Midwest Weed Resistance Recommendations

Glyphosate-Resistant Weeds
Waterhemp (Amaranthus tuberculatus (syn. rudis))
• History of developing herbicide resistance quickly
• Glyphosate resistance first confirmed in 2005
– Populations have also been confirmed with multiple resistance
to combinations of glyphosate, ALS, PPO and PSII inhibitors
• Germinates mid-May through July
• Can grow as much as 1 inch per day to an average height of 4 to 5 feet
• Dioecious species has separate male and female plants, increasing the
chances that resistant genetics are passed on to the next generation
– 1 female plant can produce 500,000 to 1 million seeds
• Heavy infestations of waterhemp:
– Season-long competition can reduce soybean yield up to 44 percent
in 30-inch rows and 37 percent in 7.5-inch rows1
– Early-season competition can reduce corn yield up to 15 percent by
the time weeds are just 6 inches tall1
Giant Ragweed (Ambrosia trifida)
• Glyphosate resistance first confirmed in 2004
• Germinates spring through July (expanded window)
• Pollen transfer between plants can increase the chances that resistant
genetics are passed on each year
• Found in patches, since its large seeds are not highly mobile
• Grows 1 to 5 feet tall, but can reach heights of 17 feet
• Season-long giant ragweed competition:
– 1 weed/110 sq.ft. can reduce soybean yield up to 50 percent2
– 2 weeds/110 sq.ft. can reduce corn yield up to 13 percent2
Horseweed/Marestail (Conyza canadensis)
• Most commonly known as marestail
• First glyphosate-resistant weed in U.S. row crops, confirmed in 2000
• Often an early sign of increasing weed control problems with glyphosate
• Germinates in fall, spring and summer
• Produces up to 200,000 seeds/plant
• Grows 5 to 6 feet tall
• Can cause yield losses of up to 83 percent in soybean3 if left uncontrolled
1Biology and Management of Waterhemp, Nordby, et.al., www.glyphosateweedscrops.org.
2Biology and Management of Giant Ragweed, Johnson, et.al., www.glyphosateweedscrops.org.
3Biology and Management of Horseweed, Loux, et.al., www.glyphosateweedscrops.org.

Fight Glyphosate Resistance
Management practices:
• Consider cultural practices such as crop rotation,
cover crops or tillage as appropriate to help control
resistant weeds
• Start clean with an effective burndown
• Apply broad-spectrum pre-emergence residual herbicides with
multiple modes of action at or prior to planting on every acre
• Use full, labeled herbicide rates
• Create your customized weed resistance management plan at