Abstract
Reduced reproductive performance of dairy cows during the summer is often associated with elevated temperature. Semen collected and cryopreserved during the summer may be of low quality and might contribute to the compromised fertility of dairy cows during this season. The present study examined the association between seasonality, semen quality and its potential to survive cryopreservation. A comparison between semen collected during the summer (July to August) and that collected during the winter (November to December) revealed the summer semen to be inferior, as reflected by low motility and high mortality of sperm. Furthermore, samples that were defined as good quality had changes in lipid concentration and fatty-acid composition in both the seminal plasma and cell compartment. In particular, semen collected during the summer had reduced levels of polyunsaturated arachidonic acid (20:4; P < 0.05) and decreased levels of linoleic acid (18:2; P < 0.05) in the cell compartment; corresponding reductions in cholesterol ( P < 0.06) and fatty-acid concentrations ( P < 0.001) were detected in seminal plasma of semen collected during the summer. In addition, we provided the first evidence for the existence of a very-low-density lipoprotein receptor (VLDLr) in bovine sperm, suggesting a mechanism for sperm utilization of extracellular lipids. Interestingly, the expression of VLDLr was three-fold greater in samples collected during the winter than in those collected in the summer ( P < 0.01) and was negatively associated with saturated fatty-acid concentration ( P < 0.018) but not with that of cholesterol. An opposite pattern was noted for samples obtained during the summer; mRNA expression of VLDLr was negatively associated with cholesterol concentration ( P < 0.01) but not with that of saturated fatty acids. Such modifications associated with extracellular lipid utilization and fatty-acid composition might explain, in part, the reduced quality of summer semen.
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