Parasitism and production in fleece‐weight‐selected and control sheep

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Abstract
In two successive years young sheep (less than 6 months old; n = 80, Year 1; n = 117, Year 2) from a line selected for increased fleece weight for 37 years and an unselected line at Massey University were either treated with an albendazole controlled release capsule (CRC), or were allowed to become subclinically infected while facing natural parasite challenge. Subclinically infected fleece‐weight‐selected (FW) sheep developed higher faecal egg counts (FEC) than control (C) sheep, but there was little evidence that FW sheep suffered greater production depression than C sheep. Blood gastrin levels tended to be less elevated in infected FW sheep than in C sheep suggesting that FW sheep may have suffered less gastric dysfunction than C sheep. There was no effect of line or infection on antiparasite antibody levels. This study demonstrates that long‐term selection for high wool production has resulted in increased FEC, suggesting that wool production and FEC may be unfavourably genetically correlated. Although CRC treatment was effective in preventing establishment of an adult worm burden, the production response was variable. During some periods production in CRC‐treated sheep (particularly rams) was lower than in subclinically infected sheep.