Sexual performance of rams as determined by maturation and sexual experience

The purpose of this study was to compare the sexual performance of rams that differed in age (maturation) and sexual experience. Twenty-four ram lambs (8 mo of age) and 21 rams (20 mo of age) were individually exposed to four females in hormonally induced estrus for 30 min on five occasions, 7 d apart. Half (12) the ram lambs and nine of the yearlings were sexually naive at the start of testing (had been denied access to females since weaning); the remaining males were sexually experienced. The sexual performance of the virgin rams was poorer during the initial exposure to females than in subsequent tests, whereas the sexual behavior of the sexually experienced males did not change over the days of testing. Sexually naive ram lambs and rams exhibited an improvement in sexual performance between Tests 1 and 2; during Test 3 to 5 the sexual performance (ejaculations per test) of sexually naive and experienced rams was similar. The only effects of age (maturation) on sexual performance during the last three test days were a higher frequency of mounts without ejaculation plus mount attempts (P less than .03) and a greater number of mount interactions per ejaculation (P less than .02) by ram lambs. In conclusion, one or two relatively brief exposures to estrous females can bring the sexual performance of virgin rams up to levels comparable to that of experienced males. Also, ram lambs in good condition exhibit acceptable levels of sexual performance.     

Luteinizing hormone, testosterone, and behavioral response of male-oriented rams to estrous ewes and rams.

During the breeding season three experiments were conducted to evaluate the LH and testosterone (T) response of rams with male sexual orientation (e.g., male-oriented homosexual rams) to female sheep, to male sheep, and to treatment with LHRH. Male-oriented rams were identified through a series of sexual performance and sexual preference tests. Treatments included exposure to estrous females and to males for 15 min (Exp. 1) and exposure to estrous females and to males for 8 h (Exp. 2). Behavioral responses to stimulus animals were recorded. In Exp. 2 homosexual rams mounted males more than females (P less than .02) and exhibited more flehmen (P less than .002) and investigatory sniffs (P less than .01) when exposed to males vs females. Acts of aggression (butting the stimulus animals) did not differ by gender (P greater than .1). Flehmen and butting were positively correlated to LH secretion (P less than .02) of rams exposed to females but not to males. In Exp. 1, LH concentration determined every 15 min for 6 h was not affected (P greater than .05) by the gender of the stimulus animal. In Exp. 2, LH pulse frequency and concentration were similar (P greater than .05) by treatment. Lack of an LH response to sexual activity in homosexual rams was not a result of pituitary or gonadal insensitivity; within 1 h of a single injection of LHRH both LH and T increased (Exp. 3).(ABSTRACT TRUNCATED AT 250 WORDS)     

Sexual Performance of Rams Sequentially Exposed to Short-tailed and Fat-tailed Ewes

The objective of the study was to compare sexual performance of pure and crossbred rams, and to evaluate whether prior exposure of rams to short-tailed females would enhance their mating ability when later exposed to fat-tailed females. Twenty-two virgin, yearling Awassi (A; n = 7), F₁ Charollais × Awassi (CA; n = 7) and F₁ Romanov × Awassi (RA; n = 8) rams were subjected to sexual performance tests on six 20-min occasions. Each ram was individually exposed to two short-tailed oestrous ewes for three 20-min occasions on three consecutive days. Following 1 day of rest, the same 3-day procedure was repeated for each ram with fat-tailed ewes. Leg kicking bout frequency increased in CA and RA rams and decreased in A rams, when they were exposed to fat-tailed compared with short-tailed ewes. No differences in anogenital sniffing were observed among rams exposed to either short-tailed or fat-tailed ewes. However, greater (p < 0.001) anogenital sniffing bouts were recorded, when rams were exposed to short-tailed females. Upon exposure to fat-tailed ewes, CA and RA rams experienced a marked increase in mounting frequency compared with a slight increase in mounting of A rams (p < 0.001). The ability of Awassi rams to raise the fat tail of Awassi ewes was greater (p < 0.001) than CA and RA rams. Mating was improved in A while declining in CA and RA, when they were exposed to fat-tailed compared with short-tailed ewes (p < 0.001). Based on the results of the current study, it seems that all yearling rams were capable of mating with short-tailed ewes, whereas only Awassi rams managed to mate with fat-tailed ewes. It appears that brief exposures of yearling crossbred rams to short-tailed ewes do not improve their mating ability when later exposed to fat-tailed ewes.     

