Bull Behavior, Sex-Drive and Management

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Bull Behavior, Sex-Drive and Management
P.J. Chenoweth
Department of Farm Animal Medicine and Production
University of Queensland, Australia
Introduction
Reproductive performance has greater
impact on beef economic returns than does
either growth rate or product quality (Trenkle
and Wilham, 1977). In most cow-calf
operations, females still conceive via the natural
breeding bull (Godfrey and Lunstra, 1989).
Thus the reproductive capabilities of bulls 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, bull performance is limited
by the fertility threshold of the female herd.
The breeding soundness examination of
bulls (BSE), as recommended (Society for
Theriogenology, 1976), is useful in identifying
bulls which have physical problems, or poor
semen quality. However, other problems do not
become apparent until bulls are exposed to some
form of mating ability assessment (Blockey,
1975). In many cases, bulls receive no form of
assessment prior to sale or use (Godfrey and
Lunstra, 1989).
Some Definitions
Libido is defined as the "willingness and
eagerness" of a bull to attempt to mount and
service, while mating ability refers to the ability
and competence of the bull in fulfilling this
aspiration (Chenoweth, 1981). Serving capacity
is a measure of the number of services achieved
by a bull under stipulated conditions (Blockey,
1976) and thus includes aspects of both libido
and mating ability. Some problems have
occurred with terminology in the past
(Wodzicka-Tomaszewska et al.,1981; Price,
1985; Morris, 1987) mostly because of
confusion between the trait and the method used
to assess it. Libido, or sex-drive, is a behavioral
trait with a large instinctive component and as
such it poses problems in its assessment and
interpretation. However, it is a trait which is
measurable and testing procedures designed for
the bull generally rely on the exploitation of
several or more of the following findings:
1. Libido in bulls has a large genetic
component (Bane, 1954; Chenoweth et
al., 1977; Blockey, 1978a; Ologun et al.,
1981).
2. Bulls are polygamous and tend to
distribute their services among receptive
females (Blockey, 1976).
3. The greatest single stimulus for a bull to
attempt mount and service is the
immobile rump of a female, or something
similar (Chenoweth et al., 1979; Blockey,
1981; Wallach and Price, 1988).
4. Prestimulation of bulls increases their
sexual response (Blockey, 1981; Mader
and Price, 1984).
5. Competition among bulls can increase
their sexual response (Mattner et al.,
1974; Mader and Price, 1984; Godfrey
and Lunstra, 1989).
Historical Aspects
Early interest in bull libido and mating
ability developed from problems encountered
with semen collection from some bulls in A.I.
centers. Work in Sweden, Africa, Scotland and
the U.S., mostly between 1945 and 1965, helped

to define testing procedures, illustrated the
strong genetic basis of both sex-drive and mating
ability, and probed the limits of bull sexual
performance (see reviews by Chenoweth, 1983a
and Price, 1985.). Osborne et al. (1971) first
described a scheme for assessment of untrained
beef bulls (libido score), using estrogenized
unrestrained females as stimuli and a short test
(5 min). This procedure was later modified with
an expanded 0 to 10 scoring system
(Chenoweth, 1976), which described degrees of
sexual interest including service, and a 10 min
test. Blockey (1981) devised a serving
capacity test which group-tested bulls with
restrained, non-estrus females for 40 to 60 min.
A score was given based only on services
performed. A composite procedure employing
elements of both systems has been described
(Chenoweth, 1980), and procedures exploiting
features of the serving capacity test have been
described (Lunstra, 1980; Boyd et al., 1989;
Godfrey and Lunstra, 1989).
Repeatability and Predictability
An ideal procedure for assessment of bull
libido would be simple, quick, highly repeatable
and very predictive of actual reproductive
performance. Unfortunately, no test is currently
available which fulfills all of these criteria
although relative differences between bulls can
be reliably estimated (Price, 1985). Chenoweth
et al. (1979) obtained moderate phenotypic
correlations (r = .67 and .60 respectively)
between libido and serving capacity scores in
yearling bulls tested on different days; reaction
times to service were not significantly correlated
with the two scores. Fifty-seven percent of the
young bulls did not achieve a service in both
serving capacity tests and thus were not scored.
