factors affecting libido in animals

Introduction

Reproductive performance has greater impact on economic returns than does either growth rate or product quality. In most operations, females still conceive via the natural breeding bull. Thus the reproductive capabilities of males are of paramount importance. These capabilities are largely influenced by one or more of the following factors:

a) semen quantity and quality,

b) libido and mating ability and

c) social interactions (dominance effects) in the breeding pasture.

In addition,male performance is limited by the fertility threshold of the female herd. The Breeding Soundness Examination of  males (BSE), as recommended (Society for Theriogenology, 1976), is useful in identifying  males which have physical problems, or poor semen quality. However, other problems do not become apparent until males are exposed to some form of mating ability assessment. In many cases, males receive no form of assessment prior to sale or use.

Some Definitions

·       Libido is defined as the "willingness and eagerness" of a bull to attempt to mount and

Service. Libido refers to sexual motivation, revealed through behaviors such as mate seeking, detection, courtship, and mating (also referred to as servicing or serving). Libido, or sex-drive, is a behavioral trait with a large instinctive component and as such it poses problems in its assessment and interpretation.

·       Mating ability refers to the ability and competence of the bull in fulfilling this

aspiration.

·       Serving capacity is a measure of the number of services achieved by a bull under stipulated conditions and thus includes aspects of both libido and mating ability.

Measurement of libido

 Libido is typically measured using the reaction time, defined as “the elapsed time between exposure to stimuli and first service”. As libido is evidently crucial for fertility, there appears to be a need for the development of a predictive, standardized libido test. This may, however, be unachievable due to the apparent conflicting effects of some of the test factors on libido, such as the use of multiple females and genetic/breed differences.

 However, libido is a trait which is measurable and testing procedures designed for the animal generally rely on the exploitation of several or more of the following findings:

1.     Libido has a large genetic component

2.     Animals which are polygamous tend to distribute their services among receptive females.

3.     Prestimulation of animals increases their sexual response.

4.     Competition among animals can increase their sexual response.

Factors Influencing Bull Libido

1.     Age and rearing effects. As mentioned previously, age and (or) experience of  males can influence their relative efficiency of mating, and consequently their libido scores and rankings. Thus, mating ability has a learning component.

In trials with young tropical beef bulls, libido score apparently increased with bull age between 16 and 31 mo,  a finding which differed from results with  males in Sweden, and with Hereford and Angus  males in Colorado. It was found that age affected sexual behavior traits in crossbred  males, with yearling  males showing lower libido and a higher proportion of mounts than older  males. Ologun et al. (1981) identified negative relationships in yearling beef bulls between sex drive and production traits, while in another study, underfeeding had no adverse effects on bull sexual behavior. McFarlane (1974) showed that Zebu males raised on open range showed tardy sexual responses compared with those reared more intensively.

2.     Bull to female ratio. The standard recommendation of using approximately one bull per 20 to 30 females has long been promoted. It is apparent, however, that this does not represent optimal bull usage and that it allows sub-standard males to go undetected.  males not selected for high libido have shown estrus detection rates from 90 to 98% when used at a Bull To Female Ratio (BFR) of 1:24 to 1:30, using ratios of 1:60 and 1:100, obtained rates of 64% and 51%, respectively.

In contrast, Rupp et al. (1977) obtained good reproductive efficiency with most males in single sire mating at BFRs of both 1:44 and 1:60 and concluded that a BFR of 1:25 represented inefficient bull usage. Comparison of single and multi-sire combinations revealed no effect of number of males in the pasture on estrus detection. The overall conclusion was that the reproductive capabilities of individual males were more important to reproductive success than either BFR or single vs multi-sire breeding combinations.

Overall, there seems little doubt that males are greatly under-utilized in many breeding programs. Although not practicable in many cases, single-sire mating is inherently more efficient than multi-sire mating.

3.     Social effects. Social ranking of males within groups can influence their sexual activity. Several studies employing blood-typing methods to determine paternity have shown that dominant males can sire the majority of calves in multi-sires. The effect of social interactions among males on herd fertility may be greater at lower BFRs than where there is higher breeding stress. Dominance rank was negatively correlated with sex-drive in one study with yearling males. If dominance and sex-drive are different traits, then the dominant male (or males) could impair herd fertility through failure to service females while preventing less dominant males from serving. In rams, the reproductive performance of subordinates was shown to be greatly reduced.

