A genetic evaluation of the Matopos Sabi sheep flock in Zimbabwe

English: A total of 4355 lamb and 4299 ewe records, obtained from Sabi sheep at Matopos Research Station from 1984 to 1994, were analysed to describe growth, carcass and reproductive traits and to investigate non-genetic factors influencing their expression. Year of birth, sex, birth/rearing status of lamb, dam age, age of lamb and slaughter age were significant sources of variation for body weights, average daily gain, carcass and reproductive traits except for exact age of lamb at 18 months, ewe age for hot and cold carcass weight. Significant (P<0.001) two-way interactions were found between year of birth and sex for pre-weaning average daily gain, 12 and 18 month weight and for birth status and sex for lamb survival. Least-squares means: 2.63 kg for birth weight; 17.2 kg for weaning weight (recorded at 120 days of age); 23.5 kg for 12 months weight; 35.7kg for 18 months weight; 124 g/day for average daily gain between birth and weaning; 21.7 kg for total weight of litter weaned and 14.6 for Kleiber ratio. Slaughter weights were low (29kg) yielding 13.6 and 13.1 kg hot and cold carcass weights respectively. Average weight of ewe at mating in May/June were 38 kg, and 35.6 kg for post-partum in October/November and 26.2 kg at weaning of lambs in February. The least squares means for reproductive traits were: 0.86,1.01,1.17,0.83,0.85 for ewes lambing to those exposed; lambs born to ewes exposed; lambs born to ewes lambing; lambs weaned to ewes exposed to the ram and lamb survival to weaning respectively. The model best describing the data were constructed and used in the subsequent analyses of (co)varaiances and heritability estimates in univariate models. Variance components of reproductive traits and lamb survival to weaning were estimated using a threshold model. The heritabilty estimates were low with 0.02, 0.06, 0.04, 0.02 and 0.04 for fertility (whether a ewe lamb or not; 0 or 1); reproductive rate (number of lambs born to a ewe exposed; 0, 1 or 2); ewe weaning rate (number of lambs weaned to a ewe lambing; 0, 1 or 2); flock weaning rate (number of lambs weaned to a ewe exposed; 0, 1 or 2); and lamb survival rate (whether a lamb born alive, was dead or alive at weaning except for prolificacy (number of lambs born to a ewe that lambed; 1, or 2). Though slow genetic progress is possible with all reproductive and survival traits, it could be faster to select for prolificacy while guarding against a higher lamb mortality. Genetic parameters for growth traits were estimated using ASREML from data consisting of 4123 lamb records from 130 sires and 1131 dams. The direct additive contribution to birth weight, 30 day weight, 60 day weight, weaning weight, pre-weaning average daily gain, 12 months weight, 18 months weight and Kleiber ratio was 0.25, 0.13,0.13,0.16,0.17,0.26,0.39, and 0.08 respectively. The direct heritability was 0.27, 0.18, 0.18, 0.12, 0.58, 0.53 and 0.67 respectively for slaughter, hot 'carcass, cold carcass, total weight of lamb weaned, mating, post partum and ewe weight at weaning. Maternal heritabilty declined from 0.12 at birth to 0.06 at 60 days of age and was negligible thereafter. The maternal permanent environmental component due to the dam contributed three to fifteen per cent of the total phenotypic variances for all the traits under consideration. Genetic progress is possible for all the growth, carcass and ewe traits considered in this study. Genetic and phenotypic correlations were estimated among lamb and ewe traits. Direct additive estimates of heritability were: 0.28; 0.17; 0.25; 0.39; 0.59; 0.50; 0.68; 0.12; 0.25; 0.11; 0.12; for birth weight, weaning weight (120 days), 12 month weight, 18 month weight, ewe mating weight, post partum weight, total weight of lamb weaned, slaughter weight, hot and cold carcass weights respectively. Genetic correlations among birth weight and other weights to 18 months were high (0.75 to 0.85) whilst the relationship among weaning, 12 month and 18 month weight was close to unity. The genetic correlation between birth weight and ewe weights (mating, post-partum and dam weight at weaning of lamb) were moderate viz 0.51, 0.40, 0.49 respectively and were close to unity at 18 months of age viz 0.96, 0.92 and 0.84 respectively. Total weight of lamb weaned was moderately correlated to birth weight (rg=0.46) but tended to be highly correlated with 18 month weight (0.92) and ewe weights (0.75-0.91). The genetic correlation between birth weight and slaughter and carcass weight was moderate and was high at 18 months. The heritability estimates from a univariate logit transformed analyses for fertility, prolificacy and lamb survival were 0.08, 0.22 and 0.01 respectively. The genetic correlation between fertility and lamb weight and ewe weights was low (- 0.08 to 0.06) and some with large standard errors. The genetic correlation between prolificacy and birth weight was negative and low. The genetic correlation between prolificacy and weaning weight, 12 month weight, 18 month weight, ewe mating, postpartum and ewe weight at weaning were 0.07; 0.12; 0.07; 0.22; 0.13; 0.24 respectively, with that between fertility and prolificacy being negative (-0.17). Genetic correlations between lamb survival and birth weight, weaning weight, 12 month weight, 18 month weight and total weight of lamb weaned were 0.18, 0.26, 0.15, 0.15, 0.13 respectively. Birth weight and total weight of lamb weaned can fruitfully be included in a selection index for the Sabi flock. Data from 1996 to 2000 of Sabi and Dorper ewes were used to evaluate within and between breed differences to internal parasite resistance as measured by packed cell volume (PCV) and faecal egg count (FEC). Dorper ewes were consistently heavier than Sabi ewes. Sabi ewes had lower FEC and higher pev at all sampling times (mating to weaning of lambs) than the Dorper ewes. Furthermore, the proportion of Dorper ewes that was treated with anthelmintic was higher than that of Sabi ewes. There was a periparturient rise in FEC from lambing up to two months post lambing for both breeds. Non genetic factors influencing internal parasite resistance in both breeds were identified. Future research need to focus on the interrelationships between 'adaptive traits' such as internal parasite resistance and both growth and reproductive traits.