However, when 26 yearling Bos taurus bulls
were assessed eight times (two tests per day on
four occasions within 2 mo), four tests were
required before test results did not differ
(Berndtson, personal communication). There
was strong evidence in these trials for expression
of a learning curve in young bulls. This
phenomenon was also observed by others (Boyd
et al., 1989; Godfrey and Lunstra, 1989) when
yearling bulls with low serving capacity scores
improved with mating experience. These latter
workers found it advantageous to conduct three
assessments prior to categorizing bulls. If the
ratio of mounts to services can provide an
indication of mating inexperience (Chenoweth,
1978a), then such inexperience was most evident
in bulls at 12 mo of age (Katz and Price, 1984).
Boyd et al. (1989) suggested that young, virgin
bulls which show poor serving capacity should
be offered sexual experience and retested to
determine their true worth.
Best success has generally been obtained
when using assessments of libido or serving
capacity to place bulls into categories or groups.
Thus Lunstra (1984) found that eight Hereford
bulls maintained their relative ranking for both
libido scores and fertility when assessed at both
16 and 40 mo of age. Blockey (1981) obtained
high correlations for mating activity rankings
between simulated pasture tests and subsequent
pen tests (r = .82 to .91) in 12 Bos taurus bulls.
In conclusion, it appears that libido and
serving capacity tests are useful in placing bulls
into groups which will then reflect their test
results in pasture mating activity. Bulls obtaining
poor to moderate results may require more than
two tests for adequate categorization. Young
bulls can improve their scores (and ranking) with
mating experience.
Relationships with Reproductive
Performance
Although cattle fertility is influenced by
many factors, there is evidence that bull libido is
of considerable importance. Blockey (1978b),
for example, obtained better first-cycle
pregnancy rates in heifers mated with higher
serving capacity bulls when compared with bulls
of low serving capacity. A more recently
published study (Blockey, 1989) showed

differences in pregnancy rates between high,
medium and low serving capacity Hereford bulls.
Other studies which have shown advantages in
expressed fertility have included those of
Makarechian and Farid (1985), and Morris
(1987). Libido and semen quality both
influenced pregnancy rates of Brangus bulls in
Florida, with libido having most effect (A.C.
Warnick, personal communication). Other
studies have indicated either that bull libido
assessment provided greater prediction of bull
fertility than did semen assessment alone (Smith
et al., 1981), or that BSE assessment alone was
lacking in predicting bull fertility (Neville et al.,
1988). Using multi-sire mating and progeny
identification by blood typing, Coulter and
Kozub (1989) showed that the number of
services performed in prior libido/serving
capacity tests was positively correlated to
fertility up to a certain point only (approximately
four services), above which fertility actually
declined with subsequent services.
Other studies have shown poor or
inconclusive relationships between bull
libido/serving capacity assessment and herd
fertility (Chenoweth, 1978b; Christensen et al.,
1982; Farid et al., 1983; Makarechian et al.,
1987; Boyd et al., 1989; Farin et al., 1989). In
some studies, although higher libido bulls
serviced more often and serviced more females
than did lower libido bulls, more pregnancies did
not result (Boyd et al., 1989; Godfrey and
Lundstra, 1989; Farin et al., 1989).
Although some of these findings may
appear contradictory, they are not necessarily so.
In a number of trials, bulls were not placed under
sufficient breeding stress to illustrate real
differences, bulls at the bottom end of the
spectrum were not included, and investigators
erred in concentrating on single trait effects on
herd fertility. As stated earlier, cattle fertility is
influenced by a number of factors of which bull
libido is but one. Breeding soundness and
examination components (scrotal circumference,
sperm motility and morphology) can separately
influence fertility (Mateos et al., 1978), but do
not appear to be genetically linked with
behavioral traits such as libido (Chenoweth et
al., 1977; Morris, 1987; Boyd et al., 1989).
