J.B. van Wyk

Genetic parameter estimates in Sabi sheep

Abstract Genetic parameters for growth traits in Sabi sheep were estimated using asreml from data records of 4123 lambs from 130 sires and 1131 dams collected between 1984 and 1994. Reproduction traits and lamb survival to weaning were analysed fitting a sire threshold model. The direct additive contribution to birth weight, 30-day weight, 60-day weight, weaning weight (WW), preweaning average daily gain (ADG), 12-months weight, 18-months weight and Kleiber ratio (KL=ADG to weaning/WW 0.75 ) was 0.25, 0.11, 0.13, 0.13, 0.17, 0.26, 0.37 and 0.08, respectively. The direct heritability for slaughter, hot and cold carcass weight, total litter weight weaned, mating and postpartum weight and ewe weight at weaning of lamb was 0.27, 0.18, 0.18, 0.12, 0.58, 0.53 and 0.67, respectively. 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 3–15% 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. The heritabilty estimates for reproduction traits were low, viz 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 mated; 0, 1 or 2); number of lambs weaned to a ewe lambing (0, 1 or 2); 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). Though slow genetic progress is possible with all reproduction and survival traits, it would be more efficient to select for prolificacy (number of lambs born to a ewe that lambed; 1 or 2) ( h 2 =0.26) but guarding against a higher lamb mortality.

Direct and maternal (co) variance components and heritability estimates for body weight at different ages and fleece traits in Afrino sheep

Abstract Data, consisting of 4325 lamb records, the progeny of 146 sires and 946 dams, collected on the Carnarvon Afrino flock over the period 1975 to 1992, were used for this study. Variance components resulting from direct additive genetic effects, maternal additive genetic effects, maternal permanent environmental effects, as well as the relationship between direct and maternal effects for birth weight, monthly body weight from weaning at 4 months to 12 months of age, 18 month body weight, 16 month clean fleece weight and 16 month mean fibre diameter were estimated by REML procedures. By ignoring or including maternal genetic or environmental effects, five different models of analysis were fitted in order to determine the most effective model for each trait. The direct heritability estimate for body weight increased from birth (0.22 ± 0.04) up to 8 months of age (0.59 ± 0.06), where it seemed to stabilise. Maternal heritability estimates for body weight, on the other hand, increased from birth (0.09 ± 0.04) to 5 months of age (0.17 ± 0.02), whereafter it decreased gradually. The maternal permanent environmental effect was significant only for birth weight (0.12 ± 0.03). Direct heritability estimates of 0.62 ± 0.04 and 0.73 ± 0.03 were obtained for clean fleece weight and mean fibre diameter, respectively. Maternal effects had no significant influence on clean fleece weight or mean fibre diameter.

Genetic parameter estimates for pre-weaning weight traits in Dorper sheep

Genetic parameters were estimated for birth-, 42-day, and 100-day (weaning) weight in the Dorper flock of the Glen Agricultural Institute in South Africa. Direct heritability estimates of 0.11, 0.28 and 0.20 and maternal heritability estimates of 0.10, 0.10 and 0.10 were obtained for body weights at birth, 42 and 100 days, respectively. The corresponding genetic correlation estimates between direct and maternal effects were 0.35, -0.63 and -0.58, respectively. Both direct and maternal genetic correlation estimates among the traits were of moderate to high magnitude and positive. It is concluded that the traits can be improved by selection with no serious antagonisms among traits studied.

Genetic and phenotypic trends and parameters in reproduction, greasy fleece weight and liveweight in Merino lines divergently selected for multiple rearing ability

