A NOTE ON THE GENETICS OF RESISTANCE OR SUSCEPTIBILITY TO RYEGRASS STAGGERS IN SHEEP

A flock of mainly Romney X Coopworth ewes was established to test sire s for genetic differences in resistance to ryegrass staggers (RGS). Th is disease is a neurotoxic condition caused by ingestion of endophyte- infected perennial ryegrass containing the mycotoxin, lolitrem B. Lamb s, 18-month males and females, and ewes of all ages were scored from J anuary to March each year for RGS (0 or 1; 0 = no staggers), while gra zing toxic pastures during natural outbreaks of RGS. Over 6 years (the autumns of 1988-93) and over all stock classes scored, age-group mean s for RGS ranged from 0 to 0.16. A breeding value for RGS was calculat ed for each animal, incorporating data from all years and all age grou ps, provided that the mean year X age group score was at least 0.04. R GS data were also recorded on another Romney flock managed at the same site over the same time period. The heritability of RGS score over bo th 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 r espectively to three elite R or three elite S rams (also selected on b reeding values for RGS). The difference in breeding value for RGS scor e (S flock minus R flock means) for the 133 lambs born in the 1993 cro p was 0.081 +/- 0.005 (P < 0.001), corresponding to an expected differ ence between lines of 8.1% in the percentage of lambs staggering; the observed difference, however, was not significant, but selection is co ntinuing. A positive correlation (0.31) was obtained between breeding values for RGS and breeding values for another mycotoxic disease, faci al eczema. These results suggest that it should be possible to change the degree of RGS susceptibility in sheep by selection using progeny-t esting under field conditions.