LAISHA Explains
Agronomy and crop production

Yield components and harvest index

How grain yield breaks into plants, ears, grains per ear and grain weight, when each is set, how crops compensate, and harvest index sums.

Exam essential6 min readUpdated
On this page
  1. The yield equation
  2. When each component is set
  3. Compensation between components
  4. Worked examples
  5. Harvest index
  6. Using yield components in practice
  7. Key points
  8. What the exams ask
  9. Questions people ask
  10. Sources
  11. Related on Laisha
  12. Discussion

A field of wheat or rice yields a certain number of tonnes per hectare, but that single number hides a sequence of events spread over the whole season. How many plants established? How many of them produced heads? How many grains formed in each head, and how heavy did each grain become? Breaking yield into these yield components lets agronomists and breeders see exactly where a crop lost yield, and what could be done about it.

Yield components and harvest index appear in almost every crop physiology and agronomy exam, often as a calculation.

The yield equation

For cereals, grain yield per unit area is the product of four components:

Grain yield of a cereal
Yield = plants per m2 × ears per plant × grains per ear × weight per grain

The first two are often combined as ears (heads, panicles) per m2. For rice, grains per panicle is often split into spikelets per panicle and filled-grain percentage, since many spikelets can stay empty. Grain weight is usually given as thousand-grain weight (TGW) in grams or as milligrams per grain.

Other crops have equivalent equations: pods per plant × seeds per pod × seed weight for pulses and soybean; tubers per plant × mean tuber weight for potato; bolls per plant × seed cotton per boll for cotton.

When the yield components of a cereal such as wheat are set: plant number at establishment; ears per square metre during tillering and stem elongation; grains per ear from stem elongation to flowering; and grain weight during grain filling; stress at a stage affects the component being formed then, and later components can partly compensate Grain yield = plants/m² × ears/plant × grains/ear × weight/grain Sowing–emergence Tillering Stem elongation Heading–flowering Grain filling Maturity Plants per m²seed rate, seedbed, establishment Ears (heads) per m²tillers formed, then some die Grains per earflorets formed, pollination succeeds Grain weightphotosynthesis and reserves fill grains Stress (drought, heat, nutrient shortage) hits the component being formed at that moment; later components can partly compensate.
When each yield component of a cereal such as wheat is formed. The bars show approximately when each component is being determined; the stages overlap, and exact timing varies with crop and climate.Diagram: Laisha, original.

When each component is set

The components are formed one after another, which is the key to interpreting them:

  1. Plants per m2 are set at establishment, by seed rate, seed quality, seedbed and early pests.
  2. Ears per m2 are set during tillering and stem elongation. Cereals form more tillers than survive; many die during stem elongation when competition for light and nitrogen intensifies.
  3. Grains per ear are set from the start of stem elongation to just after flowering: florets develop, some abort, and pollination must succeed. This is the most sensitive period in many cereals, and grain number per m2 is usually the component most closely tied to final yield.
  4. Grain weight is set during grain filling, by current photosynthesis and by reserves moved from stems and leaves.

A stress hits whichever component is forming at the time: drought or heat at flowering cuts grain number, and drought, heat or disease during grain filling cuts grain weight.

Compensation between components

Because the components form in sequence, a crop can compensate for an early loss with a later gain:

  • A thin stand tillers more, so ears per m2 partly recover.
  • Fewer ears often carry more grains each.
  • Fewer grains may fill larger.

Compensation is never complete, and it is usually strongest early (tillering cereals compensate well for low plant numbers) and weakest late (little can make up for poor grain set). It also explains why components are often negatively correlated: selecting a variety for very large grains, for example, may bring fewer of them. Breeders therefore look at the components together.

Worked examples

Worked example

Estimating wheat yield from components

At harvest, an agronomist counts 450 ears per m2, with an average of 40 grains per ear, and a thousand-grain weight of 40 g (0.040 g per grain).

  • Grain yield = 450 × 40 × 0.040 = 720 g/m2.
  • Converting: 720 g/m2 × 10,000 m2/ha ÷ 1,000,000 g/t = 7.2 t/ha (at the moisture of the weighed grain).

If heat at flowering had reduced grains per ear to 32, with no compensation: 450 × 32 × 0.040 = 576 g/m2, a loss of 20 per cent. If grain weight then rose to 44 mg because fewer grains shared the assimilates: 450 × 32 × 0.044 = 634 g/m2, recovering part of the loss.

