ISSUE 15 September 18, 2008
POST-HARVEST TIPS FOR LATER MATURING CORN
Yield potential for corn frozen during the milk stage is low. Ears are difficult to pick and shell, kernel tips may stay on the cobs, and grain will be very chaffy. Therefore, green chopping or ensiling whole plants may be the only reasonable options. Corn silage should be harvested at 60 to 70% moisture. The length of cut should be about 0.5 inch long with not more than 10 to 15% being 1 inch or longer. A bunker or horizontal silo should be crowned in the center, have a wall slope of 1:6 to 1:8, and be covered with six mil polyethylene. To be effective the plastic must be held down over its entire area. Temperatures above 120 degrees after 4 days indicate that excess air is getting into the silage.
Test weights will be much less, probably 40 to 45 lb/bu., for corn frozen in the dough stage. Although corn will eventually dry to acceptable harvest moisture, it will take at least a week longer than mature grain. Ear molds will likely develop if warm ambient temperatures follow the frost. The only means of stopping mold growth are drying the grain or ensiling.
Standing corn in the field may dry 0.6 to 0.9 percentage point per day during warm, dry fall days with a breeze. Normally about one-half percent per nice drying day is expected in North Dakota early in September, may reduce to about 0.3 to 0.4 during October and 0.15 to 0.2 or less in November (see Joel Ransom’s article). Immature, frosted corn can mold on the stalk. Based on the equilibrium moisture content for the average monthly temperature and relative humidity, corn might be expected to dry to about 15.5% moisture content during September, 16.0% during October, and 19% during November. Field drying is normally more economical until mid to late October and mechanical high temperature drying is normally more economical after then.
Shelled corn should be at 25 to 30% moisture for anaerobic (without oxygen) high moisture storage in silos or silo bags. Any tears in the plastic bag must be promptly repaired to minimize storage losses. Whole shelled corn can be stored in oxygen-limiting silos, but a medium grind is needed for proper packing in horizontal or conventional upright silos. Wet grain exerts more pressure on the silo than corn silage, so conventional concrete stave silos may require additional hoops or the silo must not be completely filled.
Natural air and low temperature drying should be completed as much as possible in October because the drying capacity is extremely poor during the colder temperatures in November. Corn above 21% moisture should not be dried using natural air and low temperature drying to minimize corn spoilage during drying. An airflow rate of 1.25 cfm/bu is recommended to reduce drying time. Adding heat does not permit drying wetter corn and only slightly increases drying speed. The primary effect of adding heat is to reduce the corn moisture content. Natural air drying in the spring is the most energy and cost effective method of drying.
Shelled corn can be stored in a grain bin at moisture contents up to about 25% if it is kept below 30 degrees using aeration. Corn kernels above about 25% moisture may freeze into a clump that causes unloading problems.
Dryers will be operated more hours than usual, so examine them carefully and perform needed maintenance before harvest. Use the maximum allowable drying temperature in a high temperature dryer to increase dryer capacity and energy efficiency. Be aware that high drying temperatures result in a lower final test weight and increased breakage susceptibility. In addition, as the drying time increases with high moisture corn, it becomes more susceptible to browning. Use in-storage cooling instead of in-dryer cooling to boost capacity of high-temperature dryers. Cooling corn slowly in a bin rather than in the high temperature dryer will also reduce the potential for stress cracks in the kernels.
In-storage cooling requires a positive-pressure, airflow rate of about 0.20 cfm/bu or 12 cfm/bu-hr of fill rate. Cooling should be started immediately when corn is placed in the bin from the dryer. Dryer capacity is increased 20 to 40% and about one percentage point of moisture is removed during corn cooling. Condensation problems can be reduced by cooling the corn in the dryer to about 90 degrees before placing it in storage.
