The study includes separate papers, which are all linked by their emphasis on the control of blowfly strike, and breech strike in particular. This summary is intended to provide readers with a broad overview of the outcomes of the study.
Part I. Management options
Chapter 3 dealt with a survey on the prevalence of blowfly strike, and the methods used to combat blowfly strike, in the Rûens area of South Africa. It was clear from the survey that breech strike is the major form of strike in the Western Cape. Mulesing was once again demonstrated to be an effective control method for breech strike. With the termination of mulesing as an acceptable management practice, this chapter highlights the need for alternative methods to be used for blowfly control. Although useful from an IPM perspective, other initiatives that could add to blowfly control failed to have the same marked impact on blowfly strike that mulesing had.
Chapter 4 reports on the effect of regular treatment with crystals derived from Aloe spp for potential use as a natural anthelmintic in yearling Merino progeny. The short-term effect of aloe treatment was also considered. Results showed no reduction in the parasite burden when sheep were treated with Aloe. The contribution of this treatment to blowfly IPM is thus limited.
Part II. Breeding options
Divergent selection for reproduction (defined as the ability of ewes to rear multiple offspring) resulted in lines that differed markedly for their susceptibility to breech strike as a correlated effect (Chapter 5). The line selected for reproduction (High line or H line) was substantially more resistant to breech strike than the line selected for low reproduction (Low line or L line).
Chapter 6 reported genetic (co)variances between wrinkle scores and the absence of breech strike in mulesed and unmulesed Merinos. This chapter suggested that breech strike on the underlying scale is partly under genetic control. Indirect selection levelled against skin wrinkle could play a role in reducing the susceptibility of sheep to breech strike in unmulesed sheep only. The significant genetic variation for absence of breech strike remaining in mulesed sheep hints at traits not associated with wrinkles and bare breeches (which are arguably being strived for during the Mules operation) also being important in breech strike resistance genetics.
In Chapter 7 subjective scores for dags, breech cover, crutch cover and belly quality were recorded for mature and maiden ewes in the divergently selected lines in an attempt to understand the reasons for the lower susceptibility to breech strike in the H line. Animals in this line displayed desirable breech and crutch characteristics compared to contemporaries selected against reproduction (L line). This generalisation held true for mature reproducing ewes as well as for two-tooth hoggets. Dag scores were accordingly improved in hoggets in the H line.
In a further study (Chapter 8) it was shown that autumn and spring dag scores; breech wrinkle score as well as the vertical and horizontal breech bare areas were all heritable in the lines divergently selected for reproduction. Genetic correlations among the breech traits were generally favourable. Yearling live weight was favourably related to breech traits on the genetic level. The only genetic correlation of breech traits with fleece traits that would cause concern was a positive correlation between clean fleece weight and breech wrinkle score. Derived breeding values in this chapter confirmed substantial genetic differences for both dag scores, breech wrinkle score and breech bare area in favour of the H line.
Results from Chapter 9 indicated that H line hoggets took substantially shorter time to be crutched than their L line contemporaries, indicating welfare benefits in favour of the former line.
Implications
The study has shown definite opportunities for the alleviation of breech strike and presents the scientific community with ample opportunities to refine and integrate existing control measures in a comprehensive IPM strategy. However, further research is needed to reach this objective.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-10172011-123547 |
| Date | 17 October 2011 |
| Creators | Scholtz, Anna Jacoba |
| Contributors | Prof T CdeK van der Linde, Prof JB van Wyk, Prof SWP Cloete |
| Publisher | University of the Free State |
| Source Sets | South African National ETD Portal |
| Language | en-uk |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.uovs.ac.za//theses/available/etd-10172011-123547/restricted/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University Free State or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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