Abstract. Decreased conception rate of dairy cows in the summer is mainly associated with the deleterious effects of environmental thermal stress on the female reproductive tract. Here, we suggest that decreased reproductive performance might be partially due to inferior-quality semen. Semen from five representative bulls was collected in summer (August to September) and winter (December to January) and evaluated with a computerized sperm-quality analyzer for bulls (SQA-Vb). No seasonal effect was found in fresh ejaculate, but sperm examined post-thawing showed lower velocity, motility and progressive motility (P<0.04) in summer vs. winter samples. Element concentrations in the seminal plasma, determined by inductively coupled plasma-atomic emission spectrometry, differed between seasons, with higher (P<0.01) concentration values of K, Mg, Na and S elements in winter vs. summer samples. Therefore, season-induced alterations in seminal plasma element concentration should be taken into account when using an extender for cryopreservation. Acrosome integrity was assessed by a triple-fluorescence test using Hoechst 33342, fluorescein isothiocyanate-conjugated Pisum sativum agglutinin (FITC-PSA) and propidium iodide. Acrosome reaction was examined by a one-step staining method using FITC-PSA. The proportion of sperm cells with a damaged acrosome post-thawing tended to be higher (P<0.07) in semen collected during the summer vs. winter. Such alterations suggest that seasonal reductions in sperm function might also be involved in the decreased conception rate of dairy cows in summer. Decreased fertility of dairy cows during the summer is mainly associated with the deleterious effects of environmental thermal stress on the female reproductive tract. However, the effects of heat stress on semen quality may also contribute to the phenomenon [1, 2].Mature mammalian spermatozoa are quiescent in the male reproductive tract. Upon ejaculation, they undergo capacitation, develop hyperactive motility and acquire acrosomal ability to react [3]. These events involve sperm membrane hyperpolarization, activation of plasma-membrane ion channels and changes in intracellular Ca 2+ concentration, [Ca 2+ ] i [4,5]. Thus, season-induced alterations in one of these events might compromise spermatozoon fertilization competence.In bovines, the sperm flagellum contains cyclic nucleotidegated (CNG) channels-nonselective cation channels that regulate Na + , Mg 2+ and Ca 2+ influx under cGMP and cAMP control [4][5][6]. CNGA3 (formerly CNGα3) ion channels are expressed along the entire sperm flagellum, whereas CNGB1 (formerly CNGβ1) channels are localized to its principal part [4,7,8]. Activation of CNG channels leads to depolarization of the membrane voltage and a concomitant increase in cytosolic cation concentrations [9]. In addition, calcium flux through sperm inositol 1,4,5-trisphosphate (IP 3 ) receptors (IP 3 Rs) located in the sperm's head and connecting piece plays a pivotal role in acrosomal exocytosis, as binding of IP 3 to IP 3 Rs trig...
