Family: Sunflower (Asteraceae)
Name: Ambrosia trifida L.
Life Span: Annual
Leaves are mostly opposite with simple blades and serrate margins. Male (staminate) flowers occupy the upper portion of the inflorescence while female (pistillate) flowers are located at the bottom. The seed is bur-like in appearance with a single short beak in the center surrounded by blunt spines. Giant ragweed has erect, branching, and coarse stems that may grow up to 13 feet in height. The most prominent feature in identifying common ragweed seedlings is the 3-5 lobed leaves. Giant ragweed is one of the first annual weeds to emerge in the spring (GDD<150), generally germinating over a period of two to three weeks. Infestations generally start in small areas along fence lines, ditch banks, and other waste areas and will move into cultivated fields over time.
Research conducted at The Ohio State University in 1998 and 1999 suggest that giant ragweed may produce at least 160 seeds/ft 2 . This same research showed that giant ragweed seed is lost from the seedbank through predation (losses to insects and rodents), premature germination, and loss of seed viability. Over a twelve-month period the seed losses from all sources ranged from 86-100%, leaving a potential maximum of 22 seeds/ft 2 added to the seedbank. This slow but steady increase in the seedbank may help explain how small infestations soon become large problems if giant ragweed is not initially controlled.
Burndown: Spring tillage is an effective control option of small seedlings because of the early emergence. Tillage becomes less effective as plants become larger and may, under moist soil conditions, be “transplanted” by tillage. Broadcast applications of glyphosate at 2.0 pt/A will control plants up to six inches in height, while 1.5 pts/A should be effective on plants up to four inches. Giant ragweed is listed as difficult-to-control on the Gramoxone Extra label and it suggests tankmixing a PSI (photosynthesis inhibitor), such as atrazine, Lorox, or Sencor for increased control. Tankmixing 2,4-D at 1 pt/A with glyphosate or Gramoxone Extra will improve giant ragweed burndown, but allow a seven day interval between application and soybean planting.
Corn: Atrazine, atrazine premixes, Hornet, and Marksman all provide fair levels of control in field corn when applied preemergence, but growers should plan for a postemergence treatment with moderate to heavy populations. The best postemergence options for giant ragweed control in field corn are Buctril/atrazine, Liberty ATZ, Marksman, NorthStar, Roundup Ultra, and ReadyMaster ATZ; however, all come with certain restrictions. Postemergence applications of Liberty ATZ, Roundup Ultra and ReadyMaster ATZ are restricted to Liberty resistant or Roundup Ready hybrids. Buctril/atrazine, Liberty ATZ, ReadyMaster ATZ, and Marksman all contain atrazine and cannot be used in atrazine prohibition areas. Marksman and NorthStar both contain dicamba and can cause stalk brittleness under certain environmental conditions. Other postemergence options available to growers that are somewhat less efficacious are, Liberty, Lightning, dicamba, 2,4-D, Hornet, Distinct, Stinger, and other premixes containing atrazine.
Soybeans: Several good options are available for controlling giant ragweed in soybeans. FirstRate applied preemergence at 0.6-0.75 oz/A (soil type dependant) or postemergence at 0.3 oz/A will provide excellent control. Be aware that FirstRate resistant giant ragweed has been reported in several fields in other states. Lorox applied preemergence will give good control. The diphenylethers, such as Cobra and Flexstar, are good postemergence options if giant ragweed has not exceeded four true leaves. It is important to realize that the diphenylethers will cause varying degrees of soybean leaf burn depending on the environment and additives, but the soybeans should recover with insignificant yield effects. Glyphosate applied postemergence to Roundup Ready soybeans is an excellent choice in that 2 pt/A will control larger giant ragweed as well as other weeds present. Postemergence applications of other ALS herbicides (i.e. Raptor, Pursuit, and Classic) will generally provided fair to good control, but giant ragweed control with Pinnacle is poor.
Alfalfa: Giant ragweed can be a problem in newly seeded alfalfa but is seldom a serious problem in established stands. Buctril/Connect and Pursuit are both postemergence options for seedling alfalfa; however, neither compound is very effective on giant ragweed and applications are restricted by alfalfa development. The best option in seedling alfalfa is an early clipping for a serious infestation.
Giant ragweed has no forage quality and may accumulate nitrates after being treated with 2,4-D. The seeds are utilized as a food source by songbirds, pheasants, and quail. Native Americans used the fiber from the stems to make thread. Giant ragweed is a common cause of hay fever during pollination in August and September.
Weeds of Nebraska And The Great Plains, Nebraska Department of Agriculture
2000 Pest Management in Wisconsin Field Crops
Weed Emergence Sequences, Iowa State University, University Extension
E.E. Regnier et.al., 1998.
Competition and Seed Production of Giant Ragweed in Corn, North Central Weed Science Society Proceedings. 53:151.
E.E. Regnier et.al., 2000.
Seed Losses in Giant Ragweed, WSSA Abstracts. 40: 99-100.
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* Senior Outreach Specialist