Soybean School: The method behind maturity ratings (and at what stage is it okay to get frost?)

It’s a colourful time of year as leaves start to turn yellow, then brown, and then drop to the ground — not only on trees, but also in soybean fields.

When moisture has not been a limiting factor, differences in days-to-maturity between varieties can be quite eye-catching heading into the final weeks of the growing season.

In this Soybean School episode, we take a look at the process of assigning maturity ratings to different varieties as we visit one of several sites where Manitoba’s provincial agriculture department gathers data on all the varieties listed in its annual seed guide.

“(The rating) is all based on your seeding date and when it reaches the R8 growth stage, which is physiological maturity,” explains Dennis Lange, pulse specialist with Manitoba Agriculture and Resource Development.

R8 is defined as when 95 per cent of the pods are brown and starting to rattle. There are usually no leaves left. At this point, the crop is usually 5-10 days away from being dry enough to harvest.

An early variety in Manitoba, as shown in the video, might reach R8 in about 105 days, says Lange, while the check or mid-season variety would typically take closer to 115 to 117 days. Longer season varieties meant for the warmest parts of the province are rated at up to two weeks longer than the check variety. For the seed guide, Lange and his colleagues give each variety a relative plus/minus days-to-maturity rating compared to the check variety.

Short-season beans are well-suited for more northern and western growing areas, but he emphasizes they also have a fit in longer-season areas, especially when spring weather isn’t ideal and growers have to look at planting late or re-planting.

Ultimately, a question for growers in northern areas when choosing which varieties to plant is always going to be: how do you maximize yield potential while minimizing the risk of frost killing the crop prematurely?

Yield losses will be significant if they occur at the R5 or R6 stages, but Lange says the risk of significant yield loss declines in R7 — when most of the pods are yellow and at least one pod has its mature brown colour.

“At R7 you probably won’t see much yield loss, maybe five percent at most,” he explains. “You may see some green seed if you get a hard frost. A minus one frost would just freeze the tops of the plants and leaves might drop off. A minus five frost, for example, might lock that green seed in, but really no yield loss.”

Related Soybean School episode: Assessing the Impact of Frost

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