Thirty years ago New Zealand sheep breeders were desperately trying to get more lambs out of their mainly Romney flocks, as the national lambing (docking) percentage stuck on 90%. The problem was low litter size measured by the number of lambs born for each ewe lambing (LB/EL). Farmers were well pleased if every ewe had a single lamb and reared it to docking (4-6 weeks old).
Then research and development successfully increased lambing percentage and now quite a few commercial sheep farmers consider they have enough fertility. This has resulted in increasing numbers of triplets, and even quads from today's modern breeds, which farmers are finding cause them management problems.
Crossbreeding was used to improve Romney fertility by crossing with the Border Leicester to breed Coopworths, and with the Cheviot to breed Perendales for harder country. This was successful and with further selection the national lambing percentage got up to over 100%.
Improving litter size or more lambs born was seen as the way to attack the problem of low fertility, and massive effort was put into breeding rams from ewes that always had twins. It was a long slow process achieving improved litter size at around 1% a year.
But farmers demanded even faster improvement and the Finnish Landrace breed was imported in the late 1970s. In it's native Finland, this breed has litters of up to 7 lambs, but they have to be hand reared indoors. It was argued that only a 'dash' of Finn would be enough to solve New Zealand's problems and it was incorporated into other breeds such as Romney, Coopworth, Texel (imported for meat) and East Friesian (imported for milking ability).
All these new mixtures are called 'composites' and some have commercial company names for marketing and promotion. The move worked well as the national lambing percentage (Lambs docked/100 ewes put to the ram) is now around 120%.
Now many farmers who have been using straight Finn rams to put over Romney ewes, or have been using high-fertility composites are having too many triples, some as many as 30% of all births and others as high as 40%.
Multiples are inevitably smaller than singles and end up being sold as stores on a depressed market after weaning, or being kept for extra time on the farm to fatten with all the extra costs of feeding, shearing, dipping, drenching and dagging.
Flushing ewes by better feeding before the ram goes out is an ancient shepherd's trick to get a ewe to shed more eggs (ovulations), but the reverse does not seem to work. Reducing feed levels at mating with high-fertility composite sheep will only reduce lambs born/ewe lambing (LB/EL) by 1-2%.
Up to 1.6-1.7 LB/EL the number of singles drop and twins increase which is very manageable. Twins simply replace the singles. But around 1.7 LB/EL, the number of twins levels off and triplets increase rapidly. This now starts to get unmanageable, as farmers have to find extra feed for them and end up with more lambs as stores instead of going prime.
Around 1.8 LB/EL, the number of twins declines and triplets show a rapid increase but at 2.2 LB/EL the number of twins declines, triplets level off around 30-40% and quads increase. This now is not manageable in hill country and high lamb mortality is inevitable, with large numbers of small lambs weaned for the store market.
The only solution is for sheep farmers to rapidly dilute the Finn genes in the flock and certainly never use more than one-quarter Finn genes in any composites. Research is working on finding ewes with genes that restrict their litter size to twins only, but this will be a long-term solution.