Parasitism and production in fleece‐weight‐selected and control sheep

New Zealand Veterinary Journal

McEwan

et al. 2001

107

Breeding sheep that are less reliant on the use of anthelmintics to maintain acceptable health and productivity is one of several options that may assist farmers to manage the growing anthelmintic-resistance problem in New Zealand and meet consumer demands to minimise drug usage in livestock. Although it has been known for many years that genetic factors contribute to the ability of sheep to cope with roundworm challenge, attempts to selectively breed for such factors have occurred only recently. Two host traits, 'resistance' and 'resilience', are thought to be involved in limiting the deleterious effects of roundworms on the health and productivity of sheep, and the associated need for treatment with anthelmintics. Over the last decade, considerable advances have been made in our understanding of the feasibility and implications of breeding for these traits under practical conditions. The experimental breeding lines developed as part of this research are now proving to be a valuable resource for further work directed at identifying genes and/or genetic markers associated with host resistance and understanding the mechanisms of host immunity to nematode parasites in ruminants. This review provides an up-to-date summary of the results of research in this field, with particular reference to dual-purpose sheep in New Zealand, and describes how this information is being applied in practical breeding programmes.

Section: Growth Rates and Wool Growth Under Roundworm Challengesupporting

confidence: 91%

Breeding sheep in New Zealand that are less reliant on anthelmintics to maintain health and productivity

Sa¹,

New Zealand Veterinary Journal

McEwan

et al. 2001

107

“…The lower fleece weights in lambs and ewes in the low FEC selection line than in the high line were consistent with our preliminary results from the same flock of Perendales (Morris et al 1997b), and with the results from the equivalent Romney selection experiment in New Zealand where animals from the two FEC lines also grazed together (Morris et al 1997a). Earlier New Zealand results (all reviewed by Morris et al 2000) from Romney fleece-weight selection lines at Massey University (Howse et al 1992;Williamson et al 1995) and at Tokanui Station (Morris et al 1996) were positive in sign (lower fleece weights associated with low FEC). Paternal half-sib data for FEC and wool growth (from midside patches over the parasite challenge period) at an AgResearch property (Woodlands) also showed large positive genetic correlations, although nearzero phenotypic correlations (McEwan et al 1992).…”

Section: Correlated Responsessupporting

confidence: 81%

Direct and correlated responses to selection for high or low faecal nematode egg count in Perendale sheep

New Zealand Journal of Agricultural Research

Wheeler

Watson

et al. 2005

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.

“…Over the last 4 years there has been concern in New Zealand about the biological significance of FEC results in the Massey University HFW selection line in Romneys (Howse et al 1992;Williamson et al 1995) and in the Woodlands HFW selection line in Romneys (McEwan et al 1992). How general are the findings that susceptibility to nematode parasite infection increased with HFW selection in those flocks?…”

Section: Faecal Egg Count Hfw Linesmentioning

confidence: 99%

“…Faecal egg counts (FEC) have previously been compared in other New Zealand Romney production selection lines (Howse et al 1992;McEwan et al 1992;Williamson et al 1995), and in Australian Merino production selection lines (Eady et al 1994). Food intake has been recorded in one small trial with New Zealand Romney lines (McClelland et al 1986) and in Australian Merino selection lines (Lee et al 1995a).…”

Section: Introductionmentioning

confidence: 99%

Faecal egg count and food intake comparisons of Romney single‐trait selection and control lines

New Zealand Journal of Agricultural Research

Clarke

Watson

et al. 1996

Two sets of correlated responses were studied in five Romney selection lines and their controls. The lines were from one experiment selecting for yearling body weight or yearling fleece weight since 1967, from a second experiment selecting for weaning weight or yearling fleece weight since 1973, and from a third experiment selecting for yearling body weight since 1973. Overall, the five selection lines were evaluated whilst being run together with controls at a single site. The responses were faecal egg count (FEC) after weaning, using lamb crops born in 1990-93, and food intake using 1993-born 7-month-old ram lambs. Four or five new sires were used per line per year, with 1471 and 232 animals being recorded for FEC and food intake respectively. The three body weight lines were each 20-22% heavier than their respective controls in March (6 months of age) and May. Average FEC over this time period was significantly greater by 63% in one of these three lines whereas there was no difference from the control in the other two body weight lines. The two fleece weight lines were 10-11% heavier in March-May, with 34-35% heavier yearling fleece weights, than their respective controls, whereas average FEC was significantly greater (by 35 and 46%) in both selection lines. Significant correlated increases in estimated food intake were recorded in all three body weight lines and in one of the two fleece weight lines, and these increases were proportional to the liveweight response. The implications of the correlated responses and their variability are discussed.