"We only retain Twins in our flock and only use rams that are twins"
If you have already read the page entitled "Breeding Philosophy", you will already know my view on this. It is my clear understanding that the genetic fecundity heritability of single and twin sheep is essentially no different.
What is important is the overall fertility potential of the animal at question. This can be determined by the following formula (2* (Dams lifetime of lambs/no of lambings) + (Sire's dams lifetime of lambs/no of lambings) +(Dam's dams lifetime of lambs/no of lambings)).
Some consider this formula rather crude and that the SIL NLBV is more accurate as it considers related siblings etc to come up with a breeding value. (Please note my comments regarding SIL later on).
For example a single ram may have a fertility potential in an excess of 200% because the dam and grand dams have had a stack of lambs, while a twin ram could conceivably have a fertility potential of 120%, ie the dam and grand dams of this ram have only singles.
Most simplistic way is if a ewe had 5 singles in a row and then set of twin ram lambs in 6th years and another ewe had 5 sets of twins or triplets and then a single lamb in 6th year; f you had to choose between these ram lambs in the 6th year and wanted the one with the most fertility, its no brainer its the single ram lamb of course.
So anyone who has such a policy like this, is just making breeding sheep more difficult than already is. I cull all ewes that have two singles in a row, I think this is more beneficial, and obviously with SIL any poor nlbbvs at mating time are culled, sometimes this is a ewe that has had two sets of twins in a row.