Sexual Preference of Crossbreed Ewes for Rams Medicated with Naloxone

Ewe sexual preference depends on several factors; ram attractiveness is important. This work was carried out in in the first week of September 2020 and repeated in November 2021. For the first trial two rams were treated with 0.5 mg naloxone im for 15 days, two control rams were treated with saline injections. Sexual preference was studied with synchronized ewes (n 0 12) displaying estrus at the end of naloxone treatment of the rams. Ewe’s proceptive and receptive behavior were noted. Plasma testosterone levels were monitored through the duration of the experiment in treated and control rams. For the second trial 4 rams and ewes were treated same as trial 1, and the number of ewes was increased to 24. It was observed that on the first week of treatment, testosterone levels were not significantly different between treated and control rams; but at the beginning of the second week of treatment testosterone plasma levels in naloxone treated rams increased until values reached levels of 15 ng/mL compared with 6 ng/mL in control rams. When ewes were exposed to treated and untreated rams during the first trial, 11 ewes showed high proceptivity and receptivity for the naloxone treated ram (p<0.01) and they were mated, for the control ram only one ewe showed complete preference. In the second trial it was observed that 19 ewes also showed high proceptivity and receptivity for naloxone treated rams (p < .001).  Hormone levels behaved following the same pattern as that observed during the first trial. It was concluded that naloxone treatment facilitated the secretion of testosterone in rams and the high concentration of this androgen motivated ewes to prefer mating with the naloxone treated ram.