A total of 31)0 Merino ewe lambs were infected with a standardised dose of infective larvae of the bloodsucking abomasal nematode Haemonchus contortus. These lambs were from a larger experiment designed to examine the effects on production of crossbreeding among Merino strains and bloodlines. Around 30? of the lambs were purebred with parents of the same bloodline, while the remainder were crossbred with parents of different bloodlines and/or strains. Liveweight gain (LWG) and blood packed cell volume (PCV) were measured before and after the 5 week infection period and worm burden was estimated from faecal egg output. There was no significant effect of crossbreeding on PCV decline or worm burden (resistence), or on LWG at any time before or after infection. During infection, resilience (LWG adjusted for initial bodyweight, worm burden and PCV decline) was 46? greater in crossbreds than in purebreds. It is concluded that this type of crossbreeding has caused increased resilience to infection.

G. D Gray, H. W Raadsma

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume XI. Genetics of reproduction, lactation, growth, adaptation, disease, and parasite resistance., , 691–696, 1986
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