This thesis addresses the issues of genetic improvement of carcass and maternal traits in hill sheep. It i) compares the performance of two genetic lines of Scottish Blackface lambs divergent for subcutaneous fat, ii) quantifies the genetic components of carcass traits in extensive hill environments, iii) explores the implications of selecting for reduced fatness in hill lambs, iv) develops and describes methods to include carcass traits in the breeding goals for hill sheep, and v) gives predicted results from index selection for maternal and carcass traits, using indexes of overall economic merit. For points i) to iii), approximately 2000 Scottish Blackface lambs were measured, sired by 32 rams divergent for subcutaneous fat depth, and born to 1660 unselected ewes in 1991 and 1992. They were reared under extensive conditions on two contrasting hill farms. Results showed that genetic differences in subcutaneous fatness arising from selection in an intensive environment are still expressed despite harsh rearing environments. Heritabilities for birth weight, marking weight (at approximately 6 weeks of age) and weaning weight (at 17 weeks) were 0.07±0.04, 0.02±0.03, and 0.14±0.05, respectively. Heritabilities for ultrasonic muscle and fat depth were 0.27±0.09 and 0.16±0.06, respectively. Heritability estimates for carcass traits were: pre-slaughter liveweight 0.36±0.13, cold carcass weight 0.39±0.14, fat class 0.13±0.08, conformation score 0.09±0.07, dissected lean weight 0.27±0.27, dissected bone weight 0.36±0.13 (constant subcutaneous fatness), dissected intermuscular fat weight 0.10±0.07, subcutaneous fat weight 0.20±0.09 (constant cold carcass weight). There was a strong maternal effect on live weight which declined with age. The rearing environment of the lambs was an important environmental effect on the heritability estimate for backfat thickness, being twice as large for animals reared on the improved pasture compared to those reared on hill pasture.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:643378 |
| Date | January 1999 |
| Creators | Conington, Joanne |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/13453 |
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