Inability to predict the adult sexual performance of bulls by prepuberal sexual behaviors

The objective of this investigation was to determine the extent to which the sexual performance of adult bulls (18 to 24 mo of age) can be predicted by prepuberal sexual behaviors. Tests were administered to 121 polled Hereford bulls to determine whether adult sexual performance (especially serving capacity or ejaculation rate) was related to 1) whether or not the subjects mounted estrous females at 6 mo of age, 2) the age at which the bulls attained their first ejaculation, 3) the frequency of sexual behaviors exhibited in heterosexual interactions and 4) the frequency of male-male mounting exhibited in all-male groups. None of the above measures reliably predicted adult serving capacity. Prepuberal interest in mounting other males and estrous females and the age at which ejaculatory competence was attained bore little relationship to serving capacity at 18 to 24 mo. Individual differences in rate of ejaculation were consistent from one age to another only after the bulls had reached 18 mo. We concluded that the sexual behaviors of prepuberal beef bulls were of limited usefulness in predicting adult sexual performance.     

Short-term individual housing temporarily reduces the libido of bulls

The objective of the present study was to determine the effects of short-term (6 wk) individual housing on the sexual interest (libido) of sexually experienced bulls. Twenty-two Hereford bulls reared with both males and females (BMF) or in all-male groups (BM) and approximately 21 mo of age were given free access for 40 min to five restrained females in hormonally-induced estrus and restricted access for 30 min to a single enclosed estrous female on two separate occasions. Free-access tests were administered to bulls in groups of five or six individuals, whereas bulls were tested individually in restricted-access tests. After testing, half the animals were housed individually (physically isolated from conspecifics) and half remained in group housing for a 6-wk period. Libido tests were administered again at the end of the treatment period and 2 wk after reinstatement of group housing for the experimental subjects. Individually housed bulls exhibited fewer mounts (P less than .001) and spent less time with females (P less than .01) in tests administered during physical isolation than before or after treatment. Individual housing did not affect BMF and BM bulls differentially. In conclusion, short-term individual housing temporarily reduced libido of beef bulls.     

Luteinizing hormone and testosterone response of sexually active and inactive rams.

The objective of this study was to identify rams exhibiting high (HP) and low (LP) levels of sexual performance and to determine whether their respective behavioral responses to ewes in estrus were related to changes in serum testosterone (T) and LH concentrations. Rams were selected on the basis of standardized serving capacity tests. Plasma T and LH concentrations in rams were measured in three experiments: 1) after 15 min of exposure to estrous ewes, 2) after an injection of 500 ng of LHRH, and 3) during an 11-h exposure to estrous ewes. During 15 min of exposure to ewes, HP rams were sexually active, whereas LP rams showed no sexual interest. Secretion of LH was similar (P greater than .05) between ram groups. Sexual arousal, copulation, and ejaculation of HP males were not related (P greater than .05) to LH secretion. Exposure to estrous ewes for 11 h, however, stimulated LH pulse frequency and elevated basal LH and T concentrations in HP but not LP rams (P less than .001). Luteinizing hormone secretion was positively correlated to the frequency of mounts (r = .19; P less than .01) and ejaculation (r = .17; P less than .03). Aggressive behavior of rams directed at ewes was negatively correlated to LH (r = -.22 P less than .003). Concentrations of LH and T after LHRH injection were similar between HP and LP rams (P greater than .05). These results show that the effects of the ewe on LH secretion of rams depend on length of the exposure period and sexual activity of the male.     

Relationship between sexual behavior classifications of rams and lambs sired in a competitive breeding environment

