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Relationship between sexual behavior classifications of rams and lambs sired in a competitive breeding environment^1,2

J. N. Stellflug^3,*, N. E. Cockett and G. S. Lewis^*

^* USDA-ARS, US Sheep Experiment Station, Dubois, ID 83423; and and Utah State University, Logan, UT 84322-4700

	Abstract

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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.

Key Words: competitive mating • ewe lambing • lambs born • paternal typing • ram • sexual behavior

	INTRODUCTION

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Sexual performance of rams is highly variable (Terrill, 1937; Price, 1987). Most mature rams readily court, mount, and mate estrual ewes, whereas the intensity of sexual behavior varies from asexuality to high sexual activity. After extensive serving capacity tests at the US Sheep Experiment Station, 15 to 20% of 18- to 24-mo-old rams showed no interest in estrual ewes. Approximately 50% of rams that are not interested in estrual ewes are classified as male-oriented (6 to 8% of all rams). At 8 to 10 mo of age, in a different population of rams, Price et al. (1988) found a similar incidence of asexual rams (18.5%; n = 54); and of 44 sexually active rams, 68.2% were female-oriented, 22.7% showed no statistically significant gender preference, and 9.1% were male-oriented.

Even though different classifications of rams are recognized, it is less clear what effect rams with different classifications have on flock fertility. In single sire mating pens, 83 to 93% of approximately 30 estrus-synchronized ewes lambed after a 9-d exposure to high sexual performing rams, compared with 21 to 48% of ewes lambing that were exposed to low performing rams for 9 d (Perkins et al., 1992). However, it is unknown what influence male-oriented rams have in a competitive breeding environment with high and low sexual performance rams. The current study was designed to 1) determine the relationship between sexual performance class (i.e., high sexual performance female-oriented, low sexual performance female-oriented, or male-oriented rams) and lambs sired in a competitive breeding environment, and 2) determine whether the male-oriented ram test is valid.

	MATERIALS AND METHODS

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General
Before breeding, all rams were kept at the US Sheep Experiment Station, Dubois, ID (located at 44° 14' N and 112° 11' W), and ewes were maintained on native rangelands. In confinement, rams and ewes were fed to meet their daily nutrient requirements depending on the stage of production (maintenance, breeding, pregnancy, or lactation; NRC, 1985).

Sexual Performance Tests
White-faced 2- to 3-yr-old rams were selected after 2 types of sexual performance tests: 1) a series of nine 30-min serving capacity tests in which individual rams were observed with 3 unrestrained, estrual teaser ewes, and 2) a series of three 30-min sexual partner preference tests in which individual rams were observed with 2 restrained estrual ewes and 2 restrained rams (Stellflug and Berardinelli, 2002). Rams were classified as female-oriented, high or low sexually active based on average number of ejaculations with ewes, and as male-oriented if they were exclusively sexually active with rams during the initial serving capacity and preference tests. Rams were given one more 30-min preference test within 2 wk after breeding. Mounts and ejaculations were recorded during all tests. Teaser ewes were ovariectomized and estrus-induced using pessaries containing 60 mg of 6-methyl-17-hydroxyprogesterone acetate (Pharmacia and Upjohn, Kalamazoo, MI) and injection of 50 µg of estradiol 17ß (Stellflug and Berardinelli, 2002). Ewes were considered to be in estrus when they would stand to be mounted by a ram.

Semen Collection and Evaluation
An electroejaculator (Ideal Instruments Inc., Chicago, IL) was used to collect semen. Sperm was evaluated for morphology and motility with a phase contrast microscope (Nikon Instruments Company, Inc., Melville, NY). Normal and motile sperm were estimated to the nearest 10%, followed with a forward progressive movement score (1 to 4; 1 < 25%, 2 = 25 to 50%, 3 > 50 to 75%, 4 > 75% motility) using a modification of Terrill’s (1937) system. Metabolic activity of spermatozoa was evaluated using a Spectra 601 spectrophotometer (Milton Roy Company, Ivyland, PA) to measure the relative absorbance at 600 nm wavelength with a resazurin reduction assay (Wang et al., 1998).