Thus bulls may be superior in one or more traits
but their fertility can be compromised by
deficiencies in others. This was illustrated in a
study by Farin et al. (1989) in which 92 beef
bulls were placed into satisfactory and
questionable BSE categories, and into high
(score 9 to 10) and medium (score 7 to 8) libido
categories prior to single-sire mating with
groups of estrus synchronized heifers (table 1).
Several points are apparent. Although
bulls at the lowest end of the BSE scale were not
used, there was a difference in pregnancy rate of
9.1% between bulls in the satisfactory and
questionable categories. There appeared to be
little relationship between the BSE and libido
categories. Bulls of high libido achieved a similar
overall pregnancy rate to that of bulls of medium
libido, despite their achievement of more overall
services and more females serviced. This
paradox was apparently due to a lower
percentage of serviced females becoming
pregnant in the high libido group. Differences in
libido between bulls can be masked by
differences in other facets of reproductive
capability such as breeding soundness
components.
In conclusion, it would appear that libido
and serving capacity tests are useful in
identifying bulls which have superior breeding
activity (i.e. serve more often and serve more
females than do other bulls). However, the BSE
classification aids in identifying differences in
ability to impregnate at those services. Optimum
prediction of bull fertility would require separate
assessment in both categories.
Factors Influencing Bull Libido
1.Age and rearing effects. As
mentioned previously, age and(or) experience of
bulls can influence their relative efficiency of

mating, and consequently their libido scores and
rankings. Thus, mating ability has a learning
component (Chenoweth, 1981).
In trials with young tropical beef bulls,
libido score apparently increased with bull age
between 16 and 31 mo (Chenoweth, 1978b); a
finding which differed from results with bulls in
Sweden (Hultnas, 1959), and with Hereford and
Angus bulls in Colorado (Chenoweth et al.,
1984). Coulter and Kozub (1989) found that
age affected sexual behavior traits in crossbred
bulls, with yearling bulls showing lower libido
and a higher proportion of mounts than older
bulls. More work is needed to differentiate the
effects of age and inexperience from the
influences of different environments and rearing
methods. In this respect, prolonged nursing was
considered to retard the expression of normal
sexual behavior in Angus bulls (Couttie and
Hunter, 1956) and Hentges (1967) reported
lowered libido in bulls fed high concentrate
levels. 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 (Wierzbowski, 1978).
McFarlane (1974) showed that Zebu bulls raised
on open range showed tardy sexual responses
compared with those reared more intensively.
No permanent sexual inhibitions attributable to
rearing methods were reported in bulls (Lane et
al., 1983) as distinct from rams (Zenchak et al.,
1981), although it is quite possible that
temporary inhibitions may compromise restricted
breeding seasons (Chenoweth, 1981). Fertility
in yearling bulls may, however, be reduced for
reasons other than those associated with libido
or mating behavior. For example, in trials
employing bulls with estrus synchronized heifers,
yearling bulls obtained lower fertility than older
bulls despite equivalent mating activity (table 2).
In this study, yearling bulls did not differ
from older bulls in either the number of services
or heifers serviced. There were differences in the
number of mounts performed. More
importantly, differences were encountered in the
percentages of females pregnant, either of those
in estrus and of those serviced. Yearling bulls
were as sexually active as the older bulls, but
less fertile.
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 bulls to go undetected.
Bulls 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 (Donaldson, 1968; Mattner et al.,
1974). Beerwinkle (1974), 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 bulls 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 bulls in the pasture on estrus
detection. The overall conclusion was that the
reproductive capabilities of individual bulls were
more important to reproductive success than
either BFR or single vs multi-sire breeding
combinations. Farin et al. (1982), mating young
bulls with estrus synchronized heifers, compared
BFRs of 1:20 and 2:40 and concluded that single
sire mating was more efficient. Heifers in single-
sire groups were serviced more times than those
in multi-sire groups, and approximately 50% of
heifers in the latter groups were serviced by both
bulls. Overall, there seems little doubt that
bulls 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. The individual
capabilities of bulls have greater impact on herd
fertility than do BFRs and these capabilities can

generally be assessed prior to the breeding
season to minimize risk.
