no exposure groups, respectively. The effect of a sterile bull on ovarian and behavioural activity of suckling beef cows indicated that in the group with the sterile teaser bull
present, the proportion of cows in which oestrus and ovulation was detected was higher Ž
. than in the control group Alberio et al., 1987 . The presence of a male causes more
intense stimulation, which decreases the rate of ovulation without expression of be- Ž
. havioural oestrus Ayalon and Weis, 1970; Ebert et al., 1972; Foote, 1974 .
Ž .
Effects of biostimulation exposure to bulls or testosterone treated cows on various reproductive variables in suckled cows indicated that cows exposed to bulls immediately
Ž after calving returned to oestrus earlier than cows isolated from bulls Burns and Spitzer,
. Ž
. 1992 . Recently, Bolanos et al.
1997 reported that bull and cow biostimulation
effectively enhanced resumption of ovarian activity in 60 anoestrus suckled zebu cows Ž
. Bos indicus in Costa Rica. The presence of the male stimulates postpartum reproduc-
tive activity, and that response can be modified effectively by the nutritional condition Ž
of the postpartum cows Alberio et al., 1987; Zalesky et al., 1984; Gifford et al., 1989; .
Monje et al., 1982; Stumpf et al., 1992 . Cows pastured with yearling or mature bulls initiated oestrous cycles 12 days earlier than cows isolated from bulls after calving,
indicating that biostimulatory effect of bulls on resumption of postpartum ovarian activity of cows is independent of the age of the bulls after they have reached 1 year of
Ž .
age Cupp et al., 1993 . There has been little or no documented information on the biostimulatory influences of bulls on resumption of postpartum ovarian activity in B.
indicus cattle. Therefore, the purpose of the present study was to determine the influence of bull biostimulation, season and parity on onset of postpartum ovarian function
following parturition in B. indicus cattle.
2. Materials and methods
2.1. Location Ž
. This study was conducted using postpartum cows n s 135 at the National Animal
Production Research Institute, Shika, Ahmadu Bello University, Zaria, Nigeria. The Ž
. study location, Shika, has two distinct seasons: dry season November to April and
Ž .
rainy season May to October . Daily minimum and maximum temperatures range from 10.58C to 23.58C and 26.38C to 37.88C, respectively. Average annual minimum and
maximum temperatures were 17.78C and 31.58C, respectively. During the harmattan period, daily temperatures range from 10.58C to 23.58C with a relative humidity of
28.8. Relative humidity during the rainy season was 67.6. Mean duration of day
Ž light per day was 11.5 h. Average monthly rainfall for the rainy season May to
. October was 153.2 mm with a range of 32.4 to 300.8 mm. The dry season was
characterised by total lack of rainfall from November to April. 2.2. Experimental cows
A total of 135 postpartum suckling Zebu cows were assigned randomly at calving to Ž
two treatments. Cows were either exposed to two matured vasectiomised bulls Mature
. Ž
. Bull Exposure, MBE or were not exposed to bulls No Bull Exposure, NBE from the
time of calving to 150 days postpartum. Cows were placed with or isolated from bulls within 48 h after calving. The two treatment groups were placed in two different
paddocks separated by a distance of 0.8 km.
The trials were conducted out in two seasons; cows calving in the dry season Ž
. Ž
. November to April and cows calving in the rainy season May to October for a period
of 2 years. In the first trial, a total of 68 postpartum cows consisting of 35 and 33 suckling cows were used for the rainy and dry season experiments, respectively. The
rainy season trials consisted of 18 MBE and 17 NBE cows, while the dry season consisted of 16 MBE and 17 NBE cows. In the second trial, a total of 67 postpartum
suckling cows consisting of 35 and 32 cows were used for the rainy and dry seasons
Ž .
experiments, respectively. The former rainy seasons experiments consisted of 17 MBE and 18 NBE cows, while the dry season consisted of 16 MBE and 16 NBE cows. Thus,
in the rainy season trials, a total of 70 cows consisting of 35 MBE and 35 NBE cows were used, while the dry season trials had a total of 65 cows consisting of 32 MBE and
33 NBE cows, bringing the total number of postpartum cows in the study to 135. The
Ž .
