BREED EFFECTS OF FOSTER LAMB AND FOSTER DAM ON LAMB VIABILITY AND GROWTH

The objective of this study was to compare maternal ability under natu ral and fostered rearing conditions in four breeds of sheep for lamb s urvival and growth to weaning age. Data included records from 1981 thr ough 1991 of 24,696 lambs of four breeds, Rambouillet, Columbia, Targh ee, and Polypay, reared by their natural dam, and 720 records of lambs fostered within their breed or across breeds. The majority (94%) crf the foster lambs were from multiple-birth litters and were approximate ly .5 kg lighter than lambs reared by their natural dams (P < .05). Vi ability 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 rear ed by Targhee darns the lowest (81%). Targhee dams were less successfu l in rearing foster lambs (75%) to weaning than the other breeds studi ed (average 84%). The lower viability of Targhee-reared foster lambs c ould not be explained by lower birth weights of foster lambs, increase d starvation, or lower milk scores for Targhee dams. Although there we re breed differences in weaning weights at 120 d of age, the foster la mbs were lighter but the ranking by breed did not change. These result s suggest that there are small breed differences in maternal ability f or a ewe to raise her own lamb or foster lamb(s), except for the Targh ee ewe, which has a lower success in rearing a foster lamb. Management practices to reduce the mortality rate of foster lambs should be inve stigated. Producers may consider the ewe's breed when selecting a fost er dam and(or) raise ewe/lamb foster pairs in a separate group so that additional care and attention can be given.