CCA Weed Identification and Control 2 — Questions and Answers
Question 1: Which weed is characterized by a triangular stem, making it easily distinguishable from grasses?
- Yellow nutsedge (Cyperus esculentus) (Correct answer)
- Crabgrass (Digitaria sanguinalis)
- Barnyardgrass (Echinochloa crus-galli)
- Foxtail (Setaria spp.)
Correct answer: Yellow nutsedge (Cyperus esculentus)
Yellow nutsedge belongs to the sedge family (Cyperaceae) and has a distinctly triangular stem cross-section, unlike true grasses which have round or flattened stems.
Question 2: What is the primary mode of action for glyphosate herbicides?
- Inhibition of acetolactate synthase (ALS)
- Inhibition of EPSP synthase in the shikimate pathway (Correct answer)
- Disruption of photosystem II
- Inhibition of acetyl-CoA carboxylase (ACCase)
Correct answer: Inhibition of EPSP synthase in the shikimate pathway
Glyphosate inhibits 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase, blocking aromatic amino acid synthesis in the shikimate pathway.
Question 3: A field scout identifies a broadleaf weed with alternate leaves, a taproot, and milky latex sap. Which weed is most likely present?
- Common lambsquarters
- Common dandelion (Taraxacum officinale) (Correct answer)
- Pigweed (Amaranthus spp.)
- Common ragweed
Correct answer: Common dandelion (Taraxacum officinale)
Common dandelion produces milky latex sap when stems or leaves are broken, and has a characteristic deep taproot with a basal rosette of leaves.
Question 4: Which herbicide class is best suited for POST-emergence control of grass weeds in broadleaf crops such as soybeans?
- Triazines (Group 5)
- Graminicides — ACCase inhibitors (Group 1) (Correct answer)
- ALS inhibitors (Group 2)
- Synthetic auxins (Group 4)
Correct answer: Graminicides — ACCase inhibitors (Group 1)
Graminicides (ACCase inhibitors) such as clethodim and sethoxydim selectively control grass weeds in broadleaf crops without injuring the crop.
Question 5: What weed management strategy involves planting competitive crop varieties and optimizing plant populations to suppress weed growth?
- Allelopathic suppression
- Cultural weed control (Correct answer)
- Integrated pest management threshold
- Biological control
Correct answer: Cultural weed control
Cultural weed control uses agronomic practices—including competitive varieties, optimum seeding rates, and row spacing—to reduce the competitive advantage of weeds.
Question 6: Which of the following is a C4 summer annual grass weed that thrives in hot, dry conditions and is a common problem in corn and soybean production?
- Annual ryegrass (Lolium multiflorum)
- Giant foxtail (Setaria faberi) (Correct answer)
- Downy brome (Bromus tectorum)
- Wild oat (Avena fatua)
Correct answer: Giant foxtail (Setaria faberi)
Giant foxtail is a C4 summer annual grass that is highly competitive in corn and soybean, producing large numbers of seeds and thriving in warm conditions.
Question 7: The economic threshold concept in weed management is best defined as:
- The weed density at which the cost of control equals the economic damage caused (Correct answer)
- The maximum weed density allowed by law in certified crop production
- The weed density at which a crop begins to show visible injury symptoms
- The population level below which no herbicide efficacy is observed
Correct answer: The weed density at which the cost of control equals the economic damage caused
The economic threshold is the weed density at which crop yield losses equal the cost of the control measure, providing a guide for when treatment is economically justified.
CCA Weed Identification and Control 2
Which weed is characterized by a triangular stem, making it easily distinguishable from grasses?