Rating soybeans based on region: heat units or day length?

Kevin Elmy planted soybeans June 15 last year, which was much later than normal on his farm.

Elmy, an experienced bean grower from Saltcoats, Sask., knows that date is too late for soybeans, but he was conducting a plot experiment on his farm.

Elmy seeded six varieties sold in Saskatchewan with similar company heat unit ratings. They were seeded in 20-inch rows using liquid and granular inoculant and the CruiserMaxx seed treatment. The same seeding rates were used in all the trials, and the plots were replicated four times.

Four varieties didn’t reach physiological maturity by Sept. 21.

“Even though those varieties were very close for corn heat unit ratings, maturity-wise there was a two and a half week spread between them,” said Elmy.

“Two of the varieties were probably six or seven days away. (The other) two varieties, thank goodness it froze, otherwise (they) would still be green.”

Other agronomic variables were the same, which prompted Elmy to conclude that the varieties that reached maturity Sept. 21 must have responded to shortening day lengths, thus accelerating their progress toward maturity.

Elroy Cober, an Agriculture Canada soybean breeder in Ottawa, said Elmy’s results are interesting but growers in Manitoba and Saskatchewan shouldn’t rely on day length sensitivity to advance the maturity of their soybeans.

He said farmers don’t need to fret about heat unit ratings or day length sensitivity.

“You should be relying on the actual values for the days to maturity. Regardless of how it gets (to maturity), when it matures in your region is what’s important,” he said.

“What (growers) have to do is look at multi location trials that are in their area. The most important thing to look at is actual days to maturity.”

Manitoba is moving away from rating beans based on company heat units and is instead adopting the approach used in the rest of North America: maturity groups that range from 00 to IX.

Manitoba and Saskatchewan are a 00 region, and there are subgroups within that group: 00.1, 00.2, 00.3 and so forth.

“Within 00 area, each .1 is equal (to) a day (till maturity), approximately,” Cober said. “If it’s two days earlier than a bench-mark, then yours is 00.1 instead of 00.3.”

Cober said maturity groupings are more useful than corn heat units, but the groupings aren’t always accurate.

“Some company could be rating (a variety) a 00.1, another 00.3.… Those ratings aren’t concrete. The real (critical) number is the days to maturity.”

Elmy’s experiment was similar to an Agriculture Canada study in Manitoba, in which Aaron Glenn grew three varieties at multiple locations in 2011, 2012 and 2013.

Company heat unit ratings were 2,325 to 2,525 and locations stretched from Morden, the warmest spot in the experiment, to Roblin, in the northern grainbelt, along the Sask-atchewan border.

“We (also) had a pretty good range from Morden to Roblin, in terms of what the day length is. Morden just gets above 16 hours (of sunlight) for a few days in June…. Roblin has over 16 hours of day length until mid July,” Glenn said.

“Besides Morden and Portage, where all three of these varieties have been grown no problem … all the other sites, the varieties were expected to vary in their ability to mature and yield… based on their heat units alone.”

Two of the varieties, 00s, performed equally well at Roblin and Morden, despite the variability in heat and latitude.

“Days after emergence were just as good at predicting when these different stages of flowering would occur than heat units or day lengths.”

The conditions in Roblin did affect the third variety, which was from the 0 maturity group.

“Every stage was longer to reach at Roblin than at Morden for that third variety,” Glenn said. “It’s a difference of … about 14 to 15 days (to reach) R7.”

Glenn said the results demonstrate that heat units aren’t a good indicator of soybean success because the two 00 varieties differed by 150 in company heat units.

“This confirms the previous and parallel findings … that we can get reasonable yields and adequate quality under what would be sub-optimal heat units (compared to) the companies ratings,” he said.

“It strengthens and validates the recent moves (in Manitoba) from using the heat units and towards using the U.S. style maturing groups.”

Horst Bohner, a soybean specialist with Ontario’s agriculture ministry, said the province abandoned heat units three years ago because it “doesn’t work.”

“The crop heat unit rating is not a good indicator of how a soybean variety will respond in a given area,” he said. “The only way you know how a soybean variety will perform in a given area is to do experimentation…. You come up with a map of which areas can handle which varieties and you have to go to the relative maturity rating.”

The challenge for growers in western Manitoba and Saskatchewan is that soybeans have a brief history in those regions. Sufficient data isn’t available on the performance of benchmark varieties and how newer varieties compare to the benchmarks.

Claude Durand, product development manager with Northstar Genetics, said Saskatchewan lacks specific information for growing regions.

“What you see in the Saskatchewan Seed Guide, they just combine all the locations, whether it’s from Redvers or from Rosthern, and just have an average of all those sites.”

Cober said more collaboration is needed between companies and public soybean breeders.

They need to pool their “resources and agree on relative maturities for benchmark varieties” and how they perform in particular regions.

He said the industry and growers should support extensive trials so that accurate data is available on benchmark varieties and newer genetics.

“The only way you get that is in big variety trials, where you’re growing everything together, so you can get days to maturity, comparing everything.”