Selection response in fecal worm egg counts in the Rylington Merino parasite resistant flock

Abstract

The Rylington Merino internal parasite resistant selection line was initiated in 1987 from a wide genetic base of 100 ewe flocks. Selection for parasite resistance was based on selecting animals with a low worm egg count in a natural parasite challenge environment. The realised annual genetic gain for estimated breeding value of worm egg count is 2.7%. The genetic gain in parasite resistance has been achieved without any adverse genetic correlations with the other economically important production traits. There is an unfavourable genetic correlation between worm egg count and scouring traits. The recommendation to industry is to select for both low worm egg count and reduced diarrhoea and combine these traits with other economically important production traits. The relative weighting applied to each component trait will vary according to the local parasite challenge situation and the time frame available to achieve sustainable parasite control.