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Bentazon

I. General Information about Bentazon
A. Selective postemergence herbicide many crops
1. Grassy crops: corn, grain sorghum, rice, small grains, turf
2. Dicot crops: soybeans, snapbeans, dry beans, peas, flax, sunflowers, mint, peanuts
B. Selective postemergence herbicide for control of many broadleaf weeds
1. velvetleaf, cocklebur, ragweed, wild mustard
2. Repeat applications can control yellow nutsedge and Canada thistle
3. Weaker control of pigweed and lambsquarters
4. use rates 0.75-2.25 lbs. a.i./acre
C. Discovered by BASF Corp., Germany; first tested in US as experimental herbicide in 1968
D. Chemical structure:

II. Physiology of Bentazon

A. Mode of Bentazon Action
1. Inhibition of photosynthesis
a. Contact-like action and symptomology: toxic effects near site on contact
b. Inhibition of PS II, probably at same site as atrazine (D-1 protein)

B. Mode of Bentazon Lethality
1. Contact injury to leaf at and near site of spray droplet contact
2. Desiccation of leaf tissue, loss of photosynthetic capacity, lead to stunting, cessation of growth, starvation, death

C. Uptake and Movement of Bentazon in plants
1. Foliar application: little apoplastic or symplastic translocation
2. Root uptake: rapid apoplastic translocation
3. Resistant and susceptible species both take up bentazon; some susceptible species take up, and translocate, bentazon more readily

D. Basis of Selective Toxic Activity of Bentazon between Plant Species
1. crop sensitivity greatest when young, small
2. Resistant plants rapidly metabolize bentazon to non-toxic metaboliste
a. Phase I: species-dependent hydroxylation
1) 6-OH and 8-OH
2) mixed function oxidases catalyze ring hydroxylation
b. Phase II: hydroxylation is followed by rapid glycosylation (glucose conjugates)
c. susceptible plants don't metabolize or do it slowly
d. Phase III: incorporation of conjugates into plant components
e. metabolic pathway:
3. Negative cross-resistance of bentazon on triazine resistant weeds (pigweed spp., common lambsquarters, Brassica campestris, common ragweed): a rate of bentazon is more effective inhibiting isolated chloroplasts of R biotypes compared to S biotypes

III. Fate of Bentazon in the Environment
A. Soil
1. Short persistence of bentazon in soil: 1-2 weeks weed control possible
2. Degradation in soil about 1 month
3. Degradation primarily microbial

B. Water: Bentazon not adsorbed to soil particles; rapid decomposition in soil limit leaching and ground water contamination threat

C. Air: low volatility, little threat of drift to adjacent, non-target, plants

D. Animal toxicology: low acute toxicity; no evidence of chronic toxic effects

IV. Plant Injury Symptomology of Bentazon in Plants
A. Rapid symptom development: 2-7 days
B. Leaf abnormalities
a. desiccation, leaf curl, chlorosis, necrosis
b. leaf scorch (copper-colored necrosis) on older leaves
C. Stunting
D. Climate factors
a. enhanced plant injury under extreme weather conditions (inhibition of detoxification metabolism): hot, cold temperatures; wet, dry weather

Bentazon injury slides
1 dry beans slight leaf necrosis, copper color
2 soybeans lower leaf necrosis, dark brown color; new trifoliates OK, no spray
3 dry beans leaf necrosis
velvetleaf severe leaf damage to older plant when sprayed
4 velvetleaf severe leaf necrosis, brown color
5 pigweed slight leaf injury on older, flowering pigweed (inherently R)
6 pigweed oldest leaves with brown spot contact injury, less injury on younger leaves
7 cucumbers slight leaf necrosis, chlorosis
8 sugar beets leaf necrosis
9 dandelion leaf necrosis


©jdekker-1999

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