Breeding Management
Rams classified as female-oriented with high sexual performance (n = 5), female-oriented with low sexual performance (n = 5), and male-oriented (n = 5) were used in a multiple-sire breeding arrangement. Five groups of approximately 200 ewes each (996 total) were used. One ram from each of the 3 classes was kept with each group for 21 d (approximately 10 ewes in estrus each day) in 1.5-ha pens. Rams with known close genetic relationships were assigned to different pens. Black-faced clean-up rams were introduced to the ewes after the study. Three groups were placed in breeding on November 3, and 2 groups were placed in breeding 10 d later. The ewes were assigned to breeding pens, stratifying for breed and previous prolificacy. The breed distribution of Columbia (range of 30 to 31 per pen), Polypay (38 to 39), Rambouillet (98 to 100), and Targhee (30 to 32) ewes provided a balanced prolificacy of 1.42 to 1.45 per pen. The age distribution ranged from 26 to 29% 2-yr-olds, 16 to 19% 3-yr-olds, 21 to 23% 4-yr-olds, 14 to 17% 5-yr-olds, 8 to 11% 6-yr-olds, and 8 to 9% 7-yr-olds in each pen.

Blood Collection and Paternal Typing
Blood from 15 rams and from 1,757 white-faced lambs and their dams was collected in 10 mL Vacutainers containing EDTA (Becton Dickinson, Franklin Lakes, NJ), stored at 4° C, and extracted within 7 to 10 d after collection. Genomic DNA was extracted from blood using a standard salt-precipitation method (Miller et al., 1988). Eight microsatellite markers were used in parentage testing of lambs: McM111 (Smith et al., 1995a) for pens 1, 3, 4, and 5; McM136 (Hulme et al., 1995) for pen 2; McM137 (Smith et al., 1995b) for pen 1; OarFCB304 (Buchanan and Crawford, 1993) for pen 3; McM63 (Smith et al., 1995b) for pen 4; OarCP34 (Ede et al., 1995) for pen 5; BMS460 (Stone et al., 1995) for pens 2, 3, and 5; and RM004 (Kossarek et al., 1993) for pens 3 and 5. Microsatellite genotyping was performed as described by Cockett et al. (1996). Sire of each lamb was determined as that ram that could exclusively provide the remaining microsatellite allele after accounting for the dam’s microsatellite allele. For some lambs, only 2 microsatellites were required to determine the sire. If a sire could not be unequivocally assigned after examination of genotypes for up to 4 microsatellites, sire was designated as ambiguous for that lamb. Of 1,757 white-faced lambs, 1,593 lambs could be assigned unambiguously to 1 of the 3 rams in their dam’s breeding pen. A range of 5 to 11% of lambs sired in each breeding pen was excluded from the data set. Their sires could not be identified because of 1) missing samples (either lamb or dam or both), 2) failure to extract DNA from the blood sample, 3) dam misparentage, or 4) failure to unambiguously distinguish among rams within the breeding pen.

Statistical Analyses
The GLM procedures of SAS (SAS Inst. Inc., Cary, NC) were used to analyze the number of ewes lambing, day of birth, percentage of normal spermatozoa, forward progressive movement of spermatozoa, and resazurin score for each ram type. Mixed model analyses of SAS were used to analyze number of lambs born from each ram type and serving capacity test scores for high and low performance rams. The Genmod procedures of SAS with the Poisson link were used to analyze mounts on ewes and rams and ejaculations in ewes and rams recorded during preference tests. The GLM and Mixed models consisted of ram class and breeding pen as main effects and ram class x breeding pen interaction as the error term. Levene’s test (Milliken and Johnson, 1984) was used to test for homogeneity of variance. Variances for ewes lambing, lambs sired, and day of birth were homogeneous. Variances for semen evaluation and sexual behavior variables were heterogeneous. The data for semen evaluation and sexual behavior variables required log transformation and were transformed back to original units for presentation of results. The SE for original units was estimated by subtracting each least squares mean from its upper confidence limit and dividing by 2. The Genmod models consisted of ram class and preference test number as main effects and ram class x preference test number interaction as the error term. If main effects were significant (P < 0.05), Fisher’s protected LSD was used as a post hoc test to detect difference between individual means.

