Weed Science: Biology, Classification, Impacts, and Crop Loss Calculations
Fundamentals and Definitions of Weeds
Definition of a Weed:
A plant out of place.
Plants whose virtues have not yet been discovered.
Plants out of place that are competitive, persistent, and pernicious.
Scope of Impact:
Weeds affect both terrestrial (land) and aquatic (water) environments.
They negatively impact land and water use, lower economic value, and diminish aesthetic appearance.
Definable Characteristics of Weeds
Weeds possess specific, definable biological features that distinguish them from other plants:
Abundant seed production.
Rapid population establishment.
Seed dormancy mechanisms.
Long-term survival and viability of buried seeds in soil seed banks.
Specialized adaptations for seed spread and dispersal.
Presence of vegetative reproductive structures.
High capacity to occupy and colonize sites disturbed by human activity.
Specialized persistence mechanisms for the spread of perennial weeds.
Benefits and Beneficial Uses of Weeds
Soil Conservation and Agriculture:
Weed foliage and root systems help prevent soil erosion.
Certain species are utilized as cover crops to stabilize bare soil.
Livestock Feed and Forage:
Downy brome and Russian thistle provide forage and feed for livestock under specific growth conditions.
Wildlife Habitat and Nutrition:
Provide essential food sources, cover, and shelter for birds, rabbits, small rodents, and other wild fauna.
Economic and Employment Opportunities:
Provide employment for weed scientists, physicians, veterinarians, and weed management professionals.
Aesthetic and Ecological Values:
Produce flowers that contribute to ecological biodiversity; some weed species like annual morning-glory are cultivated as garden ornamentals.
Resource Competition and Economic Impact
Fundamental Biological Requirements:
All autotrophic green plants compete directly for three main environmental resources: water, light, and mineral nutrients.
Competitive Timing and Dynamics:
Weeds establish earlier in the growing season than crops, conferring a major competitive advantage.
Crop yield loss due to weed competition is most severe when crop plants are young and vulnerable.
Macroeconomic Impact and Research Statistics:
In , Bridges estimated that monetary loss caused by weeds across crops in the United States totaled \format{$}{4.1}\text{ billion} dollars.
The Weed Science Society of America evaluated weed yield loss in corn, soybean, and sugarbeets in Minnesota:
Unchecked weed growth suppresses crop yields by 50\format{\text{\%}}{} to 65\format{\text{\%}}{}.
If left completely uncontrolled, between 50\format{\text{\%}}{} and 70\format{\text{\%}}{} of total crop yields would be lost, representing a combined direct cost to growers of \format{$}{2}\text{ billion} dollars annually.
1\format{\text{\%}}{}
Taxonomic and Ecological Classification of Weeds
Habitat / Environment Classification:
Terrestrial weeds: Adaptations suited to dryland or non-flooded terrestrial soils.
Aquatic weeds: Adaptations suited to aquatic ecosystems.
Botanical Systematics:
Provides precise identification by assigning plants to specific hierarchical ranks: Family, Genus, Species, and Variety.
Assigns every weed to a single distinct placement without repetition.
Binomial Nomenclature: The combination of genus and species forms a unique scientific name (e.g., Triticum aestivum for wheat).
Important Botanical Families and Major Weed Species:
Amaranthaceae: Pigweeds.
Asteraceae: Asters, Canada Thistle, Ragweeds.
Chenopodiaceae: Lambsquarter, Kochia, Russian Thistle.
Poaceae (Grass Family): Barnyard grass, Johnson grass, Foxtails, Sedges, Crab grass, Quack grass.
Polygonaceae: Curly dock, Smartweed.
Solanaceae: Jimson weed, Nightshade.
Life Cycle Classification:
Annual Weeds: Complete their life cycle from seed germination to reproductive seed production in one year or season.
Summer Annuals: Germinate in spring, mature during summer, set seed, and die in autumn.
Winter Annuals: Germinate in autumn/winter, overwinter as vegetative rosettes, flower, set seed, and die in late spring/early summer (e.g., Downy brome, Tansy mustard, Field pennycress).
Biennial Weeds: Complete their life cycle over two years. Year one is devoted to vegetative growth (rosettes), while year two involves flowering, seed production, and senescence (e.g., Queen Anne's lace, Burdock, Sweet clover, Teasle).
Perennial Weeds: Live for more than two years and persist across multiple seasons. Include the most aggressive and noxious weeds.
Simple Perennials: Reproduce and multiply exclusively by seeds (e.g., Common milkweed, Curly dock, Dandelion).
Creeping Perennials: Reproduce and multiply by both seeds and asexual vegetative structures, including underground roots, rhizomes, above-ground runners, and stolons (e.g., Johnsongrass, Purple nut sedge, Yellow nut sedge, Quack grass, Silver leaf nightshade).
Woody Perennials: Persistent multi-year trees and shrubs.
Morphological Classification:
Monocot Weeds: Broadly categorized as grasses and sedges with single cotyledons and parallel leaf veins.
Dicot Weeds: Broadleaf plants featuring net-veined leaves and two cotyledons (e.g., Ragweed, Morning glory, Missouri Golden rod, Jimsonweed, Common Sunflower).
