Harvest Index Calculators

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Harvest index (HI) is the ratio of economic yield (the harvested portion — grain, seed, tuber, or fruit) to total above-ground biological yield (total dry matter at harvest): HI = economic yield / total above-ground biomass. HI reflects the efficiency with which a crop partitions accumulated biomass into the harvested fraction. Modern wheat and rice varieties have HI ≈ 0.50–0.55 (50–55% of above-ground biomass is grain), compared to traditional varieties at HI ≈ 0.30–0.35. The Green Revolution dramatically increased yields primarily by increasing HI through semi-dwarf varieties with shorter straw. HI is approaching its biological ceiling, so future yield gains must come from increasing total biomass.

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Harvest Index Formula

HI = economic yield / total above-ground dry biomass at harvest

Example: wheat field: total above-ground biomass = 10,000 kg/ha (grain + straw + chaff); grain yield = 5,200 kg/ha. HI = 5,200 / 10,000 = 0.52.

Typical HI Values by Crop

  • Modern wheat: 0.50–0.55
  • Traditional wheat (pre-Green Revolution): 0.30–0.35
  • Modern rice: 0.45–0.55
  • Maize: 0.45–0.55
  • Soybean: 0.40–0.55
  • Potato: 0.65–0.80 (high HI due to underground storage organ)
  • Sugarcane (juice): 0.60–0.70

Green Revolution and HI

Semi-dwarf wheat and rice varieties: shorter, stiffer straw (Rht dwarfing genes in wheat; sd1 in rice) → more carbon allocated to grain rather than stem → HI rose from ~0.35 to 0.50+. This increased grain yield by 40–60% without increasing total biomass proportionally.

Biological Ceiling

Maximum theoretical HI ≈ 0.60–0.65 for cereals (constrained by minimum stem, leaf, and root biomass needed for support and function). Future yield gains must come from increasing total biomass (higher radiation use efficiency, longer growing season, more photosynthetically efficient varieties).

Glossary

Harvest Index (HI)
Economic yield / total above-ground biomass; modern wheat ≈ 0.50–0.55; traditional wheat ≈ 0.30–0.35; Green Revolution increased HI through semi-dwarf varieties; approaching biological ceiling of ~0.60.
Semi-Dwarf Varieties
Crop varieties with reduced stem height from dwarfing genes (Rht in wheat; sd1 in rice); more biomass allocated to grain (higher HI); more responsive to fertilization without lodging; core of Green Revolution.
Biological Yield
Total above-ground dry matter produced by a crop at harvest; includes grain + straw + chaff + leaves; = economic yield / HI; increasing biological yield is the primary target for post-Green Revolution yield improvement.

Frequently Asked Questions

Harvest index (HI) = economic yield / total above-ground biomass. Economic yield = the marketable, harvested portion (grain, seed, fruit, tuber). Total biomass = all above-ground plant parts at harvest (grain + straw/stems + leaves + other plant material). Example: rice field produces 6,000 kg/ha of grain and 6,500 kg/ha of straw + chaff: total biomass = 12,500 kg/ha. HI = 6,000/12,500 = 0.48. Measurement: harvest all above-ground material from known area; separate into grain and non-grain fractions; dry both at 70°C to constant weight; weigh; calculate.

The Green Revolution (1960s–70s), primarily credited to Norman Borlaug (Nobel Peace Prize 1970), transformed wheat and rice yields primarily by increasing harvest index, not total biomass: Pre-Green Revolution: traditional tall wheat varieties had HI ≈ 0.30–0.35; long straw collapsed under heavy grain (lodging) and nutrient uptake → most biomass in stems. Green Revolution: semi-dwarf wheat (Rht genes; reduced height response to gibberellin) with stiff, short straw → less carbon in stems → more available for grain → HI ≈ 0.50–0.55. Similarly, semi-dwarf rice (sd1 gene: reduced sensitivity to gibberellin → shorter internodes) → increased HI and resistance to lodging → more fertilizer-responsive. Result: wheat yields doubled or tripled in many developing countries → avoided famines predicted for Asia and Latin America in the 1970s.

HI cannot approach 1.0 because plants require structural and functional biomass: Structural: stems and roots must support grain weight and anchorage; complete elimination of stem biomass would cause lodging. Respiratory load: some biomass is consumed by respiration during grain filling. Minimum leaf area: photosynthesis requires leaves; eliminating leaves during grain fill would reduce carbon supply to grain. Roots: essential for water and nutrient uptake; cannot be eliminated. Estimates: maximum theoretical HI for cereals ≈ 0.60–0.65 (Fischer, 2007). Modern varieties at HI = 0.50–0.55 are approaching this ceiling. Implication: future crop yield improvement must primarily come from increasing total above-ground biomass (GPP, radiation use efficiency, longer growing season) rather than from further HI gains.

HI varies considerably between crops due to differences in what is harvested and plant architecture: Potato (tuber): HI ≈ 0.65–0.80 (high because underground storage organs are very efficient sinks; aboveground parts are relatively minimal). Sugarcane (juice from stems): HI ≈ 0.60–0.70 (most above-ground biomass = stem; high HI for juice). Oil palm: HI ≈ 0.25–0.35 (high carbon cost to produce fruit bunches; long-lived perennial). Wheat/rice/maize: HI ≈ 0.45–0.55. Grain legumes (soybean, peanut): HI ≈ 0.40–0.55. Forage crops (alfalfa, grasses): HI = effectively 1.0 because all above-ground biomass is harvested. Knowing HI helps set realistic yield targets for breeding and agronomy optimization.