Late season herbicide applications

Under any sort of normal conditions we would have been done spraying herbicides long before July 10. But delayed planting, replanting and a limited number of days available for spraying have resulted in many applications being made much later in the season.

If you have soybean fields yet to be sprayed there are many things you should be aware of.

1. The maximum labeled rate of glyphosate in a single application is 1.5lb/acre (44 to 64 ounces/acre depending on the formulation you use). Applying more than this in a single application is not only off label, it may damage your crop.
2. The maximum amount of glyphosate that can be applied to your soybeans after emergence is 2.25lb a.e./acre. If you are using Roundup PowerMax® or WeatherMax® that translates to 66 ounces. In other words, applying 44 oz twice is way off label.
3. The maximum growth stage for applying glyphosate in soybeans is the R2 (full flower) growth stage. This means before you have small pods on one of the 4 uppermost nodes on the plant you need to stop spraying. Many early planted soybeans in the state have already reached the R3 growth stage. Applying glyphosate after R2 (especially high rates) is not only off label, it has been shown to reduce yield.
4. Many tank mix partners used with glyphosate to help control resistant weeds can also cause yield loss when sprayed at the R2 stage and later, due to excessive crop damage caused by increased flower and pod abortion.

Roundup Ready® 2 soybeans have a very high level of tolerance to glyphosate. Even with off-label rates of glyphosate you often do not see visual symptoms of stress or injury. The most common symptom of glyphosate stress is what is commonly known as “yellow flash.” Normally, yellow flash occurs in overlap areas of high rate applications being sprayed to larger soybeans. It tends to be more pronounced when soybeans are drought stressed—obviously not the case this year. And it is more likely to occur on the second POST application to a field. The yellowing is temporary and yield loss does not always occur.

All the “burning” you see in soybean fields this year is not caused by glyphosate. Excessive rates of glyphosate can yellow and even stunt Roundup Ready soybeans, but it will not burn them. The burning is being caused by other active ingredients in the spray mix trying to improve control of possible resistant weeds.

Soybeans have been known to respond positively to being burned at the right time (typically in the early vegetative growth stages before flowering). Physical damage or tissue burn rarely results in yield loss from early season applications, but the bigger and more mature the soybeans are the more likely you are to see yield loss from crop injury.

Due to the weather many applications are being made the best or only way they can to control weeds, not the way they should. We should expect to see poorer weed control, more crop injury and possibly more yield loss as a result of some of these “less than ideal” applications that are being made. The weed competition alone is going to reduce yield potential. Be thankful if you used a good PRE this year and made your POST applications early. Those fields will have the best weed control and should have the highest yield potential.