Too much rain has been a common refrain for farmers this summer. Still, corn needs moisture late-season to fill ears with big, golden kernels, says Olan Moore, owner of High Plains Consulting near Springlake, Texas.
"When corn matures, and that starch line is one-half of the way down the kernel, and you have 90-degree-temperatures-plus, if you don't have 4.45" to 5" of soil moisture in the top 3' of the soil profile, you could lose up to 10% of your corn yield potential," Moore explains. "Just 2" to 3" applied through a pivot can make a huge difference on the tail-end of the season."
If irrigation is terminated too soon, the yield loss happens because kernels are unable to reach their full potential in size and weight, according to Erick Larson, Mississippi State University Extension grains specialist.
"Potential kernel weight is only about 75% complete at the dent stage," he writes in an online article, Late-Season Corn Irrigation and Termination. There are other down sides as well: "Early irrigation termination will also likely reduce stalk strength and promote lodging, because plants will cannibalize energy from vegetative organs to fill kernels, when they are stressed," he says.
Moore says he first learned the value of late-season irrigation-as corn approaches physiological maturity (black layer)-from his mentor and irrigation pioneer, Leon New, a now-retired agricultural engineer who was based at the Texas A&M AgriLife Research Center in Amarillo.
Farm Journal Associate Field Agronomist Missy Bauer says she then learned the practice from Moore and routinely coaches Midwest farmers that "if things are dry, there can be yield benefits from irrigating until the black layer has formed."
Bauer provides a scenario for consideration in the upper Midwest: Once corn is at 50% milk line it will take about 280 GDDs (growing degree days; heat units) to reach black layer.
"If you are at 50% milk line on September 4, you can expect black layer around September 24, so we need to have adequate soil moisture available to the crop during the 20 days between those two dates," she explains. "Depending on where your soil moisture levels are, you may require irrigating at least one more time after 50% milk line. The last irrigation could be done when 25% to 30% of the milk line remains."
Consider the weather. Because water needs in corn at the end of the season are highly temperature dependent, Moore tells growers to consult their 7- 10-day weather forecast to guide decisions. Here are a couple of scenarios farmers routinely find in late summer, depending on the year and their location, and Moore's corresponding recommendation:
- If temperatures are in the low- to mid-80s and you are at 80% to 90% of your moisture needs in the 1- to 2-foot soil profile and the starch line is halfway down, you probably can pass on adding water.
- If temperatures are 90 to 95 degrees and you are at only 40% to 50% of field capacity for moisture in the 1- to 2-foot soil profile you may need 2" of water.
"Ideally, in that 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 you watch the seven- to 10-day weather forecast and it says to expect 75- to 85-degree temperatures with humidity, you can probably get by with 3.5" to 4.5" in the top 3' of your soil profile.
Easy steps to take. Moore says he did three consecutive years of trial work, using a variety of moisture sensors, to satisfy his desire to know for certain how to evaluate water availability in the top 1', 2' and 3' of the soil profile. While he tried several different technologies, he says the Delmhorst KS-D1 digital soil moisture tester, used with GB-1 gypsum sensor blocks, has given him the most accurate results to date. He says his clients like having data provided as a % of field capacity. And, his clay loam soils have a capacity close to 6” of water in the top 3’ profile. A big added benefit is that Moore says gypsum blocks are inexpensive and easy to install.
"You want to read the sensors a minimum of once a week, starting with young corn, emerging to V3 in order to allow the root system to develop around the sensors--farmers can read the information easily; you don't need a consultant for this," Moore says. (However, if you're new to using soil sensors, get some technical help next spring on where to place them in your fields. Moore uses soil electrical conductivity [EC] data and management zones to help his customers determine placement.)
Moore adds, "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 that starch line is halfway down (50% starch line), so you can decide what amount of water that crop still needs to finish the season strong."