During a 3-year period, from April 1991 to December 1993, in a flock with four lambing periods annually, faecal samples from lambs at weaning at the age of 3 months (no. = 1756), were examined for gastro-intestinal nematode eggs by the McMaster method. Faecal samples were also examined at the age of 9 (no. = 170), 12 (no. = 157), 15 (no. = 253) and 18 (no. = 85) months from the females only. From June to December 1993 packed cell volume (PCV) was also examined from lambs at weaning. Seven genotypes were used in this study. The faecal egg counts were affected by genotype, birth season, type of birth and rearing, sex and age. First generation cross of Sumatra X Barbados Blackbelly had lowest eggs per g of faeces (EPG) followed by Sumatra, St Croix, Sumatra X St Croix Fl and F2 and Sumatra X Java Fat tail. Lambs born in February-March when rainfall was low had lower EPG (V < 0-05) than those born in May-June or August-September. The EPG of lambs born and reared singly were significantly lower (P < 0-001) than those born twin, triplet or quadruplet. Female lambs had lower EPG than male lambs (P < 0-001). The EPG of lambs at 3 months were higher than at 9,12,15 or 18 months (P < 0-001). PCV was affected by genotype, sex, type of birth and rearing and season of birth. There was a positive correlation between PCV and weaning weight of lambs and a negative correlation between EPG and weaning weight. These results show that the crosses of St Croix and Barbados Blackbelly with local Sumatra sheep are at least as resistant or more resistant to gastro-intestinal nematodes than pure Sumatra breed and therefore these breeds can be used in cross breeding programmes to improve the body size of local Sumatra sheep.

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