The objectives for this study were to 1) determine the relationship between sexual performance class and lambs sired in a competitive mating environment, and 2) determine whether the male-oriented ram test is valid. Fifteen 2- to 3-yr-old whitefaced rams classified as female-oriented, with high or low sexual performance, or classified as male-oriented were used in a multiple-sire breeding arrangement. Five groups of approximately 200 ewes each were exposed for 21 d to 3 rams per group consisting of 1 ram from each class. Rams were blocked for sexual class, and those with close genetic relationship were assigned to different pens. Genomic DNA was prepared from blood collected from 15 rams, 934 ewes, and 1,757 lambs. Up to 4 microsatellite markers were used to determine a lamb's sire. Of 884 ewes with identifiable lambs (known sires), 178 ewes had single lambs, 408 had multiples sired by 1 ram, and 298 had multiples sired by more than 1 ram. The sexual partner preference test used to identify male-oriented rams did not absolutely reflect their sexual performance during competitive breeding. In contrast to only mounting and servicing males in preference tests before breeding, male-oriented rams sired 480 lambs from 330 ewes. Serving capacity tests predicted sexual performance of high and low sexual performance rams. High performance rams impregnated more ewes (499 vs. 258; P < 0.05) and sired more lambs (756 vs. 357; P < 0.05) than did low performance rams, respectively. Low performance and male-oriented rams did not differ for ewes impregnated or lambs sired. We conclude that 1) sexual partner preference tests used to classify male-oriented rams were not absolute in reflecting their breeding performance in a competitive breeding environment; 2) serving capacity tests predicted that high performance rams would breed more ewes than low performance rams and sire more lambs than either low performance or male-oriented rams; and 3) under the conditions of this study, low performance and male-oriented rams did not have an adverse impact on the overall breeding outcome. Combined, low performance and male-oriented rams sired 81 more lambs than did high performance rams, but this required twice as many rams to obtain approximately equal breeding results. Therefore, we suggest that serving capacity tests should be used to select high performance rams, reduce number of rams with marginal sexual performance, and make decisions on ram numbers needed.     

Development of sexual and aggressive behaviors in Hereford bulls

The purpose of this study was to describe maturation-related changes in the sexual and aggressive behaviors of Hereford bulls. Two groups of 13 and 14 bulls, respectively, were tested for sexual and aggressive behaviors every 3 mo from 3 to 24 mo of age. At 3 and 6 mo of age, the bulls were exposed, as groups, to two estrous females, whereas at 9, 12, 15, 18, 21, and 24 mo these bulls were tested under each of the following several treatment conditions: 1) individual tests (single males were exposed to single females), 2) small-group tests (three or four bulls:three females), 3) large-group tests (13 or 14 bulls:3 females), and 4) male-only tests (13 or 14 bulls were placed in the test arena in the absence of females). Most bulls consistently mounted females by 9 mo of age. Twenty-one of the 27 bulls (78%) attained their first ejaculation at either 12 or 15 mo of age. The frequency of mounting without ejaculation was greatest in large-group tests at 12 mo of age, and ejaculation frequency peaked at 18 mo. The incidence of male-male mounting in large-group tests (with females) did not appreciably change from 3 to 24 mo, whereas male-male mounting in male-only tests was greatest at 9 and 24 mo. We conclude that sexual performance data obtained in serving capacity tests administered to yearling bulls before 18 mo of age may underestimate the mating potential of certain individuals.(ABSTRACT TRUNCATED AT 250 WORDS)     

Ram mating behavior after long-term selection for reproductive rate in Rambouillet ewes

Mating behavior is known to be heritable in several species, but it is not known if selection schemes for ewe reproductive traits affect mating behavior of rams. Therefore, our objectives were to determine if divergent (high vs low index) selection of a female reproductive trait (lambs born divided by age of ewe minus one) affects mating behavior patterns of male offspring during training to service an artificial vagina (AV) in a less than natural setting (Exp. 1) or during serving capacity tests (SCT) in simulated natural mating conditions (Exp. 2). The method for evaluating male mating behavior was a series of serving capacity tests to estimate sexual performance. For Exp. 1, five, 30-min observations were conducted to assess the mating behavior of 29 rams (22 mo old) being trained to service an AV. In Exp. 1, percentages of rams mounting (73 vs 36%) and ejaculating (67 vs 29%) differed (P < 0.05) between rams from high and low Rambouillet ewe selection lines, respectively. In Exp. 2, ram classification consisted of exposing each 22- to 24-mo-old ram (n = 48) to three unrestrained ewes in estrus for 18, 30-min tests. Any ram that had not mounted or ejaculated during SCT was evaluated for sexual orientation in a 30-min preference test in which each ram had access to both restrained estrual ewes and restrained rams. In Exp. 2, there was no difference (P > 0.2) in percentages of rams mounting (92 vs 78%) and ejaculating (88 vs 74%) between rams from high and low Rambouillet ewe selection lines, respectively. There was no difference (P > 0.33) between high and low ewe selection line rams for mounts or ejaculations for 18 SCT. Only one male-oriented ram was identified, which was from the high line. In Exp. 2, six SCT were necessary to obtain 95% reliability in sexual performance scores. Results of these studies indicate that long-term selection of ewes for litter size did not alter ram sexual performance after SCT under simulated natural mating conditions or affect incidence of male-oriented rams. Mating behavior observed during training to service an AV was lower for rams from ewes of the low selection line than for rams from ewes of the high selection line. We concluded that long-term selection for reproductive rate in ewes did not alter mating behavior patterns of male offspring; however, test conditions, such as restrained and unrestrained ewes and number of serving capacity tests, may affect conclusions in studies that evaluate sexual performance of rams.     

