Ryegrass staggers (RGS) is a neurotoxic disease in ruminants caused by the mycotoxin lolitrem B, found in endophyte-infected swards of perennial ryegrass (Lolium perenne L.). This paper reviews genetic studies on the incidence of RGS at Ruakura, and the early responses to selection achieved since 1993. Using a coarse 0 or 1 criterion only, the incidence of RGS each year was recorded on weaned lambs (born in 1988-92), 2-tooths and all mixed-aged ewes, between January and March of each year 1989-93. Clinical cases of RGS occurred in 17 of 25 contemporary groups, with incidences per group up to 0.16. With 3587 records from 2307 animals representing 232 sires, animal-model restricted maximum likelihood estimates of heritability and across-year repeatability for RGS incidence were 0.068 ± 0.028 and 0.24 ± 0.05 respectively. Breeding value data were used to screen ewes and 2-tooths in 1993 into resistant and susceptible selection flocks, and to allocate elite rams for mating. Over the 1993-97 mating years, 14 and 12 rams have been used in the two flocks, respectively (11 of these twice or more). There was no significant effect of gender on incidence of RGS. From the most recent completed recording year (i.e. 1996-born lambs), 82 resistant-flock and 56 susceptible-flock animals had RGS incidences of 0.20 and 0.75 respectively (P<0.001). One hundred and eighty-five first-crosses between the two selection lines were also generated in 1996 and 1997, which may eventually provide more evidence on the mode of inheritance. The selection flocks are being continued. The selection response so far indicates that genetic differences in RGS susceptibility can be achieved while using recording under extensive conditions.
Proceedings of the New Zealand Society of Animal Production, Volume 58, , 154-156, 1998
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