Seasonal variation in sperm quality parameters in Swedish red dairy bulls used for artificial insemination

• Our data suggest a seasonal effect on bull semen quality, even in Northern Europe.
• Ejaculates collected in spring had better membrane integrity than winter or summer.
• Ejaculates collected in spring had less chromatin damage than winter or summer.
• Flow cytometric analysis permits an accurate evaluation of bull sperm quality.

The aim of the present study was to investigate the extent to which seasonal factors such as temperature, atmospheric pressure or visible light length affect dairy bull sperm quality. Straws from 10 dairy bulls, from semen collections in three different seasons, were available for this study. The following quality parameters were assessed: motility, measured by computer assisted sperm motility analysis (SpermVision motility analyzer), membrane integrity (staining with SYBR14/PI (Propidium Iodide)with flow cytometric measurement of fluorescence), content of reactive oxygen species (ROS) measured using HE(Hydroethidine) and DCFDA (2′, 7′-dichlorodihydrofluorescein diacetate), mitochondrial membrane potential (using JC-1 fluorescence stain), acrosome reaction (measured with fluorescein isothiocyanate-PNA (FITC-PNA) labeling combined with calcium ionophore A23187 and PI), DNA fragmentation index (%DFI, Sperm Chromatin Structure Assay) as well as morphology, using the William׳s staining protocol. Data was analyzed by analysis of variance (PROC MIXED), using SAS software. The proportion (%) (mean±SD) of living, dying and dead sperm cells varied between seasons, with the proportion of living spermatozoa being lowest in summer, although a significant difference (P<0.05) was only observed between spring and summer. The %DFI assessed by SCSA was lowest in spring and differed significantly (p <0.04) from summer. A trend towards significance was observed between spring and summer for the motility parameters linearity (LIN) (P=0.068) and straightness (STR) (P=0.062).