Resource centerGenetics & selection4 min read
The Growth Index
One number that ranks animals by how fast their lambs will reach market weight.
The question
If you sell lambs by the kilo, which ram will make the next crop heavier at 100 days without making birth harder?
The concept
The Growth Index is a selection index: several EBVs combined into one unitless figure. Each trait's EPD (its EBV divided by two) is multiplied by a published factor and the products are added. Because traits in kilos and traits on a liability scale are mixed, the result has no unit: 0 is an animal with every EBV at the farm's genetic base, positive is above, and higher is better.
What it weighs
The declared emphasis of Growth, as shown in Sires › Indexes, is: gain 50–100 days (direct) 45%, 50-day weight (direct) 35%, birth weight (direct) 10% and lamb survival (direct) 10%. The heritabilities used in the last evaluation are shown beside each trait (0.35, 0.20, 0.20 and 0.40 on a liability scale). These percentages are the declared emphasis, how much of the selection falls on each trait, and not the exact weight the value is composed with.
- 1Growth Index (Growth) and its purpose
- 2Traits that compose the index
- 3Heritability used in the last evaluation
- 4Declared emphasis
- 5Share of selection pressure (%)
- 6'How to read these numbers'
- 7Flock Index (Flock): maternal ability, survival, reproduction
Why it matters
Growth from 50 to 100 days is the period when lambs put on kilos fastest and when feed is most expensive. Putting 45% of the emphasis there targets the kilos you sell, while the 10% on birth weight keeps the index from chasing ever-bigger lambs at birth.
Where you see it
The Dashboard ranks growing ram lambs and ewe lambs by Growth; the Animals table has a Growth column with a "FLOCK" badge (BreedIQ's own scale); the Sires page lets you compare proven sires by Growth; and every profile shows "Growth 2.37 · P100", the value and its percentile in this evaluation.
Decisions it supports
Pick terminal sires and market lambs. If a ram's the Growth Index is 0.40 and another's is -0.61, the first transmits more growth to the same ewes. For replacement ewes, the Growth Index is only half the story: read it together with the Flock Index.
Limitations
A high Growth with a low accuracy is a promise, not a fact. Indices computed on incomplete traits are flagged "partial". And the Growth Index is relative to this farm's base: it does not compare directly with a national index.