Breed effects of foster lamb and foster dam on lamb viability and growth.

The objective of this study was to compare maternal ability under natural and fostered rearing conditions in four breeds of sheep for lamb survival and growth to weaning age. Data included records from 1981 through 1991 of 24,696 lambs of four breeds, Rambouillet, Columbia, Targhee, and Polypay, reared by their natural dam, and 720 records of lambs fostered within their breed or across breeds. The majority (94%) of the foster lambs were from multiple-birth litters and were approximately .5 kg lighter than lambs reared by their natural dams (P < .05). Viability up to 3 wk of age was influenced by breed of foster dam; lambs reared by Polypay dams had the highest viability (96%) and lambs reared by Targhee dams the lowest (81%). Targhee dams were less successful in rearing foster lambs (75%) to weaning than the other breeds studied (average 84%). The lower viability of Targhee-reared foster lambs could not be explained by lower birth weights of foster lambs, increased starvation, or lower milk scores for Targhee dams. Although there were breed differences in weaning weights at 120 d of age, the foster lambs were lighter but the ranking by breed did not change. These results suggest that there are small breed differences in maternal ability for a ewe to raise her own lamb or foster lamb(s), except for the Targhee ewe, which has a lower success in rearing a foster lamb. Management practices to reduce the mortality rate of foster lambs should be investigated.(ABSTRACT TRUNCATED AT 250 WORDS)