September 16, 1999

Cool and Wet Weather Brings Late Season Wheat Drying and Storage Concerns
By Ken Hellevang, Post Harvest and Structures Engineering Specialist
NDSU Extension Service

Cool and rainy weather are causing moisture problems for producers with wheat in the field and damp grain in the bin, says a North Dakota State University agricultural engineer.

"With the cool September temperatures we've seen, the drying rate is reduced both in the field and in the bin," says Ken Hellevang of the NDSU Extension Service. "That means producers will have to do their homework and take action to avoid any additional damage to grain."

The cool temperatures require very low relative humidity to dry wheat to moisture levels low enough for long-term storage. Wheat will not dry to that level, 13.5 percent moisture, unless the outdoor relative humidity is less than 60 percent at 60 F to 70 F, 55 percent at 50 F, and 50 percent at 40 F, Hellevang notes.

Long-term average weather conditions in September are 56 F and 68 percent relative humidity in Devils Lake, and 57 F and 60 percent relative humidity in Williston, two areas where there is considerable wheat yet to be harvested.

"There may be a few hours during a warm afternoon when the wheat would dry to near storage moisture content, but the probability of wheat drying to storage moisture content in the field is poor by mid-September," Hellevang says.

Wheat already in the bin and being dried in mid- to late-September using ambient air will not dry down to the recommended storage moisture content of 13.5 percent or less. Hellevang advises producers to use air that has been warmed about 5 F. Using the average September weather conditions for Devils Lake of 56 F and 68 percent relative humidity, wheat would dry to about 15 percent moisture. Warming the air by 5 F results in the air being 61 F, with a relative humidity of 58 percent This air will dry wheat to about 13.6 percent moisture.

Hellevang notes that air will be warmed some by the fan. Systems operating at 5 to 7 inches of static pressure would be expected to heat the air by about 4 F to 5 F. This temperature rise should be considered when sizing heaters for low-temperature drying systems. Refer to NDSU Extension Service publication EB-35, "Natural Air and Low Temperature Crop Drying," for information on sizing heaters. As a rule-of-thumb, use about 1 kilowatt of heat per fan motor horsepower to provide a temperature rise for wheat of 5 F.

The publication is available from county offices of the NDSU Extension Service or from the NDSU Extension Distribution Center, Box 5655, NDSU, Fargo, ND 58105-5655.

Drying rate is primarily determined by airflow rate, Hellevang says, and final grain moisture content is determined primarily by the amount of supplemental heat provided. A system with inadequate airflow cannot be "fixed" by adding supplemental heat. Warming the air by 20 F or more will result in over-dried grain.

Natural air drying times are related to air temperature, Hellevang notes. Natural air drying times in September will be somewhat longer than in August when the average temperature is about 70 F. Heating the air by 5 F or 10 F should give similar results to natural air drying in August.

Wheat being placed into storage at moisture contents exceeding 13.5 percent moisture content must be kept cool with aeration. The expected allowable storage time approximately doubles with each 10-degree reduction in temperature. The expected allowable storage time for wheat at 16 percent moisture at 70 F is about 70 days; at 60 F, about 120 days; at 50 F, about 230 days, and at 40 F, in excess of 300 days.

Hellevang notes that some quality loss will occur during the specified allowable storage time. For example, wheat at 16 percent moisture and 70 F will have a loss in germination within about 50 days, about 20 days less than the 70-day allowable storage time. "Don't use all of your allowable storage time for the drying period and the on-farm storage period," he cautions.

Cooling stored grain also prevents insect problems. Insect reproduction is slowed at temperatures below about 70 F. Insects become dormant below about 50 F, and many insects can be killed if the grain temperature is kept below freezing for a few weeks.

"Damaged, low quality grain is more susceptible to storage problems than is clean, good quality grain," Hellevang says. "Generally, it is recommended to store damaged grain 1 to 2 percentage points drier than good quality grain. Cooling low quality grain is extremely important."

Hellevang recommends checking the stored grain every two to three weeks until the grain has been cooled to 20 F to 25 F for winter storage. Check the grain temperature and moisture content and look for insects at several locations.