A genetic study of early growth traits and ewe productivity in merino sheep

[摘要] ENGLISH ABSTRACT:The objectives of this study were to investigate genetic parameters of early growth traits,lifetime ewe productivity (total number born, number weaned and weight weaned per ewelifetime production) and testis measurements in the Merino flock maintained at the TygerhoekExperimental Farm. Non-genetic fixed factors influencing these traits were evaluated by usingappropriate models. The influence of non-genetic factors on preweaning lamb survival rateand the effect of scrotal circumference on ewe fertility was also studied. Fixed effect modelswere fitted to the data using either Procedure GLM or CATMOD in SAS (1996) dependingon the parameters being estimated. Variance components were estimated by RestrictedMaximum Likelihood (REML) procedures, fitting an animal model.Records of 8310 lambs born from 2538 ewes and sired by 681 rams covering the period 1970to 1998 were used in the analysis of birth weight (BW), weaning weight (WW) andpreweaning average daily gain (ADG). All three traits were affected (p < 0.001) by type ofbirth of lambs (single, multiple), sex, year of birth of lambs, group of animals (selection,control) and age of dam at lambing (2- to 6-yr old). Male lambs and singles were heavier bothat birth and weaning and grew faster (p < 0.001) than females and multiples, respectively.Non-selected animals were lighter than selected animals at birth and weaning with an inferiorgrowth rate. BW increased with increasing dam age at lambing until a maximum of 3.7 kgwas reached at 6-yr of age. However, WW and ADG reached a maximum at 4.5- and 4.2-yr ofage, respectively.Variance components for BW, WW and ADG were estimated using Restricted MaximumLikelihood (REML) procedures. Four different animal models were fitted, differentiated byincluding or excluding maternal effects. The direct heritability estimates (h) ranged from 0.19to 0.38, 0.25 to 0.40 and 0.26 to 0.40 for BW, WW and ADG, respectively. Estimates ofdirect heritability (h) were substantially higher when maternal effects were ignored. Thedirect heritability (h) estimate for BW decreased from 0.38 to 0.19 when both or either of thematernal effects, genetic or environmental, were fitted in the model. The h2 estimate for WWwas 0.36 when both maternal effects were ignored from the model and decreased to 0.29 and0.25 when maternal effects were fitted. A similar trend was observed for ADG. The maternalheritability (rn-) for BW ranged from 0.38 to 0.41 when only maternal additive genetic effectswere fitted in the model, and decreased to 0.25 and 0.27 when the maternal permanentenvironmental effect (c) was fitted. The m for WW and ADG ranged from 0.02 to 0.11 andfrom 0.01 to 0.10, respectively. The respective c2 estimates ranged from 0.05 to 0.10 and from0.02 to 0.09. Moderate negative genetic correlations (ram) between direct and maternaladditive genetic effects were observed in BW, while close to zero estimates were obtained forWW and ADG. The direct additive genetic correlation estimates between BW and WW andBW and ADG were 0.16 and 0.04, respectively. The corresponding maternal additive geneticcorrelation estimates were 0.93 and 0.60. The direct and maternal additive genetic correlationestimates between WW and ADG were 0.99 and 0.85, respectively.Data of 3272 lambing records collected on 818 ewes born from 689 dams and sired by 371rams were used in the evaluation of ewe lifetime productivity (total number born, numberweaned and total weight weaned). These records were collected from ewes born from 1969 to1994 and gave birth from 1971 to 1999. Year of birth of ewe and ewe two-tooth liveweightaffected (p < 0.001) the ewe reproduction traits investigated. Multiple born ewes weresuperior both in total number of lambs born and weaned than singles. Heritability estimates of0.23, 0.17 and 0.20 were obtained for total number of lambs born (TLB), total number oflambs weaned (TLW) and total weight of lambs weaned per ewe lambing over four lambingopportunities (TWW), respectively. For total weight weaned per ewe lambing over the firstlambing opportunity (TWWl) a heritability estimate of 0.02 was computed. Geneticcorrelation estimates of -0.10, 0.57 and 1.00 were obtained between TWW1 and TLB,TWW1 and TLW and TWWl and TWW, respectively. High and positive genetic correlationsranging from 0.61 to 0.92 were estimated between ewe lifetime reproduction traits.Data from 1380 rams born from 1986 to 1998 were used to investigate genetic parameters forscrotal circumference (SC), testis diameter (TD) and two-tooth liveweight (LW). The otherobjective of this study was to estimate the magnitude of the effect of SC on ewe fertility. Yearof birth, selection group and LW were significant (p < 0.001) sources of variation both for SCand TD. Heritability estimates ranged from 0.29 to 0.40, 0.25 to 0.38 and 0.49 to 0.52 for SC,TD and LW, respectively. Adjustment for LW decreased heritability estimates of SC and TDby 11.0 and 8.0 %, respectively. The genetic correlation between SC and TD was unity. Ramswith larger SC had a significant effect on ewe fertility (ewes lambing per ewe mated).Average survival rate from birth to 100 days of age was 0.79 and was influenced by lambbirth weight, year of birth, age of dam at lambing, type of birth (single, multiple) and sex oflambs. Lambs with a liveweight between 4.0 to 4.9 kg at birth showed the highest survivalrate while lambs with a liveweight of:s 2.0 kg showed the lowest survival rate.

[发布机构] Stellenbosch University