Ryegrass staggers (RGS) is a neuromuscular syndrome affecting susceptible ruminants when they graze perennial ryegrass (Lolium perenne L.) containing the mycotoxin-producing endophytic fungus Neotyphodium lolii. Genetic selection for divergence in RGS susceptibility began in 1993 at Ruakura in Romney-cross sheep, with single-trait RGS selection lines for resistance (R) and susceptibility (S) being established. Stock from both lines are repeatedly challenged when grazing together on high-toxin endophyte pastures, and scored for RGS. Lambs and `yearling` females (16 to 18 months of age) have been recorded each year, as well as breeding ewes and yearling rams in some years. Results up to the 2000 lamb crop are reported here, as well as preliminary records from the 2000 born yearling females. In the 2000 born lambs (n=61), 14% and 88% of the R and S lines showed clinical RGS, respectively (P<0.01). In the 1999 yearling ewes (n=48), corresponding percentages by line were 0 and 66% respectively (P<0.01), whilst for the 2000 yearling ewes (n=18) percentages were 0 and 63%, respectively (P<0.01). Line-crosses, born and reared together with the R- and S-line animals, had levels of clinical RGS between those of the corresponding R and S lines. From restricted-maximum-likelihood analyses, the single-record heritability for RGS in all stock classes was 0.17 ± 0.02, and the repeatability (between animals across years) was 0.28 ± 0.02. Breeding value estimates suggest that most of the divergence between lines has resulted from genetic change in the S line, but the genetic mechanism is not yet clear. Reasons for this asymmetry include: (1) greater genetic and phenotypic variation in the S line than in the R line, following a given challenge; (2) greater selection intensity applied in the S line; and (3) a major gene might be present, with an allele for resistance which is recessive or partially recessive.

NC, Aymes, CA Morris, and NR Towers

Proceedings of the New Zealand Society of Animal Production, Volume 62, Palmerston North, 191-194, 2002
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