Data/hora: 22/06/2024 02:50:43

Biblioteca(s): | Embrapa Caprinos e Ovinos.
Data corrente: | 06/04/2000
Data da última atualização: | 08/01/2024
Autoria: | MORRIS, C. A.; WATSON, T. G.; BISSET, S. A.; VLASSOFF, A.; DOUCH, P. G. C.
Título: | Breeding sheep in New Zealand for resistance or resilience to nematode parasites.
Ano de publicação: | 1995
Fonte/Imprenta: | Breeding for Resistance to Infectious Disease in Small Ruminantes, v. 29, p. 53-75, 1995.
Idioma: | Inglês
Conteúdo: | Breeding sheep for resistance or resilience to internal parasites is being carried out in commercial flocks in New Zealand and is an area of active research. Selection lines have been established in Romney sheep for resistance (low faecal egg count FEC), or for susceptibility (high FEC) to nematode parasites derived from natural challenge on pasture at two research centres, the Wallaceville Animal Research Centre and the Ruakura Agricultural Centre. Mter fourteen and eight years of selection, respectively, the divergence between the high and low selection lines, in phenotypic standard deviation units is 1.65 at Wallaceville and 0.S6 at Ruakura. These differences in FEC are strongly associated with differences in wonn burdens. Exchange of resistant and susceptible rams between these two locations has indicated no interaction between line and location. From nine estimates, the heritability of log FEC in New Zealand has a weighted average of 0.23±0.02. An ELISA assay has been developed at Wallaceville to measure antibody response to antigens from various nematode genera. The heritability of this antibody response in lambs at six months of age is 0.27±0.OS, and the response is favourably correlated genetically with log FEC (-0.56±0.lS). Screening for FEC is being provided as a commercial service to some ram breeders in New Zealand and the recording scheme (Animalplan) is able to provide breeding values to perfonnance-recording clients for log FEC and dag score. Resilience, or the ability of an animal to grow in spite of a wonn challenge, has been measured in various ways, giving heritability estimates ranging from 0.06 to 0.14. Information is presently lacking on the most appropriate way to combine resistance and production traits into a multi-trait selection objective. This is particularly important in view of the recent fmding that sheep lines that have been experimentally selected for higher levels of production in New Zealand have higher FEC levels than their controls. Diseases of importance to sheep farmers in New Zealand include facial eczema, footrot, flystrike, ryegrass staggers and internal parasitism. This review will concentrate on internal parasites but it is noted that significant research is being undertaken in New Zealand on developing breeding Breeding for Resistance to Infectious Diseases in Small Ruminants strategies for resistance of sheep to facial eczema (Morris et al. 1989, 1991); to footrot (Skerman et al. 1988); and more recently (since 1988) to ryegrass staggers (C.A Morris and N.R. Towers, unpublished). Flystrike is becoming increasingly troublesome to sheep producers but there has been no research in New Zealand on breeding sheep for resistance to flystrike. Both stud and commercial sheep farmers in New Zealand must assess the relative importance of these diseases and other production traits (e.g. wool production, meat production, reproduction) when designing their breeding programs and setting their breeding objectives. In many cases, the critical genetic parameters and relative economic values required to design more complete breeding programs are unknown and these deficiencies will be highlighted in this review. There is considerable difference of opinion about how to include parasitism in sheep breeding objectives. Briefly the issues are: 1. There is a threat to the sheep industry from anthelmintic resistance, suggesting that sheep should be selected for low faecal egg count. Z. There may be some genetic antagonism between production (especially fleece weight) and faecal egg count. 3. It is not clear whether resilience (Le. resistance to the effects of a parasitic infection) should be included in the objectives. 4. Selection for resilience without concomitant selection for resistance may fail to reduce pasture contamination. Research in New Zealand on the genetics of resistance to internal parasites in sheep has been reviewed recently by Baker et al. (1990, 1991). This review updates the 1991 paper which summarised and discussed the results of research by the Ministry of Agriculture and Fisheries (now AgResearch) at Ruakura and Wallaceville. It also discusses the application of breeding strategies on both stud and commercial sheep farms, and outlines some of our thinking on future research directions. In all experimental and progeny test flocks in New Zealand except one (the Perendale flock), resistance has been assessed using faecal egg count (FEC) after natural infection. This is in contrast to most Australian studies which have used artificial challenge with known doses of larvae from different parasitic genera (Woolaston 1990). In New Zealand, the protocol has involved drenching all lambs at weaning (about 3 months of age) and then monitoring a sample of 20-30 lambs on a regular basis, until a mean FEe of 1000-1500 eggs per gram offaeces (epg) is reached. At this time all lambs in the trial are faecal sampled, weighed, recorded for dag score and drenched. After drenching, the procedure is then repeated at least once and in the Wallaceville flocks usually twice. The first faecal sampling takes
Palavras-Chave: | Controle; Genetic control; Nematodeo gastrintestinal.
Thesagro: | Ovino; Resistência Genética.
Thesaurus Nal: | Gastrointestinal nematodes; Genetic resistance; Helminths; Sheep diseases.
Categoria do assunto: | L Ciência Animal e Produtos de Origem Animal
Registro original: | Embrapa Caprinos e Ovinos (CNPC)
Exemplares: | CNPC - (10253-1) - ADD - AP - PP - SP 04785 -  - SP 04785 - DIS