Dryeration will increase the dryer capacity about 50 to 75%; reduce energy used by about 25% and remove about 2 to 2.5 points of moisture. (0.25% for each 10 degrees the corn is cooled.) With dryeration, hot corn from the dryer is placed in a dryeration bin with a perforated floor, allowed to steep for 4 to 6 hours without airflow, cooled, and then moved to a storage bin. There will be a tremendous amount of condensation during the steeping and cooling process, so the corn must be moved to a different bin for storage or spoilage will occur along the bin wall and on the top grain surface.
A dryer that captures the heat from cooling the dry corn and a portion of the air from the final drying portion of the dryer can reduce the energy used to dry the corn by about 20%. Newer dryers typically have incorporated features to make them more energy efficient than older dryers.
Using the maximum drying temperature that will not damage the corn can also reduce energy consumption. The amount of energy required to remove a pound of water is about 20% less using a drying air temperature of 200 F than at 150 F.
Propane cost for high temperature drying corn can be estimated using the following formula. Cost/bu. – pt. = 0.022 x propane price/gal. For example, the drying cost is $0.044/ bu.-pt. if the cost of propane is $2.00, 0.022 x $2.00. It will cost about $53.00 for propane to remove 10 percentage points of moisture from 120 bushels of corn using $2.00 propane.
The estimated quantity of propane needed to dry is 0.02 gallons per bushel per point of moisture removed. For example, 24 gallons of propane is needed to dry 120 bushels of corn from 25% to 15%. (0.02 x 120 bu. x 10 pts.) This is based on 0.72 pounds of water being removed per point of moisture per bushel, 2,500 Btu of heat required to remove a pound of water in a high temperature dryer, and a propane heat content of 91,500 Btu/gallon.
The weight of water removed during drying can be calculated using the following formula. Initial Weight = (100-Final Moisture Content)/100-Initial Moisture Content) x Final Weight For 56-pound corn with an initial moisture of 25% and final moisture of 15%, the initial weight would be 63.5 pounds per bushel. The weight of moisture removal is 63.5-56 = 7.5 pounds per bushel.
Moisture shrink is the reduction in weight as the grain is dried one percentage point. Moisture Shrink Factor = 100 ¸ (100 – final moisture content). The shrink factor drying corn to 15.5% is 1.1834. The shrink drying corn from 20.5 to 15.5 would be 5 x 1.1834 = 5.92%.
Moisture meters will not provide accurate readings on corn coming from a high temperature dryer. The error will vary depending on the amount of moisture removed and the drying temperature, but the meter reading may be about 2% lower than true moisture. Check the moisture of a sample, place the sample in a closed container for about 12 hours, and then check the moisture content again to determine the amount of error. Moisture meter errors increase as corn moisture contents increase, so readings above 25% should only be considered estimates.
In addition, moisture meters are affected by grain temperature. If the meter does not automatically measure the grain temperature and adjust the value, then it must be done manually. Even if the meter does it automatically, it is recommended to cool a sample in a sealed container to room temperature before measuring the moisture content. Then compare the moisture content of the room temperature sample to the initial sample to verify that the adjustment is done accurately.
Normally, corn test weight increases about 0.25 pound for each point of moisture removal during high temperature drying. However, there will be little increase in test weight on immature or frost-damaged corn.
More fines are produced when corn is wet, because more aggressive shelling is required, which causes more kernel cracking and breaking. There is also more potential for stress cracks in kernels during drying, which leads to more breakage potential during handling. In addition, immature corn contains more small and shriveled kernels. Fines cause storage problems because they spoil faster than whole kernels, they have high airflow resistance, and they accumulate in high concentrations under the fill hole unless a spreader or distributor is used. Preferably, the corn should be screen-cleaned before binning to remove fine material, cob pieces, and broken kernels.
Immature corn has a shorter storage life than mature corn. Therefore, cooling the grain in storage to about 20 to 25 degrees for winter storage is more important than for mature corn. More frequent checking of the storage is recommended, and immature corn is not recommended for long-term storage.
Ext Ag & Biosystem Engineering