Continued selection of Romney sheep for resistance or susceptibility to nematode infection: estimates of direct and correlated responses

Abstract

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.

Environmental and genetic factors affecting faecal worm egg counts in Merinos divergently selected for reproduction

“…Morris et al (2010) reported a non-significant genetic correlation between FWEC and litter size. However, a small correlated genetic response to FWEC selection was observed in another study by Morris et al (2000), with litter size being greater (i.e. 0.11 more lambs weaned/ewe) in the parasite-resistant line than the control line.…”

The role of targeted selective treatments in the development of refugia-based approaches to the control of gastrointestinal nematodes of small ruminants

“…Although studies have shown that increased dag scores are associated with decreased FEC (Morris et al, 2000(Morris et al, , 2005, one study in the UK suggests that at mid to late summer the situation is reversed, with a positive correlation between increased dag score and high FEC in lambs (Broughan and Wall, 2007). A recent study conducted in Western Australia used scouring as the primary and weight gain as secondary indicators to target anthelmintic treatments in groups of 12-month old Merino sheep.…”

Resistência À Verminose E Suplementação Proteica No Periparto: Efeito No Parasitismo E No Desempenho Reprodutivo De Ovelhas Do Grupamento Racial Pantaneiro

“…Morris et al (7) observaram taxa de natalidade 11% maior em ovelhas resistentes (selecionadas por reprodução seletiva) quando comparadas às selecionadas para alto OPG; entretanto, os autores encontraram correlação genética desfavorável entre OPG e ganho de peso quando animais resistentes e sensíveis foram mantidos na mesma pastagem. Independentemente do grau de resistência, a concentração das mortes de ovelhas ocorreu no periparto-lactação (Tabela 2), fase caracterizada por aumento expressivo na exigência de proteína metabolizável (29) e diminuição da resistência à verminose, coincidindo com o período de transição águas-seca, época de baixa disponibilidade e qualidade das pastagens na região (Figura 1 e Tabela 3).…”

“…O desenvolvimento de linhagens de ovinos resistentes às verminoses pela reprodução seletiva tem sido utilizado na Austrália e Nova Zelândia. Investigação de campo, contudo, tem resultado tanto em relações favoráveis quanto desfavoráveis entre as características de produção e maior resistência aos parasitas (6)(7)(8) . O desenvolvimento de uma resposta imune competente contra os nematodas está associado negativamente com a eficiência nutricional como anorexia e mudanças nas rotas metabólicas, que são questões levantadas quando se busca o fortalecimento da resposta imune pela seleção genética (9) .…”