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What’s really driving high soybean yields? Yeah, someone did a study

Weather, genetics, management – or a combination of all three - help pump up yields.

High corn and soybean yields have helped keep a lot of farmers financially healthy these past few years. The storyline we keep hearing is: how? Even with some fairly lousy spring weather in 2017 yields still popped. What’s behind the winning streak?

U.S. soybean yields averaged over 4.5 bu. per acre above trend for the past four years. The last time we had such a gap was in 1994, says University of Illinois ag economist Scott Irwin.

The University looked at three factors that could be driving the winning streak: weather, genetics, management – or some combination of the three. He admits trying to disentangle these interacting factors is not an exact science. For example, for the purposes of this research, economists link genetics and management together and call it technology. 

“These factors are interactive,” he says. “For example, we breed soybeans to be more tolerant to cold and wet conditions as they emerge, so we plant earlier.”


Researchers looked at estimated impact of summer precipitation on U.S. average soy yield - specifically in August, then June and July, because rainfall in those months, in that order, has the biggest impact on yield. They looked at 10 state weighted averages for the Corn Belt from 1970-2017.

In three out of the last four years, crop weather models under predicted yields, from half to a full bushel per acre. Year 2016 was a much bigger error than the other three years, so it was considered an outlier. Conclusion? For at least three out of the last four years, weather was the main factor behind higher yields.

“We simply didn’t appreciate how good (and timely) the weather was for crops,” says Irwin.

Or was it genetics?

It’s difficult to quantify the efforts of soybean breeders. Their scientific contributions no doubt grew yields over time. So what researchers did to study the genetic impact was to plant soybean varieties released throughout time, but all at once in one year using the same management and input practices. For example, they planted a soybean line introduced in 1922, next to one introduced in 1935, next to one introduced in 1970, next to one introduced last year, and so on – all in one year.

The 1922 variety yielded 35 bu. per acre compared to today’s varieties which yielded nearly 65 bu. per acre.

After 1970 yields based on better genetics began to shoot up dramatically, gaining .4 and .5 bu. per year, notes Irwin. “But it’s next to impossible to say there was something special going on genetically in the past four years.”

Williams, a standard soybean line introduced in 1970, yielded as high as 61 bu. per acre this past year, compared to nearly 80 bu. per acre for a conventional but modern non-GMO variety.

Conclusion? “Surprisingly, genetics probably had nothing to do with the great soybean yields of the past four years. Sure, they helped bounce us up .4 and .5 bu. per acre over time, but it doesn’t explain that 4 bu. jump in the last four years.”

How about management?

Management changes have pushed yields in different parts of the country. Yields in traditional soybean states are moving higher, but the big jumps are coming from the southwest (see chart comparing Iowa to Arkansas). Soybean yields in the southeast are growing much faster than in other regions. Why? First, they’re planting earlier, trying to get soybeans in bloom by June 20 to avoid hot July weather. They also changed row width; in Arkansas, row width averaged 12 inches in 2014 and is now closer to 36 inches. They’re moving more to twin-row raised bed production. In the south they call it hipping; in the Midwest it’s called ridge till.

“They’ve made a whole suite of changes in soybean production system to make producing soybeans in the Mississippi Delta as close to producing soybeans in Illinois as you can,” says Irwin. “It’s a revolution going on right on our back door - an incredible transformation on about a quarter of the soybean acres planted in the U.S.”

So what’s the secret to those big yields? “I’d say three fourths of it was good weather, the last fourth was probably management, with special emphasis on dramatic agronomic changes in the south,” concludes Irwin.

The opinions of the author are not necessarily those of Farm Futures or Farm Progress.

TAGS: Management
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