Herbicides

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Slide 1:

Herbicide Demand When & why?

Slide 2:

Herbicide R & D Costs 13 yr to breakeven 7 yr

Slide 3:

Herbicide R & D Costs 140 mil $US

Slide 4:

Herbicide Modes of Action Contact vs systemic Acute toxicity

Slide 5:

Herbicide Modes of Action Inorganic (contact) Ex. Cu2SO4, NH4SO4 Plasmolysis 2. Auxin-like (systemic) Phenoxy-acids: 2,4-D; 2,4,5-T Rapid stem elongation Cells starve 2,4,5-T = agent orange - contam. W/ dioxin

Slide 6:

3. Photosynthesis Inhibitors Most target ‘light’ rxn

Slide 7:

Light-dependent Rxn Atrazine Diquat, paraquat Atrazine inhibits photolysis of water

Slide 9:

Ex. Triazines Contact Pre-emergent Corn, soybeans, wheat, blueberries Acute Toxicity Mammals: 1500-3000 ppm Fish: 10-15 ppm Daphnia: 7 ppm Sub-lethal Toxicity Aq. Insect comm. Structure: 20 ppb Algal growth: 1 ppb Aq. Spp richness Rel. Abundance

Slide 10:

Atrazine Fate in environment Moderate persistence 7-9 mo. Half-life “dry-year carryover” Surface water 1-70 ppb – rural creeks 1000 ppb – agric. ditches etc.

Slide 11:

4. Metabolic Inhibitors Dintrophenols – x electron transport “Round-Up”- Amino Acid Synthesis Inhibitor AA synthesis in plants vs animals 9 “essential” amino acids – dietary only for animals Histidine Isoleucine Leucine Lysine Methionine Threonine Tryptophan Phenylalanine Valine

Slide 12:

Round-Up (A.I. = glyphosate) Non-selective Post-emergent Low residual activity Water soluble- great systemic Good for rhizomes Phosphophono- amino acid

Slide 13:

EPSP enzyme Aromatic a.a. Glyphosate ‘kills’ EPSP enzyme; so no aromatic a.a. Not toxic to animals?

Slide 14:

Glyphosate Toxicity to Plants Primary: Slow, 3 weeks+ Depletes aromatic a.a. no protein synthesis NH3+ accumulates (toxic) Secondary: Aromatic a.a. sustain auxin So- glyphosate speeds auxin breakdown More lateral growth Shallow root “tillering”

Slide 15:

Herbicide Resistant Crops How? Insert constitutive EPSP gene Examples “Round-up Ready” Corn & Soybeans Pros & Cons Others: insect resistant crops (StarLink corn- Bt genes)