry air flow is cooled by evaporation of moisture. At a constant air flow the cooling effect depends on the amount of moisture present in the kernels. This effect decreases until, at a moisture content of about 14 percent, the grain temperature approaches that of the hot air. In batch dryers, grain lying next to the air inlet or plenum will dry first and may be damaged if the air temperature exceeds 60°C.
Ideal drying occurs when grain is continuously moving through a dryer against a current of hot air in such a way that each kernel receives the same treatment and the grain dries uniformly. Under such conditions higher air temperatures are permissible as the evaporative cooling is uniformly achieved and no part of the grain mass will reach 60°C during the time taken for the moisture to drop to about 15 percent. A further one percent of moisture is normally removed during the cooling phase. These conditions are approached in large dryers at terminal elevators where drying is safely undertaken with air temperatures of up to 80°C.
These conditions are partially achieved in a recirculating-type dryer - depending on the rate of recirculation and on the thickness of the grain layer. The faster the recirculation and the thinner the grain layer exposed to the drying air, the closer the conditions will approximate those in a terminal dryer. Wheat may be dried using air temperatures as high as 70°C in the best recirculating dryers. But this operating level should always be approached by drying several batches at lower temperatures, starting at 60°C and having them checked for possible damage.
Some so called "continuous" farm dryers do not achieve a uniform mingling of all of the wheat with the hot air from the plenum. The grain tends to flow along the outside or the inside wall without intermingling. As a result, the grain moving next to the plenum chamber is overdried while that on the outside is underdried. If air temperature exceeds 60°C, the grain moving next to the plenum chamber can be damaged.
b) Recommendations for operating dryers
2. Maximum recommended air temperatures for drying milling wheat:
- Non-recirculating batch dryers 60°C
- Recirculating batch dryers (depending on make and model) 60°-70°C
- Cross flow continuous dryers 60°C
- Parallel flow continuous dryers 70°C
4. Moisture content of wheat should not be reduced below 14.5 percent during the heating cycle since the cooling cycle will normally remove a further half to one percentage unit of moisture. If cooling is carried out in a separate bin following an 8-12 hour equilibration period, as much as two percentage units of moisture can be removed.
5. High moisture grain is more apt to be damaged if removal of more than six percent of moisture is attempted in one pass through the dryer. If wheat is over 20 percent moisture initially, it is advisable to reduce the drying temperature by 10°C for the last quarter of the heating cycle.
6. Outside weather conditions - temperature, wind speed and direction - may influence the relation between the plenum air temperature, as indicated by gauges, and air temperatures in other parts of the dryer. The use of several temperature sensors in the hot air plenum will assist you in determining maximum temperatures under all conditions.
Samples are used to determine the test weight, protein concentration, dockage, admixtures, moisture content, grade, and other characteristics of wheat as it moves from the combine through the delivery system to the consumer. Farmers, marketing agents, and processors also use samples to maintain inventories of different grades and monitor the condition of the grain so that the complex processes of market development, pricing, and coordination of deliveries can be carried out in an efficient, profitable manner. Therefore, it is important that samples are properly drawn to ensure that they are representative of the grain that is being handled, stored, or delivered for sale.
The Canadian Grain Commission suggests that the following method be employed to ensure that accurate samples are obtained from individual grain bins as they are filled. The equipment required for the sampling procedure consists of a long-handled cup and two pails that have been labelled A and B.
Step 1. As each truck unloads, take continuous cupfuls
from the sides and the centre of the stream of grain
using a long-handled sampling cup. Empty the cupfuls of grain
into Pail A. Take enough grain so that when the truck is empty,
the pail is about three-quarters full.
Step 2. Mix the contents of Pail A thoroughly.
Step 3. Remove a cupful of grain from Pail A and empty it into Pail B. The remaining grain in Pail A is no longer needed for the sample and can be dumped into the storage bin.
Step 4. Repeat Steps 1, 2, and 3 for each truckload that is emptied into the storage bin.
Step 5. When the bin is full, mix the contents of Pail B thoroughly.
Step 6. Place four pounds (2 kg) of grain from Pail B in a container. Label the container to identify the bin it represents.
Temperature probes and grain samples can be used to assess the grain condition inside a bin. Temperature readings and grain samples from the centre-top and bottom two to three feet (approximately one metre) of grain will provide a warning if grain starts to spoil in storage bins. However, grain samples from at least ten to fifteen widely separated probes are required to obtain a comprehensive picture of conditions within a bin and a representative sample of the grain.
Winter wheat has little or no seed dormancy and it may be used for seed without drying once it reaches physiological maturity. There is, however, a greater risk of seed damage when wheat is harvested damp. As a result, subsequent field emergence is usually highest for seed wheat that has been harvested when kernel moisture content is below 17 percent. Harvesting when the grain is dry eliminates storage problems and the need for drying when the seed is not sown immediately after harvest. These considerations are especially important because damp grain requires much closer attention when it is being dried and stored for seed rather than as commercial wheat.
The maximum temperature for safe drying of seed depends on its moisture content and the length of time it is exposed to the drying temperature. The higher the moisture content, the more sensitive seed is to high temperatures. However, moisture evaporation from damp grain causes cooling that delays the damaging effects of high dryer temperatures. Consequently, safe drying temperatures vary with the type of drying system used. As a general recommendation, the temperature for drying wheat seed should not exceed 60°C for high-speed dryers and 40°C to 50°C for batch-type dryers where the grain is held at the drying temperature for longer periods of time.
Low temperature and low relative humidity provide the best storage conditions for seed. As a general rule of thumb, the life of the seed is doubled for every one percent decrease in moisture content (between 4 and 14 percent) and every 5°C decrease in storage temperature.