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Agronomic Crops Network

Ohio State University Extension


Weeds, Weeds, and More Weeds

Every fall County Extension Educators drive through their county evaluating weed control in soybean fields to see how weed populations are changing over time.  This is my second year of doing this survey, but John Smith had done it before me.

This year I drove 95 miles of the county compared to 78 miles last year.  I started at the Darke and Mercer County lines on Rt 364 going north mostly on Tri-Township to Maier-Barber and Barber-Werner Roads, then I traveled east on Barber-Werner and Buckland-Holden Roads to Worrel road in the northeast and then straight down to New Hampshire and then down to Santa Fe line Road and Gutman then west to New Knoxville and north on Moulton-New Knoxville Road to Buckland-Holden Road.  I observed 362 soybean fields along this route.

Fields were evaluated as 100% weed-free, occasional (occasional individual plants), large patches (patch of 8 or more plants scattered in field), or widespread (numerous patches or individual plants across the field) for each species in the field. 

Only 11% of soybean fields in the county are weed-free, compared to 17% last year.  However, if you consider weed-free fields and fields having three or fewer broadleaf plants and fields only having volunteer corn, then 24% of fields are nearly weed-free.  Soybean fields have more weeds this year compared to last year.  This year 16% of fields have greater than 3 weed species compared to only 6% last year.  I observed 30% of fields having at least one species with large patches of plants and 30% of fields having at least one species with widespread plants, compared to 22% and 8%, respectively last year. The weediest part of the county is the southwest corner (Darke-Mercer County Lines and up to St. Marys) where 47% of fields had greater than 3 weed species present in a field of one individual plant or more. The area with the fewest weeds was from Cridersville east to Waynesfield then down to New Hampshire over to New Knoxville and north to Buckland-Holden road where only 8% of fields had greater than 3 weed species in a field.

The most frequent species in the county was marestail (horseweed) at 51% of fields having one or more plants followed closely by giant ragweed at 49% of fields.  Last year marestail was present in 44% of fields and giant ragweed was present in 31% of fields.  Waterhemp, the weed of greatest concern, was present in 32% of fields compared to only 21% last year.  I observed 45% of fields west of I-75 having waterhemp compared to only 14% east of I-75!  The next most frequent species in 2016 includes volunteer corn (27%), velvetleaf (22%), giant foxtail and other annual grass weeds (15%) and lambsquarters (14%), compared to 30%, 2%, 11%, and 3%, respectively last year.  The remaining species present this year at less than 5.5% included smooth pigweed, morningglory, common ragweed, jimsonweed, shattercane, yellow nutsedge, Canada thistle, cottonwood (yes, a tree!), common cocklebur, hemp dogbane, and wild carrot.  Last year, only lambsquarters, velvetleaf, morningglory and shattercane was in the less than 5.5% group.

I am very concerned about waterhemp.  This year 36% of fields with waterhemp have large patches or widespread plants, compared to only 24% last year.  In 2013 John reported only 6% of fields having a pigweed species, which probably included some waterhemp, but today 32% of field have waterhemp.  Waterhemp is a concern because it produces at least 100,000 seeds per most plants or more, germinates throughout most of the season, and causes greater herbicide costs to properly manage this weed.  We really should be treating waterhemp like Palmer amaranth.  All that can be done now is to remove plants from the field by hand before harvest and prepare to manage waterhemp better next season.

The only way to reduce the frequency of marestail in the county is to apply herbicides this fall.

Crop Observation and Recommendation Network

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.