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.”

Winning yields in dry areas

For the past three years, Larson notes, there has been a cluster of National Corn Growers Association irrigated yield contest winners in northwest Texas, Colorado, and western Kansas, where yearly rainfall averages only 14-18 inches, “showing that corn can be very productive in a dry area.”

Five of the six high yields were produced under center pivots. “We typically expect lower yields from center pivot-irrigated fields,” he says, “but those growers are achieving very high yields under pivots. There are some advantages to growing corn in that system, and I think we could benefit from incorporation of new technology in our center pivots that would allow our corn to be a lot more productive.”

One of the contest winners in southeastern Colorado, Larson says, used furrow irrigation on a 10-14 day schedule, “which is considerably more lengthy than the schedule we typically follow.”

Curious as to why that cluster of contest-winning fields was so productive, he says, “I looked at average daytime and nighttime temperatures for the region during the first 10-15 days of July, when their corn was going through its early reproductive stage, and for Mississippi’s crop during the same stage.

“They’re actually hotter in the daytime during pollination than we are, but our nighttime temperatures are a lot warmer than theirs, and that’s a big drag on our corn because it increases respiration rate — burning up energy that could be utilized to produce higher yield. That was a critical limitation for us in the last two years.”

Looking at historical June nighttime temperatures in Mississippi from 1996-2011, Larson says, “Three seasons jump out — 2010, and 2011, with reduced productivity and lower irrigated corn yields, and 1998 with its poor yields and aflatoxin problems.

“While we’ve made a lot of improvement in terms of hybrid adaptability for our region, which has allowed us to produce more consistent yields, there’s no doubt that the high nighttime temperatures the last couple of years have caused a considerable yield drag on our irrigated corn.”

There is, he says, “a lot of talk about pollination problems — and when we have a pollination failure, it can indeed be catastrophic — but I don’t really see a lot of pollination issues in our corn. Rather, a lot of the yield loss I see comes from kernel abortion due to low photosynthetic rates, high respiration rates, and the plant not having adequate energy resources to fill all the kernels on the ear that it potentially can.”

If there were pollination issues, Larson says, “we’d be seeing spots on the ear where kernels aren’t filling. But mostly what we’re seeing is kernels failing to develop out on the tip of the ear because the plant doesn’t have what it needs in its ‘gas tank’ at that particular time to fill those kernels.

“The corn plant is very dependent on photosynthetic rate and respiration rate during the first 20 days following pollination. During tasseling it can have a full tank of energy built up in its vegetation, but the problem is that when kernels pollinate and begin to develop, they’re extremely small, and they have basically zero competitive ability to draw that energy away from the vegetation, particularly during the short time period when kernel number is determined.

“If we have stress conditions, however mild they may seem — from cloudy weather, too much or too little water, nutrient deficiency, high temperatures — it can reduce photosynthetic rate, and any reduction in photosynthetic rate, particularly during the first 10-20 days after pollination, is going to reduce kernel numbers. From milk stage (roasting ear) on out, basically all you’re doing is building kernel weight.”

Important development periods

The number of kernel rows is determined when corn is about chest-high, Larson notes, while kernel number is determined in the period from tasseling to milk or roasting ear stage, and kernel weight is determined from that point to physiological maturity. “The first 20 days after tasseling are the most important, the next 20 days after that are second most important.”

In a year like 2011, he says, with great conditions prior to tassel, then stress during early reproductive stages, “We often see what I call ‘luxury ears’ — big ears with unfilled kernels near the tip. Obviously, this reduces potential, but good yields can still be produced if you can optimize irrigation and other management factors capable of minimizing environmental limitations.”

Work is under way with corn verification program cooperators to try and optimize plant spacing and uniformity in the field, he says. “One way to do that is to slow the speed of the planter going through the field.

“We’ve measured variability of plantings in side-by-side rows, and while we got basically the same overall plant populations, there were pretty significant differences in plant spacing uniformity at planter speeds of 4 mph and 5 mph, where the slower speed improved uniformity.

“In a research study we started last year, we looked at planter speed over a wider range than in the verification fields, comparing a typical John Deere planter with a standard metering system and another with an after-market metering system that’s supposed to improve plant spacing. Corn was planted at speeds from 3 mph to 6 mph.

“With the standard planting system, we were losing about 4.5 bushels per acre for each mile per hour increase in speed, but we didn’t see nearly the degree of stair-stepping response with the after-market equipment.

“Overall, the yield improvement across all the speed treatments, compared to the standard planter metering system, was nearly 9 bushels per acre better. I think these results justify our looking at this more closely as a master’s research project to determine what effects planter types and planter ground speeds have on potential corn yield.”

Yield improvement, Larson says, may also require “some philosophical changes — how producers look at management systems, how they look at corn production systems, and how they may incorporate these into their cropping programs.

“For example, we’ve always done a good job of crop rotation, but with the shift to higher market prices we’re seeing a move by some growers toward more continuous corn. There is no other practice that allows a yield advantage of the magnitude offered by crop rotation — about 15 percent. Rotation is also useful in reducing pest problems and is the No. 1 tool for managing weed resistance.”

Most producers, he says, also can benefit from improving their planting precision, and weed resistance management “is a very important issue — not just for Palmer amaranth, but for Italian ryegrass, which is a big competitor with corn because it’s growing very rapidly when corn is coming out of the ground. We don’t have foolproof postemergence tools to knock it out, so we need to try and control it 100 percent before we put corn in the ground.

“I think we can also make some improvements in irrigation scheduling, particularly in the early part of the season, by not overwatering, then maintaining plentiful water during the critical reproductive stage to be sure the crop isn’t water-stressed.

“Not only do we have to worry about water stress/water deficit, we have to also remember that we can overwater, with damaging consequences. I think the higher corn yields we’ve traditionally had in years with dry May conditions may be because it encourages root development that gives the plant a foundation for producing optimal yields, as we saw in 2007.”

With the transition toward more corn/soybean systems, and producers not as much cotton, Larson says, “We probably should consider 30-inch rows for our corn. We have two years’ research, looking at twin rows versus wide conventional rows, and 30-inch rows. The 30-inch rows are producing yields that are 8 percent to 11 percent higher than either twin rows or the single conventional rows — basically a 16 bushel to 22 bushel advantage for 30-inch rows.”

While many growers burn corn residue after harvest, Larson says, “We suggest not doing this. We need to realize that by burning residues we lose about 75 percent of the organic matter — and the value that incorporating these residues can have long term, particularly since our fields are low in organic matter.

“I’ve yet to hear any corn yield winners across the nation support burning stalks — they consistently recycle that organic matter and nutrients back into the soil.

“By burning, we also lose any nitrogen that’s in the crop residue and as much as 75 percent of the sulfur content. We don’t really know what we lose in terms of phosphorous and potash due to blowing winds or rain taking the ash away. Long term, there is potential for serious issues related to organic matter and nutrient loss on burned fields.”

Larson says research and graduate studies are under way to determine the long term effects of burning corn residues.