Sexual performance of rams in serving capacity tests predicts success in pen breeding.

The purpose of this study was to determine the extent to which sexual performance (serving capacity) tests can be used to predict the sexual behavior and reproductive success of rams in the context of pen mating. Standard serving capacity tests were used to select four low (LP) and four high-performing (HP) rams from a population of 94 yearling males. Each selected ram was then exposed to approximately 30 estrus-synchronized ewes for a 9-d period. Ejaculations observed and mounting marks left on ewes confirmed the greater (P less than .001) sexual activity of the HP rams in the field. In addition, ewes exposed to HP rams had a higher lambing percentage, more lambs born, and more live lambs born per ewe. Ram classification was not related (P greater than .60) to the number of lambs born per ewe lambing (prolificacy). It was concluded that serving capacity tests, properly conducted, can be used to predict ram mating performance and thus aid in establishing more efficient ram-to-ewe stocking rates.     

Effects of group size and the male-to-female ratio on the sexual performance and aggressive behavior of bulls in serving capacity tests

The purpose of the following study was to determine the effects of group size and the male-to-female ratio on the sexual and aggressive behaviors of bulls in the context of serving capacity tests. Tests were administered to two groups of 13 and 14 polled Hereford bulls, respectively, under four test conditions in which the number of bulls and the ratio of males to females was varied. The test conditions were as follows: 1) individual tests (single males were exposed to single females), 2) small-group tests (three or four bulls:three females), 3) large-group tests (13 or 14 bulls:3 females), and 4) male-only test (13 or 14 bulls were placed in the test area in the absence of females). In addition, the sexual performance of 44 Hereford and 12 Angus bulls was compared when competing (small-group tests) vs not competing (individual tests) for females. Ejaculation rate (serving capacity) was similar in individual and small-group tests in which a male-to-female ratio of approximately 1:1 was maintained. Increasing the male-to-female ratio to approximately 4.5:1 enhanced levels of aggression and reduced overall sexual performance. Angus bulls were more aggressive than their Hereford counterparts when competing for females and exhibited lower (P less than .05) ejaculation rates in small-group tests than when evaluated individually. We concluded that the sexual performance of polled Hereford bulls can be reliably evaluated when tested in small groups as long as a 1:1 male-to-female ratio is approximated.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sexual behavior of medium-ranked rams toward non-estrual ewes is stimulated by the presence of low-ranked rams

In sheep, access to receptive females is affected by dominance relationships. The aim of the first experiment was to determine how the presence of high- or low-ranking rams affects sexual behavior of medium-ranked rams toward non-estrual ewes in pen tests. A complementary objective was to determine whether the presence of 1 or 2 rams has stronger effects. Social rank of 13 rams was determined using the food competition test. The number of courtship behaviors, mounts, and ejaculations was recorded in pen tests with non-estrual restrained ewes. In the first experiment, medium-ranked rams were subjected to 5 tests with: (1) the ewe alone, (2) the ewe and 1 restrained tied high-ranked ram, (3) the ewe and 2 restrained tied high-ranked rams, (4) the ewe and 1 restrained tied low-ranked ram, and (5) the ewe and 2 restrained tied low-ranked rams. The number of high- or low-ranked rams (1 or 2) present in the tests did not affect the results. There were no differences on courtship behaviors, but the number of mounts and mounts with ejaculation was greater when low-ranked rams were present. In a second experiment, it was determined whether the presence of medium- or low-ranked rams stimulates sexual behavior of high-ranked rams. The high-ranked rams were tested with: (1) the ewe alone, (2) the ewe and 1 restrained tied medium-ranked ram, and (3) the ewe and 1 restrained tied low-ranked ram. No differences were recorded in relation to presence of other rams during the tests. It was concluded that the presence of low-ranked rams stimulates sexual behavior of medium-ranked rams toward non-estrual ewes in pen tests. This effect was not related with the number of rams included in the test. The presence of medium- or low-ranked rams did not alter the sexual behavior of high-ranked rams toward non-estrual ewes.     

