📊Weed Indices, Seed Dispersal and Key Terminology

Seeds That Travel Thousands of Kilometres

The previous lesson covered allelopathy, annidation, and crop rotation effects — the chemical and ecological dimensions of weed science. This final lesson completes the weed science module with practical tools and terminology that tie everything together: how weed seeds disperse, how we measure weed control effectiveness, when weeding matters most, and the key terms that appear in competitive exams.

In the vast grasslands of Argentina, mature plants of Mexican prickly poppy (Argemone mexicana) dry up and break free from the soil. The wind rolls the entire dead plant across the plains like a tumbleweed, scattering seeds with every bounce. This dramatic censer mechanism can carry seeds for kilometres. Understanding how weed seeds travel is the foundation of preventive weed management — if you know the pathways, you can block them.

This lesson covers:

1. Seed dispersal mechanisms — autochory, wind, water, human, animal, bird
2. Weed indices — Weed Index, WCE, WSE formulas and interpretation
3. Critical periods of crop-weed competition for all major crops
4. Herbicide bioassay — indicator plants for residue detection
5. Key terminology — adjuvant, safener, epinasty, and more
6. Herbicide combination responses — synergistic, antagonistic, additive

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Dispersal of Weed Seeds

Seed dispersal is classified into two broad categories:

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1. Wind Dispersal (Anemochory)

Seeds possess special floating or flight organs:

Censer Mechanism: Wind detaches the whole plant and rolls it over long distances like a tumbleweed, dispersing seeds along the way. Common with Salsola (Russian weed) and Argemone mexicana.

2. Water Dispersal (Hydrochory)

Seeds with waterproof coats or air-filled structures float and are carried by irrigation channels, floods and rainfall runoff. Canal-bank weeds are major seed sources.

3. Human Dispersal (Anthropochory)

Through contaminated seed lots, farm machinery, transport of produce and movement between fields. This is the most significant dispersal agent in modern agriculture.

4. Bird Dispersal (Ornithochory)

Birds eat fleshy weed fruits and deposit viable seeds in droppings far from the parent plant. Loranthus (birdvine) is a classic example.

5. Animal Dispersal (Zoochory / Biotic)

0.2 to 9.6% of ingested weed seeds pass in viable form through animal excreta — a significant pathway through grazing and manure application.

6. Other Mechanisms

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Weed Indices — Quantitative Measures

Once we understand how weeds disperse, we need ways to measure the severity of infestation and evaluate how well our control methods are working. Weed indices provide standardised formulas for this purpose. These formulas are frequently asked in competitive exams.

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1. Weed Infestation (%)

Measures the percentage of weeds in total plant population:

Weed Infestation (%) = (Total weeds / (Total weeds + crop plants)) x 100

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2. Weed Index / Weed Competition Index (RRB 2019)

Measures yield reduction due to weeds compared to a weed-free plot.

WI = (Yield_weed-free - Yield_treated) / Yield_weed-free x 100

Lesser the weed index = better the herbicide efficiency. A weed index of zero means the treatment was as effective as keeping the field completely weed-free.

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3. Weed Control Efficiency (WCE)

Measures percentage reduction in weed population (number) under treatment vs untreated control.

WCE (%) = (Weed count_unweeded - Weed count_treated) / Weed count_unweeded x 100 Higher WCE = better weed control.

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4. Weed Smothering Efficiency (WSE) (RRB SO 2020)

Measures the effect of intercropping on weed suppression compared to sole crop.

WSE (%) = (Weed dry wt_{sole crop} - Weed dry wt_intercropped) / Weed dry wt_{sole crop} x 100

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Comparison of Weed Indices

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Critical Period of Crop-Weed Competition

Weed indices tell us how much damage weeds are causing. The next practical question is when to intervene. The critical period is the specific time window during which weed removal is most essential for preventing significant yield loss. Weeding outside this window has relatively less impact.

Sugarcane has the longest critical period (2-120 DAS) — nearly four months. This is because sugarcane establishes canopy cover very slowly, leaving abundant space and light for weeds.

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Herbicide Bioassay

A bioassay determines if herbicide residues are present in soil at concentrations that affect plant growth. Sensitive indicator plants are grown in suspect soil — toxicity symptoms confirm residues.

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Key Terminology

The following terms cover herbicide application, plant responses, and formulation additives. They appear frequently in competitive exams and are essential for precise communication in weed science:

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Response Types in Herbicide Combinations

In IWM practice, herbicides are often tank-mixed for broader weed spectrum control. Understanding how two herbicides interact when combined is critical for avoiding wasted money and ensuring effective control. When two or more herbicides are mixed, their combined effect can take different forms:

Important Synergistic Combinations

Important Antagonistic Combinations (Avoid)

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Autotoxy (Revisited)

Autotoxy = self-toxicity — where a plant’s own allelochemicals inhibit its own species. Classic example: Parthenium daughter plants suppress parent plants, limiting colony density.

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Summary Cheat Sheet