Genetic parameters for reproduction rate in the Tygerhoek Merino flock. 1. Heritability

Abstract

Data of the Tygerhoek Merino flock were used to obtain halfsib heritability estimates for the components of reproduction rate within age groups as well as averaged overa numberof lambing opportunities. Two separate sets of data were considered. The first set of data included progeny of all the selection group rams retained for two breedihgs seasons, while the second set of data included progeny of all the available rams. Reproduction traits Investigated were ewes conceived/ewe mated (Ec/Em), lambs born/ewe conceived (Lb/Ec), lambs born/ewe mated (Lb/Em), the maternal component of pre-weaning mortalities/Iamb born (Ld/Lb), and lambs weaned/ewe mated (Lw/Em). The obtained heritability estimates suggested that selection for Ec/Em is unlikely to yield substantial genetic progress in the Tygerhoek Merino flock. MUltiple birth rate, on the other hand, was heritable and it was contended that LblEc would respond to selection, a finding wh ich is in agreement with literatu re cited. The heritability estimates obtained for the maternal component of Ld/Lb were discouragingly low. Based on literature cited, however, itwasconcludedthatthe rearing ability of ewes should also be considered in a selection programme for LblEc, if the objective Is to markedly improve the composite trait of Lw/Em. Methods for the acceleration of the genetic progress in LblEc and LwlEm by the use of modern technology are proposed.

Data van die Tygerhoek-Merinokudde Is gebruik om halfsib-oorerflikheidsberamings vir die komponente van reproduksletempo binne ooi-ouderdomsgroepe en van die gemiddelde reproduksie oor 'n aantallamgeleenthede te ondersoek. Twee afsonderilke datastelle is geanaliseer. Die eerste datastel het nageslag van aile selekslegroepramme wat vir twee paarselsoene behou is, Ingesluit, terwyl nageslag van aile beskikbare ramme opgeneem is In die tweede dataste!. Reproduksle-eienskappe wat geanaliseer is, was oole beset/ooi gepaar (Ec/Em), lammers gebore/ooi beset (LblEc), lammers gebore/ooi gepaar (Lb/Em), die maternale komponent van voorspeense vrektes (Ld/Lb) en lammers gespeen/ool gepaar (Lw/Em). Die verkree oorerflikheidsberamings dui daarop datseleksie vir Ec/Em waarskynlik nie noemenswaardige genetiese vordering in die Tygerhoek-kudde sal veroorsaaknle. Meerlinggeboortes, aan die anderkant, was tot 'n mateoorerflik, en Lb/Ec sal waarskynlik reageer op seleksle. Hlerdie bevinding is in ooreenstemming met aangehaalde literatuur. Die oorerflikheldsberamings vir die maternale komponentvan Ld/Lbwasontmoedigend laag. Volgensaangehaaide literatuur, blyk dit nogtans dat die vermoe van oole om hulle nageslag groot te maak in 'n seleksieprogram vir Lb/Ec oorweeg behoort te word, Indien noemenswaardige verhoglng In die saamgestelde eienskap van Lw/Em ten doe I gestel word. Metodes waarvolgens genetiese vorderlng In Lb/Ec en Lw/Em deur die gebruik van moderne tegnologie versnel kan word, word voorgestel.

Keywords: Paternal halfslb heritability, reproduction rate, Merino sheep