Abstract :
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
