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Will soybeans turn yellow?By Richard C. Snell Barton County Extension agent, agriculture Kansas Green beans are not always a good thing. I have had several questions this year about whether soybeans would turn yellow in the field, in the pod or after storage following harvest. I will attempt to answer that today. Now, I know your mother told you to eat your vegetables and one of our most common vegetables out of the garden is green beans or snap beans, as they should be called. When it comes to soybeans in the field however, yellow is the desired color. Snap beans are picked or should be before the seed develops. You are really eating the bean pod. In the case of soybeans, we want the seed for the oil inside and for the meal from the dry part of the seed. We throw away the pod because it is mature and dried up at harvest. Normally if soybeans are allowed to mature completely in the field, the seeds will be a pale yellow. This year, we had some soybeans that were hit by freezing temperatures while the plants, seeds or pods were still green. This resulted in a few entire fields having green beans or in some cases, a few pods or a few plants having green beans. Again, here we are talking seed color. You see grain elevators and oil seed crushers don't like these green beans because it gives an off flavor to the oil. Thus there can be heavy discounts or absolute rejections of a load plus a price dock to the farmer. I had a hard time getting a straight answer on this when I called K-State or contacted other sources. Jim Shroyer, K-State agronomist, said at one time he understood that they would change color after a period of time but then recently was told at a conference in Iowa by an Iowa State University agronomist that they wouldn't. Several people said to me, call your friends in Minnesota. Well, I have friends in Minnesota but they wouldn't know and the ones that know, I don't know. But, I stumbled onto some information from Ontario, Canada and Purdue University that may shed some light as to why this is not a black and white answer to a yellow and green question. Later planted soybeans are always at risk of frost/freeze injury in the fall. When soybeans are planted later in the spring than usual, due to wetter weather patterns in May and June, killing frosts are a threat when occurring earlier than usual or even at the average date. (I might also mention that a cooler, cloudier late summer and fall contributed to this problem by slowing the maturing process). Frost-damaged soybeans may have any of the following affects: --Will have green, or elongated yellow seeds that will shrink to smaller than normal size after drying; --Will have reduced oil contents with poor oil quality and poor extract ability; --Will have fairly normal levels of seed protein; --Will cause a moisture meter to read low (i.e. seed will be wetter than indicated); --Will dry down slower in the field. Producers will see brown and blackened soybean leaves. By definition, a killing frost occurs when temperatures drop to freezing for several hours and terminates plant growth. The critical freezing temperature for soybeans is 29 F. (At my house, it never got above 30 degrees most of the day on either the Oct. 3 or Oct. 10, I forget which). Yield losses will depend on the stage the soybeans were in when freezing temperatures occurred. Points to consider when assessing a field include: 1. If all the seed has turned yellow (physiologically mature) there are no yield or quality impacts due to frost. R7 fields that have not completely turned yellow may have green beans that will remain green at harvest. Yield impact is minimal. (0 to 5 percent reduction) 2. Yellow or brown pods should be opened to determine if the seed is detached from the pod. If the beans have not detached from the white membrane inside the pod, the beans will stay green. If the seed has detached from the pod the seed should turn yellow over time. 3. If all the pods were green before the frost a large percentage of the seed will remain green even after dry-down. (Frosted pods may turn black due to frost.) 4. Even if the stem is still green, once the temperature gets below -2 C essentially no translocation occurs from the stem to the pods. The majority of the seed will stay green. Beans that are still green and soft will shrivel. Stems will rapidly turn brown/black and not recover. Beans in pods that have turned yellow will most likely mature normally. Some of the green beans will turn yellow after 30 to 40 days of storage. By the way, if you let snap beans get too mature in the garden, that seed that swells up inside will be from white to gray in color, depending on how mature it gets. So you see, this whole issue is kind of gray depending on if the beans separate from the pod membrane or not.
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