The predominant pigweed species that has emerged in fields in May and early June in Minnesota has been redroot pigweed. However, growing degree day accumulation are now sufficient for waterhemp to germinate and emerge. Matter of fact, in southern and west central Minnesota, any new pigweed germinating and emerging in fields probably is waterhemp.
What makes waterhemp so problematic for corn, soybean and sugarbeet farmers? Waterhemp is a summer annual weed that germinates much later than other pigweed species, mid-June and into July in fields in southern and west central Minnesota. Waterhemp can germinate and emerge from the soil surface to one-half inch deep in the soil and can remain viable for at least four years in soil. A unique feature about waterhemp is male and female flowers are on separate plants (dioecious). That is, male plants produces pollen and female plants make seed. This unique biology creates tremendous genetic diversity in populations and results in plants that are biologically and morphologically unique. It also has contributed to development of biotypes that are resistant to several families of herbicides including ALS, triazine, PPO, and glyphosate. Waterhemp’s competitive advantage is in its ability to produce tremendous quantities of seed that potentially germinate and emerge after a farmer has completed postemergence herbicide applications. A few weed escapes in ‘year one’ can lead to a severe weed problem in a field by ‘year three’. Prevention of seed production can pay dividends if maintained for approximately a four-year time period. A field experiment conducted in Urbana, IL, from 1997 to 2000 allowed a heavy field infestation of waterhemp (approximately 40 plants per square foot) to set seed in 1996 and was not allowed to go to seed after 1996. In 1997, 1998, 1999, and 2000, the percentage of the original waterhemp seed bank that remained was 39, 28, 10, and 0.004%, respectively. As waterhemp, kochia, common and giant ragweed biotypes develop resistance to multiple herbicide sites of action, weed seed bank management will become increasingly important.
The diversity of biotypes has led to populations that have differential glyphosate tolerance. Control of susceptible biotypes and failure to control more tolerant biotypes can very quickly lead to weed shifts that will result in the Roundup Ready system being less effective or ineffective in fields planted to corn, soybean or sugarbeet.
The best control strategies in corn, soybean and sugarbeet are a soil-applied herbicide followed by a postemergence tank-mix of herbicides with unique site of action (SOA) that are efficacious on waterhemp. Please refer to the PRE and POST Herbicide Diversification Options for Glyphosate-Resistant Corn and Soybean Crop Rotations for more details.
Thomas J. Peters, Extension Sugarbeet Agronomist, University of Minnesota and North Dakota State University and Jeffrey L. Gunsolus Extension Weed Scientist, University of Minnesota