Kochia has been a weed of concern in South Dakota for almost a century. First introduced to the U.S. as an ornamental plant in 1900, kochia has had plenty of time to become an aggressive weed.
"In the dry and droughty times it has even been put up for feed for livestock and its nickname is 'poor man's alfalfa,'" said Paul Johnson, SDSU Extension Agronomy Field Specialist.
In the 1980s, the concern of resistance kochia to ALS herbicide changed some of the cropping patterns in the James River Valley of South Dakota.
"In soybeans, control options are very limited when it comes to kochia without the use of ALS herbicides," Johnson said.
At that time, Johnson said the best option left was the use of Cobra during the early stages of kochia growth and even then, control was only about 80 percent he explained.
Then in 1996, the introduction of Roundup Ready soybeans solved the kochia problem in soybean fields. "South Dakota led the nation in the adaption of this (Roundup Ready) technology," Johnson said. "Then, with the adoption of no-till, the landscape changed from wheat, soybean rotation to a corn, soybean rotation."
And, he added, with the price and effectiveness of glyphosate, it was the herbicide of choice for both corn and soybeans. Roundup remained a good solution until 2009, when the first glyphosate resistant kochia was found in South Dakota.
History repeats itself
Today, the options to control kochia in soybeans are once again limited.
Johnson explained that research on new herbicide resistant technologies includes work with dicamba and 2,4D, but these will not be available for at least a couple more growing seasons. Johnson also stated that growers should be aware that in states near South Dakota, dicamba resistant kochia has been present for about 20 years, and he said 2,4-D is not very good for kochia control.
For partial control of kochia, Johnson suggests in-season tillage. The best strategy he said is to look at a good crop rotation of at least three crops.
"In addition to rotating crops, growers also need to rotate chemistries in each crop, to help keep kochia under control," Johnson said. "The good news is that kochia seed only survives a few years in the soil with the majority of it germinating the first year."
After five years of management, Johnson said most of the seed will be germinated and should be cleaned up - unless some kochia seed blows in from neighboring fields.