Repeatability of lambing rate.

Repeatability of number of lambs born using the life-time records on 263 crossbred ewes produced in 1971 and 1972 were estimated by the intraclass correlation method and by the regression of subsequent lamb production on number of lambs born alive at the first and second lambings. The ewes were of Finnsheep (F), Dorset (D) and Rambouillet (R) breeding and first lambed at 1 yr of age. Five breed combinations were represented 1/2D 1/2R 1/4R 3/4R, 1/4F 1/2D 1/4R, 1/4F 1/4D 1/2R and 1/4F 3/4R. Ewes were bred to yearling Hampshire, Suffolk or reciprocal cross rams at varied intervals in the course of study. The repeatability of number of lambs born estimated by intraclass correlation was .138 +/- .067. Repeatability of number of lambs born estimated by the regression of subsequent lamb production on initial lambing rate was .121 +/- .025 lambs/ewe season and that estimated by the regression of subsequent lamb production on second lambing rate was .046 +/- .020 lambs/ewe season. Ewes producing twins at their first lambing subsequently produced an average of .11 more lambs/lambing than ewes producing singles and ewes producing singles subsequently produced an average of .16 more lambs/lambing than ewes than produced no lambs. Differences between these lambing classes at the second lambing were small (.05 and .06). The first lambing at 1 yr of age was a much better predictor of the ewes' lifetime lambing rate.