Corn yield potential has been reduced in Mississippi the past couple of years because of “serious environmental limitations,” says Erick Larson, associate Extension and research professor of plant and soil sciences at Mississippi State University.

That has been the case not only for dryland corn, but irrigated acreage as well, he said at the annual meeting of the Mississippi Agricultural Consultants Association. The main culprits have been drought stress from lower than normal rainfall and heat stress during the critical reproductive stage.

“The timing of water stress on corn is extremely critical for corn productivity,” he says, “with early reproductive stages, including tasseling and silking the most critical. The last couple of years, we’ve also had more early season stress and wilting than normal.

“We typically have a full profile of soil moisture early in the season, but we also tend to produce higher irrigated corn yields when it’s somewhat dry early in the season.

“I think the rapid temperature changes we had during May 2011, particularly the first 20 days of the month, played a significant role in the wilting symptoms we saw. We had some days with highs in the low 50s, followed by days in the high 80s and 90-degree range, and corn plants had a difficult time dealing with those rapid changes.”

It’s important, Larson says, to “probe the soil and evaluate the moisture content, and make sure you don’t initiate irrigation until the corn actually needs it. With our predominant furrow irrigation systems, we’re saturating the soil every time we irrigate — and that’s not necessarily a good thing for corn plants in the short term, particularly for root development.” Corn doesn’t like wet feet, he says, noting that the nation’s highest corn yields have been in northwest Texas, Colorado, and western Kansas, areas with only 14 inches to 18 inches of annual rainfall.

“We’re creating some negative situations with our furrow irrigation systems over the course of the season —particularly in the early and late periods — by overwatering our fields,” Larson says.

“We need to try to manage irrigation so the supplemental water we provide during the season follows a bell-shaped curve, rather than just a routine every-Monday watering program during the entire growing season, which will provide more than the crop needs at certain times and limit plant productivity.”

In that bell-shaped curve, he notes, water demand reaches maximum during corn’s early reproductive stage, then falls off substantially as it reaches physiological maturity.

“The mid-vegetative growth stages determine the number of kernel rows on the ear, whether there are 14, 16, or 18 rows, and environmental stress can affect that.”

While “it’s very infrequent” that this happens, Larson says, “in 2009, when it stayed wet and some areas of the state got 15 inches to 20 inches of rainfall during the first 20 days of May, we saw an adverse impact on the number of kernel rows.

“As a general rule, I think wet conditions at this time of year can play a larger role in reducing overall yield potential than dry conditions. Corn can tolerate a bit more stress during the early and later parts of the season than we typically think. During my tenure at Mississippi State University, the years when we’ve produced the highest irrigated corn yields generally have been when we had a dry May.”