Genetic evaluation of production, reproduction and survival in a flock of Ethiopian Horro sheep

[摘要] English: Genetic analyses of economically important traits were carried out using data collectedfor 20 years (1978-1997) from a flock of Horro sheep at Bako Research Center, Ethiopia.(Co)variance components and genetic and environmental parameters were estimated forpre- and post- weaning average daily gain (ADGl, ADG2) and Kleiber ratio (KRl, KR2),birth weight (BWT), weaning weight (WWT) and bi-monthly weights to 12 months ofage (WT2 to WTI2), and weight at 18 months of age (WTI8). using ASREML. Twelvemodels, formed with inclusion or exclusion of the maternal genetic, permanentenvironmental, and temporary (litter) environmental variance components and thecovariance between the direct and maternal additive effect on the basic direct additivegenetic model, were used. Bivariate analyses were also done. Maternal genetic andtemporary environmental components were found to be important (P<0.05) sources ofvariation for ADG 1 and KRl while only the temporary environmental component wasfound to be important for ADG2 and KR2. Total heritability estimates for ADGl, ADG2,KRl and KR2, were 0.13±0.04, 0.04±0.03, 0.13±0.03, and 0.01±0.02 respectively. Forweights to about six months of age, the inclusion of the temporary maternal effect in themodels was found to have significant (P<0.05) contribution to the total variance.Maternal genetic components were important for weights until about eight months of age,while the direct-maternal covariance was important for BWT, WT2, WWT and ADGl.Direct genetic correlations of ADGl with BWT, WWT and WT6 were 0.04±0.21,0.96±0.02 and 0.92±0.09 while with KRl they were -0.41±0.21, 0.74±0.10 and 0.66±O.16respectively. The inclusion of maternal genetic, temporary and permanent environmentaleffects in analytical models for early weights improved model fit. Heritability estimatesof average daily gains and Kleiber ratios are low to moderate. The correlations withweight traits, with the exception of BWT, are moderate to high. Thus, it seems morepractical to select on weight traits to improve gain and efficiency.Genetic and maternal environmental parameters of perinatal, pre- and post-weaningsurvival in Horro lambs were studied using 3864 lamb records. The perinatal periodcovered the first three days after birth (3DS) while pre-weaning (3MS) and post- weaning(3-6MS) survival were taken as survival in the pre- and post-weaning three-monthperiods respectively. Cumulative survival to six months of age (6MS) was alsoconsidered. The same twelve linear genetic models used for growth traits were applied.Additionally logit and probit analyses were carried out. The most appropriate model waschosen which was consistently superior in all of the analytical methods and used toestimate genetic and environmental parameters. A bivariate analyses between survival onthe observed scale and birth weight was carried out. For survival to all ages, the model,which included the direct genetic and maternal temporary (litter) environmental effectwas found to be the most consistent of all the models compared. Heritability estimates for3DS, 3MS, 6MS and 3-6MS were 0.00±0.00,O.02±0.03, 0.05±0.03 and 0.07±0.03respectively. Genetic correlations with birth weight were 0.20±0.23, 0.68±0.34,0.26±0.17 and 0.45±0.26 respectively, for 3DS, 3MS, 6MS and 3-6MS. The lowheritability estimates of survival in this study suggest acceptable progress with respect tothis trait through selection will be difficult to attain. The relatively higher geneticcorrelation of 3MS with birth weight indicates survival may be improved indirectlythrough selection for birth weight. Despite the low rate of prolificacy (34% twinning)there exists a sizeable litter effect on survival. This should be looked into, since this mayentail a separate management for twin born lambs.Genetic parameters of productivity expressed as ewe fertility, incidence of twinning,number of lambs weaned, and total lamb weight at birth (TBW) and at weaning (TWW)per ewe per lambing were estimated using direct additive and repeatability models.Fertility and twinning were analyzed by a logit analysis, using similar models. In theanalysis of fertility, service sire was included either as random or fixed effect.Additionally cumulative total weight of lamb weaned in the first four parities (TWW4)was also analyzed, using the direct additive model. Except for fertility where service sirewas considered as random effect, the repeatability model was found to have a better fitthan (P<0.05) the direct animal model. Bivariate analyses were done, using the directanimal model. Direct heritability estimates for fertility, twinning and total weight oflambs at birth