Is There a Guaranteed Inheritance of Lit Asize Within Sheep Populations?

Lowland sheep excret something called swint, which is part lan lin an and a few other things. But that attracts blow flies then lay their eggs on the fleece. That's why i want hare sheep and to mitigate the shetlands poor litesise. I could quite y breed from more prolific females, but the inheritance of lit asize is quite low. So in my pure shetlands, we'll scan approximately a hundred and 20 to a hundred and forty %. And it would take upwards of ten geneation to increase that too a undred and 80%. Which is thought to be the most optimum for an outdor lambing system.