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Type: Thesis
Title: Breeding for robustness in maternal sheep.
Author: Walkom, Samuel Felix
Issue Date: 2014
School/Discipline: School of Animal and Veterinary Sciences
Abstract: A favourable phenotypic relationship between ewe body condition and reproduction has been well reported in literature with producers managing ewe condition to improve reproductive performance. This thesis explores the genetic variation of ewe body composition across her production life and the ability to select for it to improve the maternal performance of the ewe in variable environments. The quantitative genetic analyses were carried out on four data sets; 1. Maternal Central Progeny Test provided data on 2,846 first cross ewes that were the progeny of Merino ewes crossed to multiple maternal sire breeds. Weight, fat score, fleece traits and reproductive traits were recorded over three production cycles. 2. The Trangie D flock incorporated 3,300 Merino ewes from five strains (Fine-wool, Medium-wool Peppin, Medium-wool non-Peppin, High fertility Peppin and South Australian strong wool). The weight and body condition score of the ewes were recorded over five production cycles. 3. An industry stud flock (Lambpro, Holbrook NSW) provided ultrasound scan measurements on 2,796 adult maternal composite ewes from 2010 to 2013. The industry flock is based around a high fertility maternal composite flock focussed on providing self-replacing ewes to the prime lamb industry. 4. The final data set includes 6,347 Scottish Blackface ewes which were managed in the Scottish highlands, UK. The ewes were ultrasound scanned as lambs before being managed as part of the breeding flock for an average of four production cycles. The body condition score and reproductive performance was recorded across their production life. Multiple analysis techniques were used to demonstrate that the genetic rank of individuals was consistent across maturities, measurements times, parities, environments and reproductive status. An individual's genetic rank for weight, fat and condition were highly consistent across the ewe's production life. Consequently, the results suggest that selection against fluctuations in the weight and body composition profiles of the breeding ewe will not be effective. Adult fat and muscle are moderately heritable whilst body condition score and fat score were low to moderately heritable providing scope to improve the ewe's condition during the production cycle. Post-weaning weight and body composition traits were strongly correlated with the corresponding adult traits' Therefore selection practiced on young animal traits will improve the lifetime condition of the ewe. Neither fibre diameter nor fleece weight influenced the magnitude of weight and fat fluctuations across the production cycle. Fibre diameter was moderately correlated with fat so finer fleeced sheep were leaner. The genetic relationship between body composition traits and maternal productivity was weak. Under "tough" conditions the relationship was positive and yet when faced with favourable conditions the relationship was weakly antagonistic. Selection on post-weaning fat depth will result in improved genetic merit for body condition in the breeding ewe. However, selection for fat will not alone improve reproductive performance and in favourable nutritional conditions can become antagonistic to maternal productivity. ln highly variable environments or feed limited production systems there maybe merit in selecting for increased condition to improve both the welfare and productivity of the ewe.
Advisor: Brien, Forbes
Hebart, Michelle Leanne
Pitchford, Wayne Scott
Dissertation Note: Thesis (Ph.D.) -- University of Adelaide, School of Animal and Veterinary Sciences, 2014
Keywords: quantitative genetics; fat; liveweight; maternal performance; reproduction
Provenance: This electronic version is made publicly available by the University of Adelaide in accordance with its open access policy for student theses. Copyright in this thesis remains with the author. This thesis may incorporate third party material which has been used by the author pursuant to Fair Dealing exceptions. If you are the owner of any included third party copyright material you wish to be removed from this electronic version, please complete the take down form located at:
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