CCA Weed Identification and Control 3 — Questions and Answers
Question 1: Which mechanism of herbicide resistance involves weeds metabolizing the herbicide more rapidly than susceptible biotypes?
- Target-site resistance
- Metabolic resistance (Correct answer)
- Sequestration resistance
- Exclusion resistance
Correct answer: Metabolic resistance
Metabolic resistance occurs when resistant plants possess enhanced enzyme systems (e.g., cytochrome P450s) that break down the herbicide before it can cause damage.
Question 2: Common cocklebur (Xanthium strumarium) is best identified by which characteristic?
- Milky sap and deeply lobed leaves
- Spiny burr fruits and rough triangular leaves (Correct answer)
- Triangular stem and grass-like leaves
- Yellow flowers and pinnately compound leaves
Correct answer: Spiny burr fruits and rough triangular leaves
Common cocklebur produces distinctive spiny burs (involucres with two seeds) and has large, rough, triangular to heart-shaped alternate leaves.
Question 3: A grower applies an ALS-inhibiting herbicide and achieves poor control. Testing reveals the target enzyme has a single amino acid substitution. This is an example of:
- Metabolic resistance
- Target-site resistance (Correct answer)
- Reduced absorption resistance
- Cross-resistance from detoxification
Correct answer: Target-site resistance
Target-site resistance occurs when a mutation in the herbicide's target enzyme reduces binding affinity, preventing effective inhibition at labeled rates.
Question 4: Which of the following best describes the 'critical weed-free period'?
- The time after herbicide application when weeds cannot re-emerge
- The period during crop growth when weeds must be absent to prevent significant yield loss (Correct answer)
- The minimum interval between sequential herbicide applications
- The dormancy period of weed seeds in the soil seedbank
Correct answer: The period during crop growth when weeds must be absent to prevent significant yield loss
The critical weed-free period is the portion of the growing season during which the crop must be kept free of weeds to prevent economically significant yield reductions.
Question 5: Horseweed (marestail, Conyza canadensis) has become a major management challenge primarily because it:
- Produces allelopathic compounds that inhibit herbicide uptake
- Has developed resistance to both glyphosate and ALS-inhibiting herbicides (Correct answer)
- Reproduces vegetatively through underground rhizomes
- Only germinates under no-till conditions
Correct answer: Has developed resistance to both glyphosate and ALS-inhibiting herbicides
Horseweed biotypes resistant to both glyphosate and ALS inhibitors are widespread, necessitating use of alternative modes of action such as PPO inhibitors or synthetic auxins.
Question 6: Which cultural practice has been shown to reduce weed pressure by depleting the weed seedbank over time?
- Stale seedbed technique (Correct answer)
- Deep primary tillage
- Reduced fertilizer application
- Increasing crop row spacing
Correct answer: Stale seedbed technique
The stale seedbed technique stimulates weed seed germination before crop planting, then destroys emerged seedlings with shallow tillage or contact herbicides, depleting the seedbank.
Question 7: Palmer amaranth (Amaranthus palmeri) is distinguished from other pigweeds primarily by:
- Its preference for wet, poorly-drained soils
- Extremely rapid growth rate and petioles longer than the leaf blade (Correct answer)
- Presence of spiny stems and alternate leaf arrangement
- Its inability to develop herbicide resistance
Correct answer: Extremely rapid growth rate and petioles longer than the leaf blade
Palmer amaranth can grow over 2 inches per day under ideal conditions and has characteristically long petioles that often exceed the length of the leaf blade.
CCA Weed Identification and Control 3
Which mechanism of herbicide resistance involves weeds metabolizing the herbicide more rapidly than susceptible biotypes?