The wide genetic variability between and within sheep breeds for litter size allows genetic improvement (i.e. litter size shows an important genetic component ranging from 0.05 to 0.11 according to breeds or populations).
Genetic improvement of a population/breed requires a selection program i.e a collective organization of farmers for:
- performance recording;
- pedigree recording;
- exchange of breeding animals by natural mating and/or artificial insemination.
The portion of farmers directly involved in pedigree and performance recording (breeders) should be as large as possible representing at least 10-20% of the whole population submitted to selection.
The rest of farmers (commercial farmers) exploit the genetic improvement generated by breeders by using breeding animals (mainly males) coming from breeders.
Figure 1 – Pyramidal management of the population (Source: Barillet F., 2001)
Cost/benefits analysis of the selection scheme should be carefully performed in order to assure the profitability of this organisation i.e long term incomes must be higher than costs at population level.
Litter size is the number of lambs born (alive and dead) when pregnancy reaches full term.
Hormonal treatments affect the rate and quality of ovulation and consequently litter size. So in the definition of the breeding objective, prolificacy based on induced estrus and the prolificacy based on natural estrus should be considered as two different objectives.
Improving litter size may lead to triplets and/or extreme litters that are encountered in prolific ewes (Romanov; Finnish) with five or even more lambs per lambing.
Even triplets are often unacceptable for ewe and lamb viability.
Breeders would like to have litter sizes of two exactly – and not on average – or as often as possible. In many situations, twins are the most profitable but the increase of twins rate leads to an increase of lambings with triplets or more lambs.
Selection objective should be more sophisticated than litter size including a penalization for extreme litters.
In any case, simultaneous selection for maternal ability as well as strategies to manage triplets should be applied.
SheepNet solutions for selection
Selection criterion i.e: a trait highly correlated with litter size which is easier and cheaper to record
Litter size is easy to measure but laborious to handle since lambings are often concentrated so mis-mothering and/or recording errors may often occur.
Alternative selection criteria
- The number of lambs weaned per lambing ewe is easy to measure but its correlation with litter size is low since it is affected by the maternal ability and other management or environmental factors. In any case it should be recorded to monitor the sustainability of the genetic improvement realised for litter size. Large deviations between litter size and number of weaned lambs may suggest that litter size improvements are not sustainable by the farm management.
The number of ova per joined ewe and/or the number of foetuses per joined ewe. Pregnancy scanning at around 30d of pregnancy may be useful for both management and genetic purposes.
- to assess the number of ova and the prolificacy ability of the ewe;
- to predict litter size and define groups of ewes for management.
The recording of pregnancy scanning results may be facilitated by the use of EID and application tools on PC or Smartphone.
SheepNet solutions for pregnancy scanning
Pedigree recording can be traditionally performed by single sire natural mating and/or artificial insemination. Both practices are laborious and costly and may discourage farmers to participate to the selection programs.
EID is a promising tool to allow an early identification of mother/lamb couples and pedigree recording. It is important to follow the evolution of costs for a parental assignment based on DNA analysis. In a next future it may allow pedigree recording with multiple sire mating.
Other factors affecting the breeding objective
Flock management, climatic conditions, time of mating and individual features such as age, health, nutritional condition are the main factors affecting litter size.
Data to collect
- Type of mating (hormonal treatment, artificial insemination, single sire natural mating, natural mating)
- Date of mating (date of artificial insemination, date of rams introduction, date of potential mating)
- List of mating ewes
- Date of lambing
- Abortion events
- Results of reproduction (pregnant or not, lambed or not lambed)
SheepNet solutions for abortion events recording
Further data related to factors affecting reproductive results could be useful to increase the accuracy of the estimated breeding values:
- Farm management (feeding, flushing, groups management, shearing, milk urea)
- Body condition of mating ewes (BCS and weight)
- Rams reproductive ability
- Health status of flock during mating and gestation (vaccinations, parasite or antibiotic treatments)
- Environmental stress
SheepNet solutions for BCS
All recorded data must be used to produce estimated breeding values of reproducers.
It is crucial for the breeders organization to plan the availability of scientists able to manage large datasets and modern methodologies (BLUP) for the genetic evaluation.
Single genes affecting large part of the genetic variability of litter size in specific breeds/populations (major genes). There are several detected mutations in genes that result in increased ovulation rates. It is crucial to verify the existence of these mutations in each breed/population and check the cost/benefits of a selection assisted by the genotype determination of reproducers at these mutations. It should be carefully modelled taking into account the evolution of DNA analysis costs.
A International Committee for Animal Recording () to draw up guidelines on prolificacy recording is ongoing. of Sheep, Goat, Small Camelids Working Group of the