Soybean Plant Uniformity: Weighing Costs Versus Gains

Editor’s Note: The author, Matt Hagny with Pinnacle Crop Tech, has been an independent crop consultant specializing in no-till systems since 1994. He also is the founder and president of Exapta Solutions, which focuses on improving no-till seeding results.

We tend to be rather complacent about soybean stand uniformity. Let’s be clear that two types of uniformity can be factors. One has to do with spacing between plants in the row, and the other takes into account the uniformity of timing of emergence.

As an agronomist, I long ago stopped worrying about uniformity of soybean emergence. However, I am super-concerned about uniform timing of emergence for corn, cotton, sunflowers.

Soybeans are very forgiving, it seems.

A recent Argentine study1 with good methodology attempted to identify and measure any yield effects. The scientists planted replicated plots with a small-plot planter in 20-inch rows at higher-than-desired populations, then hand-thinned them 6 to 8 days after emergence. The result was a set of plots with either uniform or non-uniform spacings among plants.

But they didn’t stop there.

To create a non-uniform timing of emergence effect within some of those plots, they also removed certain plants at first trifoliate and placed a seed where the plant had been. Next, they applied a small irrigation to ensure those seed germinated. Over a 3-year period at two locations, they used early and late planting dates, two different maturity groups of soybeans, and two populations.

Again, I’m not exaggerating to say this was an extremely thorough study.

How Much Difference?

The research team in Argentina found:

About a 6% yield loss from spatial non-uniformity, meaning uneven distances between plants. In their study, the average plant spacing across all treatments was 8.2 inches, with standard deviation of uniformity at 4.7 for the ‘uniform,’ but 9.0 in non-uniform plots.

About an 8% yield loss from non-uniform timing of emergence. Admittedly, this wide difference in emergence – half the plants don’t germinate until the rest are at first trifoliate – isn’t a common situation but it’s not impossible.

The Group IV variety was less sensitive to non-uniformity than the Group III. That’s understandable since shorter maturity means less time for branching to fill in gaps. Also, their Group III variety had particularly low branching capability for that maturity, whereas their Group IV had high branching ability.

Higher plant populations didn’t overcome the yield loss from non-uniform timing of emergence.

Practical Implications

How economically advantageous it is to try to eliminate these yield losses depends, of course, on the cost of doing a better job with planting. Costs are fairly minimal if it’s simply a matter of paying more attention to detail in adjusting the planter or drill for the field conditions. Those adjustments usually pay large dividends.

If the planter or drill is also used for other crops that are more sensitive to non-uniformity, that should be factored in when upgrading equipment (including attachments).

As noted earlier, corn, cotton and sunflowers are, in fact, far more sensitive to non-uniformity, and many farmers reading this do indeed grow at least one of those other crops.

Planter Versus Drill

In general, assuming you cover any significant acres with a piece of seeding equipment, many well-chosen upgrades can provide a handsome ROI, especially if also used for corn, cotton, or sunflowers. Remember that other costs can be incurred, too, with non-uniform stands, including greater weed emergence, non-uniform ripening, etc.

This upgrading becomes more complex for farmers who have other reasons to own both a planter and a drill — which one to use? In the Corn Belt, we have seen a strong migration towards split-row planters for soybeans, although drills haven’t completely gone away in this region, especially among farmers growing soft red winter wheat or cover crops.

Is it okay to use the drill to help out with soybean planting if tractors and labor are available? The answer depends partly on what drill design we’re talking about. Something like a JD 50/60/90 or Case PD-500 drill with some upgrades can do a fairly good job on soybeans in no-till. That said, it will never quite equal the precise placement of a planter. In dry and loose soils, that’s especially true.

As for in-row spacing variability with a drill, it ain’t pretty. However, farmers who use both – sometimes in the same field at the same time – report little to no difference in yields. Sometimes this is due to the drill being on narrower row spacing than the planter, which overcomes some of the erratic spacing.

