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Keep It Cool

Thursday, November 13, 2003
filed under: Harvest/Storage

Unseasonably warm temperatures makes harvesting sunflower and other late-season crops a more pleasant task, but it can also lead to post-harvest storage problems.

The key is cooling sunflower and other grain down to the right temperature for adequate storage, and preserving quality until it’s sold later.

“We lose more grain to quality loss when we don’t control the temperature of that grain, than for any other reason,” says Ken Hellevang, extension ag engineer at North Dakota State University.

Roughly every time the temperature of stored grain is cooled by 10 degrees, the allowable storage time of that grain is doubled. The ideal temperature for insect and mold growth in stored grain is about 80 degrees. Cooling grain below 70 degrees reduces insect reproduction, and cooling it below 50 degrees causes insects to become dormant. Mold growth stops at temperatures below 40 degrees. Hellevang recommends that grain be cooled to about 20 to 25 degrees for winter storage, or as close as climate allows.

“Cooling” simply means using an aeration system or fans to moving air through the stored grain. How long should you cool grain? A good rule of thumb is to divide air flow rate by 15. Thus, if you have a fan that has an air flow rate of two-tenths cubic feet of air per bushel, required cooling time would be 15 ÷ 0.2 cfm/bu, or 75 hours.

Now, that formula is based on a 60-lb bushel weight, so while it’s perfect for figuring wheat and soybeans, it needs to be adjusted for oil sunflower, which at 32 lbs is about half that of wheat and soybeans (shelled corn is 56 lbs/bu). Half of 15 used in the wheat/soybean formula is 7.5. Thus, the formula to figure the time needed to cool oil sunflower with the same size fan used in the wheat example: 7.5 ÷ 0.2 cfm/bu, or 37.5 hours. It would be about 35 hours for confection sunflower, which is 24 lbs/bu.

Sunflower can be stored for short periods during cool weather at 12% moisture with adequate airflow to keep the seeds cool, but needs to be brought down to 10% moisture or below for safe storage over the winter.

If needed, add heat with a natural air/low temperature grain dryer to dry sunflower down to the safe storage moisture content. Generally enough heat to warm the air 10 degrees is the maximum amount required. As a rule of thumb, about 2 kilowatts of heat will be required per fan motor horsepower. The equation for calculating the heat requirement in BTUs is: BTU/hr = cfm x 1.1 x temperature increase.

Keep in mind that when harvesting grain during warmer temperatures, there is a greater possibility for moisture migration. Grain is an insulator, and can hold a tremendous amount of heat. When temperatures cool in the fall, grain harvested when it was close to 80 degrees outside may remain warm, particularly in the center of the bin. The warmer air can move up and deposit moisture as it comes into contact with cooler air toward the top of the bin (see graphic).

“Any time we see roughly a 20-degree temperature differential between the walls of the bin and the center of the grain mass, we’ll have a greater potential for moisture accumulation,” says Hellevang. “And the larger the size of the bin, the more likely we’ll see moisture migration. So it’s imperative we aerate, so that as outdoor temperatures cool, we can cool the grain mass down.”

Flattening the top of a grain pile in storage reduces the concentration of moisture and heat in the middle and peak of the grain mass, helping to prevent crusting and the potential for insect and spoilage problems.

Cooling gradually in 10 to 15-degree steps to avoid a large difference between the bin and outside temperature, along with adequate bin vents, will help minimize condensation. Make sure bin roof openings are adequate: One square foot of bin vent opening for every 1,000 cfm of airflow is recommended.

Be mindful that condensation moisture can form into ice that can seal bin vents, resulting in bin roofs pushed up if air is pushed through the grain, or flattened roofs if air is pulled downward through the grain.

“This can happen with late-season crops like sunflower and corn harvested in October under fairly warm temps, which then drop below freezing. If you’re operating the fans and looking at a considerable temperature difference between the stored grain and outdoor temperatures that are freezing or below, open the fill hole of the bin or crack the door to allow for pressure release in case the vents freeze up. Then once the grain is cooled, don’t forget to close the bin back up.”

