Corn Populations and Row Width Deserve Review

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By Bill Mahanna and Ev Thomas

Summarizing corn population research can be daunting. There are reams of studies that address specific maturities in unique geographies, making it somewhat difficult to draw sweeping conclusions across diverse growing environment s. However, it comes as no surprise that, over the last 25 years, the average U.S. corn planting population has risen from 23,000 plants per acre (PPA) to about 30,000 PPA. High-yielding environments even allow populations to be pushed closer to 36,000 to 38,000 PPA depending upon individual hybrid genetics.

Higher populations stiffen competition among plants for water, sunlight, and soil nutrients. Pioneer has conducted studies comparing hybrids sold during previous decades. There has been modest improvement in grain yield production from individual plants. To a lesser extent, these improvements are due to: higher leaf area index; leaf photosynthesis efficiency; kernels per ear; and weight of each kernel. However, the major gain has come from the genetic selection of corn hybrids for stress tolerance which has accounted for gains of 1 to 1.5 bushels per year in yield over the past 80 years. The corn plants improved ability to handle stress has resulted in higher plant populations which boosts the number of ears per acre. More precise soil fertility practices and technology traits which improve resistance to insect and weed pressure also have significantly improved yields per acre. Further driving yield is the fact that the average grower is planting corn about two weeks earlier than 10 years ago, somewhat the result of improvements in seed treatment.

Two different discussions
It is important to differentiate between grain and silage production when discussing this topic. Let's start with grain production.

In low-yielding environments (less than 130 bushels per acre), response to plant population is more significant, although grain yields tend to drop off gradually with higher populations. This is much different than the drastic drops encountered among hybrids 30 years ago. These hybrids were more prone to barrenness under high-plant densities. Higher yield environments' (greater than 130 bushels per acre) yield response to plant population is similar regardless of field yield potential.

In the central Corn Belt, optimum populations are about 35,000 to 36,000 PPA with grain yields leveling off rather than falling at even higher populations. This presumably makes variable-rate seeding more beneficial in lower-yield environments.

With improved hybrid stress tolerance, many seed companies are routinely evaluating hybrids at plant populations as high as 42,000 PPA. There also appears to be slight differences in optimum plant populations by maturity (CRM ). Shorter-season hybrids (less than 100 CRM ) tend to show a greater grain response to higher populations, followed by 101 to 113 CRM hybrids, and, finally, longer-season hybrids (greater than 113 CRM ).

Researchers theorize that higher populations overcome some of the disadvantages of smaller stature and lower leaf area index exhibited by shorter-season hybrids. Some seed companies provide planting rate calculators on their websites to determine optimum economic grain planting rates based on hybrid genetics, yield environment, seed cost, and grain price.

What about silage?
Silage is a more complex situation. Many universities recommend boosting plant populations by 10 to 20 percent per acre in hybrids destined for silage. Although higher plant populations tend to reduce stalk diameter and increase potential for lodging, this is much less of a concern for silage than for grain corn harvested at a much later maturity. The research is quite consistent that higher populations (upwards of 40,000 to 42,000 PPA, depending upon field yield potential) significantly enhance silage yield and slightly reduce quality.

The drop in quality is caused by higher stover yield diluting the grain (starch) portion of the plant, resulting in slightly higher fiber levels. Some earlier research suggested that the smaller-diameter stalk found in higher populations altered the rind:pith ratio, causing slightly lowered fiber digestibility. More recent research conducted in 2008 and 2009 by Cornell University with conventional, leafy, and BMR hybrids planted at populations ranging from 25,000 to 40,000 PPA showed no significant effect of raising populations on fiber digestibility.

There are some silage growers who prefer to plant at lower populations, more optimal to grain yield, in an attempt to boost the starch content of silage in response to rising supplemental grain costs. A healthy corn crop can lay down as much as 1 percent unit additional starch per day from one-third milkline to physiological maturity (blacklayer). Research among newer hybrids with technology traits helping to deliver excellent late-season plant health shows that delaying harvest until three-quarters milkline (or later) will result in higher-starch corn silage without a significant reduction in fiber digestibility. If the crop is stressed or diseased, there is greater tendency to have a lowered fiber digestibility from delaying harvest to these later stages.

As for width
What about row widths? Corn planted in narrow rows has more equidistant plant spacing down and across the row to reduce plant competition for available water, nutrients, and light. In 2009, the US DA reported that about 85 percent of the corn crop was planted in 30-inch rows or wider. Only about 4 to 5 percent of the crop was planted in 15- or 20-inch rows. This is consistent with the location of major corn-producing regions and research showing significant grain or silage yield response (3 to 10 percent increase) to narrow rows occurs primarily in northern Corn Belts limited in solar radiation and/or lacking for fertility or moisture.

Twin rows accounted for less than 0.2 percent of the 2009 corn crop; yet the practice is gaining interest as a way to potentially boost yields without the machinery cost associated with switching to narrow-row production. Pioneer twin-row research conducted in 2010 on 179 paired comparisons across 31 locations showed no overall grain yield advantage to twin rows over 30-inch rows. This supports the accumulated body of university and industry research concluding that a transition from 30-inch rows to twin rows would not provide a wide-scale yield benefit across the majority of the Corn Belt. There is also a lack of evidence that new hybrids and higher plant populations will broadly favor twin-row production in the near future.

Yield environment does not appear to affect twin-row yield response, although research data from low-yield environments are limited. The most promising applications for twin-row corn appear to be where narrow rows have been most successful, such as the northern Corn Belt and in silage production, as well as in southern wide-row systems.

Obviously, both the crop consultant and seed supplier should be an integral part of any dairies' management team. Don't forget to also talk with your nutritionist so that you can communicate to the cropping side of your operation the priorities for silage or grain yield versus quality (and if quality is defined by starch or fiber digestibility).

At that point, reputable seed suppliers should be able to provide data as to how they expect individual hybrids to respond to the challenge of planting population or row spacing. Finally, don't forget the option of manipulating harvest dates (maturity) and high chopping as tools to further manage yield and quality.