The decrease in dairy cow fertility during the summer is mainly associated with the deleterious effects of heat stress on the female reproductive tract. The present study suggests that the decreased reproductive performance is, in part, a result of using semen of inferior quality. Evaluated parameters included (1) ionic concentrations in the seminal fluid of bull semen, including [Ca²⁺], [K⁺], and [Na⁺]; (2) expression of ion channels CNGB1, CNGA3, and IP3R; (3) parameters of semen quality such as volume, concentration, motility, and progressive motility; and (4) acrosome integrity. Semen was collected from 5 representative bulls throughout the summer (August and September) and winter (December and January), and was evaluated according to a computerized sperm quality analyzer for bulls (SQA-Vb, Medical Electronic Systems, Caesarea, Israel). [Ca²⁺], [K⁺], and [Na⁺] in the seminal fluid were determined by inductively coupled plasma-atomic emission spectrometry. A semiquantitative PCR and the computer program Scion Image (Scion Corporation, Frederick, MD) were used to determine RNA expression of ion channels CNGB1, CNGA3, and IP3R. Acrosome integrity was assessed by a triple-fluorescence test, which included Hoechst 33342 (h33342), fluorescein isothiocyanate-conjugated Pisum sativum agglutinin (FITC-PSA), and propidium iodide (PI) staining. Data were presented as mean ± SD and analyzed by t-test (JMP-in 5.1). Findings revealed seasonal differences in [K⁺], [Na⁺] (P < 0.01), and [Ca²⁺] (P < 0.08) in the seminal fluid. These were associated with differences in RNA expression for ion channels CNGB1, CNGA3 (P < 0.01), and IP3R (P < 0.09), which are known to be involved in acrosome reactions. Although no differences were found in fresh semen, a progressive decrease in motility was noted for post-thaw semen collected in the summer (P < 0.05). Furthermore, the proportion of sperm cells with damaged acrosomes was higher in post-thaw semen collected in the summer than in its counterpart collected in winter (54.2 ± 3.5% v. 51.4 ± 1.9%, respectively; P < 0.08). The results suggest that semen collected during the summer is less able to survive cryopreservation, as reflected by its inferior vitality post-thawing. Further examination is required to determine whether such alterations are involved in the low summer fertility of dairy cows.