Divergent breeding lines of Romney sheep, selected as lambs for consistently high or low faecal worm egg count (FEC) following natural multi-species challenge by nematode parasites, were established in New Zealand at Wallaceville Animal Research Centre in 1979 and at Rotomahana Station in 1985. In 1988 the Rotomahana lines, including an unselected control line maintained under the same management conditions, were transferred to Tokanui Station where they remained for 4 years. In 1993 elite high and low FEC animals from Tokanui, along with the controls, were transferred to Wallaceville, where merged lines have since been managed together. Selection responses from the lines at Rotomahana and Tokanui, and from a further 5 years of divergent selection in the merged lines, are reported here. For the two most recent lamb crops (1996 and 1997 birth years), log-transformed FECs of the high and low lines were 1·27 and -1·46 phenotypic standard deviation units from the control. After back-transformation to the original scale, where the FEC for control line lambs averaged 1255 eggs per g, the means for the high and low lines were 3Ό5 and 0·27 times the control mean. Animal-model restricted maximum likelihood estimates of her it ability and repeatability for single-record FEC (following separate infections) were 0·28 (s.e. 0·02) and 0·42 (s.e. 0Ό1), respectively. Correlated responses in production traits include significantly decreased post-weaning weight gain and increased dags (breech soiling) in lambs, and decreased fleece weight in yearlings and ewes in the low FEC line, compared with those in the high line. However the low FEC line had proportionally 0·11 more lambs weaned per ewe mated than the high FEC line (F < 0·01). It is concluded firstly that selection for high or low FEC in Romney s has achieved an 11-fold difference between the divergent lines. Secondly, it will generally be necessary in a commercial environment to apply index selection for a combination of increased productivity, decreased FEC and possibly decreased dags, when potential candidates are recorded under conditions of nematode challenge.

A selection experiment with Perendale sheep was established in 1986, with lines selected solely on the basis of high or low faecal nematode egg count (FEC) in lambs after weaning. Ranking for FEC involved a natural mixed-species challenge in all years, although in early years this was augmented by an artificial challenge with Haemonchus contortus larvae. Faecal samples were taken from each animal for FEC on two occasions, separated by an anthelmintic drench. A total of 1840 lambs were recorded for FEC in the high and low selection lines from 1986 up to the 2002-born lamb crop. Direct responses to divergent selection for or against FEC were estimated, along with indirect responses in live weights (lambs, yearlings, and adult mixedage ewes), in fleece weights (yearlings and ewes) and in breech soiling ("dag") scores. Analyses of both lines across all years were carried out using animal-model restricted maximum likelihood techniques and also fixed-effects models. The realised heritabilities of log e (FEC + 100) at the two sampling times were 0.22 ± 0.03 and 0.16 ± 0.03; the genetic correlation estimates between log e (FEC1 + 100) and yearling live weight and fleece weight were 0.36 ± 0.17 and 0.54 ± 0. back-transformed FEC means averaged 556 and 114 eggs/g for the high and low lines, respectively, representing a 4.9-fold line difference. When exposed to equal parasite challenge, lambs from the high FEC line were heavier than low-line lambs by 6-12% (breeding ewes 8%), with all differences being significant (P < 0.001); corresponding figures for fleece weight were 24-26% (breeding ewes 15%), again with all differences significant (P< 0.001), and dag scores averaged 0.55 units higher in lowline animals (P < 0.001). It is concluded that, under natural mixed-species parasite challenge on pasture, small rates of genetic change for FEC were achieved in small, closed populations of Perendales. There were unfavourable correlated responses to selection to reduce FEC, comprising lower weights, reduced fleece weights, and more dags. The prospect of index selection to break the unfavourable genetic correlations with FEC is discussed.

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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 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.