Selection for or against facial eczema susceptibility in sheep

C.A. Morris, N.F. Towers, C. Wesselink, M. Wheeler

AgResearch, Ruakura Agricultural Research Centre, P B 3123, Hamilton, New Zealand.

Proceedings of the New Zealand Society of Animal Production 1994, 54: 263-266

Facial Eczema (FE) in sheep is associated with liver damage and considerable production losses. Romney flocks, resistant (R) and susceptible (S) to FE were established at Ruakura in 1975; a control (C) and a second resistant (R2) flock were added after 1981, the R and R2 flocks were merged in 1988, and three flocks (R, C, and S) still exist. Progeny testing was carried out in early years for ram selection, and performance testing was used after 1981. The R flock was dosed with sporidesmin in most years since 1981 at 0.15 mg/kg live weight, with the dose rate for the S flock initially 0.066 mg/kg and now reduced to 0.04 mg/kg. The C flock was divided into two groups, with one group dosed at 0.15 and one at 0.04 mg/kg, to provide an indirect comparison of the R and S flocks. Blood samples were taken after dosing to monitor liver damage via plasma gamma-glutamyltransferase (GGT) concentration. Loge GGT levels increase in proportion to increasing severity of liver damage. Thus, changes in mean loge GGT value reflect changes in relative susceptibility or resistance, with negative changes indicating increasing resistance.

Two distinct peaks appear in the loge GGT distribution, centred around values of about 40 and 500 international units (i.u./l) for both the R and S flocks. The selection flocks differ in the proportions of animals at each peak, suggesting that the R and S flocks may be indirectly compared via the C flock although different dose rates were used. Annual divergence in loge GGT in the R flock relative to the C flock was -0.081±0.047 i.u./l, and the R-flock difference from control in the most recent years averaged -1.24±0.14 i.u./l. Divergence of the S flock from the C flock was more variable from year to year and the S-flock difference from control for the last 6 years averaged 0.48±0.11 i.u./l.

A total of 101 industry flocks have used the commercial FE testing service, Ramguard, with 9 of them testing for at least 10 years. A comparison with the Ruakura R-flock response was obtained from one such flock selecting for production traits and FE resistance since 1979. Their GGT trend was 34% of the R-flock trend, indicating that less but worthwhile genetic progress has been obtained from using a selection programme with a multiple objective.

Keywords: NZSAPAB; sheep; facial eczema; breeding; selection; resistance; susceptibility.

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Last Updated 25-01-1997