	RESULTS

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Of 996 ewes used in this study, 32 died before lambing, 35 (3.6% of ewes present at lambing) were bred with clean-up rams, and 30 (3.1% of ewes present at lambing) did not lamb. A total of 899 ewes were used for paternal typing, but 15 of them did not have any lambs with identifiable sires. Consequently, data for lambs from these 15 ewes and for unidentifiable lambs from ewes that only had some of their lambs with identifiable sires were deleted from the data set (n = 164 lambs). Of 884 ewes with identifiable lambs, 178 ewes had single lambs, 408 had multiples sired by one ram, and 298 had multiples sired by more than one ram. In Table 1, the total count for ewes lambing was 1,087 because the same ewe was included 2 or 3 times if she had identifiable lambs from 2 or 3 sires, respectively. The number of ewes lambing after exposure to the 3 classes of rams varied (P < 0.05) according to ram class but not among breeding pens. High sexual performance rams impregnated 46% of the ewes (n = 499), compared with 24% (n = 258) and 30% (n = 330) for low sexual performance rams and male-oriented rams, respectively (Table 1).

Sperm morphology, progressive forward motility, and resazurin score did not differ among ram classes used for breeding. The 5 high sexual performance rams had 3.2 ± 0.2 ejaculations per 30-min test, compared with 0.4 ± 0.2 ejaculations for the 5 low sexual performance rams in 9 serving capacity tests performed before breeding. The 5 male-oriented rams did not mount or ejaculate during 9 serving capacity tests performed before breeding. Table 2 shows the combined data for the 3 preference tests before breeding and the 1 preference test after breeding that were used for analyses. During 3 preference tests before breeding, high performance rams averaged 11.5 ± 1.5 mounts and 3.4 ± 0.4 ejaculations compared with 5.7 ± 3.1 mounts and 0.9 ± 0.4 ejaculations for low performance rams. Male-oriented rams averaged 20.9 ± 7.1 mounts and 0.6 ± 0.3 ejaculations on males and no mounts or ejaculations on estrual females in the preference tests before breeding.

	DISCUSSION

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION IMPLICATIONS LITERATURE CITED

 
The individual sexual partner preference tests did not absolutely reflect the breeding performance of the male-oriented rams. The sexual activity of all 5 of the male-oriented rams required to impregnate ewes and sire lambs differed from the lack of sexual activity in 4.5 h of serving capacity tests, and sexual activity toward only male partners in 1.5 h of sexual partner preference tests before the breeding period. However, the individual serving capacity tests accurately predicted the breeding performance of high and low performance, female-oriented rams.

Data are not available to explain the inability of preference tests to absolutely reflect the actual breeding performance of male-oriented rams in a competitive mating environment. Some possibilities for the inability of the ram test to predict the breeding performance are restraint of teasers, lack of competition, and ability to acclimate to the test environment. Restraint of estrual ewes, as used in sexual partner preference tests, does not have a major impact on sexual performance in preference tests (Price et al., 1993). However, restraint of teaser rams may result in different behavioral effects than restraint of ewes. Restrained rams may divert attention from estrual ewes similar to the influence of anestrous ewes in serving capacity tests (Zenchak et al., 1988). Lack of competition in the tests or slow acclimation of rams to the test conditions may lead to lower sexual activity than normal. Sexual activity was enhanced when rams were exposed to a ram that had recently mated (Maina and Katz, 1997), and this influence along with competition and breeding activity of other rams may stimulate the male-oriented rams to breed ewes that have been recently bred by other rams because of pheromones or the presence of semen. The increased breeding activity, male to female ratio, and pheromones may improve the acclimation of some male-oriented rams. Familiarization to surrounding conditions may not be required for high performance rams to the same extent as for male-oriented rams in serving capacity tests.

The competitive, natural breeding experience did not alter male mounting activity of 2 of the 3 male-oriented rams that exhibited sexual activity during the preference test after breeding. This observation indicates that the breeding experience did not completely change sexual partner preference behavior in all male-oriented rams. However, the fact remains that sexual partner preference tests did not absolutely reflect breeding performance of male-oriented rams during competitive breeding. In contrast, serving capacity tests accurately predicted the increased breeding performance of the high sexual performance rams, compared with the low performance and male-oriented rams. High sexual performance rams impregnated approximately twice as many ewes as low sexual performance rams and sired nearly as many lambs as low performance and male-oriented rams combined.

