Herbicide Mode of Action Celestine Duncan Weed Management Services
Mode of Action Sequence of events from absorption into the plant until the plant dies • Systemic herbicides – translocated from site of entry to area in plant where they are active • Contact herbicides - kill only the area where they are applied.
Classification by Mode of Action • Photosynthesis Inhibitors • Plant Growth Regulators • Amino Acid Biosysthesis Inhibitors • Growth Inhibitors • Pigment Inhibitors • Grass Specific herbicides • Membrane Disrupters
How many MOA are represented in this list of herbicides? • Tordon 22K • Transline • Curtail • Banvel (Clarity) • Garlon • 2,4-D • Redeem ?
Range and PastureHerbicide MOA • Plant Growth Regulators • Amino Acid Biosynthesis Inhibitors
Plant Growth Regulators (PGR) Tordon 22K, Transline, Curtail, Redeem, Banvel, 2,4-D • Referred to as synthetic auxins (regulate growth in plant tissues) • Translocate in both xylem and phloem. • Can act at multiple sites in a plant to disrupt hormone balance and protein synthesis • Abnormal growth resulting in twisting stems • Stems swelling due to rapid cell division
Plant Growth Regulators (PGR) • Accumulation at growing points • Specific biochemical change responsible for killing plant is not known (probably several processes) • Grasses are not susceptible (may be due to differences in vascular tissue structure or differences in translocation or metabolism)
Common Leaf Symptoms from PGR Exposure normal cupped and blistered from PGR exposure
Foliar PGR Application shortinternode accumulation In new growth
Root Absorption of Plant Growth Regulator Herbicides translocation to growing point swollen hypocotyl callus formation on roots lack of root development
Amino Acid Inhibitors (Glyphosate, Escort, Plateau) • Very important group of herbicides • Roundup introduced 1971 • First sulfonylurea introduced in 1979 (Telar)
Amino Acid Inhibitors • Prevent synthesis of certain amino acids produced by plants but not animals. • Excellent foliar and root absorption • Broad weed spectrum • Translocates to shoot and root new growth • Plants stop growing shortly after application • Plant death may be slow (10 days+)
Plateau or Escort Injury Symptoms Stunting Chlorosis of youngest tissue
Glyphosate (Roundup) Drift chlorosis shortened internodes stem proliferation