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When Green, Not Brown, Is the Color of Harvest

Tuesday, September 1, 1998
filed under: Harvest/Storage

Green is a nice color to have in your wallet, but it's not exactly the color sunflower producers want in their fields at harvest.

Or is it?

Traditionally, "physiological maturity" of sunflower has been defined as the stage at which the back of the head has turned from green to yellow, bracts are turning brown, and the seeds are below 35-percent moisture. Of course, the plants will not be harvested until seed moisture drops into the 'teens or lower. Growers commonly have watched for green stems and heads to turn yellow and brown as an indicator that seed moisture was reaching an appropriate level for harvesting.

That's not automatically the case anymore, however. While a number of commercial hybrids still put on their "fall colors" in the traditional sense, some newer hybrids retain their green summertime hues - even though the seeds may be ready for harvest.

These hybrids possess what plant breeders call the "stay-green" trait. Along with stems and head tissue which do not senesce (turn brown) like other hybrids, these stay-green plants also tend to retain their green leaves.

The stay-green characteristic means the plant is still taking up water and nutrients and enjoying good overall late-season plant health. That's especially important when stress factors like drought, disease or insects are present. Stay-green typically translates into a lower incidence of lodging, stem breakage, stalk rots and other yield-robbing developments under extreme (dry or wet) environmental conditions. That, in turn, means more seed in the combine hopper.

Jerry Miller, USDA-ARS sunflower research geneticist at Fargo, N.D., says the stay-green trait first caught his eye during the drought years of 1988-90. "We noticed that some sunflower [hybrids and parental breeding liens] were starting to ripen prematurely despite no evidence of disease or insects," he recalls. "They just senesced before they should have." Others "behaved" very differently, however - their stems and leaves remaining green for an unusually long period.

An NDSU graduate student, Belgir Cukadar, investigated this phenomenon for her thesis project. She studied hybrids and inbreds across five different environments and found the stay-green trait to be genetically consistent regardless of location or environment. She also discovered that the characteristic does not correlate to seed moisture levels, i.e., seed moisture did not stay high just because a plant remained green at maturity.

"A major conclusion of her thesis," Miller relates, "was that the lack of correlation between stem color at physiological maturity and seed moisture content indicated it was possible to develop hybrids with the stay-green trait and low harvest moisture content. So genetically, we can retain the stay-green in the stem but still have drydown."

If that sounds like a case of being able to "have your cake and eat it too," it is - to a degree. There is, however, a very pragmatic complication. While the seeds of stay-green hybrids may be sufficiently dry for harvesting, the head tissue shares the stay-green characteristic. Therefore, the farmer would be threshing plant heads which still contain a significant amount of sticky gum - the amber-colored sap which can goo up a combine.

"The gum is a major problem," Miller affirms. "So from a practical standpoint, we concluded that we wanted something that has stay-green . . . but only to the point where it will give up that stay-green character and then dry down after a frost - or, perhaps, not be too frost-tolerant."

The result? "We had to compromise," Miller explains. The USDA group actually selected against those breeding lines that displayed the strongest stay-green inclination and instead focused on the "in-between" ones. "We would prefer to have no disease, no lodging or effect on the stalk," he says, "but I don't think we'd be able to harvest those plants because of the moisture content and gumming inside the combine."

(The seeds themselves, however, would be low in moisture and thresh without difficulty, Miller adds.)

How should a grower be managing a stay-green hybrid as harvest draws near? Miller has several suggestions.

First, the back of the head on a stay-green hybrid will not be yellow at maturity; it's still green. "So then we have to look at the bracts," he advises. "If the bracts are starting to turn brown, we know the seeds are physiologically mature."

Second, "the grower needs to monitor his seed moisture content much better than simply looking at a field and saying, 'Well, that field is ready.' He needs to be taking seed samples and testing for moisture percentage - and probably doing so earlier than he otherwise would" with a standard hybrid.

A little experimentation may be in order, too, if the producer finds the seeds to be sufficiently dry - but is concerned about the wet head tissue going through his combine. "Try it and see what happens (i.e., whether there's any indication of gumming)," Miller suggests. "Try to find a stage where there's no gumming, yet the seed moisture is low enough." The USDA geneticist knows of at least one North Dakota producer, for example, who harvested a quite green field last fall to save it from blackbird depredation. His yields were very good, and he experienced no gumming problems.

While stay-green hybrids carry obvious benefits for late-season plant health, they're not for everyone, Miller points out. If you're planting late - or if you farm in the northern reaches of the sunflower belt, your main concern will be getting the crop matured and dried down, not extending its vegetative life. Also, growers in areas where diseases have not been a problem and where it's typically quite dry in the fall likely would not want or need this trait.

If, however, one grows sunflower in an area where late-season diseases, insects or environmental extremes tend to cause significant stalk rot or lodging, a stay-green hybrid could be a real ally. That in itself may be worthy of investigation when you're considering the hybrids you'll plant in 1999. - Don Lilleboe

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