Results of the current study with multiple sires are in agreement with a previous report using single-sire mating that indicated high performance rams service more ewes and produce more lambs than low performance rams (Perkins et al., 1992). The current study also agrees with reports that sexual behavior of rams influences flock fertility (Mattner et al., 1971; Kilgour, 1993) but contrasts with others (Kelly et al., 1975; Kilgour and Wilkins, 1980; Mickelsen et al., 1982) in which little difference was found. One of the key differences among the studies relates to the number of ewes that are in estrus daily (i.e., high sexual performance only manifests itself under conditions providing sufficient breeding challenge). The greater breeding success of high performance rams compared with low performance and male-oriented rams probably is related to their greater sexual motivation as documented by more mounts and ejaculations in sexual performance tests, rather than being attributable to their general dominance. This concept is supported by results from a study specifically designed to determine differences in competitiveness for food and estrual ewes between high and low sexual performance rams (Erhard et al., 1998).

Evaluation of date of birth indicated there was no significant difference among the 3 ram behavior classifications, with only a 1- or 2-d difference. Thus, one ram class did not seem any slower to start breeding ewes than the other ram classes. This observation differs from work in cattle demonstrating that calves sired by high serving capacity bulls were born earlier (Blockey, 1978); however, there was no competition between high- and medium-libido bulls in the same breeding pen that could explain the differing results. In addition, cattle become less receptive to males after several services (De Alba et al., 1961), unlike sheep that remain receptive for many hours regardless of previous services (Parsons and Hunter, 1967). Our short breeding exposure of 21 d did not allow for a second breeding in all ewes and might have masked the difference among rams regarding time of breeding, but the low percentage (<3%) of ewes bred to clean-up rams or that failed to get pregnant suggests that a longer breeding period would have had minimal impact on our results.

With paternal genotyping data, we were able to document the considerably large number of ewes that gave birth to lambs sired by more than 1 ram. This agrees with observations that rams do not necessarily lose their opportunity to service ewes when they are not among the first to find and breed estrual ewes. Ewes remain receptive for many hours even when frequently bred (Parsons and Hunter, 1967; Fletcher and Lindsay, 1971). The current study also demonstrated that a similar percentage of ewes gave birth to singles, twins, and triplets regardless of which classification of ram impregnated them. There was no indication that any of the 3 classes of rams was able to disproportionately select and impregnate the more prolific ewes. Thus, the greater number of lambs sired by high performance rams, compared with low performance and male-oriented rams, was primarily because high performance rams impregnated more ewes. Rams selected for this study represented extremes of a continuum for serving capacity test results. Thus, we cannot conclude that average-scoring rams would be inferior to high performance rams under these same conditions.

The combination of high, low, and male-oriented rams in the same breeding pen did not adversely impact overall reproductive success during the breeding period. Approximately 3% of ewes present at lambing time were bred to clean-up rams, and 3% of the ewes did not lamb. Together, low performance and male-oriented rams sired 81 more lambs than high performance rams. Thus, twice as many low performance and male-oriented rams were needed to obtain approximately equal breeding results as those for high sexual performance rams in a competitive environment.

	IMPLICATIONS

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION IMPLICATIONS LITERATURE CITED

 
This research indicates that individual sexual partner preference tests do not absolutely reflect breeding performance of male-oriented rams. However, individual serving capacity tests do predict breeding performance of high and low sexual performance female-oriented rams. Exclusively male-oriented rams, as identified in preference tests before a breeding trial, will breed ewes under competitive conditions and sire as many lambs as low performance rams, but will not sire as many lambs as high performance rams. Combined, 1 low performance and 1 male-oriented ram may breed as many ewes and sire as many lambs as a single high performance ram. Therefore, we suggest that serving capacity tests should be used to select high performance rams, reduce the number of rams with marginal sexual performance, and make decisions on the number of rams needed.

	Footnotes

 
¹ Acknowlegments: V. A. LaVoie and M. A. Williams are cordially thanked for their excellent technical assistance. S. Wang is recognized for assistance in evaluation of semen, B. E. Mackey for statistical advice, and T. S. Hadfield, E. Dent, and S. Eng for microsatellite analysis.

² Mention of a proprietary product does not constitute a guarantee of warranty of the product by USDA-ARS, Utah State Univ., or the authors and does not imply its approval to the exclusion of other products that may also be suitable.

³ Corresponding author: jstellflug{at}pw.ars.usda.gov

Received for publication May 2, 2005. Accepted for publication September 14, 2005.

	LITERATURE CITED

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