Last year, we had more soybean injury from pre-emergence herbicides than I’ve ever seen. It was an epidemic that many farmers across the state experienced, brought on mostly by the prolonged periods of cool and wet weather before and after planting.
This year, there are already some calls and samples that have started to come in with these same issues. I don’t know if we’re going to see as much soybean injury as last year or not (I hope not), but I do think that if we are going to continue to have resistant waterhemp as our number one weed problem, our growers are going to have to get used to seeing some degree of early season soybean injury. And the good news is, early season injury as a result of pre-emergence herbicides doesn’t always translate into soybean yield loss. Of course whether or not you have yield loss will depend on a lot of factors, most notably the growing conditions after herbicide application, but we conducted one study last year that has helped us understand this issue a little better.
In this study, we measured soybean height and biomass reduction in response to pre-emergence soybean herbicides 5 weeks after emergence and then followed this through to see what impact these injury levels had on yield at the end of the season. A small portion of the results from the study are shown in the graph below (to see all of the results go to (http://weedscience.missouri.edu/weedtrials/graphs/PPO%20and%20ALS.pdf ). Basically, what this one year of data would suggest is that soybean plants can recover from a substantial amount of early season injury and, as mentioned above, that early season injury doesn’t always translate into soybean yield loss. As shown in the graph below, we measured as much as 23% soybean height reduction and 28% biomass reduction in response to these pretty standard Authority XL and Authority Maxx herbicide treatments, but neither resulted in any statistically significant yield loss when compared to the weed-free, non-treated control. I think these results can be used as a guide for what we could experience in the next several weeks--we may see some pretty substantial injury that looks significant, but just wait for the soybean to get some better growing conditions and grow through it and chances are you will be much better off to keep the original stand than to tear it all up and start over. We have a new student that is beginning his research this season and will be tackling this issue for the next several years. We hope to be able to provide more results about how early season soybean injury correlates with yield loss in the future.
REVISED: September 30, 2015