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ARS Home » Pacific West Area » Dubois, Idaho » Range Sheep Production Efficiency Research » Research » Publications at this Location » Publication #159909
Title: TESTOSTERONE AND LUTEINIZING HORMONE RESPONSES TO NALOXONE HELP PREDICT SEXUAL PERFORMANCE IN RAMS

Author
item Stellflug, John
item PERKINS, ANNE - CARROLL COLLEGE
item Lavoie, Verne

Submitted to: Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/5/2004
Publication Date: 11/1/2004
Citation: Stellflug, J.N., Perkins, A., LaVoie, V.A. 2004. Testosterone and luteinizing hormone responses to naloxone help predict sexual performance in rams. Journal of Animal Science. 82:3380-3387.

Interpretive Summary: Levels of sexual performance (libido) range from rams with no interest in mating to those exhibiting high capacity to mate repeatedly with many ewes. Rams can be ranked from low to high libido by counting the number of ejaculations achieved during serving capacity tests that involve exposing rams to estrual ewes. A series of serving capacity tests are a good predictor of breeding performance but are a labor intensive, expensive, and often impractical test to conduct. Clearly an easier more practical method would be advantageous. Naloxone (an opioid inhibitor) did not increase sexual activity when injected into sexually inactive rams however, there appeared to be differences in LH response to naloxone in sexually active and inactive rams. Thus, the first objective was to determine if LH and testosterone respond differently to injected naloxone in relation to varying libido levels in rams. If differences occurred, the second objective was to determine if the differences would predict libido levels. Sexually active, sexually inactive and male-oriented rams were treated with 1.5 mg/kg BW naloxone in December of year 1 and the same rams were treated with either 0.75 or 1.5 mg/kg BW naloxone in November of year 2. For year 1, sexually active rams had increased LH values 15 min after naloxone and testosterone increased early for sexually active female- and male-oriented rams compared to a delayed, minimal increase for sexually inactive rams. For year 2, both LH and testosterone profiles depicted an earlier increase in sexually active than that observed for sexually inactive rams and a greater increase in testosterone for active rams with no significant difference between naloxone doses. These hormone values were used to predict sexual activity of the rams. In year 1, greatest accuracy was 73.5 % based on testosterone at 60 min after naloxone injection. In year 2, greatest accuracy was 85% based on LH values at 15 min multiplied by testosterone values at 60 min after naloxone. Test repeatability for both years on the same rams was 76%. During the fall breeding season, naloxone can assist with identifying sexually active and inactive rams ranging from 1.5 to 3 yr of age but it cannot distinguish between sexually active female- and male-oriented rams. The potential exists that this test could be used in the sheep industry to identify and exclude sexually low performing rams from breeding. It would be advantageous to only buy rams that are active breeders and possibly increase pregnancies early in the breeding period.

Technical Abstract: The first objective of this study was to determine if LH and testosterone respond differently to injected naloxone in relation to varying libido levels in rams. If differences occurred, the second objective was to determine if the differences would predict libido levels. From a group of 1.5- to 3.0-yr-old rams, 20 sexually active and 39 sexually inhibited rams were selected based on previously observed sexual behavior. Each ram was exposed to three estrual ewes for 18, 30-min sexual performance tests and those found to be inactive were given two 30-min sexual partner preference tests. Final distribution was 28 sexually active, 22 inactive, and 9 male-oriented rams. Rams were treated with 1.5 mg/kg BW naloxone in December of year 1 and the same rams were treated with either 0.75 or 1.5 mg/kg BW naloxone in November of year 2. Plasma values of LH and testosterone were evaluated with mixed model analyses for repeated measures separately for each year to coincide with logistics procedures for modeling the probability that rams were sexually active. For year 1, a sexual activity x age x time interaction for LH after naloxone was observed (P < 0.03). Sexually active rams had increased LH values 15 min after naloxone. For testosterone, there was a sexual activity x time interaction (P < 0.03) with a similar, early increase for sexually active female- and male-oriented rams compared to a delayed, minimal increase for inactive rams. For year 2, when all rams were over 2.5 years of age, a sexual activity x time interaction for both LH and testosterone (P < 0.02) depicted an earlier increase in sexually active than that observed for sexually inactive rams and a greater increase in testosterone for active rams. No significant difference was observed between naloxone doses of 0.75 mg and 1.5 mg/kg BW. Testosterone and LH values were used as explanatory variables and sexual activity as the response variable in logistics procedures. In year 1, greatest accuracy was 73.5 % using testosterone at 60 min after naloxone injection. In year 2, greatest accuracy was 85% using LH values at 15 min multiplied by testosterone values at 60 min after naloxone. Test repeatability for both years on the same rams was 76%. In conclusion, a single injection of naloxone to 1.5 to 3.0-yr-old rams during the breeding season stimulates an increase in LH followed with an increase in testosterone that will assist with identifying sexually active and inactive rams with an accuracy of 69 to 85%.