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Abstract
A flock of mainly Romney × Coopworth ewes was established to test sires for genetic differences in resistance to ryegrass staggers (RGS). This disease is a neurotoxic condition caused by ingestion of endophyte‐infected perennial ryegrass containing the mycotoxin, lolitrem B. Lambs, 18‐month males and females, and ewes of all ages were scored from January to March each year for RGS (0 or 1; 0 = no staggers), while grazing toxic pastures during natural outbreaks of RGS. Over 6 years (the autumns of 1988–93) and over all stock classes scored, age‐group means for RGS ranged from 0 to 0.16. A breeding value for RGS was calculated for each animal, incorporating data from all years and all age groups, provided that the mean year x age group score was at least 0.04. RGS data were also recorded on another Romney flock managed at the same site over the same time period. The heritability of RGS score over both flocks was 0.07 ± 0.02 and the repeatability over years was 0.24 ± 0.04. In March 1993, 170 ewes were allocated to a resistant (R) or susceptible (S) line according to their breeding values, for mating respectively to three elite R or three elite S rams (also selected on breeding values for RGS). The difference in breeding value for RGS score (S flock minus R flock means) for the 133 lambs born in the 1993 crop was 0.081 ± 0.005 (P < 0.001), corresponding to an expected difference between lines of 8.1 % in the percentage of lambs staggering; the observed difference, however, was not significant, but selection is continuing. A positive correlation (0.31) was obtained between breeding values for RGS and breeding values for another mycotoxic disease, facial eczema. These results suggest that it should be possible to change the degree of RGS susceptibility in sheep by selection using progeny‐testing under field conditions.
