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.