The effect of breed and breed-by-flock interaction on summer mortality of free-ranging lambs in Norwegian sheep

We studied breed and breed by environment interaction effects on lamb mortality during the summer grazing period. One hundred forty-six Norwegian sheep farms that stocked the two most common breeds - Norwegian White Sheep (NWS) and Spælsau together and grazed both in the same free-range grazing areas were used. Average summer mortality of lambs on the study farms was 8.7% for NWS and 6.3% for Spælsau. For 110 of the 146 sheep flocks Spælsau had the lower lamb mortality during summer. The higher mortality observed amongst NWS lambs may suggest a higher environmental sensitivity for this breed, which coincides well with results previously obtained for lamb autumn weights of the same two breeds. Analysing genotype by environment interactions with a probit threshold model revealed that the breeds differed significantly in lamb mortality, and that they ranked their flock environments in a slightly, albeit significantly, different way, i.e., breed by environment interactions did affect lamb mortality. The consequences of the threshold model's assumptions and the constraints on interpretation of results are discussed.