Rumen function and digestion of Merino sheep divergently selected for genetic difference in resistance to 'Haemonchus contortus'

The aim of this experiment was to investigate the effect of 'Haemonchus contortus' infection on rumen function and digestion of Merino sheep from lines divergently selected for genetic resistance to 'H. contortus'. Rumen function and whole-tract digestibility were determined in 29 Merino weaner wethers from the CSIRO 'Haemonchus' selection flock, increased resistance to 'Haemonchus' (IRH), decreased resistance to 'Haemonchus' (DRH) and random bred control (C) selection lines. Wethers were fed a restricted low quality roughage diet (5.9 MJ ME/kg DM, 29 g MP/kg DM) to maintain weight and were either uninfected (NIL) or infected with 'H. contortus' (INF). Measurements were taken at week 7 of infection, while animals were housed in metabolic crates. Animals were then euthanased to determine abomasal worm counts. IRH had significantly lower worm egg count than DRH and C lines however, adult worm and larval counts, though lower in IRH animals, did not differ significantly. DM and OM digestibility and in sacco degradability of IRH and DRH selection lines were greater than C animals. Rumen function of animals from the IRH line was altered in response to 'H. contortus' infection with an increase in fluid outflow and turnover rate and a decrease in propionic acid concentration. These changes may be a component of a greater host resistance and begin to explain the failure of IRH animals to translate lower worm egg count into greater bodyweight gain.