and at weaning were 0.02±0.02, 0.15±0.03, 0.08±0.02 and 0.04±0.02respectively. Where service sire was considered as fixed effect, heritability estimate offertility from a direct model increased to 0.06. Heritability estimates for number of lambsweaned were 0.03 and 0.00 for the direct and repeatability models respectively.Repeatability estimates were 0.10, 0.07, 0.16, 0.06, 0.02 and 0.08 for TBW, TWW,twinning, litter size weaned and fertility under random and fixed sire models respectively.Estimate of the variance ratio in fertility attributed to service sire was 0.16±0.02.Heritability estimate for TWW4 was 0.05. Genetic correlations among TBW, TWW,twinning and number of lambs at weaning were in the range of 0.57 to 0.86. Except fortwinning, heritability estimates were very low. Twinning was found to have mediumheritability and repeatability, and moderate to high genetic correlation with number oflambs weaned, TBW, and TWW. These suggest that twinning can be used as a selectioncriterion for improvement in productivity despite increase in lamb mortality with increasein twinning.Weight (kg)-age (days) data of 524 Horro sheep surviving past 3.5 years of age werefitted to Brody's function to estimate growth curve parameters. Genetic and phenotypicparameters of the curve parameters and their relationships among themselves and withbirth weight (BW), weaning weight (WW), 6-month weight (WT6) and yearling weight(YW) were estimated using direct univariate and bivariate animal models. Least squaresmeans of growth curve parameters: A (asymptotic mature weight, kg), B (proportion ofmature weight attained after birth) and k (rate of maturity, kg gain k-1 body weight day-1)were 37.6, 0.88 and 0.27x10-2 respectively. Female and male Horro sheep haveasymptotic mature weights of 33.3 kg and 41.8 kg respectively. Heritability estimateswere 0.29±0.10, 0.18±0.09 and 0.14±0.09 for A, Band k respectively. Geneticcorrelations between A and B, A and k, and B and k were 0.39±0.31, -0.07±0.34 and0.25±0.39 respectively. Genetic correlations of A with BW, WW, WT6 and YW were0.27±0.34, 0.34±0.27, 0.44±0.20 and 0.67±0.17 respectively. Genetic correlations of kwith BW, WW, WT6, and YW were -0.13±0.69, 0.37±0.35, 0.61±0.25 and 0.66±0.22respectively. Presence of medium heritability for A and k indicates that progress inimproving these traits can be made through selection. WT6 and YW have mediumgenetic correlations (with reasonable standard error) with the growth curve parametersand these may allow the use of these weights as an indirect selection criterion to shortengeneration interval.Weight data from birth to about one year (maximum of 408 days) were analyzed usingthe random regression model. A total of22149 weight records of 1951 lambs were used.Three different orthogonal (legendre) polynomial orders (three, four and five) of fit foradditive genetic and animal permanent environmental effect were considered along withtwo different error measures. The residual error for weight at the different ages wasassumed to be either homogeneous (one error measure) throughout the growing period orheterogeneous (four error measure classes: birth, 10 to 90 days, 91 to 180 days and >180days). Likelihood ratio test (LRT) and Akaike's Information Criteria (AIC) were used formodel comparison. Model fit improved with increased order of fit and with assumption ofheterogeneity of error variance. Sizeable differences were found in heritability estimatesfrom the different models, particularly for weight at birth and early age. Components forboth additive genetic and animal permanent environmental covariances increased with anincrease in age. Heritability estimates from the 'best' model have shown declining andincreasing values at different parts of the trajectory. The lowest estimate was 0.14 forweight at birth, while the highest estimate was 0.40 for weight at about 400 days of age.The ratio of permanent environmental variance to the total variance increased from 0.42at birth to 0.67 at about 270 days and decreased slightly thereafter. With the exception ofweight at birth and at about yearling, heritability estimates from this study were lowerthan estimates from univariate studies where maternal effects were considered. Thedifference between estimates decreased with age, implying the higher estimates from thisstudy are a result of ignoring maternal genetic and environmental (temporary andpermanent) effects. Therefore consideration of maternal genetic and environmentaleffects needs attention. Choice of appropriate order of polynomial has significantinfluence and increased order of polynomial may need to be tested to refine estimateswith a larger data set (additional weights per animal).

[发布机构] University of the Free State