Variation in daily teed intake and growth between Merino sheep lines selected for differences in resistance to nematodes

This trial was designed to characterise daily feed intake and growth responses of animal s selected for genetic difference in resistance to nematode infection when fed different quality diets, with and without an artificial infection with 'Haemonchus contortus'. Fifty four Merino 10-months old rams from the CSIRO flock with three divergent lines selected for either increased (IRH) or decreased (DRH ) resistance to 'Haemonchus', or unselected (C) were stratified within selection line on the basis of liveweight measured following adjustment to experimental conditions and faecal egg counts from previous experimental challenge and individually housed during a two week non-infected period and nine weeks of trickle infection with McMaster strain of 'H. contortus'. Throughout infection, there were no significant differences in daily feed intake and body weight gains between selection lines, but rams on the high quality diet had significantly high er (P<O.OOO1) feed intakes and liveweight gains than those fed the moderate quality diet. Average faecal egg counts per gram at week 9 of infection followed the expected differences between selection lines; the counts were 3, I02 for IRH , 14.417 for DRH and 9,674 for C. It is assumed from the lower faecal egg counts that resistant animals were carrying significantly lower worm infections. The results suggest a difference in supply and/or partitionng of peptides and amino acids between selection lines, the IRH compared with DRH and C directing more of their protein supply into immune functiois and less to production functions such as weight gain.