Martin R. Dally

Effect of early experience on the sexual performance of yearling rams

The following study examines the effect of early sexual experience on the sexual performance of yearling rams. Forty-eight rams were given four individual 30 min exposures to estrous ewes at 10 months of age (experimental Ss). Forty-seven male counterparts were given no early heterosexual experience (control Ss). All subjects were maintained in a single all-male group from weaning (3 months of age). At 22 months of age, experienced and control rams were individually exposed to four estrous ewes for 30 min on four occasions at weekly intervals. Sexual behaviors observed included courship, mounting and ejaculation frequencies. Significantly fewer experienced rams than control rams (P < 0.01) failed to show sexual interest in ewes on Test Day 1 (1 of 48 vs. 13 of 47 rams, respectively). In addition, sexually active experienced rams exhibited higher (P < 0.01) rates of ejaculation than sexually active control rams on all four test days (x = 4.17 vs. 3.45 per 30 min, respectively). Follow-up studies on non-performing rams revealed that continuous exposure to ewes usually activated sexual interest in females. It was concluded that lack of exposure to ewes during early development can inhibit the expression of sexual behavior in rams. This effect has not been reported in the sexual development of male cattle and swine.

In vitro fertility evaluation of cryopreserved ram semen and its correlation with relative in vivo fertility

The present study was designed to evaluate zona-free hamster ova assay conditions for cryopreserved ram semen and to investigate the correlation between ability to penetrate zona-free hamster ova and in vivo fertility. In vivo fertility was estimated for cryopreserved semen from 5 Merino rams using heterospermic insemination. Equal numbers of postthaw motile spermatozoa from a Merino ejaculate and pooled Suffolk ejaculates were mixed prior to insemination. Each Merino ejaculate was paired with the same pool of cryopreserved Suffolk semen. Relative in vivo fertility for each Merino ram was calculated as the proportion of offspring that were sired by the Merino (range 42 to 100%). These ejaculates also differed in their ability to penetrate zona-free-hamster ova (3.6 to 9.0 penetrated spermatozoa per ovum). Differences in penetration rate were correlated with in vivo fertility (P < 0.002, R2 = 0.69). Results of these studies suggest that the zona-free hamster ova bioassay may be a useful test in the assessment of cryopreserved ram sperm fertility.

Artificial selection of rams for sexual performance and its effect on the sexual behavior and fecundity of male and female progeny

The objective of this study was to determine the effect of a single generation of artificial selection for sexual performance in rams on the sexual behavior and fecundity of their male and female progeny. Ninety-two ram lambs born to sires selected for either high or low sexual performance were evaluated for their sexual behaviors at approximately 8 months of age when individually exposed to four estrous ewes for 30min in four weekly serving capacity tests. Number of mounts and successful matings (ejaculations) were recorded. Fourteen of the 17 high-performing ram lambs identified were sired by high-performing sires, whereas 22 of 37 low-performing ram lambs were sired by low-performing sires (P<0.01). Sons of high-performing sires exhibited more ejaculations (P<0.04) and more mounts without ejaculation (P<0.02) than sons of low-performing sires. The two groups of ram lambs did not differ in mating efficiency (ratio of ejaculations to total mounts). Daughters of high-performing rams (N=79) exhibited their first behavioral estrus approximately 8 days earlier than daughters (N=61) of low-performing sires (P<0.005). Ovulation rates for the two groups of ewe lambs did not differ (P=0.55). It was concluded that there was sufficient genetic variation in the population of sheep studied to obtain a significant response to selection for ram sexual performance in both male and female offspring in a single generation.

Heterosexual experience differentially affects the expression of sexual behavior in 6- and 8-month-old ram lambs

Abstract The sexual performance of 6- and 8-month-old ram lambs (175 and 26 subjects, respectively) was quantified when individually exposed to four estrous ewes for 30 min on four occasions 7 days apart. In addition, half of the animals in each age class (experimental subjects) were exposed to 16 estrous females for 6, 16 or 24 h after testing on Test Days 1, 2 and 3. Control subjects received no additional exposure to females. Sexual performance improved for all groups over the 4 weeks of testing. However, 6-month control rams improved at a slower rate than 6-month experimental rams. By the fourth weekly test, 58% of the control rams and 85% of the experimental rams had attained ejaculatory competence. Treatment did not affect the sexual performance of the 8-month-old subjects; both control and experimental groups had attained 100% ejaculatory competence by the fourth week. Length of the extended exposure to females did not affect the results. It was concluded that the sexual responsiveness of ram lambs toward females is sufficiently undeveloped at 6 months (i.e. puberty) that extended exposure to sexually receptive ewes is needed for many males to exhibit adult levels of sexual performance. At 8 months, the sexual development of ram lambs has sufficiently matured so that relatively brief encounters with estrous females releases the full expression of adult sexual behaviors. These findings may be of benefit to sheep breeders who wish to use ram lambs in their breeding programs.

