To prevent development of Roundup-resistant weed population on your farm, many farmers are planning to use something besides glysophate in soybeans.  As a result, I have been getting questions about whether Flexstar applied post emegence will burn the soybean leaves and, if it doesn,  how much yield loss can be expected?

Soybeans may suffer leaf burn at this stage from some contact herbicides.

I assume the underlying precept from this question is lack of experience from a previous era of Blazer and Cobra use. Growers with this concern have either forgotten or were not around in the pre-Roundup Ready soybean era when soybean leaf burn from contact herbicides was just a normal routine in weed control. Growers have been “conditioned” by complete soybean safe weed control using the Roundup Ready soybean technology. Management of herbicide resistant weeds now require addition of another herbicide and a quick review of the few potential herbicides show PPO “contact burner” herbicides as clear options. PPO stands for protoporphyrinogen oxidase, an enzyme in a plant’s cell involved photosynthesis.

Many academic researchers quantified yield loss from PPO herbicide leaf burn in the 1980s and 1990s. Much of the data did not show any yield loss because of soybean’s plastic growth nature – being able to recover from stress conditions. Cobra burned soybean plants back to only stems but plant quickly recovered from growing points and at harvest there was no yield loss.

Leaf burn from Flexstar does not always happen and predicting when and to what extent it may happen is impossible. Certainly, temperature, humidity, type of adjuvant and rate of adjuvant all factor into the risk. The higher the temperature and humidity the more likely (but not always) leaf burn will occur.

A Methylated Seed Oil (MSO) adjuvant is more effective than petroleum based Crop Oil Concentrate (COC) adjuvant but also can cause more leaf burn. Applying oil adjuvants on an area basis (1 to 1.5 pints per acre) rather than on a volume basis (1% volume to volume), which has become more popular, will also result in better weed control but can also increase risk of leaf burn. Fortunately, soybean leaf burn from Flexstar seems to be the least aggressive compared to Ultra Blazer and Cobra.

The most impacting factors on yield potential will be the growing conditions after leaf burn has occurred. If soil moisture is not lacking, if temperatures remain warm, if there is no pest interference, if fertility is not limiting, if growing conditions remain good, then soybean plants can quickly recover with no yield loss penalty. Soybean has this almost unique quality of rapid recovery from early season stress whether from herbicides or environmental conditions.

Zollinger is a North Dakota State University Extension weed specialist.