MISSISSIPPI STATE -- Most of the state's corn is entering a critical growth period in serious need of water, but a good rain now still can help boost yields at harvest.

Erick Larson, grain crop specialist with Mississippi State University's Extension Service, said most corn was in the pollination stage by mid-June.

"Corn has the highest demand for water during pollination," Larson said. "Rainfall now could significantly improve corn yield potential in most of the state."

Larson said the state has had erratic rainfall since late May, and a lot of areas are very dry with depleted subsoil moisture reserves, particularly south of Highway 8. About 75 percent of the state's corn is grown without irrigation, so drought stress can substantially reduce yield potential.

"Some areas that got rain in May still have moisture reserves to draw on, but the areas that didn't receive rain are in marginal condition and are very dependent on a timely rainfall," Larson said. "June is a critical time for corn grain production. Any severe stress during this time causes irreversible yield loss."

Corn is a determinant crop, which means it grows according to an internal time clock and cannot generate new growth to compensate for stress during key growth periods.

Larson said ideal weather for corn is a daytime high temperature of 86 degrees with very cool temperatures and low humidity at night.

Overall, Larson said the crop is on schedule this year, and if it rains soon, the only production problem will be higher-than-normal numbers of corn borers.

Don Parker, Extension entomology specialist, said the Southwestern corn borer was a serious pest in the 1960s when the state's corn acreage was high. It became less of a problem when the state's acreage dropped, but has become a problem again with recent increases in corn acres.

There are typically three generations of corn borers each growing season. This year's first generation was higher than expected, and the second appeared in large numbers by mid-June.

"Normally we have not worried much with the first generation, but this year we had numbers high enough that several fields were sprayed for first-generation corn borers," Parker said.

He said no-till farming is partly responsible for the increase in numbers.

"We're not destroying the overwintering site of the corn borer, so they are overwintering in the corn stubble that remains in the field," Parker said. "We may be raising higher numbers of corn borers in the winter than we would with conventional tillage."

Bt corn is the best control for corn borers, but Parker said farmers are allowed to plant just 50 percent of the state's corn in this insect-resistant variety. The rest is treated with insecticides or growth regulators.