Too much rain has been a common refrain for farmers this year. Still, corn needs late-season moisture to fill ears with big golden kernels.
“When the starch line is one half of the way down the kernel and you have 90°-temperatures-plus, you could lose up to 10% of your yield potential if you don’t have 4.45" to 5" of soil moisture in the top 3' of the soil profile,” explains Olan Moore, owner of High Plains Consulting near Springlake, Texas.
Figure Growing Degree Days
According to Farm Journal Associate Field Agronomist Missy Bauer, once corn is at 50% milk line it takes about 280 growing degree days or heat units to reach black layer.
“If you are at 50% milk line on Sept. 4, you can expect black layer around Sept. 24 in the upper Midwest, so we need to have adequate soil moisture available to the crop during the 20 days between those two dates,” she explains. “Depending on your soil moisture levels, you might need to irrigate at least one more time after the 50% milk line. The last irrigation could be done when 25% to 30% of the milk line remains.”
Consider The Forecast
Because water needs for corn at the end of the season are highly temperature dependent, Moore says to consult your seven- to 10-day weather forecast to guide decisions. Here are two late-summer scenarios and Moore’s recommendations:
- If temperatures are in the low- to mid-80s and you’re at 80% to 90% of your moisture needs in the 1' to 2' soil profile and the starch line is halfway down, you probably can pass on adding water.
- If temperatures are 90°F to 95°F and you’re at 40% to 50% of field capacity for moisture in the 1' to 2' soil profile you might need 2" of water.
“Ideally, in the top 3' of the profile we want 5" of soil moisture just to mature the corn and not have any yield loss, especially if daytime temperature is in the 90s,” Moore says.
At the other end of the weather spectrum, Moore says if the forecast says to expect 75°F to 85°F temperatures with humidity, you can probably get by with 3.5" to 4.5" of soil moisture in the top 3' of the profile.
Use A Soil Moisture Sensor
After three years of trials, Moore says the Delmhorst KS-D1 digital soil moisture tester, with GB-1 gypsum sensor blocks, has given him the most accurate results on water availability in the soil profile.
”You want to read the sensors a minimum of once a week, starting with young corn, emerging to V3, to allow the root system to develop around the sensors,” Moore says. ”When you get to the tail end of the season, read the sensors twice a week and convert the information to percent of field capacity. That will tell you exactly what moisture is in the soil profile when the starch line is halfway down, so you can decide how much water the crop needs to finish the season strong."
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