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

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dc.contributor.advisor Schoeman SJ, Prof en
dc.contributor.advisor Cloete SWP, Mr en
dc.contributor.author Jaleta GD en
dc.date.accessioned 2016-09-22T08:34:26Z
dc.date.available 2016-09-22T08:34:26Z
dc.date.submitted 2001 en
dc.identifier.uri http://hdl.handle.net/20.500.11892/30928
dc.description.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 ewe lifetime production) and testis measurements in the Merino flock maintained at the Tygerhoek Experimental Farm. Non-genetic fixed factors influencing these traits were evaluated by using appropriate models. The influence of non-genetic factors on pre-weaning lamb survival rate and the effect of scrotal circumference on ewe fertility were also studied. Fixed effect models were fitted to the data using either Procedure GLM or CATMOD in SAS (1996) depending on the parameters being estimated. Variance components were estimated by Restricted Maximum Likelihood (REML) procedures, fitting an animal model. Records of 8310 lambs born from 2538 ewes and sired by 681 rams covering the period 1970 to 1998 were used in the analysis of birth weight (BW), weaning weight (WW) and pre-weaning average daily gain (ADG). All three traits were affected (p less than 0.001) by type of birth 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 both at birth and weaning and grew faster (p less than 0.001) than females and multiples, respectively. Non-selected animals were lighter than selected animals at birth and weaning with an inferior growth rate. BW increased with increasing dam age at lambing until a maximum of 3.7 kg was reached at 6-yr of age. However, WW and ADG reached a maximum at 4.5- and 4.2-yr of age, respectively. Variance components for BW, WW and ADG were estimated using Restricted Maximum Likelihood (REML) procedures. Four different animal models were fitted, differentiated by including or excluding maternal effects. The direct heritability estimates (h2) ranged from 0.19 to 0.38. 25 to 0.40 and 0.26 to 0.40 for BW, WW and ADG, respectively. Estimates of direct heritability (h2) were substantially higher when maternal effects were ignored. The direct heritability (h2) estimate for BW decreased from 0.38 to 0.19 when both or either of the maternal effects, genetic or environmental, were fitted in the model. The h2 estimate for WW was 0.36 when both maternal effects were ignored from the model and decreased to 0.29 and 0.25 when maternal effects were fitted. A similar trend was observed for ADG. The maternal :heritability (m2) for BW ranged from 0.38 to 0.41 when only maternal additive genetic effects were fitted in the model, and decreased to 0.25 and 0.27 when the maternal permanent environmental effect (c2) was fitted. The m2 for WW and ADG ranged from 0.02 to 0.11 and from 0.01 to 0.10, respectively. The respective c2 estimates ranged from 0.05 to 0.10 and from 0.02 to 0.09. Moderate negative genetic correlations (ram) between direct and maternal additive genetic effects were observed in BW, while close to zero estimates were obtained for WW and ADG. The direct additive genetic correlation estimates between BW and WW and BW and ADG were 0.16 and 0.04, respectively. The corresponding maternal additive genetic correlation estimates were 0.93 and 0.60. The direct and maternal additive genetic correlation estimates 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 371 rams were used in the evaluation of ewe lifetime productivity (total number born, number veaned and total weight weaned). These records were collected from ewes born from 1969 to 1994 and gave birth from 1971 to 1999. Year of birth of ewe and ewe two-tooth live-weight affected (p less than 0.001) the ewe reproduction traits investigated. Multiple born ewes were superior both in total number of lambs born and weaned than singles. Heritability estimates of 0.23, 0.1 7 and 0.20 were obtained for total number of lambs born (TLB), total number of lambs weaned (TLW) and total weight of lambs weaned per ewe lambing over four lambing opportunities (TWW), respectively. For total weight weaned per ewe lambing over the first lambing opportunity (TWWI) a heritability estimate of 0.02 was computed. Genetic correlation estimates of -0.10, 0.57 and 1.00 were obtained between TWWI and TLB, TWWI and TLW and TWWI and TWW, respectively. High and positive genetic correlations ranging 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 for scrotal circumference (SC), testis diameter (TD) and two-tooth live-weight (LW). The other objective of this study was to estimate the magnitude of the effect of SC on ewe fertility. Year of birth, selection group and LW were significant (p less than 0.001) sources of variation both for SC and 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 TD by 1.0 and 8.0 percent, respectively. The genetic correlation between SC and TD was unity. Rams with 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 lamb birth weight, year of birth, age of dam at lambing, type of birth (single, multiple) and sex of lambs. Lambs with a live-weight between 4.0 to 4.9 kg at birth showed the highest survival rate while lambs with a live-weight of 2.0 kg showed the lowest survival rate. en
dc.language English en
dc.subject Agriculture en
dc.subject Animal production en
dc.title A genetic study of early growth traits and ewe productivity in Merino sheep en
dc.type Masters degree en
dc.description.degree MSc en


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