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Waterhemp has Emerged in Row-Crop Fields (06/18/15)

Part of the weeds management strategy is properly identifying weeds in the field. Pigweed identification is especially difficult in the early seedling stages since many species look the same. A ‘pigweed identification guide’ developed at Kansas State University is available.

Waterhemp has Emerged in Row-Crop Fields

Part of the weeds management strategy is properly identifying weeds in the field. Pigweed identification is especially difficult in the early seedling stages since many species look the same. A ‘pigweed identification guide’ developed at Kansas State University is available.wdsci.peters.1.redroot pigweed near foxhome mn

The predominant pigweed species that emerged in fields in May was redroot pigweed. However, growing degree day accumulation are now sufficient for waterhemp to germinate and emerge. Matter of fact, in west central Minnesota and southeastern North Dakota, any new pigweed germinating and emerging in fields probably is waterhemp.

What makes waterhemp so problematic for row crop farmers? Waterhemp is a summer annual weed that germinates much later than other pigweed species, June, July and in some cases, early August in fields in west central Minnesota and southeastern North Dakota. Waterhemp seed 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.

There are a great number of waterhemp biotypes in fields; plants that may look the same but genetically are slightly different. In some cases, ‘biotypes’ respond differently to glyphosate herbicide, ranging from susceptible to tolerant. 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 sugarbeet. So what is the solution? First, use full rates and tank-mixes of at least two herbicides that are efficacious on waterhemp and spray when waterhemp is no more than two or three inches tall. Second, use the appropriate adjuvants with selected herbicides. And finally, be vigilant since waterhemp will continue to emerge into July and early August, especially after rainfall events. Consult your ag-retailer, crop consultant or extension specialist/agent for control options in various row crops.wdsci.peters.2.waterhemp seedling hillsboro

wdsci.peters.3.waterhemp herman mn

Tom Peters

 Extension Sugarbeet Agronomist

NDSU & U of MN

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