Contextual Dual-Role Plants:
Annual morning-glory: Classified as a competitive weed by cotton farmers, but grown as an ornamental plant by gardeners.
Johnsongrass, Canary grass, and Bermuda grass: Cultivated as high-value forage crops, but classified as noxious weeds in croplands.
Parasitic and Poisonous Weeds
Parasitic Weed Biology:
Definition: Plants that depend on host plants for survival by forming protoplasmic connections (haustoria) to derive food, water, or organic nutrients.
Economically Significant Parasitic Weeds in the USA:
Witchweed (Striga asiatica):
Native to and a major problem in Africa and Asia; introduced to the USA in .
Parasitizes corn (Zea mays) and other grass crops.
Relies on host root exudates to germinate and derive water and nutrients during subterranean growth; later produces photosynthetically active shoot growth.
Economic Impact: Known to reduce corn grain yield by 40\format{\text{\%}}{} to 50\format{\text{\%}}{}.
Dodder (Cuscuta species):
Completely non-photosynthetic parasitic vine, deriving all required water, sugars, and nutrients from host plants.
Severe problem in small-seeded legumes like alfalfa, as well as horticultural and floricultural crops.
Mistletoe:
Parasites of woody tree species that extract water and inorganic minerals from hosts while synthesizing their own photosynthates.
Poisonous Weed Pathology:
Cause severe toxic reactions in humans and livestock (livestock poisonings are especially prevalent in western US rangelands).
Notable Poisonous Species: Wild black cherry, Chokecherry, Cocklebur, Whorled milkweed.
Poison Hemlock and Water Hemlock: Cause human and livestock fatalities due to physical misidentification with edible, non-toxic plants of similar appearance.
Jimsonweed (Datura stramonium): Causes human poisonings when consumed for medicinal purposes or as a recreational substitute for marijuana.
Poison Ivy and Poison Oak: Produce contact dermatitis and severe inflammatory skin rashes.
Quantitative Crop Yield Loss Calculations
Standard Agronomic Benchmarks:
Corn Yield Baseline: Average corn yield benchmark is ( national average is ).
Soybean Yield Baseline: Average soybean yield benchmark is (USDA soybean yield average was ).
Soybean Field Scouting and Loss Calculation (Example 1):
Field Scouting Density Data: Average of ragweed plants (15\format{\text{\%}}{} loss), velvetleaf plants (8\format{\text{\%}}{} loss), and yellow foxtail plants (5\format{\text{\%}}{} loss).
Total Cumulative Yield Loss Percentage:
15\format{\text{\%}}{} + 8\format{\text{\%}}{} + 5\format{\text{\%}}{} = 28\format{\text{\%}}{}\text{ yield loss}
Expected Gross Crop Value per Acre (at expected yield and \format{$}{12.00}\text{/bu} price):
60\text{ bu/acre} \times \format{}{12.00}\text{/bu} = \format{}{720}\text{/acre}
Calculated Monetary Loss per Acre:
\format{}{720}\text{/acre} \times 0.28 = \format{}{201.60}\text{/acre}
Net Gain from Treatment (with treatment cost of \format{$}{30.00}\text{/acre}):
\format{}{201.60}\text{/acre} - \format{}{30.00}\text{/acre} = \format{$}{171.60}\text{/acre}
Enterprise Scale Calculations ( of soybeans):
Total Expected Revenue Loss from Weeds: 3000\text{ acres} \times \format{}{201.60}\text{/acre} = \format{}{604,000}
Uninhibited Income: 3000\text{ acres} \times 60\text{ bu/acre} = 180,000\text{ bu} \times \format{}{12}\text{/bu} = \format{}{2,160,000}
Net Crop Income After Uncontrolled Weed Impact: \format{}{2,160,000} - \format{}{604,000} = \format{$}{1,550,000}
Corn Field Scouting and Loss Calculation (Example 2):
Field Scouting Density Data: Average of cockleburs (10\format{\text{\%}}{} loss) and pigweeds (16\format{\text{\%}}{} loss) per of corn.
Total Cumulative Yield Loss Percentage:
10\format{\text{\%}}{} + 16\format{\text{\%}}{} = 26\format{\text{\%}}{}\text{ yield loss}
Expected Gross Crop Value per Acre (at expected yield and \format{$}{6.00}\text{/bu} price):
225\text{ bu/acre} \times \format{}{6.00}\text{/bu} = \format{}{1,350}\text{/acre}
Calculated Monetary Loss per Acre:
\format{}{1,350}\text{/acre} \times 0.26 = \format{}{350.00}\text{/acre}
Net Gain from Treatment (with treatment cost of \format{$}{35}\text{/acre}):
\format{}{350.00}\text{/acre} - \format{}{35.00}\text{/acre} = \format{$}{315}\text{/acre}
Enterprise Scale Calculations ( of corn):
Total Expected Revenue Loss from Weeds: 3000\text{ acres} \times \format{}{350.00}\text{/acre} = \format{}{1,050,000}
Uninhibited Income: 3000\text{ acres} \times 225\text{ bu/acre} = 675,000\text{ bu} \times \format{}{6}\text{/bu} = \format{}{4,050,000}
Net Crop Income After Uncontrolled Weed Impact: \format{}{4,050,000} - \format{}{1,050,000} = \format{$}{3,000,000}