Harvest Index

Harvest index is the fraction of a crop's above-ground biomass that ends up as harvested grain: 6 t/ha of grain in 13 t/ha of total dry matter is $6/13 = 46\%$. It is a plain ratio, and it is also the single number that describes what plant breeding did to cereals in the twentieth century.

The Green Revolution is very largely a harvest index story. The semi-dwarf wheat and rice varieties that Norman Borlaug and his colleagues released did not photosynthesise more than the tall landraces they replaced — they grew less straw and put the same total production into grain instead, taking harvest index from roughly 0.30 to 0.45–0.50. Shorter straw had a second benefit that mattered as much: a dwarf plant could take a heavy nitrogen rate without lodging, so the fertiliser response that had previously flattened a crop now filled it.

There is a ceiling, and it is not far above where modern varieties already sit. A plant still needs the stem and leaves that made the grain, and the practical limit is generally reckoned at around 60%. Values reported above about 55% almost always mean the biomass was cut at combine height and the stubble left out of the measurement — the ratio is only meaningful when both terms cover the same plant, on the same dry matter basis, with roots excluded from both. That last convention is universal but rarely stated, and it is why harvest index is an above-ground figure by definition.

Read the other way, the ratio is a practical straw estimate. At a harvest index of 0.45, straw is $(1-0.45)/0.45 = 1.22$ times the grain yield, so an 8 t/ha wheat crop leaves close to 10 t/ha of straw — a real number for anyone budgeting bedding, or deciding whether to bale it. That decision is a nutrient decision as much as a cash one: straw carries most of the potassium the crop took up, so baling it can triple potassium removal, while the grain half of the plant carries most of the nitrogen and phosphorus.