Seasonal variation in testis diameter and sperm output of rams of breeds of different prolificacy

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Abstract
SUMMARY: Seasonal variation in testis diameter and sperm output was studied in five Finnish Landrace (Finn) and five Tasmanian Merino (Merino) rams, together with five crossbreds between these breeds (Finn-Merino), five selected phenotypically from the Finn-Dorset breed type for high juvenile testis diameter (Finn-Dorset-High) and four for low juvenile diameter (Finn-Dorset-Low). There was marked seasonal variation in testis diameter for each of the breed types. The time of the seasonal increase in testis diameter varied among the breed types, the Merino rams starting earliest—49 days before the Finns. The characteristics of seasonal variation in testis diameter thus showed similarity with the characteristics of seasonal variation in the reproductive performance of females of the breed type. Breed differences in response to season were also demonstrated by a significant breed by season interaction for sperm production, but there were no indications of a useful association between variation in sperm production and variation in the reproductive performance of related females. Overall, differences in adult testis diameter were small relative to those among juveniles.