NEWS for North Dakotans
Agriculture Communication, North Dakota State University
7 Morrill Hall, Fargo, ND 58105-5665
September 14, 2000
Denise McWilliams, extension crop production specialist for North Dakota State University and the University of Minnesota, answers late season questions from soybean producers.
Q. I had two varieties of soybeans in the field by my house, but when the light frost came through one variety seemed to recover better than the other one. Why?
A. Soybeans are easily damaged by frost, but at 32 degrees or just above, some soybeans may come out of the cold just a bit better than other varieties or individual plants. Location in the field may make a difference as cooler temperatures may linger in low spots longer. Also, preconditioning makes a big difference. A few early frost scares may condition some varieties or individual plants more than others.
Q. What temperature will result in a quick kill on soybeans?
A. Temperatures at or below 28 degrees for any length of time will usually result in complete soybean kill.
Q. A few of the soybeans I have harvested are green. If I slowly dry down the beans, will this color disappear?
A. Usually green beans harvested will remain green and may even be soft and will shrivel. If you can't pinch the green bean between your finger and thumb and mush it down, drying will result in less shriveling and occasionally, with a month of storage, some green beans may change more to the yellow color you would like.
Q. Can't I just determine the best harvest time by the color change in the soybean leaves and pods to brown?
A. Actually, a moisture reading should be done to see if soybeans are at least at 18 to 16 percent moisture, if possible. Keying color change in leaves and pods to harvest timing doesn't always work as different varieties under different moisture and temperature conditions may vary in their "color" signs of maturity. As proof, many soybean varieties remained green toward the end of the season this year, partially due to end of the season rains.
Q. Can I somehow look at the soybeans and estimate maturity?
A. Yes. Open a few pods (at random over the plants because remember that beans usually mature from the top down) and check the shrinking of the beans from the pod. If the beans are completely separated from the white membrane inside the pod, they will not gain any more test weight in the field.
Q. Is there a stage in soybeans when I know a frost will result in yield loss?
A. Yes. A frost at or before R6, or full seed, stage will result in yield loss over 50 percent. The previous stage, R5 or beginning seed, can have drastic yield losses (greater than 75 percent).
Q. Once a frost occurs, does dry-down in soybeans slow?
A. Not necessarily. Temperatures and moisture after the frost determine dry-down timing as does the stage of maturity of the beans at frost.
Q. The pods on my soybean plants were still green when the frost hit. What should I do?
A. Check to make sure the beans were at or past R6 or full seed and wait for the pods to mature before combining if conditions allow it.
Q. Does row-spacing affect soybeans sensitivity to frost?
A. Right around the 32 to 28 degree temperatures, row-spacing may greatly affect the potential of frost on soybeans. Narrower row spacings can have slightly more tolerance to light frosts than wider rows, depending on the stand. A more complete plant canopy can hold in soil heat better.
Q. If the soybeans are just at beginning maturity, will a frost affect yield?
A. You can have up to 5 percent yield loss. Only at full maturity, R8, are soybeans not usually affected by frost. Delaying harvest too long, however, can result in yield loss due to pod splitting or dropping or stalk lodging as well as possible deterioration from field weathering.
Q. If my soybeans don't lose the green color in storage, can I still sell the beans?
A. You might be able to find a buyer that will purchase the beans at a discounted price, probably to be used in soybean meal production. Green soybeans used in processing soybean oil create problems. The green color has to be masked or removed--both taking more time and expense. Refiners check soybeans routinely for green beans (or the chlorophyll remaining in soybeans).
NDSU Agriculture Communication
Source: Denise McWilliams, (701) 231-8160
Editor: Gary Moran, (701) 231-7865