I have a lot of broadleaf weed escapes in my soybeans. The question that I have is will there be a soybean yield loss from spraying a diphenyl ether (think Cobra®) at this stage of soybean growth? Also, will I be able to control these weeds?
Let us review the current stage of soybean growth: we are between R3 and R4, which is beginning pod and full pod. There are a lot of flowers on top of the soybean plants and pods on the middle branches.
The diphenyl ether products are PPOs, contact-type herbicides. The diphenyl ether products kill the weeds by burning the leaf area, which stops photosynthesis and the weeds die. So spray coverage is very critical. You will need to increase the volume of spray solution to cover the whole plant for good control.
When using a diphenyl ether at this stage you will burn the top part of the soybean plant and hurt the flowers, which will lower the pod count and hurt the yield. Some studies have shown as much as a 15% yield reduction, but if you have good growing conditions after application you may only see about a 7% yield loss.
Most of the weeds coming through the soybean canopy are waterhemp and you can only see about 20% of the plant sticking above the canopy. When using a contact-type herbicide you will only kill the growing point that you can cover with the spray solution. So the answer to the second question is generally that you will not be happy with the control of the waterhemp because it will regrow from the growing points below the soybean canopy.
When spraying escapes, you have to reset your control expectations with weeds like ragweed, waterhemp and Palmer amaranth. While you may burn off the tops of these weeds they will just regrow from below and come back. The lesson here is that you have to control these tough weeds when they germinate or are under four inches tall.
We are in a no-win situation. We want to be proactive as farmers to raise good clean soybean fields. But, I think this year we will have to live with some weedy soybean fields.