🌽 Seed Production of Cross-Pollinated Crops

This lesson builds core elective concepts in BSc Agriculture with practical applications and exam-oriented clarity.

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Seed Production of Cross-Pollinated Crops

Cross-Pollinated Crops — Overview

Cross-pollinated (allogamous) crops are those where pollination predominantly occurs from pollen of a different plant of the same species. Natural selfing is minimal or impossible (due to protandry, protogyny, self-incompatibility, or physical separation of anthers and stigma).

Major cross-pollinated crops: maize, bajra (pearl millet), sunflower, sorghum (partially), rye, sugar beet, many vegetables (cabbage, onion, carrot)

Challenges for seed production:

• Pollen can travel hundreds of metres by wind → large isolation distances required
• Varietal purity easily lost if pollen from other varieties contaminates the plot
• Roguing must be done before anthesis to be effective
• Strict synchronization of flowering needed for hybrid seed production

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General Principles of Cross-Pollinated Crop Seed Production

1. Spatial Isolation

The most critical requirement. The seed plot must be separated from other fields of the same crop species (or closely related species that can cross) by the prescribed minimum isolation distance.

Crop	Foundation Seed Isolation	Certified Seed Isolation
Maize	200 m	200 m
Bajra (Pearl Millet)	300 m	300 m
Sunflower	300 m	300 m
Sorghum	200 m	200 m

Isolation means: no other variety of the same species should be flowering within the stated distance during the period when the seed crop is flowering.

2. Temporal Isolation

When spatial isolation is not feasible, time isolation can be used:

• Sow the seed plot at a different date so its flowering period does not overlap with other fields of the same crop in the vicinity
• Used as an alternative or supplement to spatial isolation
• Requires knowledge of local crop calendar

3. Roguing — Critical Before Anthesis

• Must remove off-type plants before they shed pollen
• Off-type plant shedding pollen contaminates the entire plot → the lot may be rejected
• Roguing schedule: seedling stage (morphological markers), vegetative stage (height, leaf characters), pre-flowering (most critical)

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Maize (OPV Variety) Seed Production

Land Selection and Sowing

• Avoid fields where maize was grown in the previous season (volunteer plant risk)
• Deep tillage; well-prepared seed bed
• Sow within the recommended planting window for the region

Isolation

• 200 m from any other maize variety (OPV or hybrid) in all directions
• Taller obstacles (trees, buildings) can reduce effective isolation distance to 100 m in some standards (check SCA guidelines)

Roguing

• Seedling stage: remove plants with markedly different leaf colour, width, or growth vigour
• Vegetative stage (before tasseling): identify off-types by plant height, leaf shape, leaf angle
• Pre-tasseling (most critical): remove off-types identified by tassel structure, silk colour, plant architecture — must be done before pollen shed
• Off-types remaining after pollen shed cannot be corrected

Detasseling (for Hybrid Seed Production in OPV plots — not applicable to pure OPV seed)

• Not required for OPV variety seed production (both male and female on same plant are of the same variety)
• Required only for hybrid seed production (see Lesson 5)

Harvest

• Harvest ears at physiological maturity (~35% grain moisture); black layer at tip of grain
• Dry ears to ≤ 12% grain moisture before shelling (natural or mechanical drying)
• Shell and clean seed; avoid mechanical damage (damage reduces germination)
• Store in cool, dry conditions
• Seed yield: 2–3 t/ha

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Bajra (Pearl Millet) Seed Production

Pearl millet is predominantly wind-pollinated and cross-pollinated. It is the most widely grown millet in India (Rajasthan, Haryana, Gujarat, Maharashtra).

