De genetica achter de scrotumomtrek en gewichtseigenschappen bij Black Hereford runderen
De Black Hereford is een relatief jong vleesveeras, ontstaan aan het einde van de 20ste eeuw in de Verenigde Staten. Het ras vindt zijn oorsprong in kruisingen tussen het Hereford en Angus ras, twee beroemde vleesrassen. Gedurende de eerste jaren van het fokprogramma, hebben de fokkers zich vooral toegelegd op productiekenmerken zoals de voederconversie en de vleeskwaliteit. Nu het ras aan bekendheid toeneemt, wensen de fokkers ook enkele reproductiekenmerken in hun fokprogramma op te nemen. Een van deze reproductiekenmerken is de leeftijd waarop een vaars (een eerste kalfskoe) voor de eerste maal kalft. In optimale commerciële omstandigheden kalven vaarzen wanneer ze twee jaar oud zijn. Op dit moment ligt de gemiddelde leeftijd in veel kuddes echter hoger.
In andere vleesveerassen in de Verenigde Staten wordt de omtrek van het scrotum van de vader gelinkt aan de leeftijd van eerste kalving van de dochter. De nakomelingen van een stier met een grotere scrotumomtrek op jaarling leeftijd bereiken gemiddeld vroeger de puberteit. Vaarzen die vroeger de puberteit bereiken, kunnen vroeger gedekt of geïnsemineerd worden en dus op jongere leeftijd kalven.
De scrotumomtrek van een stier kan eenvoudig gemeten worden. Dit gebeurt door middel van een circulaire tape die rond het scrotum gespannen wordt. De meting dient voor alle dieren op dezelfde leeftijd (1 jaar) te gebeuren.
Een belangrijke parameter voor een bepaald kenmerk in de selectie is de erfelijkheidsgraad. Deze erfelijkheidsgraad geeft weer in hoeverre de geobserveerde variantie van het kenmerk kan worden toegeschreven aan de genetica van de dieren. Enkel kenmerken met een matige tot hoge erfelijkheidsgraad kunnen direct worden gebruikt in een fokprogramma. In deze studie werden de erfelijkheidsgraden voor het Black Hereford ras voor geboorte -, speen - en jaarling gewicht en de jaarling scrotumomtrek geschat als matig tot hoog. Dit wijst erop dat selectie voor deze vier kenmerken mogelijk is.
Daarnaast is het belangrijk om de invloed van de selectie voor een grotere scrotumomtrek op andere kenmerken te bestuderen. In deze studie werd dan ook de genetische correlatie tussen de jaarling scrotumomtrek en het geboorte-, speen- en jaarling gewicht bestudeerd. Voor deze drie gewichtskenmerken werd de genetische correlatie als positief geschat. Dit duidt erop dat wanneer er geselecteerd wordt voor een grotere scrotumomtrek op jaarlingleeftijd dit geen negatieve invloed heeft op de verschillende gewichtskenmerken.
In de fokkerij wordt vaak gebruik gemaakt van fokwaardeschattingen. Deze fokwaardeschattingen geven een beeld over de genetische aanleg van potentiële ouderdieren. In deze studie werden fokwaardeschattingen berekend voor het geboorte -, speen - en jaarling gewicht en de jaarling scrotumomtrek voor 4099 Black Hereford runderen. Deze fokwaardeschattingen kunnen worden gebruikt in het fokprogramma van het ras.
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