Responses achieved in Romney flocks selected for or against susceptibility to facial eczema, 1975–87

Abstract

Genetic selection foror against resistance to the mycotoxicosis, facial eczema (FE), was begun in a Romney flock in 1975. Randomly selected ewes were mated with rams which had been previously identified by progeny testing as resistant (R) or susceptible (S) to FE. In subsequent years (until 1982), the same ewes and their daughters were again mated within flock to progeny-tested rams. From 1983 until 1987, performance testing was substituted for progeny testing. The most resistant R rams and most susceptible S rams by performance test were then used for mating in their respective flocks. Over these 5 years, young rams bred in the trial were performance-tested using a challenge with sporidesmin, the mycotoxin which causes FE. The severity of the liver damage induced was assessed by measuring plasma gamma-glutamyltransferase (GGT) activity. In 1982, two further flocks were established from a common Romney source as demonstration flocks; they were managed together with the R and S flocks. Six years of matings (1982–87) were carried out with selected performance-tested rams in one flock (P) and with randomly sampled rams in a second flock as a control (C). A best linear unbiased prediction (BLUP) analysis of GGT data from all four flocks over the 1982–87 lamb crops was used to determine the effects of selection. This showed that, in 1982, the R flock had 16% more animals resistant to FE than the P flock. From estimates of subsequent rates of genetic change in the R and P flocks, performance testing achieved 0.9 and 2.4% more animals resistant per year respectively. The different rates of change from performance testing in the R and P flocks partly resulted from different flock sizes, but there was also considerable between-year variation in response. By 1987, 21 and 15% more R and P animals respectively, were resistant than before selection began. There were 27 ± 13% more animals from the S flock susceptible in 1987 than in the C flock. Comparisons of weaning weight, autumn weight (4–6 months of age), yearling weight, fleece weight, litter size, and lamb survival from up to 2426 lamb records and 2065 ewe lambing records showed few significant differences between R and S flocks, or between P and C flocks.

Keywords:

• facial eczema
• sheep
• Pithomyces chartarum
• mycotoxin
• breeding
• selection
• disease
• susceptibility
• genetic change
• correlated response