Influence of divergent selection for reproduction on the occurrence of breech strike in mature Merino ewes
A. J. Scholtz A B F , S. W. P. Cloete A C , J. B. van Wyk D , A. C. M. Kruger A and T. C. de K. van der Linde EA Institute for Animal Production: Elsenburg, Private Bag X1, Elsenburg 7607, South Africa.
B Centre for Sustainable Agriculture and Rural Development, Faculty of Natural and Agricultural Sciences, University of the Free State, PO Box 339, Bloemfontein 9300, South Africa.
C Department of Animal Sciences, University of Stellenbosch, Private Bag X1, Matieland 7599, South Africa.
D Department of Animal, Wildlife and Grassland Sciences, University of the Free State, PO Box 339, Bloemfontein 9300, South Africa.
E Department of Zoology and Entomology, PO Box 339, University of the Free State, Bloemfontein 9300, South Africa.
F Corresponding author. Email: ansies@elsenburg.com
Animal Production Science 50(3) 203-209 https://doi.org/10.1071/AN09123
Submitted: 23 September 2009 Accepted: 28 January 2010 Published: 8 April 2010
Abstract
Divergent selection resulted in Merino lines that differ markedly for reproduction. These lines were selected from the same base population from 1986 to 2009. Selection was initially based on maternal ranking values for reproduction in both ewe and ram progeny. The maternal ranking scores were augmented by breeding values from a single-trait repeatability model since 2003. The site and severity of flystrike were recorded for mature breeding ewes in the flock from 2007 to 2009. The following details were captured: animal number, site of the strike (body or breech) and the severity of the strike (1 = mild to 5 = severe). Breech strikes amounted to 92.1% of all strikes and this trend was consistent across years. High-line ewes were less likely (P < 0.05) than low-line ewes to be suffering from breech strike. Ewes that suffered from breech strike during a reference year were more likely to be struck again during subsequent years, when compared with contemporaries not affected by breech strike in the reference year. When breech strike and body strike were assessed in a joint preliminary analysis, it seemed that body strike was generally more severe when diagnosed than breech strike. It was concluded that Merino sheep selected for improved reproduction for >20 years were less likely to be affected by breech strike than contemporaries selected for low reproduction.
Additional keywords: blowfly strike, frequency, repeated strike.
Acknowledgements
We are grateful to Messrs D. Marang and Z. Stentyi for their involvement in the maintenance and recordings of the research flock. The establishment and maintenance of the breeding flock would have been impossible without the financial support of the South African Wool Industry and the THRIP program of the South African Department of Trade and Industry.
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