Preliminary results of selecting sheep for resistance or
susceptibility to ryegrass staggers
C.A. Morris, N.R. Towers, N.C. Amyes and M. Wheeler
AgResearch, Ruakura, PB 3123,Hamilton
Proceedings of the New Zealand Society of Animal Production. 1998,
58: 154-156
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.
Keywords: NZSAPAB;
sheep; ryegrass staggers; selection; resistance; Lolium
perenne L.
Last Updated 12-09-1998