Genetic and phenotypic trends and parameters were estimated for reproduction, fleece weight and liveweight in a South African Merino population, divergently selected from 1986, either for (H line) or against (L line) maternal multiple rearing ability. Annual reproduction, ewe greasy fleece weight and pre-joining liveweight data were recorded on 809 Merino ewes, from 1986 to 2002. Phenotypic trends indicated divergence in reproduction traits between the H and L lines. The direct additive variance ratio (h2 ± s.e.) for day of lambing was 0.08 ± 0.02. Estimates of h2 for reproduction traits were: 0.10 ± 0.02 for number of lambs born per ewe; 0.04 ± 0.02 for number of lambs weaned per ewe; and 0.04 ± 0.02 for weight of lamb weaned per ewe, corrected for the gender of the lamb. Corresponding h2 estimates for annual production were 0.57 ± 0.06 for greasy fleece weight and 0.48 ± 0.06 for ewe liveweight at joining. Service sire only exerted a significant (P<0.05) effect on day of lambing, but it accounted for merely 2% of the overall phenotypic variation. Ewe permanent environment variance ratios (c2ewe) for the reproduction traits were: 0.07 ± 0.03 for number of lambs born per ewe; 0.11 ± 0.03 for number of lambs weaned per ewe; and 0.11 ± 0.03 for total weight of lamb weaned per ewe. Corresponding c2ewe estimates for annual production traits were 0.14 ± 0.05 for greasy fleece weight and 0.27 ± 0.06 for ewe joining weight. Genetic and ewe permanent environmental correlations between measures of reproduction exceeded 0.7. Genetic correlations of reproduction traits with greasy fleece weight were low and variable in sign. Genetic correlations of reproduction traits with ewe joining weight were positive and particularly high for weight of lamb weaned. Permanent environmental correlations of reproduction traits with greasy fleece weight and joining weight were generally low to moderate. Genetic trends for the H and L lines (derived from averaged direct breeding values within birth years) were divergent (P<0.01) for all reproduction traits. Expressed as percentage of the overall least squares means of the respective traits, breeding values in the H line increased annually, with 1.3% for lambs born per ewe, 1.5% for lambs weaned per ewe and by 1.8% for weight of lamb weaned per ewe. Corresponding trends in the L line were, respectively, –0.6%, –1.0% and –1.2% per year. Substantial genetic progress in annual lamb output was attainable, despite relatively small h2 estimates. This response was achieved without unfavourable genetic changes in wool and liveweight.

Genetic parameter estimates for body measurements and growth traits in South African Bonsmara cattle

Abstract Data of 7266 records of Bonsmara bull calves participating in on-farm growth tests were used to estimate genetic parameters fitting both univariate and multivariate models. Linear body measurements analyzed were shoulder height (SH), body length (BL) and scrotal circumference (SC). Growth traits included birth weight (BW), weaning weight (WW), final weight (FW) measured at the end of test, and average daily gain (ADG) from the beginning to the end of test period. A complete covariance structure was estimated amongst all the traits fitted. Heritability estimates for body measurements from univariate and multivariate analyses showed a general agreement though there was a tendency for an increase in estimates when multivariate models were fitted. Heritability estimates found in this study indicate that improvement through selection is possible for all the traits considered. Genetic correlations were favourably positive for body measurements and growth traits. The high genetic correlation (0.76±0.06) between shoulder height and body length suggests that selection for shoulder height may lead to rapid progress in body length since direct selection for this trait may be hampered by its low heritability.

Genetic parameter estimates for total weight of lamb weaned in Afrino and Merino sheep

A study was undertaken to estimate genetic parameters for total weight of lamb weaned in Afrino and Merino sheep. Data collected on the Carnarvon Afrino flock, the Carnarvon Merino flock and the Grootfontein Merino stud were used. For Afrino sheep, heritability estimates of 0.061 (0.043), 0.054 (0.055) and 0.170 (0.074) were obtained for total weight of lamb weaned after the first (TWW1), second (TWW2) and third (TWW3) parities respectively. Corresponding estimates for Carnarvon Merino sheep ranged from 0.090 (0.025) for TWW1 to 0.257 (0.047) for TWW4. Similar estimates of 0.084 (0.027), 0.045 (0.029), 0.132 (0.048) and 0.100 (0.054) were obtained for Grootfontein Merino sheep. High positive genetic (rG) and phenotypic (rp) correlations were estimated between first parity and lifetime reproductive performance in all three flocks. For the Carnarvon Afrino flock, rG increased from 0.609 (0.367) between TWW1 and TWW2 to 0.791 (0.206) between TWW1 and TWW3. The corresponding rp, however, decreased from 0.735 (0.019) to 0.613 (0.026). Similar decreases in rp from 0.708 (0.011) and 0.696 (0.013) (Between TWW1 and and TWW2) to 0.549 (0.017) and 0.510 (0.022) (between TWW1 and TWW4) were observed for the Carnarvon and Grootfontein Merino flocks respectively. Genetic correlation estimates of 0.866 (0.127), 0.913 and 0.736 (0.272) were obtained between TWW1 and TWW2, TWW3 and TWW4 for the Grootfontein Merino flock. The results of this study, obtained with two different breeds and in two different environments in flocks with a high and a low reproductive rate, indicate that selection for increased lifetime reproductive performance could be based on total weight of lamb weaned after the first parity.

