Variance components and genetic parameters for body weight and fleece traits of Merino sheep in an arid environment

Abstract

Data collected on the Carnarvon Merino flock over the period 1962 to 1983 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 genetic effects for several body weight and fleece traits, were estimated by DFREML procedures. Traits analysed included 120-day weaning weight (WW), body weight (W6) and greasyfleece weight (GFW6) at six months of age, as well as body weight (W18), clean fleece weight (CFW) and mean fibre diameter (MFD) at 18 months of age. By ignoring or including maternal genetic or environmental effects, four different models of analysis were fitted to determine the most suitable model for each trait. WW and GFW6 were significantly influenced by both maternal additive genetic and permanent environmental effects. For W6, W18 and CFW, the most suitable model included a direct as well as a maternal additive genetic effect, while only the direct additive genetic effect had a significant influence on MFD. Direct and maternal heritability estimates for the various traits were as follows: 0.14 and 0. 11 for WW; 0.18 and 0. 10 for W6; 0.20 and 0.06 for GFW6; 0.43 and 0.04 for W18; and 0.26 and 0.04 for CFW. A direct heritability of 0.60 was estimated for MFD. Standard errors for these estimates ranged from 0.01 to 0.04. Positive genetic correlations between direct and maternal genetic effects were estimated for WW (0.57), W6 (0.86) and W18 (1.00).

Data wat van 1962 tot 1983 op die Carnarvonse Merinokudde ingesamel is, is vir hierdie studie gebruik.  Variansie-komponente vir direkte additiewe genetiese effekte, maternale additiewe genetiese effekte en maternale permanente omgewingseffekte vir verskeie liggaamsmassa en vageienskappe is deur middel van DFREMl-prosedures beraam. Die volgende eienskappe is ontleed: 120-dae speenmassa (WW), liggaamsmass (W6) en rouvagmassa (GFW6) op ses maande-ouderdom, sowel as liggaamsmassa (W18), skoonvagmassa (CFW) en veseldikte (MFD) op 18 maande-ouderdom. Om die mees geskikte model vir elke eienskap te bepaal, is vier verskillende modelle gepas deur maternale genetiese en omgewingseffekte te ignoreer of in te sluit. WW en GFW6 is betekenisvol deur maternale additiewe genetiese effekte sowel as maternale permanente omgewingseffekte beïnvloed. Die mees geskikte model in die geval van W6, W18 en CFW het direkte en maternale additief genetiese effekte ingesluit terwyl slegs die direkte additief genetiese effek 'n invloed op MFD gehad het. Direkte en maternale oorerflikheidsberamings vir die verskillende eienskappe was soos volg: 0.14 en 0.11 vir WW; 0.18 en 0.10 vir W6; 0.20 en 0.06 vir GFW6; 0.43 en 0.04 vir W18; en 0.26 en 0.04 vir CFW. 'n Direkte oorerflikheid van 0.60 is vir MFD beraam. Standaardfoute vir hierdie beramings het gewissel van 0.01 tot 0.04. Positiwe genetiese korrelasies tussen die direkte en maternale genetiese effekte is vir WW (0.57), W6 (0.86) en W18 (1.00) beraam.

Keywords: Merino sheep, body weight, fleece traits, maternal effects, variance components, arid environment.