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Southwestern corn borer cuts into Mississippi crop

High populations of southwestern corn borers in the Delta and northeast regions of Mississippi will certainly cause substantial harvest losses this year if the corn crop is not harvested promptly.

Third-generation southwestern corn borers cut stalks down by girdling the stalk internally a few inches above ground level. This girdling action predisposes stalks to nearly certain lodging, particularly when followed by moderate wind.

Girdling activity normally begins about 30 days after egg-laying. Since the third generation southwestern corn borer moth flight normally occurs in early August, producers should try to finish corn harvest before Labor Day weekend.

Corn may be harvested any time after grain reaches physiological maturity, which occurs around 30 percent moisture. However, corn may not be safely stored until after considerable moisture loss.

Grain elevators discount wet corn to account for drying expenses and moisture weight loss during drying. Moisture dockage schedules between elevators may vary significantly, so compare rates. Most schedules discount about 2.5 percent per each percent moisture above the standard and may increase as moisture content rises.

Water evaporated during drying (shrinkage) accounts for 1.18 percent of the dockage per each percent moisture. The producer loses this weight whether he sells wet grain to the elevator, dries it mechanically or lets the grain dry in the field. A producer should subtract this value from the dockage rate to show the realized or actual dockage.

Harvest losses are as important as moisture dockage rate in making harvest decisions. The longer corn stays in the field, the greater the likelihood of substantial field losses. Factors such as stormy weather and southwestern corn borer damage can cause considerable lodging in unharvested fields. Abundant July rainfall has also promoted morningglory growth in some corn fields, which can greatly inhibit harvest efficiency. Each of these factors may cause substantial field loss, which would considerably outweigh moisture savings.

Producers should consider their harvest capability — the longer it takes to complete harvest, the earlier harvest should start.

Growers should harvest non-Bt hybrids infested with corn borers, early-maturing hybrids, and those with below-average stalk quality promptly and before other hybrids.

Producers should closely check for loss while the combine is harvesting and make adjustments accordingly. Two corn kernels per square foot or one dropped ear per 100 feet of row equals about 1 bushel per acre yield loss.

Research generally indicates combine efficiency is best (harvest losses are lowest) when corn grain moisture is about 20 to 22 percent. Growers seeking maximum profitability should always strive to finish harvest before grain moisture falls below 15 percent.

Producers selling corn at less than 15 percent moisture are giving away profit. A producer harvesting 150-bushel corn at 14 percent moisture is losing $3.54 per acre and $7.08 per acre at 13 percent moisture (at $2 per bushel). This loss is solely from reduced grain weight due to lower moisture content. This moisture weight loss closely approximates the actual dockage most elevators charge for high-moisture corn.

Since corn loses approximately 0.6 percent per day during the harvest season, begin harvest early enough to guarantee all corn is harvested before it reaches 15 percent.

Improper grain handling, particularly with high-moisture grain, can quickly promote aflatoxin development after harvest. Wet grain should be dried immediately to below 15 percent moisture or hauled to an elevator (which will dry the grain). Wet grain should not be stored in trucks, combines, bins or any non-aerated site more than four or six hours before drying begins.

Fungal growth which causes aflatoxin will reach excessive levels very quickly in wet, warm grain. Conversely, fungal growth becomes dormant when grain moisture drops below 15 percent. Producers should thoroughly sanitize handling and storage facilities before and during harvest.

Erick Larson is a grain crops specialist with Mississippi State University. e-mail:

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