Year-round breeding of crossbred Dorset or Finnish landrace ewes using a synthetic light regimen

A group of crossbred Dorset or Finnish Landrace ewes maintained under a synthetic light regimen (Light Treatment, 4 mo long days - 16 h light, 4 mo short days - 8 h light) and exposed to rams every 8 mo (January 1, September 1, May 1) was compared for lamb production over a period of 4.5 yr with two other comparable groups under natural daylight conditions, one exposed to rams once a year in the fall (Control I) and the other every 8 mo (Control II). Conception rates across breed type of ewe were 92% for Control I, 66% for Control II and 83% for Light Treatment ewes. Conception rates for May breedings only were 16% in Control II and 88% in Light Treatment ewes. Prolificacies of crossbred Finn ewes were higher (P less than .05) than those in crossbred Dorset ewes of Control I (258 vs 193%), Control II (187 vs 165%) and Light Treatment (238 vs 163%). The annual production at lambing from the crossbred Finn and Dorset ewes in Control I was 251 and 206 lambs/100 ewes exposed to rams, respectively. Corresponding productions were 231 and 178 for Control II and 296 and 211 for Light Treatment ewes. Mortality of lambs from the crossbred Finn ewes (27%) was higher (P less than .01) than that from the crossbred Dorset ewes (12%). Annual attrition of the crossbred Dorset ewes in Control I, Control II and Light Treatment groups was 5.2, 5.4 and 5.4%, respectively. Corresponding percentages of the crossbred Finn ewes were 7.0, 8.8 and 11.2.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of morphine and naloxone on LH response and sexual behavior of rams (Ovis aries)

Effects of the opiate agonist, morphine, and antagonist, naloxone, on LH release, courtship behavior and ejaculation frequency of mature, sexually active or sexually inactive rams were investigated. Plasma LH concentrations were monitored from blood samples collected every 15 min for 10 hr (0800 to 1800 hr) from eight rams that were isolated from or in contact with estrous females. Plasma LH concentration was higher (P<.05) in sexually active rams exposed to receptive females compared with hormone concentration of rams isolated from ewes. Intravenous infusion of morphine sulphate (1 mg/kg) into rams 4 and 6 hr after exposure to ewes reduced (P<.05) plasma LH concentration as compared to rams given saline. Morphine did not affect (P>.05) courtship behavior (investigatory sniff, mount attempt, foreleg kick, flehmen, vocalization) but diminished (P<.05) number of ejaculations. In another trial, LH concentrations were higher (P<.05) in seven sexually active rams given naloxone iv or when given to three rams through an intracerebroventricular cannula (icv) as compared to LH response of sexually inactive rams. LH did not differ (P>.05) in seven sexually inactive rams before or after administration of naloxone. Investigatory sniffs by sexually active rams were increased (P<.03) after treatment with the opiate antagonist. Four of the seven sexually active rams had more ejaculations after naloxone compared with the pretreatment period, but mean ejaculation frequency after treatment did not differ (P=.31). Naloxone did not stimulate courtship behavior of sexually inactive males. These data suggest that the effect of opiates on sexual behavior and LH secretion depends upon the inherent level of sexual activity among rams.     

