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Corn crop enters critical irrigation time

Where it has been irrigated properly, the Mid-South corn crop looks good — make that very good.

“The irrigated crop is doing great,” says Pioneer agronomist William Johnson, just outside Eudora, Ark. “Arkansas corn — and the crop here is largely irrigated — looks good overall and the replanted crop looks fantastic. After the Easter freeze, those that replanted seem pleased because the fields they tried to salvage have been shaky.”

Even shakier are cornfields that haven't received proper moisture. Erick Larson can attest to that in Mississippi, where a large percentage of the corn crop is dryland.

“The main issue in Mississippi corn is subsoil moisture levels have been depleted,” says the Mississippi Extension corn specialist. “We haven't had sufficient rainfall to recharge the soils since early March. That has put our dryland corn crop in extreme jeopardy.”

But the irrigated portion of Mississippi's corn crop is also under the gun. If, like last year, the state continues to experience dry conditions through the summer, “it will be very, very difficult to maintain water demands of the crop. We certainly learned that in 2006 when we had to supply virtually 100 percent of the water demands of our irrigated corn.”

The fact that Mississippi has such depleted subsoil moisture “will, without doubt, stress our crop. We have 950,000 acres of corn in the state. Traditionally, about 75 percent of Mississippi corn is dryland. This year, though, with the large acreage spike, that percentage should be much lower.

“I expect over 50 percent of the state's corn will be irrigated to some extent. The reason for that is a majority of the corn acreage bump is in the Mississippi Delta. Most of that is set up for some sort of irrigation.”

On some Mississippi corn, pumps have been running since March. It's normal to have no more than two furrow irrigations prior to the first week of June. This year, some producers have at least four furrow irrigations behind them.

Without rainfall, “producers can generally plan to have one furrow irrigation per week,” says Larson. “That means we could have as many as eight irrigations to go.”

As summer nears, Louisiana corn has finally received a long-needed rain. “Some parts of the state could still use more rain, but most areas won't need any for a few days,” says David Lanclos, LSU AgCenter corn specialist. “I doubt anyone would turn a rain down, but the weather is actually prohibiting our soybean acres from being planting.”

Until the recent downpour, Louisiana corn had already been irrigated for a few weeks. “Down here, we're mostly using furrow irrigation with polypipe and center pivots. Some producers flush — they send water from one end of a field to the other. With diesel and electricity costs so prohibitive, farmers are looking for the most efficient system they can.

“The rain hitting right now is absolutely optimum for the corn. But it isn't good for our soybean crop. That isn't to say the beans already in the ground aren't benefiting, but the beans sitting in the warehouse certainly aren't. We have quite a bit of soybean acres to plant. There are still a few thousand acres of wheat to harvest before farmers can put in double-crop beans.”

Most of Louisiana should begin seeing drier weather soon. “We're supposed to see heat indexes above 100. If conditions are typical, it may take only three days to dry out, and we'll be hoping for another rain.”

The need for irrigation seems to catch new corn growers by surprise, says Jason Kelley. And with plenty of first-time corn producers in Arkansas, this year is no exception.

“Everyone is so busy, corn requirements sneak up,” says the Arkansas Extension corn specialist. “It's like, ‘corn's fine, corn's fine, wheat needs harvesting, weeds need controlling in rice, bean planting needs to be finished.’ It's so easy to get behind. In driving around the state, there's a clear difference in the corn that's been irrigated properly and the fields that haven't been kept as wet.

“I hope no one is thinking, ‘irrigating once a week is plenty.’ Some years that may be true. But on ground that seals over easily, essentially the soil gets harder and there is less infiltration as the season wears on. In those situations, the first furrow irrigation may have taken 24 hours to water out the end of the field. After a few irrigations, though, it'll take less and less time for water to reach the ends because it isn't percolating down. That means the frequency of irrigation must increase.”

Most Mid-South corn has entered the most critical period of water need: the end of vegetative growth — around V12/V13 to dent.

“We tell Louisiana corn growers they need about 22 inches to 26 inches of water during the season, total,” says Lanclos. “And from V12 to dent, the crop needs a little over 1.5 inches of water per week. The crop cannot afford any slack in water availability. The majority of our corn crop is in that critical window right now.”

An irrigation scheduler is available through the University of Arkansas and is being used by corn producers across the Mid-South. Larson laments more in Mississippi aren't set up to use it.

“We know what corn water demands are at different growth stages. Corn water use is at maximum level during the four weeks following tassel. Right now, we're at the beginning of that period. So, maximum water use will be during the month of June.

“The furrow irrigators usually run a weekly program. The center-pivot owners must run their systems 24/7 to even attempt to keep up. Unfortunately, most of our center-pivot systems aren't designed to supply enough water to keep up with corn needs without rainfall to help.

“That our soil moisture is so short compared to the norm could affect irrigators more this year. A corn crop, during maximum water use, will use about 1.75 inches of water per acre per week.”

All interviewed agree that farmers should consider keeping center pivots running. “I wouldn't turn the pivot off until it got stuck,” says Johnson. “I've been suggesting farmers slow their pivots down so more water gets to the crop. Just make sure you don't slow down so much that there's too much run-off.”

The key to pivots on corn is to make a round every four days. Johnson says corn will “burn up” under pivots that take five to seven days to make a round.

Kelley suggests producers place rain gauges in corn under center pivots. “Growers need to know just how much water is reaching the crop. A producer often believes he's putting out 1.5 inches of water, but in reality that isn't the case. Read those rain gauges and find out the truth.

“And to keep ahead of the game — especially if the weather stays hot and dry — you've got to turn the pivots on and leave them on. That's hard to do with the outrageous price of diesel. But I can attest that in recent hot, dry years, the folks that left their pivots running — maybe only shutting them down for maintenance — were much happier at harvest.”

Larson says there are several common irrigation-related mistakes.

Don't irrigate based on the weatherman's prediction

“Never rely on weather forecasts to make management decisions — especially with irrigation. If your corn crop needs water, don't worry about a 30 percent, or even 60 percent, chance of rain. Go ahead and crank the system up and let any rainfall dictate whether you turn the pump off.”

If using a center pivot, begin a bit early

“Center pivot irrigators must stay ahead of the curve. It's just not feasible to put on 3 inches of water with a pivot and recharge the soil profile. The pivots don't have the capability to fully meet the maximum water needs of the crop. So you must begin irrigating ahead of schedule to charge the soil a bit before the crop even reaches maximum water demand.”

Don't terminate irrigation too early.

“I understand the urge to finish, but too often farmers terminate irrigation early. Because the crop wasn't completely mature and needed more water, yield is reduced considerably. Even though water needs decrease as the crop reaches maturity, the plants need water until they reach the black-layer stage.

“An extra irrigation or two can mean an additional 10 to 15 percent yield. Irrigation termination is very important, and it's relatively easy to determine by checking cross-sections of kernels as maturity approaches.”

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