Effects of sexual stimulation on the sexual performance of rams

Abstract The sexual performance of bulls and male goats is improved if they are allowed to view the hetero-sexual behavior of other males as a prelude to mating. The purpose of the following study was to determine whether sexual stimulation enhances the sexual performance of rams. In Experiment 1, 11 sexually experienced ram lambs (∼ 9 months of age) and 18 sexually experienced yearling and 2-year-old rams were individually exposed to 4 unrestrained, hormone-induced estrous ewes for 60 min after viewing the courtship and mounting behaviors of a male conspecific for 20 min (two tests) and in the absence of stimulator animals (two tests). In contrast to the results with bulls and bucks, the rams were hardly influenced by the sexual stimulation treatment. Latencies for first mount and first ejaculation were shorter for sexually stimulated ram lambs; otherwise, treatment differences were negligible. A similar follow-up experiment was administered to 12 mature rams using restrained females in the sexual performance tests. Again, treatment differences were minor. It was concluded that sexual stimulation does not functionally enhance the sexual performance of rams. Species differences in response to sexual stimulation are discussed in terms of female sexual behaviors that may result in a selective (competitive) advantage to males that are stimulated to locate and mate with females early in the estrous period.

Competitive ability of rams selected for high and low levels of sexual performance

The objective of this study was to determine any differences in the competitive ability of rams differing in levels of sexual performance. Twenty sexually experienced Targhee-type rams were preselected from a population of 95 males for exhibiting relatively high and low ejaculation rates when individually exposed to oestrous ewes for 30 min. Each of 10 high performers (HP) were paired with each of 10 low performers (LP) on three occasions when competing for food (after food deprivation) and twice when competing for an oestrous female. Tests were 5 min in duration. Food-deprived HP and LP rams were equally aggressive when competing for a source offood that could only be accessed by one ram at a time. Time offeeding by HP and LP rams did not differ. HP rams spent more time with the oestrous ewe and attained more mounts in a greater number of pairings than LP rams even though HP and LP rams did not differ in number of successful agonistic interactions (active displacements and successful defences) when competing for the ewe. It was concluded that HP and LP rams, taken as groups, do not differ in their basic competitive ability and that the greater mating success of HP rams can be explained by their higher levels of libido.

Social preferences of domestic ewes for rams (Ovis aries)

Abstract A series of four experiments was performed to investigate the social preferences of 16 domestic ewes for potential mating partners. Ewe preferences were assessed by measuring the time spent in proximity to each of 2 restrained rams and/or 1 or 2 restrained ewes, in 5- or 20-min tests. In the first experiment, it was shown that oestrous ewes preferred to be in proximity to rams, while anoestrous ewes preferred other ewes. The second experiment showed that ewe preferences were strongly correlated with the rates of ram sexual solicitations and that ewes preferred older, larger rams over younger, smaller rams. The third experiment demonstrated that ewes preferred rams that had a history of higher sexual performance, independent of their rates of sexual solicitation. The fourth experiment demonstrated that the presence of other oestrous ewes interacting with rams can enhance the attractivity of some, but not all rams. Taken together, these results suggest that ewes can have strong preferences for potential mates and that these preferences may account, in part, for the findings that older, larger, more sexually active rams tend to be more reproductively successful. These findings suggest specific management practices which could improve breeding efficiency in domestic flocks.

Effect of early fenceline exposure to estrous ewes on the sexual performance of yearling rams

Abstract Previous research has demonstrated that early heterosexual (mating) experience improves the sexual performance of yearling rams. The present study examines the effect of early fenceline (FL) contact of young ram lambs with estrous females on their sexual performance as yearlings. A total of 104 rams, 7–8 months of age, were given either no contact (NC), direct contact (DC) or FL exposure to estrous ewes during a 3-week period in the fall breeding season. When initially tested for sexual performance as yearling rams at 18–19 months, 8 of 21 (38%) FL rams exhibited no sexual interest in estrous females (did not court or mount females), a result that was intermediate to and not significantly different from (P>0.05) the incidence of sexual disinterest exhibited by NC (59%) and DC rams (15%). The difference between NC and DC rams was significant (P 0.05) the incidence of mounting by NC and DC males (41% vs. 85%, respectively). Again, NC and DC rams differed from one another (P

Sexual responses of rams to ovariectomized and intact estrous ewes.

This investigation compares the sexual performance of rams when exposed to ovariectomized and intact estrous ewes. In Experiment 1, 22 sexually experienced rams were sequentially exposed to hormone-induced ovariectomized estrous ewes or intact estrous ewes that were cycling naturally or induced to exhibit estrus using progestin sponges. The sexual performance of the rams was not affected by ewe treatment. In Experiment 2, 95 rams were simultaneously exposed to ovariectomized and intact ewes induced to exhibit estrus via hormone treatment. Rams mounted ovariectomized ewes more frequently than intact females, but ejaculation rates were similar for the two treatments. It was concluded that the ejaculation frequency of rams in sexual performance tests (i.e. serving capacity) is not biased by the mode of estrus induction of the stimulus females.

Effect of ewe restraint on the libido and serving capacity of rams

Abstract The following study examines the effect of ewe restraint when assessing the libido and serving capacity of rams. Twenty-five domestic rams were exposed to both unrestrained and restrained estrous ewes under circumstances in which copulations were permitted and prevented. Ewe restraint did not affect the rate of ejaculation or measures of precopulatory behavior when copulation was permitted. When rams were exposed to ewes whose perinea had been covered to prevent copulations (libido tests), bouts of leg-kicking and anogenital sniffing were not affected by female restraint but mounting occurred more frequently when ewes were unrestrained. The latter measures of libido correlated with ejaculation rates (serving capacity) observed in tests permitting copulation and, in four out of six cases, were slightly higher when covered females were restrained. Rams were particularly aggressive (butts and vigorous pawing) toward ewes that were covered and restrained. It was concluded that the advantages of ewe restraint are not great enough to justify its use when assessing the libido and mating potential of rams with estrous ewes.