Lamb mortality of domestic sheep (Ovis aries) in Suldal, Norway, 1998.
Domestic lamb mortality on open summer range in the municipality of Suldal, Rogaland county, Norway was investigated in 1998. Two hundred and seventy-one lambs in five of seven herds (consisting of a total of 363 lambs) were fitted with silent mortality transmitters and monitored throughout the grazing season from May to September. Total losses in the seven herds were 26 (7.2%) lambs. The carcasses of 16 lambs were recovered, all of which were equipped with radio collars. Of the recovered lambs, four (25%) died as a result of disease, four (25%) died in accidents, three (19%) were killed by wild predators, and four (25%) were killed by domestic dogs. The most commonly revealed disease among the lambs was pasteurellosis, while accidents included fall from cliffs and into crevices. Implicated wild predators included golden eagle (Aquila chrysaetos) and red fox (Vulpes vulpes). The remains of one lamb were too decomposed for necropsy. Carcasses were recovered throughout the entire grazing season, from the end of May to mid September. However, most were found in the last half of the summer. Most carcasses were located in the northwestern part of the range. The percentage of lamb mortality varied from 0.0% to 36.4% between the herds in the grazing unit. Factors associated with losses were identified by use of logistic regression as a statistical analyzing tool. The results of these analyses showed that mortality was higher among lambs with low birth weight, and of the spel breed than the dala breed. In addition, mortality was high for lambs very young and very old at release. An optimal intermediate age at release was estimated to be 48 days. Male lamb mortality was independent of ewe age, whereas female lamb mortality decreased with increasing ewe age. The negative association between lamb mortality and birth weight may be related to better resistance to infections, or improved abilities to avoid accidents for heavier lambs in good condition. The high mortality of the spel breed might be due to lower birth weight. The reason why age at release is optimal at an intermediate level may be due to that very young lambs are too immature when released, while older lambs may be too independent on their dam. The reason why only female lamb mortality is dependent on ewe age may be due to behavioural differences between the sexes. Male lambs obtain earlier independency, and are, regardless of the age of their dam, grazing at a larger distance from them. Female lambs, on the other hand, tend to remain at a shorter distance to their dam, and may therefore be more dependent on the maternal care provided. The quality of such care seems to differ with ewe age, as older ewes provide better care for their offspring.
Candidates Scientiarum (equivalent to a MSc) thesis. 2002. University of Oslo. Department of Biology. Division of Zoology.