Records from two flocks were used to look for sires whose daughters more consistently produced twins.
Fourteen thousand lambing records
A study published on 16 July 2026 analysed 8,061 Polypay litters recorded from 1999 to 2025 and 5,948 Rambouillet litters from 2005 to 2025 at the US Sheep Experiment Station. Its models considered both average litter size and variation in twinning, seeking breeding values linked to twin births rather than more triplets.
Sire breeding-value results
Among sires with daughters represented in the records, 35.8% of Polypay rams and 51.5% of Rambouillet rams had the favourable combination of a positive estimated breeding value for twinning and a negative value for variation in twinning. The authors based these figures on the two recorded flocks and proposed considering the trait in wider genetic evaluations.