Six years of selection responses for resistance or susceptibility to ryegrass staggers in sheep

Abstract

Ryegrass staggers (RGS) is a neurotoxic disease in ruminants. It is caused by the mycotoxin lolitrem B, found in endophyte-infected perennial ryegrass (Lolium perenne L.). This paper reviews genetic studies on the resistance of sheep to RGS at Ruakura. Genetic differences in susceptibility among sire groups were evaluated in lambs born in 1988-92 (Phase 1). Selection lines were then formed and responses to selection for resistance or susceptibility are described for the 1993-98 birth years (Phase 2). The incidence of RGS was recorded on a presence or absence basis from January to March or April each year, in experimental flocks with animals of known pedigree. For estimating genetic parameters (Phase 1), there were 3587 RGS records available from 2307 animals representing 232 sires. Clinical cases of RGS occurred in 17 of 25 contemporary groups, with incidences per group up to 16%. Estimates of heritability and acrossyear repeatability for RGS incidence were 0.068 ± 0.028 and 0.24 ± 0.05 respectively. Breeding value data were used at the beginning of Phase 2 to screen ewes, two-tooths and ewe lambs into resistant and susceptible selection flocks (lines). Breeding values were also used to select rams that were elite for RGS, and to allocate these to each line for mating. At least some natural RGS challenge was experienced by the resulting selection-line stock born in five of the six years of Phase 2. Susceptible-line lambs had a mean incidence of 39% clinical RGS cases over these five years, whilst the resistant-line lambs grazing the same paddocks had a much lower mean clinical RGS incidence (P<0.001) at 15%, a difference of 24%. Breeding ewes from the resistant RGS line also experienced significantly less clinical RGS than those in the susceptible RGS line (P<0.001). Evidence is presented from another experiment at Ruakura, in which selection pressure was applied for increased or decreased resistance to facial eczema. The facial eczema-resistant line was also more resistant to RGS than the facial eczemasusceptible line, suggesting that there is a common biochemical pathway explaining some of the resistance to both sporidesmin and lolitrem. Studies of the RGS selection flocks are being continued in order to investigate the underlying genetic mechanisms. The selection response so far indicates that genetic differences in RGS susceptibility can be achieved by using records collected under extensive conditions. Keywords: Lolium perenne, resistance, ryegrass staggers, selection, sheep