What I do hear a lot is that seed costs per acre are more with the drill than a split-row planter when aiming for a decent stand. Some of this also comes down to how well the drill is maintained and if any components have been upgraded with well-chosen aftermarket parts.  But some is inherent in the less-precise placement with drill openers versus planters.

Beyond that, some of this is also perception, since there’s a tendency to overpopulate soybeans anyway, and they look so much thinner when in narrow rows with a drill. On the other hand, air drill owners frequently talk about how many more acres they get done in a day versus split-row planters—even with central-fill. And the JD drills are certainly more robust than planters (less downtime, and more ability to handle faster ground speeds).

Row Spacing – Another Variable

Row spacing is also important, especially for Canada and the northern-tier states of the United States, where a 15-inch spacing, in fact, has often been shown to be too wide to capture enough sunlight to maximize yields. That usually means going to 7.5- or 10-inch drills for soybeans.

As for using 30-inch planters, I think the evidence is pretty solid that these are too wide for soybeans, even in the southern U.S. You’re not only giving up too much yield, but also opening the door for too many late-season weed escapes.

South and/or west of the Corn Belt where cool-season cereals are a significant part of the crop mix, farmers typically have far greater drill width than planter width (or maybe no planter at all). So, it becomes a matter of choosing a good drill model for precise placement, and doing the maintenance and upgrades to get the most out of it.

It is in these areas where it becomes a question of whether it’s worth buying a planter with less than 30-inch spacing. You might gain whatever percentage of yield potential is still left on the table for soybeans, but will that be enough to justify the cost compared to imprecise drills (even after you add-on all the goodies to a JD 50/60/90 or Case P-500)?

This percentage of yield loss is likely a bit less than the 14% cumulative effect shown by the Argentine study, if for no other reason than we usually don’t have half the soybeans germinating when the other half is at first trifoliate. That is, unless you simply weren’t getting half the seeds to moisture and needed a rain to sort it out.

On the other hand, a small percentage of beans almost always emerge late, and our spatial distribution might be even worse than in the Argentine study. That’s especially the case at the low seeding rates we use in central Kansas and Oklahoma, plus going over our “speed bumps” – a.k.a. terraces.

Also, there is some evidence that poor spatial distribution has a greater percentage effect in dry environments, probably because the plants don’t branch as much. When it’s dry, competition for resources may be a factor, too. Paul Jasa, University of Nebraska-Lincoln, has found that non-uniform timing of emergence for corn results in much greater yield loss in droughty years.

Weighty Matters

One of the issues with using split-row planters in central Kansas or other parts of southern U.S. is that you often can’t get enough downforce due to lack of frame weight. And on Deere’s splitter planters, there’s really not much of anyplace to add significant amounts of weight, at least not without some serious engineering involved.

Automatic hydraulic downforce helps this somewhat, since it wastes less of the frame weight from over-applying downforce on the rows that don’t need it. However, there are still times it simply doesn’t exert enough downforce due to lack of frame weight in tough soil conditions.

South of Interstate 80, in continuous no-till, there’s also a bit of concern regarding erosion and how much stubble and stalks are being trampled by all those gauge wheels on splitter planters. Although that’s not an issue if cover-crop cereal rye is used ahead of the beans. Granted, some drier regions can’t do this.

Assuming these things don’t dissuade you, there may be opportunity to spread the cost of a split-row planter over additional crops on your farm, such as grain sorghum, dry beans, etc., assuming that narrowing up the rows doesn’t result in seeds being farther apart in the row (on average) than the distance between the rows.

The Argentine study does confirm what we’ve kinda known — that soybeans are more forgiving than some other crops. But, it also shows that it’s worth trying to do better for this crop, especially given the price of soybean seed.

1 A. Masino, P. Rugeroni, L. Borrás & J. Rotundo, 2018, Spatial and temporal plant-to-plant variability effects on soybean yield, Eur. J. Agron. 98: 14-24.

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