Although waterhemp is native to Iowa, it has only been a major weed problem for the past three to four years. Waterhemp is concentrated south of I-80, but we are seeing more of it in northern Iowa. We believe that the major factors involved in the increase in waterhemp populations are the adoption of reduced tillage, changes in herbicide programs, and the reduction in the use of cultivation as a weed management tool.
Populations of waterhemp, and other small-seeded weeds, tend to increase in reduced tillage. Thus, the widespread adoption of conservation tillage has created an ideal habitat for this weed. Changes in herbicide use patterns and the decline in mechanical weed control also favor waterhemp. Many populations of waterhemp in Iowa have been confirmed to be resistant to ALS-inhibiting herbicides. However, other factors besides resistance are involved in the growth of waterhemp as a problem. Waterhemp seedling leaves are long, narow and shiny.
There has been a move away from products with long residual control (atrazine, Treflan, etc.) to postemergence herbicides with little or no residual control. This, in combination with the decrease in row cultivation, has effectively shortened the length of control provided by weed management programs. Waterhemp is ideally suited to these management systems because of its late emergence.
Left: date of emergence for woolly cupgrass and waterhemp.
The dates of emergence of waterhemp and woolly cupgrass at an Ames site in 1996 ( see graph) indicated that a higher percentage of woolly cupgrass seedlings emerged than that of waterhemp, but waterhemp emerged much later than cupgrass. The majority of woolly cupgrass emerged between May 10 and May 20, whereas peak waterhemp emergence did not begin until June 24. Giant foxtail and velvetleaf emergence were intermediate in timing to cupgrass and waterhemp. The late emergence of waterhemp has caused serious problems this year due to poor crop canopy development and the dilution of preemergence herbicides by excessive rain.
How can waterhemp be better managed in the future? Steps must be taken to extend the length of control provided by the weed management program. This usually will involve using multiple tactics, such as a combination of preemergence and postemergence herbicides and the use of row cultivation. No-till fields in which consecutive waterhemp control failures have occurred may benefit from tillage to dilute the seedbank within the soil profile. Waterhemp will continue to cause problems in the future, but the implementation of integrated management systems should eliminate most of the control failures that have occurred the past few years.
To order a poster about waterhemp identification, see separate article.
This article originally appeared on pages 115-116 of the IC-476(17) -- July 8, 1996 issue.