3.Social effects. Social ranking of bulls
within groups can influence their sexual activity
(Chenoweth, 1981). Several studies employing
blood-typing methods to determine paternity,
have shown that dominant bulls can sire the
majority of calves in multi-sire groups (Farid et
al., 1983). Dominance is expressed more
strongly and linearly in older bulls (>3.5 to 4 yr)
and appears to be more related to seniority than
to age or weight (Blockey, 1979). The effect of
social interactions among bulls on herd fertility
may be greater at lower BFRs than where there
is higher breeding stress (Blockey, 1979).
Dominance rank was negatively
correlated with sex-drive in one study with
yearling bulls (Ologun et al., 1981). If
dominance and sex-drive are different traits, then
the dominant bull (or bulls) could impair herd
fertility through failure to service females while
preventing less dominant bulls from serving.
Evidence has been presented for such effects
occurring in extensive beef operations
(McCosker et al., 1989) where it was also
shown that social dominance ratio of bulls had
some relationship with herd fertility. In rams,
the reproductive performance of subordinates
was shown to be greatly reduced (Fowler and
Jenkins, 1976). Such effects are probably most
evident when older and younger males are
combined in the breeding pasture (Blockey,
1979), although mixing different bull genotypes
has apparently caused similar effects.
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 < 3 yr), 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 (Amann and
Almquist, 1976). Zebu bulls have a reputation
for "sexual sluggishness" and a tendency to
mount females in full estrus only (Anderson,
1948; Hafez, 1960). When comparing libido
scores of six breed groups in Queensland, those
bulls with a Brahman component in their
breeding had consistently lower scores than
either British or Africander derived breed types
(Chenoweth and Osborne, 1975). 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 bulls than with the Bos taurus bulls. 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 bulls were equal to the
best of the other genotypes. Interestingly, when
different trials employing either Bos taurus or
Bos indicus bulls mated with synchronized
females were compared (table 3), the Bos
indicus bulls achieved similar fertility although
they displayed less sexual activity.
Pitfalls in Libido/Serving Capacity Testing
In general, successful testing of bulls for
libido and mating ability requires careful
planning and lots of patience. Some of the
pitfalls which may be encountered (Chenoweth,
1983b) are described as follows:
1. Testing of bulls 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 bulls 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 bulls in groups in which one
or more bulls are markedly dominant,
such as with mixed-age groups of
bulls. The exposure of only two bulls 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 bulls 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 bulls 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 (Foote et al., 1976; Chenoweth et
al., 1979). Difficulties were posed by the
episodic nature of hormone release and the
inhibiting effects of handling or restraint of the
animal (Rhodes et al., 1979). By inducing LH or
T release with parenteral administration of
gonadotropin releasing hormone (GnRH), a
number of these difficulties have been avoided.
Post et al. (1987) reported 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.
A Fertility Index for Bulls?
It is apparent that a number of factors
influence bull fertility, including BSE values and
behavioral factors such as dominance and libido,
and that these do not always work in harmony.
Attempts have been made to combine a number
of these factors to best predict bull performance.
Perry et al. (1989) assessed a number of traits in
young tropical beef bulls at varying intervals
from single-sire matings (10 wk). The traits
assessed included BSE values, sex-drive (libido
and serving capacity), production traits (ADG
and body weight), tick resistance and LH and T
responses to GnRH. A step-wise regression
procedure was used to select the most suitable
combinations of traits highly correlated with
pregnancy rate. Fertility indices were calculated
from this analysis (table 4).

The fertility indices derived were, in
general, highly predictive of pregnancy rate,
even at 11 mo prior to mating. The lowest
correlation was the one obtained at the
examination just prior to breeding (r = .45;
p < .05) when the only trait included in the index
was libido score. Genotype differences had little
influence on these indices. Overall, the most
important measurements were GnRH induced
LH levels, testicular volume, libido and body
weight. Interestingly, these factors are derived
from each of the main categories separately
known to influence bull reproductive
performance, i.e. sex-drive, endocrine status and
sperm production. These results also showed
that relationships change with bull age and
interval to mating and perhaps illustrate the
variability of semen traits during the post-
puberal year and a growing importance of sex-
drive during this period.
