For dairy farmers, the goal for corn silage is simple: harvest the most tons possible of a highly digestible hybrid. But that’s easier said than done when extreme weather conditions invite pests and diseases that can compromise yield and quality of what you’ll feed in the coming year.

Drought conditions provide an ideal environment for destructive spider mites to thrive. Conversely, northern corn leaf blight (NCLB) can quickly get a damaging toehold when cool, moist conditions exist. The common denominator is that, when uncontrolled, both can significantly reduce yield and total digestible nutrients (TDN) in corn silage.

Growers must be proactive to stop these yield robbers. This is especially true when growing brown midrib (BMR) corn, a favored feedstuff of dairy producers, thanks to its high fiber digestibility that can boost milk production. BMR corn must be carefully managed to guard against yield-depleting foliar diseases like leaf blight. Spider mites also pose a serious threat to tonnage, though they are an equal-opportunity pest – no corn hybrid, conventional or BMR, can claim resistance.

Mitigating mites

Spider mite damage can reduce corn silage yield by up to 6 tons per acre when not controlled, according to large-scale research conducted by the University of California – Davis. Mite damage can reduce acid detergent fiber and neutral detergent fiber, as well as TDN.

Spider mites are problematic in hot, dry weather. They are a constant problem in arid climates in the western U.S.

These mites live in colonies on the underside of corn leaves and can rapidly reproduce on drought-stressed plants. One female produces 45 to 100 eggs. It takes about 10 days for them to go from egg to adult.

The two most common species of spider mites are Banks grass mite and two-spotted spider mite. The Banks grass mite is more common early in the season and stays on the lower portion of the plant, moving up as the population develops. The two-spotted mite is spread out on the plant throughout the growing season.

Heavy rain or overhead irrigation can wash mites off of foliage. However, widespread furrow irrigation practices in the West shield mites during the growing season.

Growers must be proactive, rather than reactive, to mitigate spider mite damage. I recommend the following:

1. Have a plan. Low water supply is a consistent problem in the West, so growers must always be ready for spider mite pressure. Without a good water supply, mites are a huge threat, so you must go into the season with a miticide program in place.
2. Go low. Ground application is always superior to air application for spider mite control. If there are mites in the field, even at low populations, treat by ground before lay-by. You always get more bang for your buck by using a ground rig.
3. Spray smart. Improve application coverage by getting it down into the canopy. Use multiple drop nozzles and at least 20 gallons of water per acre. It should almost create a fog behind your sprayer.
4. Try tank mixing. Tank-mixing miticides is a new practice that can provide benefits. Spider mites are building resistance to some miticides, so mixing different mode-of-action products can achieve greater control. Check labels on miticides and see what can be mixed. Also, remember to mix up chemistries if more than one application is required during the growing season. Tip: Know which mite species you are trying to control. There are several miticides and insecticides labeled to manage spider mites, but some products are labeled for certain species while others are not.

Beating blight

Northern corn leaf blight is a perennial problem in the Northeast and throughout the Corn Belt.

The disease survives in corn residue, spreading to new crop plants by rain splash and wind that distributes spores. Infection occurs when the corn leaf surface remains wet for six to 18 hours in temperatures ranging from 65ºF to 80ºF.

Infections generally begin on lower leaves and progress up the plant. New spores can be reproduced in as little as a week, so NCLB spreads much faster than many other corn leaf diseases.

Unlike some conventional corn hybrids, BMR corn is susceptible to the foliar disease. Both dry matter yield and starch content will take a hard hit in untreated corn silage fields, says Art Graves, a Mycogen Seeds agronomist in New York.

“If 95 percent of the plants have lesions at tasseling, you can lose 3 to 5 tons, or about 150 dollars, per acre,” he explains.

Despite its leaf blight susceptibility, BMR corn is popular with dairy producers seeking highly digestible feedstuffs to maintain or increase their herd’s milk production.

“Nobody complains because getting digestible forages is the most important thing,” Graves says, explaining the value of BMR corn silage far outweighs the cost of a fungicide application. “It’s not even close. We’re talking hundreds of dollars in added inputs versus thousands of dollars return in milk production.”

Graves offers these tips to beat blight:

1. Use the latest hybrids. “Every new generation of corn hybrid gives better leaf blight resistance than the one before,” he says. Talk with your agronomist and review research plots in your area to seek out new hybrids that may perform better in your fields.
2. Keep the fungicide handy. “If you have leaf blight every year, you should be prepared to spray,” Graves advises. Have fungicides factored into input costs and know what you’ll want to use, if it’s needed.
3. Boost nitrogen. Proper fertilization is crucial to establishing a healthy plant and protecting where leaf disease may be an issue. Graves recommends boosting the recommended nitrogen rate by 10% to improve disease defenses.
4. Spray for economic return. Apply fungicides only when it makes dollars and sense. “If lesions are visible on the ear leaf at tasseling, that’s a pretty easy decision-maker. Spraying a fungicide then will give you a good return on your investment,” Graves advises.

Proactive management is the key to stopping pests and disease from wrecking a BMR corn silage crop. Do the math to weigh the risks versus rewards regarding timely chemical treatments. Then, scout your fields early and regularly to know when to act.