Corriedale

Abstract: This thesis is based on seven related studies on Ethiopian indigenous Horro and Menz sheep breeds and crossbreds of Menz with exotic breeds to test the general hypothesis that there exist genetic and environmental dependent variations among and within breeds that could be utilised to improve overall productivity and health of the Ethiopian sheep population. The specific studies deal with growth, survival, causes of mortality, risk factors for major causes of mortality, outbreak investigations, maedi-visna, and economics of anthelmintic treatment and supplementation. Results of studies on lamb growth and survival revealed that Horro lambs were heavier than Menz lambs both at birth and weaning. Birth weight increased significantly from the first to third parity; was higher for lambs born as singles than multiples, and for male than female lambs. Lambs born in the wet season had higher birth weight, pre-weaning average daily weight gain and weaning weight than their contemporaries born in the dry season. Pre- and post-weaning mortalities were 33.1% and 54.5% for the Horro and 19.2% and 25.9% for the Menz sheep. Cumulative mortality up to yearling was more than twice as high for Horro than for Menz lambs (69.6 vs. 30.2%). Mortality was higher for lambs born in the dry season compared to those born in the wet season, for multiple-born lambs than singles, and for male lambs than females. There was a positive relationship within breed between birth weight and survival at all ages. Causes of mortality were similar in Horro and Menz, pneumonia accounting for more than half of all deaths, followed by digestive and gastrointestinal problems, endoparasitism, starvation-mismothering-exposure complex and septicaemia. Within breed, sires were a significant source of variation for lamb growth and survival. A retrospective case-control study conducted on 6718 sheep of the Horro and Menz breeds on risk factors for mortality associated with respiratory diseases revealed that 54.4% of total mortality was due to respiratory diseases. Annual mortality associated with respiratory diseases ranged from 6.3 to 19.0%, and breed, sex and month of the year were significant sources of variation. Mortality associated with respiratory diseases was higher for the Horro than for the Menz breed (16.5% vs. 12.4%), and between October and March than between April and September. There was a significant relationship between monthly mortality associated with respiratory diseases and monthly average minimum air temperatures and with the average monthly daily deviation between maximum and minimum air temperatures. Estimation of genetic and environmental parameters for growth traits showed that the maternal genetic component was important for birth weight, weaning weight and pre-weaning average daily gain. The contribution of the permanent environmental component in the models was also substantial but less important than the common (litter) environmental component. Total heritability estimates for Menz and Horro were generally low to moderate at 0.22 vs. 0.26 for birth weight, 0.15 vs. 0.12 for weaning weight, 0.21 vs. 0.04 for yearling weight, 0.14 vs. 0.11 for pre-weaning average daily gain, and 0.11 vs. 0.11 for post-weaning average daily gain. Estimates of genetic parameters on lamb survival from the mixed Linear Model and Survival Analysis were compared. For the mixed Linear Model, survival defined as a binary trait measured at different pre-determined time, and for the Survival Analysis, survival defined as time to respective periods for lamb surviving (censored records) and time to death (uncensored records) was used. The maternal genetic effect was important for lamb survival at all survival periods. The heritabilities from Survival Analysis (0.3% to 18.5%) were higher than those obtained with the mixed Linear Model (0.5% to 5.6%). The accuracies of predicted breeding values were also higher for the traits analysed with Survival Analysis. Some limitations of Survival Analysis are discussed. An investigation into a respiratory diseases outbreak in Menz and Awassi × Menz crossbred sheep revealed that multi-factorial causes were involved. These include peste des petits ruminants (72.3%, serologically confirmed), lungworms, maedi-visna, bacterial bronchopneumonia, enzootic pneumonia and fungal infections. A follow-up serological study revealed that 74% were positive for maedi-visna antibodies in sheep of two ranches, but antibodies for maedi-visna were not detected in sheep and goats from elsewhere in Ethiopia. The maedi-visna was detected in the indigenous Menz and imported pure Awassi and crossbreds of Menz with Awassi, Hampshire, and Corriedale with a significant breed difference in prevalence. This result suggested that the maedi-visna might have been introduced into Ethiopia through sheep importations. The profitability of anthelmintic treatment and supplementation was evaluated in a 2×2×3 factorial experiment under natural sub-clinical helminthosis challenge using partial budget analysis. Supplemented sheep had significantly higher marginal profit per sheep than non-supplemented sheep. Likewise, the anthelmintic treated sheep performed significantly better than their non-treated contemporaries. The indigenous Menz and 50% Awassi × Menz were significantly more profitable during the experimental period than the 75% Awassi × Menz crosses, but ranking of genotypes changed with age. Timely health and management interventions on identified key factors and utilising genetic variation through selection would improve lamb survival and growth. Life-time assessment of economic returns helps to draw early decisions in sheep improvement programmes. Sheep breeding objectives are discussed in the context of reducing risks of genetic loss in low-input systems and improving productivity of indigenous breeds. Breeding programmes are proposed to be based on open-nucleus flocks utilizing government ranches at the top of a three tier system of flocks. Such schemes could be used for conservation and improvement of indigenous breeds as well as for crossbreeding.