Ž 135 cows were comprised of 61 cows three to five calvings; P
and 74 cows one to
1
. two calvings; P . In all the trials, randomization of the experimental cows was
2
accomplished by a pair-wise procedure that involved randomly assigning the first cow that calved to either the MBE or NBE treatment groups. The next cow that calved was
assigned to the alternate treatment and this procedure was repeated for each successive pair until a total of 67 and 68 postpartum cows for the MBE and NBE treatment groups,
respectively, were assigned.
2.3. Management of postpartum cows The experimental cows in the MBE and NBE treatments were maintained in
improved pasture paddocks separated by a distance of 0.8 km to minimize influence of exteroceptive stimuli associated with each other. In addition to grazing improved sown
Ž .
pasture fields Stylosanthes, Brachiaria and Digitaria , the cows were given supplemen- tary concentrates consisting of maize and cotton seedcake of about 1–2 kg per cow per
day. Cows were also given baled hay, free access to a salt lick and water ad libitum throughout the experimental period. All cows were ear-tagged with large plastic ear tags
Ž
. Ritchey Europe, Yorkshire, England to enable identification of the cows from a
distance during observations for detection of oestrus. Before commencement of the study, the cows were screened for blood and helminth parasites and appropriate
treatments and vaccination against endemic diseases were performed.
2.4. Body measurements Cows were weighed and body condition scored within 72 h of calving. Cows were
subsequently weighed every 4 weeks until onset of ovarian activity or day 150 postpartum. Cows were body condition scored using a scale of 0 to 5 from the most
Ž .
emaciated to the fattest Pullan, 1978 . Only cows with a body condition score of 2.5 and above were involved in this study. The average liveweights of the treatment groups
at the start of the experiments for cows calving in the dry and rainy seasons were 303.1 8.6 and 300.6 7.6 kg, respectively.
2.5. Assessment of the reproductiÕe tract Postpartum cows were rectally examined for functional reproductive status according
Ž .
to the method of Zemjanis 1970 . Briefly, this involves retraction and subsequent examination and evaluation of the tubular genitalia and ovaries. Following parturition,
rectal examination of the genitalia was conducted every 3 days until involution was considered complete and then weekly, until oestrus was detected or 150 days postpar-
tum. Involution of the uterus was recorded as being complete when the uterus and its
Ž contents were easily retractable and palpable within the pelvic cavity Custer et al.,
. 1990 .
2.6. Detection of oestrus and blood sampling Postpartum detection of oestrus began 10 days after the first cow was assigned to any
of the treatments. In both the MBE and NBE groups, cows were observed visually for signs of oestrus twice daily between 0730 and 0830 hours and 1730 and 1830 hours by
herdsmen and inseminators. Vasectomised bulls were harnessed with chin-ball mating Ž
. device containing red paint The Great Outdoor, New Zealand to aid in the detection of
oestrus. Blood samples were collected from cows after parturition from the jugular vein once a week during the 150 d experimental period. Blood samples were placed
immediately on ice and allowed to clot for 24 h. Serum was separated then by centrifugation at 3000 rpm for 15 min and stored at y208C until concentrations of
Ž . progesterone P
were determined by radioimmunoassay.
4
Serum P concentrations were determined using a no-extraction, ACoat A CountB
4
Ž .
solid-phase P , RIA kit Bergfeld et al., 1996 . The sensitivity of the assay, defined as
4
twice the standard deviation away from the zero standard was 0.075 ngrml. The within and between assay coefficients of variation was 7.4 and 8.6, respectively. An
increase in serum P concentration of 1 ngrml for two consecutive weekly samples
4
was taken to indicate the presence of functional luteal tissue, and this was used to Ž
characterize resumption of postpartum ovarian activity or estrous cycles Oyedipe et al., .
1986 . 2.7. Statistical analyses
The number of days from parturition to initiation of cyclic ovarian function was calculated for individual cows by subtracting the date of parturition from the date at
which the first rise of P or first onset of ovarian activity was detected. Cows with an
4
increase in serum progesterone concentration of 1 ngrml from the weekly blood samplings were used to statistically analyse the number of days from parturition to the
time of resumption of ovarian activity. Presence of bulls, season and parity of postpar- tum cows were the main effects, while interval to completion of uterine involution,
interval to resumption of ovarian activities, and liveweight changes as dependent
Ž .
variables analyzed by ANOVA using General Linear Model Procedures SAS, 1990 .
Proportion of postpartum cows that exhibited cyclic ovarian activities after calving were analyzed using the Categorical Data Modelling.
3. Results
