A number of producers and crop advisors have reported several cases of “yellow” soybeans this year. There are a number of reasons why soybeans might not have the deep green color we normally expect.
This year we’ve been seeing more severe “yellow flash” in response to glyphosate applications than in most years. We always see a bit of this where there is overlap or a particularly high rate of glyphosate was applied for some reason. Stressed beans will flash more readily, and with the stress that soybeans have been under, it may be that many fields are flashing even in response to typical rates of glyphosate this year. Soybeans usually come out of it quickly, but one of our demonstration plots that received a particularly high rate of glyphosate is still showing yellowing several weeks after the application. Monsanto has a nice summary of causes of yellow soybeans, including yellow flash from herbicide application, here.
It is important to distinguish a case of “yellow flash” due glyphosate application from possible nutrient limitations. Deficiency symptoms that can be similar to “yellow flash” are iron chlorosis, and manganese and possibly other deficiencies. However, these nutrient deficiencies are more common early in the season and soybeans would normally have grown out of these conditions by this point in the season, except in very severe deficiency situations. Nutrient deficiencies in general will add a level of stress to the plants, which could make potential glyphosate flash symptoms more likely.
Many fields had not set many pods until recently because of the extreme heat during July and the first week of August. Pods are setting rapidly in many fields now that temperatures have moderated a bit. The impact on yield is a bit hard to predict because we don’t know what the rest of the season will be like, but it probably will reduce yields somewhat for several reasons.
* If pods had been set earlier, when flowering started, they would have had more time to fill beans. With a shorter period for filling, bean seed size and consequently yields may be reduced.
* It is likely that final pod number may be less with delayed pod set than it would have been with more optimal conditions.
* Initial indications are that we have a higher-than-normal incidence of 2-bean pods among those that were set earlier. Anything that reduces final seed number (e.g. fewer pods or fewer seeds per pod) is likely to result in lower yield because seeds per acre is the most important component for determining yield.
All that said, soybeans are an amazingly responsive plant with a tremendous capacity to adapt to changing conditions. Now that the weather has improved, additional pods can be set in a short period of time. There are still several weeks left in the growing season, so most pods set now should be able to fill well, provided we don’t get an early fall or return to extreme heat or drought.
In the summer of 1980, Kansas farmers experienced very similar conditions to what we have had this year. However, in 1980 the onset of scorching temperatures did not start until July, and most areas had adequate rainfall up until then. By the first week of August, the corn was decimated and most was going to silage wherever possible.
Soybean prospects were grim and much talk had begun about when to pull the trigger and hay, greenchop, or ensile the soybeans. The first week/weekend in August that year, temperatures dipped into the high 60’s to low 70’s at night and stayed in the 80’s during the day for about a week. In addition, most areas received from 0.5" to slightly more than 1.0" of precipitation. Not much more precipitation was received after that, but the air temperatures did not climb back into the 100’s. This made a soybean crop.
In 1980. a strip plot on the bottom ground just east of the Jackson County Shop/Yard in Holton illustrates soybeans’ ability to bounce back from mid-season drought and heat stress. The plot averaged near 30 bu/acre, with the top yielder at 36-39 bu/acre. The whole field made 30 bu/acre as well. That wasn’t a bin-buster, but it was a crop. The majority of the pods were set at the top, then at the bottom of the plants -- with the least amount of soybeans produced at the middle nodes. This may be what we could expect, in general, from the 2011 crop, providing the weather the rest of the summer is relatively normal.