Coulter and Kozub (1989) also used a
regression model to predict bull fertility in multi-
sire breeding where paternity was determined by
blood typing. The most important traits were
found to be scrotal circumference, backfat,
sperm morphology (particularly primary
abnormalities) and sex-drive. In yearling bulls,
the model accounted for 37% of the variance in
bull fertility and in 2 yr-old bulls it accounted for
22%. In conclusion, the best prediction of bull
fertility is obtained when bulls are assessed for a
number of traits, including sex-drive. The
development of fertility indices, which combine a
number of important traits, should improve
predictive ability. They already provide
considerable advantage over single trait
measurements.
Summary
Bull libido is a measurable trait, largely
under genetic control, and is an important
component of bull reproductive performance. It
can be measured in a number of ways but a
formalized procedure is necessary to quantify
this trait. Libido does not necessarily work in
concert with other traits known to separately
influence bull fertility (e.g. BSE traits and social
dominance). Until a single procedure is found
that can adequately assess all of these factors,
optimal bull appraisal requires separate
evaluation of each factor. The development of
bull fertility indices promises to combine the best
of these measures for optimal bull fertility
prediction.
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Table 1. Least-Square Means of Mating Performance within Breeding Soundness Examination
and Libido Classifications
Exam classifications Libido
Sat. Quest. High Medium
No. bulls
No. mounts
No. services
Mounts:serivces
Serivced/estrus (%)
Pregnant/serviced (%)
Pregnant/estrus (%)
Total pregnancy rate (%)
80
146.3
47.8
5.8
73.5
56.1
44.8
45.6g
12
120.7
42.4
4.8
71.4
50.8
36.7
36.5h
69
112c
52.8e
3.1e
81.3e
51.8
43.7
41.5
23
155d
37.5f
7.5f
63.5f
56.1
37.8
40.6
c,d Means differ (P < .05) Farin et al. (1989)
e,f Means differ (P < .01)
g,h Means differ (P < .10)
Table 2. Mating Performance as Affected by Age of Hereford and Angus Bulls
Age (yr)
One Two Three+
No. bulls
No. mounts
No. services
Mounts:services
Serviced/estrus
Pregnant/serviced
Pregnant/estrus
Total pregnancy rate
29
207.1
54.5
6.6:1
69.4
39.6c
30.2
30.9c
36
120.0d
37.6
5.4:1
73.8
59.4d
40.3d
41.5d
27
85.8d
40.5
4.5:1
72.0
62.2d
50.7e
49.9e
c,d,e, Means differ (P < .05). Pexton, Farin, Rupp and Chenoweth (in press)

Table 3. Comparison of Bull and Synchronization Trials with Bos taurus and
Bos indicus Cattle
Bos taurusaBos indicusb
SMB PGF SMB
No. groups 39 53 31
BFRc1:7 to 51 1:15 to 20
Females in estrus (%)
Estrus/served (%)
Total females served (%)
Avg. services per bull
Pregnant/estrus (%)
Pregnant/served (%)
Pregnant/total (%)
90.8
73.3
66.1
45.1
42.4
56.4
41.3
78.3
70.4
55.1
41.0
56.1
42.7
77.2
72.0
55.7
23.6
40.6
57.3
32.6
aPexton et al. (1989)
bWilliams (1988)
cBFR=bull to female ratio
Table 4. Fertility Index Correlations with Pregnancy Rate in Young Beef Bulls
Interval to mating
(mo) Within or Among
mating Trait in
index Correlation with
pregnancy rate
-11
-8
-6
-2
-0.5
Within
Among
Within
Among
Within
Among
Within
Among
Among
LH
LH, D, Tvol
Lib, Bwt, Tvol
Lib, Bwt, Tvol
Bwt, Tvol
Bwt, Tvol
LH
LH, Age
Lib
0.75**
0.71**
0.89**
0.76**
0.86**
0.73**
0.80**
0.66**
0.45*
Legend: LH = induced LH level (* = P < .05) Perry et al. (1989)
D = dominance value (** = P < .01)
Lib = libido score
Bwt = bodyweight
Tvol = testicular volume
Age = bull age (d)