Can Beans Take a Beating?

A growing number of producers believe roughing up or stressing soybean plants may boost yields by causing the plant to branch and set more pods.

Some agronomists, myself included, want to increase pod numbers, especially since soybeans abort 50% to 60% of their flowers or pods during the course of the summer. Soybean breeders have been increasing soybean yields at a rate of about a third of a bushel per acre per year for the last 80 years, and that needs to increase if we want to keep soybeans competitive.

The thinking behind abusing soybean plants is that shortening the internodes can create more branching points and more potential pod-bearing nodes on the main stem and branches. The damage has to be done early in the season, by V3 growth stage, when the plant is determining its node number. Planting soybeans early also allows the plant to set more nodes and branches before summer solstice, the time when flowering becomes the driving force in plant development.

In 2006, when soybean yield king Kip Cullers, Purdy, Mo., won the national soybean yield contest with 139 bushels per acre and again in 2007 (156 bushels), I visited his farm and counted pods in his plots and recorded numbers like 100, 150 and 200 pods per plant. An average soybean field would have 30 to 50 pods per plant. Cullers attributed his yield increase primarily to increased pod setting along with slightly greater seed weight and more seeds per pod.

Cullers has experimented with many different ways of reducing soybean height and increasing soybean branching. He delights in telling audiences about using a weed whacker to cut the soybean down to size with the hope it would respond by more branching. His more practical strategy to attain a similar result is to use a diphenyl-ether herbicide like Cobra (lactofen) over the top of vegetative-stage, irrigated soybeans.

Paul Stephens, DuPont Pioneer senior soybean research director, told DTN in an interview this week that such tactics elicit phytoalexin production, a physiological defense mechanism in the plant. Some growers have counted on that response when using Cobra at R1 growth stage to help fend off white mold, a disease that can occur in high yield environments. However, Stephens said the white mold application is different than when used as a yield-boosting strategy. "It's not something we encourage growers to do," said Stephens of intentionally damaging soybeans earlier in the season. "As a practice, it's just too easy to overdo."

Even Cullers warns that the practice doesn't work all the time and urged farmers to be careful because such an early application can damage the plant to the point where it can hurt yield, particularly if the plant is already under stress.

Not all PPO (protoporphyrinogen oxidase) herbicides have the same impact on soybean plants either. Bryan Young, Southern Illinois University weed scientist, told DTN that while Cobra, Flexstar and Blazer are similar in many ways, only Cobra provides faster and more complete necrosis on soybeans. "It appears it's the physiological response of soybeans to lactofen that results in some beneficial growth for soybeans just as much as the branching from defoliation. We don't see the same physiological response of soybeans to Flexstar or Blazer," Young said.

Emerson Nafziger, agronomist with the University of Illinois, isn't a huge fan of beating up on beans. "My biggest concern about intentional damage to young soybean plants is their recovery -- or the ability to yield -- requires a certain amount of favorable weather, which we can't depend on. Related to this is the fact that damage usually has the effect of delaying crop development to some extent. This can also be positive, neutral, or negative, depending on subsequent weather. It's well enough known that Cobra can sometimes increase yield, but repeated research shows that it lowers yield probably as often as it increases yield," Nafziger said.

Larry Purcell, soybean researcher with the University of Arkansas, agrees. "We have done experiments with Cobra, Aim, crop oil, etc. At best, we can say that the results have been inconsistent and unpredictable. We will know more this fall as the treatments that we did this spring had a more dramatic effect (on soybeans) than anything that we had done before," he said.

Rolling soybeans is another method used to inflict damage. Today, growers can purchase large field rollers that come in widths up to 90 feet. Rolling was done first for the purpose of pressing rocks and stones back into the soil, firming the seedbed, improving emergence under some soil conditions and leveling the field for easier harvest (a level field allows the combine to cut lower and gather more pods). Nafziger considers rolling more of a crop protection practice to protect yield potential than a way to increase yield potential. "I haven't seen any good data on positive (increases in yield) from rolling an emerged crop, but on the face of it this seems counter-intuitive that rolling will increase yield."

So the definitive answer on roughing up soybeans is an inconclusive maybe. Many farmers have tried stressing soybean plants, and results remain inconsistent and unpredictable. Some years it works and others it doesn't, and there is always the risk of damaging the plant and even hurting yield. "I don't think it's helpful to know that a practice might increase or decrease yield, depending on what happens after it's been done, when we don't know probabilities and can't predict a response based on a certain set of conditions," Nafziger said.

If stressing soybeans to increase yield appeals to you as a potential yield-boosting practice, I recommend a lot of discovery and testing.

(GH/BAS/AG)

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