Weatherproof for the Homestretch

When silks are growing well, corn can tolerate a moderate population of silk-clipping insects. But if silks are growing poorly and pollen viability is short, even a small number is a threat. ( Darrell Smith )

Planting has come and gone and your corn is standing knee-high. At this point, it’s turned the corner and is entering the home stretch. Now you must weatherproof corn through the pollination stage and soybeans during their reproductive and pod-filling stages, R1 to R6.

You want to keep plants as strong and vigorous as possible, so they can resist adverse weather during the pollination period. “Stress during pollination costs yield,” says Farm Journal Field Agronomist Ken Ferrie. “That’s why market forecasters who try to predict yield focus on the month-long period beginning two weeks prior to pollination. Two or three days of stress, including drought or cloudiness, which reduces photosynthesis, can cause plants to abort a lot of pods or kernels.”

Because pest attacks create stress, which multiplies the plants’ vulnerability to the weather, your farm’s pest management team now steps into the spotlight. Since the amount of damage from all insects is higher in dry years, your scouts’ and applicators’ job is to weatherproof the crop against drought. Think of them as superheroes, clearing the tracks for the oncoming pollination train. Every car is crammed full of kernels holding all the starch produced by however much photosynthesis Mother Nature has provided.

Prioritize your scouting based on the hybrids likely to attract aphids. “Adult aphids look for the best-tasting hybrids,” Ferrie explains. “This is influenced by sugar content. If you split your planter and plant two hybrids, one hybrid might be free of aphids, while the other is 70% or 80% infested. Your seeds rep might be able to tell you which hybrids are most attractive to the insects, so you can manage around that weakness.”

Ensure pollen is available. Monitor growing degree units, and 10 to 14 days before pollination, while tassels are still snug in the whorl, pull some, unwrap them and search for aphids.

“Instances when aphids affect pollination are rare, but they do occur, especially when few predators are present,” Ferrie says. “Aphids secrete a waxy waste product called honeydew, which can coat the tassel and prevent the pollen sacs from opening. If you find a high percentage of plants with aphids, treat them before the insects reach the tassel. This will be in the loose-whorl stage, before tassels are visible from the road.”

Scout for silk clippers. “When pollen drops, you need silks to receive it,” Ferrie says. “Look for insects such as rootworm beetles and Japanese beetles. For pollination, you must maintain ½" of fresh silks on the tip of the ear. If scouts find silks clipped shorter than ½", peel off the husk and shake the ear. If silks fall off, those kernels are pollinated, and silk-clipping insects are no longer a concern. But if the ear isn’t pollinated, you must stop the feeding within days.”

Ideally, ears need to pollinate from butt to tip in three to five days. “Under good growing conditions, when silks grow 1" to 1½" per day, corn can tolerate a moderate level of silk-clipping insects,” Ferrie says. “But under stressful conditions, when silks are not growing well and pollen viability is short, even a small number of silk clippers is a problem.”   

When pollination is complete, focus on grain fill. “Aphid colonies left unchecked can coat the upper three leaves with honeydew and shut down photosynthesis,” Ferrie says. “This damage can reduce ear fill by a third to half.”

Now your goal is to keep the ear and plant intact. “Scout for earworm and western bean cutworm eggs,” Ferrie says. “There’s only a short window from when eggs hatch until the insects get under the husk and into the ear, where they are harder to kill.”

During this stage of growth, plants translocate sugar from the stalk and leaves into the ear. “To make sure this process happens, continue scouting for European corn borers and southern corn borers,” Ferrie says. “Use pheromone traps to alert you to the presence of borers, and look for borer eggs and feeding damage. Shortly after hatching, the insects enter the stalk, so you must react quickly. Focus on your non-GMO hybrids that are not genetically protected.”

Also scout for diseases. “Diseases don’t influence pollination, but they do interfere with starch production that fills kernels,” Ferrie says. “The pest team must know what weather conditions trigger which diseases in their environments. That requires understanding the pest pyramid, which is how the presence of a host crop, disease organisms and environmental conditions interact to produce a disease outbreak. Focus your scouting on fields that have had disease problems in the past or contain susceptible hybrids. Be ready to spray if a disease reaches threshold levels.”

Spraying a fungicide on corn helps plants stay green and improves stalk quality. “In our trials, even if a fungicide didn’t improve yield, it added two weeks of standability,” Ferrie says. “This can be important in years such as 2018, when we had to deal with a lot of down corn. In other words, applying a fungicide can weatherproof harvest.”

Be sure the product you apply will control the disease in your field. “For example, a bacteria such as Stewart’s Wilt can’t be controlled by a fungicide,” Ferrie explains. “Keeping good field records will help you weatherproof against that disease, by selecting resistant hybrids for those fields in the future.”

