New Zealand Journal of Agricultural Research abstracts
Direct and correlated responses to selection for high or low faecal
nematode egg count in Perendale sheep
C. A. Morris
M. Wheeler
T. G. Watson
B. C. Hosking
D. M. Leathwick
AgResearch
Ruakura Research Centre
Private Bag 3123
Hamilton, New Zealand
Abstract 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 mixed-age 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 loge(FEC + 100) at the two sampling times
were 0.22 ± 0.03 and 0.16 ± 0.03; the genetic correlation
estimates between loge(FEC1 + 100) and yearling live weight
and fleece weight were 0.36 ± 0.17 and 0.54 ± 0.16,
respectively. Averaging the breeding value results for the 1998–2002
crops, the 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 low-line 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.
Keywords sheep; internal parasite; faecal egg
count; selection
A04012; Received 18 February 2004; accepted 25 November 2004; Online
publication date 21 March 2005
New Zealand Journal of Agricultural Research, 2005, Vol. 48:
1–10
0028–8233/05/4801–0001 © The Royal Society of New Zealand 2005
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