Do you need to be concerned about temperature fluctuations during the winter? Not if your grain is cooled down properly, at the right moisture for storage.

“If you have sunflower at 10% moisture cooled down to around 20 degrees and we get a string of 40 to 50 degree days in the winter, I wouldn’t be concerned. But if you’ve got sunflower sitting there at 12 or 13% moisture, pushing the margins of recommended storage, there could be problems. A string of warm sunny days can warm the outside walls of a steel bin to 50 or 60 degrees, resulting in hot spots and possible insect problems.”

The best solution in that scenario is to get fans going again, to cool the grain. “Sometimes people might be tempted to use a fumigant, but you need a certain temperature for the chemical to volatilize, and the best we can hope for in a bin with hot spots of grain is partial control. A fumigant sitting there that hasn’t been fully volatilized can also be a health hazard to grain handlers, when the chemical volatilizes later when temperatures warm up.” —Tracy Sayler

Sunflower Storage Do’s and Don’ts

• Do run fans and cool sunflower to about 20 to 25 degrees, and then hold them at that point. Safe storage of oil and confection sunflower over the winter is 10% or below.

• Do consider cleaning sunflower before putting it in storage to help maintain quality.

• Do dry sunflower seeds before storage if they’re harvested wetter than 10%.

• Do remember that sunflower is an oil-based crop, and fine fibers from sunflower seeds pose a constant fire hazard. Prevent dust and “fines” from accumulating, and keep a fire extinguisher on hand when harvesting and drying sunflower.

• Don’t turn fans off too early. Sample the last exit point to make sure moisture has been pulled or pushed through the grain adequately. If you’re pushing it through from the bottom, then check the grain at the top. If you’re sucking the air from the top, then sample at the bottom of the bin.

• Don’t be fooled by sunflower coming from a dryer with shells that are drier than the kernels inside. For example, a moisture meter may give a reading of 10%, then climb back up to 12% again the next morning. Moisture rebound can be estimated by placing a sample from the dryer in a covered jar and rechecking the moisture after 12 hours.

• Do monitor the moisture and temperature of seed in storage. Check the condition of stored grain about every two weeks while grain is cooling, then about monthly after grain has cooled. A check should include measurements of moisture content and temperature at several locations.

Quality Discounts Can Add Up

A good contract price (c/p), along with an attractive oil premium of 2% of the c/p for each 1% over 40%, can be erased in a hurry by quality discounts.

Following is the discount schedule at ADM’s Northern Sun in Enderlin, N.D. (as of late October; it is subject to change) for oil sunflower (contract basis of 40% oil, 10% moisture, 25 lb test weight) with grade and quality determinations and standards set forth by the USDA’s Federal Grain Inspection Service.

Moisture: Between 10% to 14% will be discounted at a rate of 2% of the c/p for every 1% of moisture. Moisture over 14% is subject to 4% of the c/p for each 1% over 14%, and subject to rejection.

Test weight: 1% of the c/p for each one-half lb under 25.

Heat damage: 0.5% is allowed; discount applies from the c/p of 2% for every 1% over the 0.5%, up to 2%, prorated by fraction. Over 2% heat damage is subject to rejection.

Reg. damage: Amount allowed is 5%. Over 5%, there is a discount of 2% from the c/p for each 1% over, and over 10% damage is subject to rejection.

Infested: Discount is 3% of the c/p. No seeds treated or “COFO” (commercially objectionable foreign odor) accepted.

Stones: Discount of $.05 cwt for 1-10 stones; for every stone after 10 there is a $.01 cwt discount.

Other: Discount of 2% from the c/p for sour and/or musty, heating or low quality.

Foreign Material: All FM is deductible from the gross. If FM exceeds 12%, a discount of 1% from the c/p applies for each 1% over 12%.

Discount for oil is 2% of the c/p for each 1% under 40-38%; 3% of the c/p for each 1% under 38-32%; and 3.5% of the c/p each 1% under 32%, prorated if a fraction.

Online Resources For Postharvest Grain Storage Tips And Information (search publications for “grain storage”)

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