Worked example

Rice yield with filled-grain percentage

A rice crop has 300 panicles per m2, 120 spikelets per panicle, 85 per cent filled spikelets and a 1,000-grain weight of 25 g.

  • Filled grains per m2 = 300 × 120 × 0.85 = 30,600.
  • Grain yield = 30,600 × 0.025 g = 765 g/m2, or about 7.65 t/ha.

Cold or heat at flowering in rice can cause spikelet sterility; if filling fell to 60 per cent, yield would drop to about 5.4 t/ha, which is why rice seasons are timed so flowering avoids temperature extremes.

Harvest index

Only part of what a crop grows is harvested. Harvest index (HI) measures that part:

Harvest index
HI = economic (grain) yield ⁄ total above-ground biological yield

It is usually given as a fraction or a percentage, on a dry-weight basis. Typical values for modern varieties are about 0.40 to 0.55 for wheat, rice and maize, often lower for traditional tall varieties and for pulses and oilseeds.

Worked example

Harvest index from a field sample

A wheat sample has 7.2 t/ha of grain and 9.0 t/ha of straw and chaff (same moisture basis).

  • Biological yield = 7.2 + 9.0 = 16.2 t/ha.
  • HI = 7.2 ÷ 16.2 = 0.44, or 44 per cent.

A tall traditional variety producing the same 16.2 t/ha of biomass with an HI of 0.30 would give only 4.9 t/ha of grain.

Much of the yield gain of the Green Revolution came from raising harvest index: semi-dwarf wheats and rices put less into straw and more into grain, and resisted lodging under high nitrogen. Since then, gains have come increasingly from higher total biomass, because harvest index in cereals cannot rise much further (a theoretical ceiling of about 0.60 to 0.65 is often cited).

Using yield components in practice

  • Diagnosing yield loss: comparing components of a poor field with a good one shows which stage failed, for example low ear number (poor establishment or tillering) versus low grain weight (late drought or disease).
  • Crop management: seed rate, nitrogen timing and irrigation scheduling target specific components; for example, nitrogen at tillering supports ear number, and water at flowering protects grain number.
  • Breeding: selecting for grain number, harvest index and stay-green (longer grain filling).
  • Yield forecasting: counting ears and grains before harvest gives an early estimate.

Key points

  • Cereal yield = plants per m2 × ears per plant × grains per ear × grain weight; ears per m2 combines the first two.
  • Components are set in sequence: establishment, tillering, floret formation and pollination, grain filling.
  • Grain number per m2, set around flowering, is usually most closely linked to yield.
  • Later components can partly compensate for earlier losses; components are often negatively correlated.
  • Harvest index = grain yield ÷ total above-ground biomass; about 0.40–0.55 in modern cereals.
  • Semi-dwarf varieties raised harvest index in the Green Revolution.

What the exams ask

  • School-leaving and entrance papers: naming the yield components of a crop and defining harvest index.
  • Degree and research-fellowship entrances: calculating yield from components, filled-grain percentage in rice, compensation, critical periods, harvest index and biological yield.
  • Professional and breeding examinations: diagnosing yield loss from component data, physiological traits for breeding, and correlations among components.

Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.

Questions people ask

Which yield component matters most?

In most cereals, grain number per square metre explains more of the variation in yield than grain weight, because it varies much more. But all components multiply, so a large loss in any one reduces yield.

Is a higher harvest index always better?

Not on its own. A crop with a high harvest index but little total biomass still yields poorly. Yield = biomass × harvest index, and both must be high.

How do I measure yield components in the field?

Sample a known area, such as a 1 m2 quadrat or several metres of row, count plants and ears, thresh a subsample of ears to count grains, and weigh a counted number of grains, correcting to a standard moisture.

Sources

  1. Slafer, G. A., Savin, R. and Sadras, V. O. "Coarse and fine regulation of wheat yield components in response to genotype and environment." Field Crops Research 157, 71–83 (2014).
  2. Fischer, R. A. "Number of kernels in wheat crops and the influence of solar radiation and temperature." Journal of Agricultural Science 105, 447–461 (1985).
  3. Yoshida, S. Fundamentals of Rice Crop Science. International Rice Research Institute, 1981.
  4. Hay, R. K. M. "Harvest index: a review of its use in plant breeding and crop physiology." Annals of Applied Biology 126, 197–216 (1995).
  5. Gardner, F. P., Pearce, R. B. and Mitchell, R. L. Physiology of Crop Plants. Iowa State University Press, 1985.

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