Variation in post-thaw sperm quality of white-tailed deer bucks (Odocoileus virginianus) during rut

Abstract

White-tailed deer farming is a growing industry in the United States, with breeding operations contributing significantly to the industry’s economic impact. Artificial insemination with frozen semen allows for selection and dissemination of valuable genetics, yet surprisingly little is known regarding the best time throughout rut to perform semen cryopreservation. The objective of this study was to compare semen quality following cryopreservation of white-tailed deer bucks collected early in the breeding season (September, n = 6), at peak rut (December, n = 8), and late season (March, n = 7). We hypothesized that post-thaw semen quality would be enhanced at peak rut. Mature bucks were anesthetized with tiletamine-zolazepam and xylazine administered intramuscularly via projector. Semen was collected by electroejaculation and cryopreserved using Optixcell extender. Overall and progressive sperm motilities were assessed for each sample before and after cryopreservation using a computer-aided sperm analyzer. Flow cytometry was used for post-thaw assessment of sperm viability (SYBR-14/PI), acrosome integrity (FITC-PNA/PI), and DNA stability (acridine orange). Analysis of variance was applied to normalized data using a general linear mixed model with buck ID as a random variable, and a Tukey HSD test was performed as needed for post-hoc analysis. Pre-freeze overall and progressive sperm motilities were lowest in March, intermediate in September, and highest in December (p ≤ 0.04). Post-thaw overall and progressive motilities were lowest in September (p ≤ 0.02), but did not differ between December and March (p ≥ 0.12). The DNA Fragmentation Index was lowest in December, intermediate in September, and highest in March (p ≤ 0.05). The percentage of spermatozoa with intact plasma membrane was higher in December than September (p < 0.01), but the percentage of intact acrosomes per sperm with intact plasma membrane was highest in September (p = 0.03). This study confirms that post-thaw semen quality appears to be superior during peak rut (December) in bucks. Though semen collected early or late in rut may present acceptable motility, DNA stability is impaired, which could adversely affect fertility rates. This data suggests that semen cryopreservation during transitional periods should be avoided, though field studies evaluating the translation of these results into satisfactory pregnancy rates are warranted.