Influence of flock size on annual genetic gain for selected weight traits and inbreeding in pure Barbados Blackbelly flocks and the impact of genetic make-up when producing crossbreed Katahdin and Barbados Blackbelly rams on a Government operated nucleus farm : a thesis presented in partial fulfilment of the requirements for the degree of Master of Animal Science, Massey University, Palmerston North, New Zealand
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In developing countries, an interest by governments and farmers in improving production by using genetic gain in indigenous animals and utilizing the benefits of crossbreeding exists. This has been plagued by poor genetic improvement and high rates of inbreeding when these decisions have been left solely to the farmer. As a result, nucleus breeding schemes that are centrally located or are linked with village-based nucleus schemes have served as viable options to address the deficiencies of village level management of genetic improvement. In the tropics, particularly the Latin America and Caribbean region, (LAC), producers are beginning to realize the benefits of crossbreeding and the introduction of exotic breed genetics into their tropical hair sheep flocks is increasing, though there introduction has low levels of success and not very sustainable in the long term. This study was carried out to determine appropriate systems to improve the sheep breeding program at the government operated Central Livestock Farm (CLF), in the Caribbean Island of Dominica. To achieve this, three aspects were addressed, firstly methods to increase the rate of annual genetic gain, secondly to consider the annual rate of inbreeding within the indigenous Barbados Blackbelly breed and finally to investigate production of crossbreed rams from exotic and indigenous breeds. This study initially focused on the evaluation of pure breeding schemes for a nucleus flock of varying population sizes using the Barbados Blackbelly. Pure breeding schemes were simulated for a nucleus that consisted of either 50 or 100 ewes, each with five or ten rams respectively. These breeding schemes were evaluated based on annual rate of genetic gain for birth weight (BW), weaning weight (WW) and average daily gain (ADG) for different scenarios: age, selection intensity and accuracy of selection. The mating strategy to produce crossbreed rams also involved the Barbados Blackbelly as well as the Katahdin. The effect on a nucleus herd when producing crossbreed rams of different genetic makeup was considered. The results of the present study indicated that annual rate of genetic gain for BW, WW and ADG when changes are made in generation interval, selection intensity and accuracy of selection was always greater when accuracy of selection was done objectively compared to subjectively. Overall, the 100 ewe, five rams breeding flock when analysed objectively had the greatest annual rate of genetic gain. Inbreeding coefficient was greater in breeding schemes with five rams; however, these rates were still acceptable because they were at the recommended rate of < 1%. It is also important to note that when simulations were done in 50 ewe, five rams self-replacing flocks meant to produce crossbreed rams, the rate of inbreeding was > 1%. Production of crossbreed composite rams was quicker than producing upgraded rams, four and six years respectively and conversion of ewe flock from pure to crossbreed also occurred quicker in composite ram production. Additionally, the production of crossbreed composite rams produced the most rams during the period of study, when no replacements was considered the number of rams produced was 58 and 12 respectively, and in self replacing flocks, 43 and nine respectively. This study suggests that the most appropriate nucleus flock size would be a 50 ewe, five rams flock because of the limitations due to space at the at the CLF. The CLF should also concentrate on producing the crossbreed composite male and not incorporate self-replacement as a component of its crossbreeding program.
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