A genetic evaluation of the Matopos Sabi sheep flock in Zimbabwe

[摘要] English: A total of 4355 lamb and 4299 ewe records, obtained from Sabi sheep at MatoposResearch Station from 1984 to 1994, were analysed to describe growth, carcass andreproductive traits and to investigate non-genetic factors influencing their expression. Yearof birth, sex, birth/rearing status of lamb, dam age, age of lamb and slaughter age weresignificant sources of variation for body weights, average daily gain, carcass andreproductive traits except for exact age of lamb at 18 months, ewe age for hot and coldcarcass weight. Significant (P<0.001) two-way interactions were found between year ofbirth and sex for pre-weaning average daily gain, 12 and 18 month weight and for birthstatus and sex for lamb survival. Least-squares means: 2.63 kg for birth weight; 17.2 kgfor weaning weight (recorded at 120 days of age); 23.5 kg for 12 months weight; 35.7kgfor 18 months weight; 124 g/day for average daily gain between birth and weaning; 21.7kg 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. Averageweight of ewe at mating in May/June were 38 kg, and 35.6 kg for post-partum inOctober/November and 26.2 kg at weaning of lambs in February. The least squaresmeans for reproductive traits were: 0.86,1.01,1.17,0.83,0.85 for ewes lambing to thoseexposed; lambs born to ewes exposed; lambs born to ewes lambing; lambs weaned toewes exposed to the ram and lamb survival to weaning respectively. The model bestdescribing 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 wereestimated 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 oflambs weaned to a ewe lambing; 0, 1 or 2); flock weaning rate (number of lambsweaned 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 thatlambed; 1, or 2). Though slow genetic progress is possible with all reproductive andsurvival traits, it could be faster to select for prolificacy while guarding against a higherlamb mortality.Genetic parameters for growth traits were estimated using ASREML from dataconsisting of 4123 lamb records from 130 sires and 1131 dams. The direct additivecontribution to birth weight, 30 day weight, 60 day weight, weaning weight, pre-weaningaverage 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, coldcarcass, 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 wasnegligible thereafter. The maternal permanent environmental component due to the damcontributed three to fifteen per cent of the total phenotypic variances for all the traitsunder consideration. Genetic progress is possible for all the growth, carcass and ewetraits considered in this study.Genetic and phenotypic correlations were estimated among lamb and ewe traits. Directadditive 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, 18month weight, ewe mating weight, post partum weight, total weight of lamb weaned,slaughter weight, hot and cold carcass weights respectively. Genetic correlations amongbirth weight and other weights to 18 months were high (0.75 to 0.85) whilst therelationship among weaning, 12 month and 18 month weight was close to unity. Thegenetic correlation between birth weight and ewe weights (mating, post-partum and damweight at weaning of lamb) were moderate viz 0.51, 0.40, 0.49 respectively and wereclose to unity at 18 months of age viz 0.96, 0.92 and 0.84 respectively. Total weight oflamb weaned was moderately correlated to birth weight (rg=0.46) but tended to be highlycorrelated with 18 month weight (0.92) and ewe weights (0.75-0.91). The geneticcorrelation between birth weight and slaughter and carcass weight was moderate andwas high at 18 months. The heritability estimates from a univariate logit transformedanalyses 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 betweenprolificacy and birth weight was negative and low. The genetic correlation betweenprolificacy and weaning weight, 12 month weight, 18 month weight, ewe mating, postpartumand 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 correlationsbetween lamb survival and birth weight, weaning weight, 12 month weight, 18 monthweight 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 selectionindex for the Sabi flock.Data from 1996 to 2000 of Sabi and Dorper ewes were used to evaluate within andbetween breed differences to internal parasite resistance as measured by packed cellvolume (PCV) and faecal egg count (FEC). Dorper ewes were consistently heavier thanSabi ewes. Sabi ewes had lower FEC and higher pev at all sampling times (mating toweaning of lambs) than the Dorper ewes. Furthermore, the proportion of Dorper ewesthat was treated with anthelmintic was higher than that of Sabi ewes. There was a periparturientrise in FEC from lambing up to two months post lambing for both breeds. Nongenetic factors influencing internal parasite resistance in both breeds were identified.Future research need to focus on the interrelationships between 'adaptive traits' such asinternal parasite resistance and both growth and reproductive traits.

[发布机构] University of the Free State