Testosterone and luteinizing hormone responses to naloxone help predict sexual performance in rams

The first objective of this study was to determine whether LH and testosterone respond differently to a naloxone injection in relation to varying sexual performance in rams. If differences occurred, the second objective was to determine whether differences would predict variation in sexual performance. From a group of 1.5- to 3-yr-old rams, 20 sexually active and 39 sexually inactive rams were selected based on previously observed sexual behavior with estrual ewes. 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. The final distribution was 28 sexually active, 22 inactive, and nine male-oriented rams. Rams were treated with 1.5 mg of naloxone/kg BW in December of Year 1 and again with either 0.75 or 1.5 mg of naloxone/kg BW in November of Year 2. Plasma concentrations of LH and testosterone were evaluated with mixed model analyses for repeated measures separately for each year to coincide with logistic 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). 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 with a delayed, minimal increase for inactive rams. For Year 2, when all rams were over 2.5 yr of age, a sexual activity x time interaction for both LH and testosterone (P < 0.02) seemed more related to an earlier increase of both hormones for sexually active rams than the increase observed for inactive rams. In addition, sexually active rams had a greater increase in testosterone than inactive rams. No significant difference was observed between 0.75 and 1.50 mg of naloxone/kg BW. Testosterone and LH were used as explanatory variables and sexual activity was used as the response variable in logistic procedures. In Year 1, greatest prediction accuracy was 73.5% using testosterone at 60 min after naloxone injection. In Year 2, the greatest prediction accuracy was 85% using LH at 15 min multiplied by testosterone at 60 min after naloxone. Test repeatability for both years on the same rams was 76%. In conclusion, pattern and magnitude of naloxone-induced changes in endocrine function may facilitate identification of sexually active and inactive rams during the breeding season. Prediction accuracy of the naloxone-based test was 69 to 85%.     

High frequency of lymphocyte chimerism XX/XY and an analysis of hereditary occurrence of placental anastomoses in Booroola sheep

Cytogenetic studies of a closed herd of Booroola Merino sheep were carried out over six consecutive years and covered 167 (90 females and 77 males) sheep originating from 77 heterosexual multiple births. Lymphocyte 54XX/XY chimerism was revealed in 24 litters including 20 (11.98%) ewes and 22 (13.17%) rams, i.e. 25.14% of the karyotyped group of animals and 11.17% of all animals weaned in the herd. Familial and pedigree relations revealed in the study indicate a hereditary tendency to develop placental anastomoses in sheep. Fifteen chimeric litters were sired by six rams. Three rams were carriers of XX/XY chimerism and each sired a chimeric litter. Coefficients of inbreeding ranged from 0.06525 to 0.125 with a mean of 0.094. The pedigree analysis up to the fifth generations showed that two rams were ancestors of all chimeric litters. Based on the analysis of the reproductive performance of dams and sires which produced 32 litters of which three-quarters were chimeric suggested, that the formation of anastomoses is controlled by a dominant gene.     

Behavior and endocrine changes in high-performing, low-performing, and male-oriented domestic rams following exposure to rams and ewes in estrus when copulation is precluded.

High-performing, low-performing, and male-oriented rams were used to investigate behavior and neuroendocrine correlates of sexual interest and discrimination. Treatment consisted of visual and olfactory contact with stimulus animals through a woven wire fence, which inhibited copulation (either ewes in estrus or other rams), for 4 h on each of three consecutive days. Before exposure to stimulus animals on d 1 and during the final 1 h of exposure on d 2, blood samples were collected every 15 min for 1 h to determine concentrations of LH and testosterone. During exposure to stimulus animals, rams were continuously observed and investigatory behaviors were recorded. There was no day effect for any behavior. Groups of rams differed (P < .05) in amounts of behaviors exhibited, but behaviors were not influenced by sex of stimulus animals. High-performing rams exhibited more (P < .05) investigatory behaviors toward stimulus animals than low-performing or male-oriented rams. Plasma concentrations of LH increased (P < .05) in high-performing rams following exposure to estrous ewes, but not following exposure to rams. In low-performing and male-orientated rams, concentrations of LH were unchanged regardless of sex of the stimulus animal. Change in plasma concentrations of testosterone from pre- to posttreatment did not differ between high-performing, low-performing, and male-oriented rams. However, low-performing rams exhibited an increase (P < .05) in plasma concentrations of testosterone following exposure to rams. In conclusion, high-performing rams exhibit a high degree of investigatory behaviors toward estrous ewes and other rams. High-performing rams seem to discriminate sex of stimulus animals and exhibit a neuroendocrine response (i.e., increased plasma LH) only when exposed to ewes in estrus. The sensory signals provided by estrous females are either not detected by low-performing or male-oriented rams or are not sufficiently provocative to elicit further investigation by these rams.     

