Resistance vs. resilience: A solvable dilema?

Abstract

Selecting for resistance to gastrointestinal parasites in sheep provides an epidemiological advantage, but the anticipated improvements in productivity have been far from consistent. The timing of immune development, as judged by faecal egg counts (FEC), in lines of Romney sheep that had been selected for either resistant or resilience to gastrointestinal parasites was compared. Peak FEC and the time from which FEC remained at less than 200 eggs per gram (epg) occurred at a younger chronological age and lighter live weight in resistant compared with resilient animals (P≤0.02 for all) but at a similar relative maturity of circa 0.45 (P=0.26 and P=0.16 for peak FEC and FEC <200epg, respectively). These results indicate that physiological age, rather than chronological age, should be considered when comparing the immune competence of animals to ensure they are at an analgous stage of development. The implications for the timing of selection for resistance on mature live weight, and thus growth potential are discussed.