Reproductive and Maternal Behavior of Livestock - Genetics and the Behavior of Domestic Animals - page 162

objective manner because: (a) bulls are polygynous and tend to distribute
their services among receptive females; (b) the greatest single stimulus for
bulls to mount and service is an immobile object that resembles the rear end
of a female; (c) prestimulation of bulls increases their sexual response (and
semen harvest); and (d) competition among bulls can increase their sexual
response ( Chenoweth, 1997 ). Despite these underlying principles, however,
one study showed that six to eight tests were required of 2-year-old Angus
and Hereford bulls to achieve reasonably repeatable results for both libido
score and serving capacity, although fewer tests were required to identify the
“best” bulls ( Landaeta-Hern´ndez et al., 2001 ). For young bulls, a learning
process is often necessary before competent mating is achieved ( Boyd et al.,
1989; Chenoweth, 1994a; Landaeta-Hern ´ ndez et al., 2001; Lunstra, 1984 ).
Such considerations probably contribute to study differences in which bull
sex-drive was either favorably associated with aspects of herd fertility
( Blockey, 1989; Blockey et al., 1978, Godfrey and Lunstra, 1988 ) or not
( Boyd et al., 1991; Farin et al., 1989 ).
Although there are anecdotal reports of bull breeds differing in sex-drive,
relatively few definitive studies have been conducted on this question. Bulls
of dairy breeds may generally be more sexually active than those of beef
breeds ( Amann and Almquist, 1976; Chenoweth, 1997; Hafez 1987 ) and Bos
indicus bulls are generally considered to be more “sexually sluggish” than
Bos taurus bulls ( Chenoweth and Landaeta-Hern´ndez, 1998 ) with one study
from an A.I. center in India reporting that 22.6% of Sahiwal bulls were
culled due to inadequate sex drive ( Mukhopadhyay et al., 2010 ). This is in
contrast to the findings of a Brazilian survey of beef bulls of largely Bos
taurus extraction (n

5.2% were rejected for sexual
behavior problems ( Menegassi et al., 2011 ). In both Australian and U.S.
studies, Bos taurus bulls performed better in sex-drive assessments than did
Bos indicus bulls ( Chenoweth and Landaeta-Hern´ndez, 1998 ) even though
the latter can be equally effective in detecting, serving and impregnating
estrous females ( Chenoweth, 1994a ). This discrepancy may be explained in
part by observations that Bos indicus bulls tend to be selective and shy bree-
ders, and do not perform well in pen tests to assess sex drive ( Chenoweth
et al., 1996 ). Zebu bulls are reputedly reluctant to breed with females of
other breeds ( Chenoweth and Landaeta-Hern ´ ndez, 1998 ). Selectivity in
breeding partners has also been reported among Bos taurus breeds in bull
mating activity ( Trautwein et al., 1958 ). Although these studies have gener-
ally implicated the bull as being the partner that initiates and controls sexual
contact, increasing evidence suggests that female choice might play a major
role in such outcomes. Although it as yet unclear as to which gender plays
the major role in such selectivity, observations on interactions between Bos
indicus (Gyr and Brahman) and tropicalized Bos taurus (Criollo-Limonero)
cattle in Venezuela indicate that female rejection of males of different breed-
types does occur (Landaeta-Hern´ndez, unpublished).

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