Breed and environmental influences on weaner lamb production in Zimbabwe

Performance of the Dorper and Mutton Merino breeds and their crosses was assessed from 1,305 ewe and 1,453 lamb records from six lambing seasons. Dorper and Mutton Merino were mated in all combinations under an accelerated lambing system and crossbred females were mated to Suffolk rams. The Dorper and Mutton Merino breeds were not significantly different for fertility, pre-weaning lamb growth and survival. Suffolk sired lambs from crossbred ewes grew rapidly and were heaviest at birth and at weaning. They also survived better than lambs from the other breeding groups and this contributed to the higher weaning rate (P less than 0.05) in crossbred ewes. Weight of weaned lambs per ewe exposed to the ram per season was 12% higher (P less than 0.05) in the Dorper than the Mutton Merino ewes. However lifetime production was 28% higher (P less than 0.01) in Mutton Merino ewes due to significantly superior longevity. Crossbred ewes were highly productive. Individual heterosis was not significant but estimates were positive for most traits. Growth rate and survival of lambs increased as age of dam increased to 48 and 40 months respectively. Lambs born to previously non-lactating ewes were heavier and survived better than lambs born to ewes lactating in the previous season. Lambs born and reared as singles were 25% heavier at birth (P less than 0.001) and 52% heavier at weaning (P less than 0.001) than those born and reared as twins. Male lambs grew faster and were 7% heavier at weaning than females (P less than 0.001).     

Competition Between Different Social Ranked Rams has Similar Effects on Testosterone and Sexual Behaviour Throughout the Year

Dominant rams have preferential access to females, as they frequently interrupt sexual behaviour from subordinated. Testosterone concentrations are directly linked to sexual and aggressive behaviour and have important variations along the year. Therefore, it may be expected that the effects of dominance relationships on reproductive behaviour differ according to testosterone concentrations, and thus to the period of the year. The aim of this study was to compare the effects of dominance relationships on testosterone and sexual behaviour in different moments of the year in rams. Twelve rams were maintained in a single group. Social rank was determined in January (maximum reproductive development), May (regression of the reproductive status) and August (lowest reproductive activity), and the four rams with higher (HR) and the four with lower (LR) success index were used. Testosterone serum concentration was weekly measured four times during each experimental period. Sexual behaviour was evaluated in each period with an oestrous ewe, and with the oestrous ewe and another ram from the other social rank (each HR with each LR ram). Testosterone concentration was greater in HR than LR rams in January (p = 0.03), and all the behaviours were displayed more frequently in non‐competitive than in competitive tests (p < 0.05). Rams modified their sexual strategy in competitive environments decreasing the display of sexual behaviour independently of their social status. This effect was observed consistently throughout the year: high‐ranked rams have greater testosterone concentrations than LR rams only during the pre‐rut, when they naturally compete to join the groups of ewes.     

Sexual and aggressive behaviors of Hereford bulls: interrelationship of variables and problems with the use of composite scores to describe overall sexual performance

The purpose of this investigation was to determine the interrelationships of the various sexual and aggressive behaviors exhibited by Hereford bulls in sexual performance (serving capacity) tests and to determine the extent to which these interrelationships change with age (maturity). Correlation matrices were constructed for the frequencies of sexual and aggressive behaviors exhibited by two groups of 13 and 14 bulls, respectively, during 120-min tests with females when the bulls were 12, 18, and 24 mo of age. The frequencies of selected sexual behaviors recorded during individual tests at 18, 21, and 24 mo were factor-analyzed using a principle components analysis. Also, a comparison was made of the length of the refractory periods following various sexual behaviors at 12 and 24 mo of age. Bulls varied greatly in the way they responded to sexual stimuli. Few sexual behaviors were intercorrelated and the factor analysis of selected sexual behaviors was inconclusive. Frequencies of aggressive behaviors were more highly correlated. Age (maturation) did not affect the interrelationship of the sexual behaviors recorded. Aggressive behavior variables were more highly correlated at 12 mo than at 18 or 24 mo of age. The average length of the refractory period following ejaculation was longer (112 s; P less than .05) than the refractory period following mounts without ejaculation (50 s), and the latter had a longer (P less than .05) refractory period than mount attempts, mount intentions and head throws (26 to 38 s).(ABSTRACT TRUNCATED AT 250 WORDS)