"Two things are happening at the same time in local corn and soybean fields in the Federated service area," said Kevin Carlson, Federated's senior agronomist, "the spread of waterhemp, and the development of herbicide-resistant biotypes. Both are very problematic for our growers."

The issue of herbicide resistance in waterhemp can be narrowed down to two main challenges.

Challenge #1: Waterhemp has spread significantly in the last several years, caused by waterhemp seeds being moved by equipment, birds, wind, and other environmental factors, all of which are very difficult to control. How can anyone keep the seeds out of a combine, or geese out of the fields?

Challenge #2: Once waterhemp is introduced into a field, controlling it becomes an issue because, according to Carlson, "most often it is already a herbicide-resistant biotype of some kind." And, if it is not, he added, "it will quickly become resistant to one in particular: glyphosate (Round Up®)."

To address these challenges and start to take control of waterhemp, growers need to think differently, first about the weed's characteristics, and secondly about the nature of the biotypes that have become resistant and how to select the right herbicides.

Waterhemp is a dioecious species, and thus cross pollination must occur to make seed (male plants + female plants = mixing of the gene pool.) Also, female plants are capable of producing large amounts of seed  (photo at right shows small but prolific waterhemp seeds).

Other Midwest states have been dealing with the waterhemp issue longer than Minnesota and Wisconsin; Illinois has documented waterhemp to be resistant to six different site-of-action (see article below) classes of herbicides to date.

1. ALS (e.g., Pursuit)
2. Triazines (e.g., Sencor)
3. PPO inhibitors (e.g., Flexstar)
4. Glyphosate (e.g., Roundup)
5. HPPD inhibitors (e.g., Callisto)
6. Auxinic herbicides (e.g., 2,4-D)

Choosing the right herbicide, or combination of herbicides, becomes increasingly complex with one additional factor, according to Carlson: "These six different herbicide resistances that are known in waterhemp can also be stacked in the biotypes that become resistant." In other words, a grower could conceivably spray all six herbicides in a field at the same time in a tank-mix and not kill some or all of the population of waterhemp in a field.

Thus, said Carlson, "It comes down to management. Herbicide management." And it is no simple task.

Federated Agronomists are ready to help growers face the challenge of waterhemp and other herbicide resistant weeds. Call with your questions and concerns. Also watch for more information on this important topic at the Soybean Grower Workshops.