Accurate and diverse genetic indexes
Sheep breed indexes

Summary of the article

French genetic evaluations on sheep breed stud animals offer a core set of 18 indexes on a wide range of sharply-defined criteria: 7 core–composite indexes put together based on results of individual on-farm and on-station performance testing; 10 core–composite indexes put together based on results of progeny testing.

For dairy sheep breeds, a further 9 production, morphology and composite net merit indexes are calculated.

This enables a breeder to make the best-informed choice based on the combination of traits that best fits their performance targets and priorities.

The Institut de l’Elevage is the official authorized publisher of the French genetic merit indexes for all assessed stud animals in cattle, sheep and goat breeds.

 Meat sheep breeds

Improving carcass year traits is one of the top-priority selection objectives on specialized sheep-for-meat breeds. The aim is to improve farmed lamb weight gain (which is tied to dam-ewe milk yield value) and carcass quality (fat depth and muscling).

For hardy breeds, which are often exploited as terminal crosses with meat breeds, the selection objectives tend to hinge on improving maternal quality merit traits.

The various different production traits are measured through on-farm performance testing (on maternal value and weight gain traits), individual on-station testing (weight gain, fattening, muscling), plus progeny testing (carcass merit traits and maternal value traits) for certain breeds.

Building on the measurements taken during on-farm performance data recording, the indexes calculated focus on maternal value and weight gain type traits:

  • Reproduction index, according to average litter size,
  • Milk value index, an indirect measurement based on the 30-day weight gain of the dam’s lambs (coded ’PAT30’ for typical weight at age 30 days),
  • Weight gain index, according to ’GMQ 30-70’ (for mean daily weight gain [MDWG], in grams, from day 30 to day 70), which can be used to estimate how early the lambs start to show growth

Building on the measurements taken at individual on-station testing on young rams, indexes are computed for 4 core meat value-related type traits:

  • Weight gain index, according to MDWG at 8 weeks, which indicates lamb weight gain and typical weight at age 8 weeks at the end of the on-station recordings.
  • Back fat index: thickness of the back fat cover, which is used to evaluate fattening status at a standard weight.
  • Conformation index: eye muscle depth at standard weight, combined with shoulder, rump and leg scores, gives an estimate of muscling and muscle growth.
  • Composite (net merit) index, combining these three core indexes, each of which is given a breed-specific weighting according to the selection objectives.
  • These indexes are computed based on a simplified BLUP for animal models that only integrates parentage between half-sibs and fathers within the same flock.

Building on the measurements taken on at least 30 carcasses post-progeny testing, the indexes calculated focus on the following type traits:

  • weight gain (MDWG 0-30 days, MDWG 0-70 days, MDWG 30-70 days and MDWG birth-to-slaughter)
  • carcass quality (weight, yield (or kill-out percentage), conformation, bone fraction, eye muscle surface, fat depth)

A composite (net merit) index (’ISS’) can then be computed from the results for MDWG birth-to-slaughter, conformation, thickness of the back fat cover, and – on option – carcass yield .

 Dairy sheep breeds

There are four indexes on the Lacaune, Pyrenean (Basco-Béarnais and the two Manechs) and Corsican breeds for typing milk value traits:

  • ’ILAIT’: quantity of milk produced (in litres).
  • ’ITB’: milk fat content (in g/L) ― Lacaune and Pyrenean breeds only
  • ’ITP’: milk protein content (in g/L) ― Lacaune and Pyrenean breeds only
  • ’IPROD’: composite index combining these core indexes under a weighting scheme geared specifically to each breed.

Furthermore, the Lacaune breed has its own additional set of core and composite indexes:

  • angle (’ANG’), udder arch (’SIL’), udder-to-hock distance (’PLJ’): core udder morphology indexes
  • ’IMAM’: composite udder morphology index
  • ’ICELL’: index that gives an indirect measure of mastitis resistance, and which is computed based on somatic cell count analysis in the ewe’s milk.
  • ’ISOL’: a composite net value index for dairy sheep, combining the production, udder conformation and mastitis resistance indexes.

Key figures

  • 3 elementary indexes after on-farm performance recording
  • 3 elementary indexes after on-station individual testing
  • 1 composite index after on-station individual testing
  • 10 elementary indexes after progeny testing
  • 1 composite index after progreny testing
  • 3 elementary yield indexes
  • 1 composite yield index
  • 3 elementary udder traits indexes
  • 1 composite udder traits index
  • 1 global composite index