Yellow soybeans and delayed pod set
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.