A note on the genetics of resistance or susceptibility to ryegrass staggers in sheep

Abstract

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 w...