Continue to monitor diseases and insects, such as aphids, spider mites and later generations of corn borer, all the way to harvest.

Although soybeans have longer pollination and pod-filling windows, the pest team can’t stop to rest. Any stress during these periods can cause abortion of pods or beans, or small shriveled beans.

 “While we monitor insect pressure and defoliation all season, we must be especially sensitive and quicker to react during the pod filling stage,” Ferrie says. “Watch for Japanese beetles, aphids and spider mites.”

Stress can result not only from insects, diseases, drought and cloudiness, but also from herbicide damage, Ferrie points out. “If you apply a rescue herbicide, be sure the improved weed control is worth the stress on the soybean plants and the damage caused by the applicator,” he says. “Soybean plants run over during a rescue herbicide application are lost, and there is no time for the remaining plants to compensate. Our trials in solid-seeded soybeans showed wheel tracks on a sprayer with a 60' boom cost 2 bu. per acre.”

Damage from rescue herbicides can be reduced by using tramlines, aerial application or the widest boom.

As with corn, planting soybeans of various maturity ranges can mitigate the effect of weather stress. Unlike corn, staggering planting dates is not effective because soybean maturity is triggered by the hours of darkness, rather than growing degree units. 


Stagger Planting to Weatherproof Pollination

Spreading pollination over two or three weeks protects against bad weather and reduces pressure on your pest team and applicators. Staggering planting dates is easiest in regions with a long planting window. You might stop planting corn and plant soybeans for awhile.

“Staggering maturities is easier, but there are two things to remember,” says Farm Journal Field Agronomist Ken Ferrie. “First, check each hybrid’s growing degree units to flowering—two hybrids might have the same maturity, but one will flower a week later than the other. Second, if you decide to plant early, midseason and late-maturing hybrids, plant them in that order, from earliest to latest. If you do the opposite, they might all flower in the same week.”


Manage Water to Weatherproof Pollination

The ability of silks to receive pollen after it falls from the tassel is affected by weather. “Hot, dry conditions reduce the viability of both the pollen and silk,” says Farm Journal Field Agronomist Ken Ferrie.

“Most pollination problems during drought are caused by poor viability of the silk.”

At the other extreme, prolonged wet periods slow pollination because tassels must dry out in order for the pollen sacs to open. “If you irrigate, try to apply water in the afternoon and evening, if possible, so the pollen sacs will have time to dry out during the peak of the day,” Ferrie advises.

“Pre-loading water before pollination starts might reduce the need to water in the morning,” he adds.

If you don’t have irrigation, you can still identify problems that affect future management decisions. “For example, don’t drop a hybrid because of low yield if the cause was poor ear fill resulting from unusual weather during pollination,” Ferrie says.

With soybeans, too, managing irrigation can help mitigate the effect of stress. “But be careful,” he says. “Irrigation decreases the evapotranspiration rate, by raising the humidity and lowering the temperature in the canopy. That can affect soybean plants’ ability to hold pods.

“In years when you don’t have a high evapotranspiration rate, it might be better to apply water from late afternoon through early morning and avoid the peak of the day. It’s not uncommon to see dryland corners outyield irrigated portions of a field because the evapotranspiration rate was lowered too much during the reproductive stage,” Ferrie adds.


Evaluate Nutrients While Scouting for Pests

Midway through the growing season, a scout’s primary focus is on insects and diseases. But that’s also the perfect time to evaluate your nutrient planning.

“Carry a scouting manual to help identify nutrient deficiencies,” says Farm Journal Field Agronomist Ken Ferrie. “Collect tissue and soil samples and check nitrate content to back up your observations.”

If you discover low nitrate values in soil and plant tissues, and see deficiency symptoms, applying nitrogen as late as tasseling might reduce kernel abortion. “Plant uptake at this stage is only about 2 lb. of nitrogen per acre per day,” Ferrie says. “But the grain-fill process lasts for about 60 days, so you don’t want to run out.

“In 2015, when central Illinois received 16" of rain in June, growers who applied nitrogen at tasseling still lost some yield, but the application produced 20 bu. to 60 more bushels per acre than they would otherwise have harvested.”

If you document a nitrogen deficiency, consider why the deficiency occurred—was it specific to certain fields, management zones or hybrids? What should you do differently next year?

Nitrate testing is just as key in a dry season, when you might discover unused nitrogen in the soil. Plant a cover crop to help carry it over to next season and prevent it from ending up in water supplies.

With soybeans, the nutrient deficiencies you spot will be mostly micronutrients. “There’s no good soil test for micronutrients, so rely on tissue testing to confirm your observations,” Ferrie says.


Too dry, too wet and everything in between. This story is the seventh in an eight-part series on weatherproofing your crops. Follow along at bit.ly/weatherproof-crops

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