Isolation

• 300 m (Foundation), 300 m (Certified)
• Pearl millet pollen is light and can travel long distances

Roguing

• Roguing is critical before earhead emergence
• Off-types identified by: plant height, leaf colour, bristle on earhead, grain colour
• Downy mildew (Sclerospora graminicola) infected plants must be removed immediately:
  • Downy mildew is seed-borne — infected plants produce infected seed → spreads disease to next crop
  • Infected plants: yellow-green leaves, white downy growth on lower leaf surface, malformed earhead ("green ear" symptom)
  • Destroy infected plants; do not compost

Key Points

• Bajra is highly prone to downy mildew — seed health inspection is critical
• Use Metalaxyl seed treatment to reduce seed-borne downy mildew
• Seed yield: 0.8–1.5 t/ha

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Sunflower (OPV) Seed Production

Sunflower is insect-pollinated (primarily honeybees) and cross-pollinated.

Isolation

• 300 m from other sunflower varieties or wild sunflower relatives
• Place bee colonies (1–2 hives per hectare) to improve pollination and seed set

Roguing Schedule

Stage	Characters to Check
Seedling (cotyledon)	Cotyledon shape, size — remove distinctly different seedlings
Vegetative	Leaf shape, size, stem pubescence, branching
Pre-flowering	Plant height, head diameter, ray flower (ligule) colour
Flowering	Disc flower colour, pollen colour

Harvest

• Harvest individual heads when the back of the head turns yellow-brown and bracts dry
• Dry heads in sun before threshing (reduce moisture)
• Thresh heads; clean seed on air-screen cleaner
• Seed yield: 0.8–1.2 t/ha

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Sorghum Seed Production

Sorghum is partially cross-pollinated (open panicle; wind-pollinated; ~5–30% natural crossing).

Isolation

• 200 m from other sorghum varieties

Off-Type Identification

• Very easy in sorghum: off-types distinguishable by:
  • Head type (compact vs. loose; erect vs. drooping)
  • Grain colour (white, red, brown, yellow)
  • Plant height (dwarf vs. tall)
  • Glume colour and coverage

Notes

• Johnson grass (Sorghum halepense) is a serious weed in sorghum seed plots — must be removed (it can cross with sorghum)
• Seed yield: 1.5–2.5 t/ha

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Detasseling Techniques in Maize

Detasseling is the removal of tassels (male flowers) from designated female-parent rows before pollen shed. This prevents self-pollination of the female parent and forces cross-pollination from the male parent rows.

Hand Detasseling

• Workers move through rows and pull out the emerging tassel before it sheds pollen
• Must be done daily as new tassels emerge over 5–7 days
• Labour-intensive; 30–40 person-days/ha
• Efficiency: >99.9% detasseling required

Chemical Hybridizing Agents (CHAs)

Also called gametocides — chemicals that cause male sterility when applied at specific growth stages:

• Gibberellic acid (GA₃): applied at booting stage; promotes panicle exsertion in female parent (used in rice)
• Ethephon (ethylene-releasing): causes male sterility in wheat, sorghum
• Sodium Methyl Arsenate: used in rice hybrid seed production (controversial)
• CHAs reduce labour cost but require precise timing; phytotoxicity risk

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Importance of Strict Roguing Schedule

In cross-pollinated crops, a single off-type plant that sheds pollen before roguing can contaminate the entire seed lot. This is why:

• Roguing must be completed before 5% of plants in the field begin shedding pollen
• Multiple roguing rounds are needed
• Rejected seed lots cannot be "cleaned" of off-types post-harvest (genetic mixture is irreversible)
• Field inspection by SCA at pre-flowering stage is the critical checkpoint

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Cross-Pollinated Crops Seed Production Standards

Crop	Isolation (m)	Critical Roguing Stage	Key Off-Type Characters	Seed Yield (t/ha)
Maize (OPV)	200	Pre-tasseling	Plant height, tassel type, grain colour	2–3
Bajra	300	Pre-earhead emergence	Plant height, bristle, downy mildew	0.8–1.5
Sunflower	300	Pre-flowering	Head size, ligule colour, height	0.8–1.2
Sorghum	200	Pre-heading	Head type, grain colour	1.5–2.5

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Summary Cheat Sheet

Topic	Key takeaway
Main focus	Principles and practices for seed production in cross-pollinated crops including maize, bajra, sunflower, and sorghum — isolation, roguing, and detasseling.
Section context	Revise this lesson with the rest of Seed Production Technology for stronger conceptual continuity.