- Know the breed standard. Sheep that have severe faults (off-color, overshot jaws, etc) should not be retained for breeding. Strengths and benefits of sheep with lesser faults (e.g., legs that are not quite straight) should be carefully evaluated prior to keeping them in the breeding flock. If it is possible to cull them while meeting the other goals of the breeding program, it should be considered.
- Be sure your management reflects your production goals. If your flock prolificacy (number of lambs born per ewe exposed to a ram) is low, look first to your management. Prolificacy has a relatively low heritability, so that it is hard to increase the number of lambs born per ewe exposed through selection. However, better feed during breeding (i.e., flushing) and during late pregnancy, as well as close attention during lambing, can greatly improve the number of lambs born, and the survival of those lambs. (Regular vaccination and worming are also involved in successful herd management.)
- Choose rams from good ewes. An old saying states that “the ram is half the flock”, since his genes will provide one-half of the genetic makeup of his lambs. You are most likely to keep his ewe lambs, so pay attention to what sort of ewes he is likely to produce. Strong, productive ewes with a strong history of twinning (or triplets), consistent lamb production, and good conformation will produce the best ram lambs. The ram itself may be a single, twin, or triplet – many sheep production manuals suggest keeping rams only from twin or triplet births, but this is because many commercial breeders to do not keep records for each ewe. It is the ewe’s production record that matters, not how many lambs she had in this year — and for a purebred flock, this production information is readily available.
- Strive to keep inbreeding below 5% per generation. As the coefficient of inbreeding (COI, the degree of relatedness) increases in a flock, inbreeding depression may occur, during which fertility, growth, and other reproductive traits tend to decline. Such declines are especially dramatic when the COI reaches or exceeds 30%. Various computer programs are available to calculate COIs from pedigree records (see list of some available programs). The article What is conservation breeding? has more information about the use of COI’s.
- Regularly evaluate your stock in as impartial a manner as possible (so that you don’t give extra points to your favorites). For instance, the Swans use a scoring sheet of 6 traits (length, topline, thickness, muscle, bone, feet/legs) rated from poor (1) to perfect (5) for each animal. Consider culling sheep with lower than average scores. Another possible selection criteria is productivity. For instance, one can calculate pounds of lamb produced per ewe (there are correction factors available in the Sheep Production Handbook to correct for number of lambs born per ewe, ewe age, and lamb sex; or groups of breeders can elect to join the National Sheep Improvement Program, NSIP, in which case NSIP will calculate scores for several possible selection criteria for each animal in the flock). Producers should consider culling the ewes that are the poorest producers in the flock.
- Keep ewe lambs from good ewes. While the genetic impact of an individual ewe is likely to be less than that of an individual ram, keeping daughters of good ewes will tend to increase the positive maternal qualities in the flock.
What is conservation breeding? by Karen Gerhart. Strategies for keeping rare breeds viable.
Inbreeding: Its meaning, uses and effects on Farm Animals. by Vogt, Swartz and Massey. A technical disussion of inbreeding.