Southdown versus Suffolk sire breed effects on numbers and traits of crossbred lambs

Abstract Oestrus was synchronised in 36 groups of 25 ewes in each of 2 years. At and for 3 weeks following the second synchronised oestrus, an intact Suffolk and an intact Southdown ram were run with each group of ewes. For Year Ilambings, Suffolk and South down rams sired approximately equal numbers of lambs, but in Year 2, Suffolk rams sired nearly two-thirds of the lambs. Of 597 sets of twins that could have been of mixed-breed paternity, only 200% had both a Suffolk-sired and Southdown-sired lamb. There was no evidence that being born or reared with a co-twin of different breed paternity affected a lamb's pre-natal or pre-weaning growth.