 In conclusion, social effects should be considered in both the breeding pasture and during libido/serving capacity tests. With multi-sire mating programs, more efficient breeding and sire utilization would occur if the bull groups were young              (preferably < 3yr), of similar age, size, genotype and social background.

4.     Genotype differences. Anecdotal evidence for breed differences in bull sex-drive has long been reported. For example, differences between beef and dairy breeds in semen collection ease are noted. Zebu males have a reputation for "sexual sluggishness" and a tendency to mount females in full estrus only. When comparing libido scores of six breed groups in Queensland, those males with a Brahman component in their breeding had consistently lower scores than either British or Africander derived breed types. With subsequent trials, Bos indicus genotypes generally did not perform successfully in serving capacity type tests employing restrained females. More success was obtained with the use of unrestrained estrus-induced females and single bull tests. It was obvious that the testing procedure was more unsettling to a number of these males than with the Bos taurus  males. Even in the breeding pasture, difficulties were encountered in observing mating behavior because of apparent wariness by the bull(s) towards the observer. Despite this, the best performing Bos indicus  males were equal to the best of the other genotypes. Interestingly, when different trials employing either Bos taurus or Bos indicus  males mated with synchronized females were compared, the Bos indicus  males achieved similar fertility although they displayed less sexual activity.

Other factors include:

1.     Systemic diseases

Any diseases causing depression, anorexia, fever, loss of weight and pain etc. would affect the sexual desire of the bull e.g. tumors, severe parasitism traumatic gastritis, etc. the extent of loss of sexual desire depends on the severity of the case.

2.     Injuries and diseases of the reproductive organs

Injuries affecting the joints, muscles, nerves and tendons particularly affecting the rear quarter may affect the libido. Also the tumors or other affection of the penis, prepuce, the scrotum etc. may affect it.

3.     Endocrine factors

Deficiency of the circulating androgens may cause reduction of sexual desire without other clinical symptoms.

Pitfalls in Libido/Serving Capacity Testing

In general, successful testing of  males for libido and mating ability requires careful planning and lots of patience. Some of the pitfalls which may be encountered are described as follows:

1.     Testing of  males that are excessively apprehensive or agitated.

Apart from taking precautions to handle cattle quietly and to avoid distractions, there is no easy solution to this problem, which can lead to depressed scores.

2.     Testing of  males immediately following their subjection to other procedures such as electroejaculation, vaccination and parasite control measures.

3.     Testing under adverse weather conditions, such as in extreme heat, cold, or inclement weather.

4.     Testing of  males in groups in which one or more  males are markedly dominant, such as with mixed-age groups of  males.

The exposure of only two  males to test at a time, and subsequent retesting with a different bull, helps to minimize this problem. It should be noted, however, that a dominant bull can exert an inhibitory effect from a distance (eg. from an adjacent pen).

5.     Use of inadequate stimuli.

Restrained females should be incapable of excessive movement or some  males may be deterred. The service crates used should not impede mounting and service. If unrestrained females are used, they should be in full estrus.

6.     Spreading of venereal diseases.

Every precaution should be taken to ensure that diseases such as vibriosis and trichomoniasis are not transmitted by such procedures.

7.     Injury or undue stress to restrained females.

Humane considerations mandate that females be closely monitored for signs of stress and be replaced if these become evident. Mild sedation of females and prior lubrication of their genital regions are also recommended.

Alternative Assessment Procedures

The indirect determination of bull libido as reflected by blood concentrations of hormones has some attraction as it could reduce or eliminate the time, labor and aesthetic concerns which occur with libido/serving capacity testing. This would also allow assessment of males which did not respond well to yard or pen testing. Earlier attempts at linking luteinizing hormone (LH) or testosterone (T) levels with bull sex-drive were, however, disappointing. Difficulties were posed by the episodic nature of hormone release and the inhibiting effects of handling or restraint of the animal. By inducing LH or T release with parenteral administration of gonadotropin releasing hormone (GnRH), a number of these difficulties have been avoided. A significant  relationship between induced T levels and bull fertility, while Perry et al. (1988) obtained positive relationships with induced LH levels. Work is proceeding with such indirect tests to define  dosages, sampling times and the effects of bull age, genotype, season and nutrition. Although criticism has been levelled at such procedures for not allowing concurrent assessment of mating ability, the additional use of the BSE and judicious pasture observation should minimize these concerns.

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