Voluntary feed intake and diet selection of Merino sheep divergently selected for genetic difference in resistance to 'Haemonchus contortus'

This research was designed to determine if divergent selection for resistance to 'Haemonchus contortus' had produced correlated changes in voluntary feed intake and diet selection. Voluntary feed intake, diet selection and production were determined in 54 Merino weaner rams from the CSIRO 'Haemonchus' selection flock, increased resistance to 'Haemonchus' (IRH), decreased resistance to 'Haemonchus' (DRH) and random bred control (C) selection lines. Weaner rams were fed ad libitum either a high (9.2 MJ ME/kg DM, 90 g MP/kg DM) or moderate (6.3 MJ ME/kg DM, 30 g MP/kg DM) quality diet and given the choice between the two diets, when uninfected (NIL) or infected with 'H. contortus' (INF). Symmetrical response to divergent selection for worm egg count (WEC) was not matched by a symmetrical change in feed intake and there was no difference in diet selection between selection lines. Feed intake, growth and wool production of DRH animals remained the same as that of IRH, yet DRH animals had five times greater WEC than IRH. This study begins to explain the mechanisms that allow resistant animals to effectively prevent establishment and/or development of 'H. contortus', by maintaining a greater immune response to infection through higher circulating eosinophils, plasma globulin and IgG₁ antibody titres. Susceptible animals have displayed resilience by improving feed conversion efficiency and increasing protein synthesis.