Genetic parameter estimates for fitness traits in South African Merino sheep

Abstract The main objectives of this study were to estimate heritabilities and the range of sire breeding values for reproductive traits and survival rate in two Merino flocks using a threshold model. A GFCAT set of programmes was used to analyse reproductive data collected on the Grootfontein Merino stud (from 1968 to 1996) and the Carnarvon Merino flock (from 1964 to 1983). Fitness traits analysed were fertility (whether a ewe lambed or not; 0 or 1), fecundity (number of lambs born to a ewe that lambed; 1, 2 or 3), litter size at birth (number of lambs born per ewe mated; 0, 1, 2 or 3), litter size at weaning (number of lambs weaned per ewe mated; 0, 1, 2 or 3) and survival rate (whether a lamb born alive, was dead or alive at weaning; 0 or 1). The estimated heritabilities on the underlying scale for the respective traits were 0.072, 0.173, 0.131, 0.092 and 0.000 for the Grootfontein Merino stud and 0.203, 0.311, 0.201, 0.183 and 0.000 for the Carnarvon Merino flock. The heritability estimates and the range in sire breeding values indicate that it would be possible to improve reproduction, but not survival rate, genetically through selection.

Resistance of Sabi and Dorper ewes to gastro-intestinal nematode infections in an African semi-arid environment

Abstract Sabi and Dorper ewes were evaluated for resistance to gastro-intestinal nematode parasites (predominantly Haemonchus contortus ) at the Matopos Research Station in Zimbabwe. Live weights (LWT), blood packed red cell volume (PCV) and faecal egg counts (FEC) were recorded at mating, 1 month before lambing and 1–3-month post-lambing for five separate lambings between 1996 and 2000. Dorper ewes were heavier ( P

A description of growth, carcass and reproductive traits of Sabi sheep in Zimbabwe

Abstract A total of 4355 lamb and 4299 ewe records, obtained from Sabi sheep of Zimbabwe 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 age ( P >0.05) of lamb at 18 months of age, and ewe age for hot and cold carcass weight (HCW and CCW). Significant ( P

Genetic (co)variances between wrinkle score and absence of breech strike in mulesed and unmulesed Merino sheep, using a threshold model

Heritability estimates for, and genetic correlations among neck wrinkle score, body wrinkle score, breech wrinkle score and the absence of breech strike were determined on 2918 16-month hoggets from a selection experiment with South African Merinos. Data were obtained from the lamb drops of 9 years between 1998 and 2007, born as the progeny of 247 sires and 1250 dams. All progeny born until 2002 were subjected to the Modified Mules operation as lambs (n = 1799). However, due to international pressure to phase out the Mules operation, animals from 2003 onwards were not mulesed (n = 1119). During both periods, the experimental animals were maintained in single flocks (separated on sex) after weaning. Progeny were subjected to visual appraisal of wrinkle scores (range) on the neck (1–6), body (1–5) and breech (1–6) at an age of ~16 months. The occurrence of blowfly strike in the breech area (breech strike) was recorded in all animals during the wool growth period of a year from shearing as weaners. Two four-trait animal models were fitted to obtain (co)variance components and ratios from the separate datasets with neck wrinkle score; body wrinkle score, breech wrinkle score and the absence of breech strike, all defined as threshold traits. Estimates of heritability for breech wrinkle score amounted to 0.27 and 0.45 for the mulesed and unmulesed groups, respectively. The absence of breech strike on the underlying scale was also heritable for the mulesed (0.33) and unmulesed (0.46) groups; translating to heritability estimates of 0.074 and 0.157, respectively, on the observed scale. The genetic correlations of wrinkle scores with absence of breech strike were generally favourable, but significant only for neck wrinkle score (–0.57) and breech wrinkle score (–0.45) in the unmulesed group. It was concluded that South African Merino sheep would respond to selection against breech strike under conditions of adequate challenge. Based on results from unmulesed sheep, selection for plainer sheep is also expected to result in a favourable correlated response in breech strike.