PBL 4: Weeds and Herbicides: Plant diseases/pesticides Flashcards
Weeds
Compete with crops and can transmit diseases and pests. Can result in more than a 30% loss of yield on good land:
1. Annual weeds
2. Biennial weeds
3. Perennial Weeds
Annual weeds
plants that complete their life cycle in one growing season.
-Thrive on frequently disturbed sites (i.e. corn+soybean production).
-Often have high levels of seed production
-Tillage+herbicide are effective at killing annual plants but often develop from soil seed bank.
Summer annual weeds: germinate+grow spring/summer
Winter annuals: germinate fall/vegetative in winter, germinate in spring.
Biennial weeds
Plants that require two growing seasons to complete their life cycles.
-Are vegetative in the first year, flower in the second year.
-Intolerant of tillage in the first/second year
-Mowing in the second year before they flower can eliminate seed production.
-Persist in the permanent pastures, waterways, and roadside areas.
Perennial weeds
Plants that live for 3+ years. They have some specialised reproductive structures that allow for:
-overwintering
-long-term persistence (crowns rhizomes, stolons)
-Once established, difficult to control except by herbicides.
-Best adapted to survive under a diversity of cropping systems.
Grasses and Broadleaves
Monocot weeds: grasses (seed remains underground, only leaves emerge)
Dicots: Broadleaves, may have compound leaves. Most seeds energy from below ground during germination and are easier to kill compared to grass.
Impacts of Weeds
-Reduce agricultural productivity and profitability
-Reduces yield: competing with crops
-increase production costs during their control
-Some are allelopatheic
-Some are parasitic
-May contain toxins that are harmful to livestock
Allelopathy
the adverse effect of one plant on another due tot production of a chemical inhibitor.
-Living or decaying plant tissue may excrete the chemicals
Grain contamination
Mixing weed seed with grain can result in economic loss if separation is not possible at harvest or later during processing.
-may decrease its value for livestock feeding or human use: wild oat contamination of wheat, barley, wild garlic, etc.
Unique weed traits: Seed production
Many weeds produce small seeds and very high numbers of seed.
-provides an opportunity for weed populations to rapidly increase.
-ensures the survival of species
Unique weed traits: Seed dormancy
Weed seeds can have a long dormancy period and a staggered rate of seed germination.
-enables weed seed to persist and remain viable in soil for long periods.
Seed bank: total dormant+nondormant seed accumulated in the soil, provides a reserve of seed.
Unique weed traits: Seed dispersal
-Dispersed by numerous means: wind, water, animals, humans, appendages
-e.g.: purple loosestrife, aquatic weed that attaches to feet of ducks/geese
Unique weed traits: negative (asexual) reproductive:
-enables spreading the weed from the original plant without using seed.
-Rhizomes
-creeping roots
-stolons
Biotypes
Populations of weeds that develop special traits like herbicide resistance.
Herbicides
-primary method of weed control in conventional agriculture.
-used for killing or injuring plants: modern herbicides are synthetic chemical compounds with specific mechanisms for killing plants.
Herbicide Advantages
-Production costs per acre reduced vs mechanical weed control/hand weeding.
-Herbicides provide greater flexibility in timing of weed control than mechanical approaches.
-Selectively can remove weeds from turf+forages without damaging the crop.
-Reduce/eliminate the need for mechanical tillage
-Can persist in soil to provide long-term weed control
Herbicide Disadvantages
-Can potentially kill/injure non target plants
-Contamination of water supplies
-Can lead to development of resistant weed species
-Human health.
Glyphosate
A nonselective herbicide, most widely used herbicide.
-Chemical fallowing
-Resistance built from GMO (Roundup Ready crops).
-Low-cost herbicide.
-when used properly, has minimal environmental impact.
LD50
Lethal dose: amount ingested where 50% of test mice die. Acute toxicity.
Acute toxicity: instant result
Chronic toxicity (more dangerous for humans: accumulation of long-term result
Routes of entry:
Exposure to: mouth, nose, eyes, skin
Modes of Action
Biochemical mechanism that herbicides use to kill plants. Can be selective or or nonselective related to the weeds they affect:
- Growth regulators
- Amino acid synthesis inhibitors
- Lipid synthesis inhibitors
- Nitrogen metabolism inhibitors
- Seedling growth inhibitors
- Photosynthesis inhibitors
- Cell membrane disruptors
- Pigment inhibitors
Modes of Action: Growth regulators
Disrupt plant growth by interfering with hormones+protein synthesis. Mostly taken up by the foliage but can be absorbed by the roots. Translocated in the plant’s vascular system
