The ability to exogenously regulate FSH concentrations in cattle is important to synchronise follicle wave development in both cyclic and anoestrous cattle. Although
progress has been made in recent years on improving the efficiency of oestrous synchronisation regimens, variability in oestrous response and pregnancy rates after
treatment are still major limitations preventing its greater use in cattle. It is necessary to control follicle wave dynamics as well as luteal function in order to have a ‘healthy’ DF
Ž .
present at the end of treatment Mihm et al., 1994 . When used together, E and
2
Ž .
progesterone can synchronise follicle waves, with variable success Bo et al., 1995 . However, the optimum dose of E necessary to suppress FSH is not clear, neither is the
2
effect of route of administration of E on blood concentrations of E known. Hence, the
2 2
aim of this experiment was to determine the effect of dose and route of E administra-
2
tion at time of progesterone administration on circulating concentrations of FSH, in order to select optimum dose to use in subsequent studies to manipulate follicle waves.
In this experiment, ovariectomised animals given progesterone were used to prevent confounding effects of endogenous E production.
2
2. Materials and methods
2.1. Animals and treatments Thirty-four long-term ovariectomised crossbred beef heifers, 18–24 months of age
and weighing 350–480 kg were used. All heifers received a Progesterone Releasing Ž
. Intravaginal Device PRID, Sanofi France for 7 days in order to simulate luteal phase
concentrations of progesterone before treatment. Four days after removal of the first PRID, a new PRID was inserted for a further 7 days. Animals were randomly allocated
Ž .
Ž . to receive one of eight oestradiol benzoate ODB treatments at this time; 1 0.0 mg
Ž . Ž .
Ž . Ž .
Ž . Ž .
Ž . Ž .
Control; n s 3 , 2 0.5 mg n s 4 , 3 1 mg n s 4 , 4 2.5 mg n s 6 , 5 5 mg Ž
. Ž . Ž
. n s 4 , 6 10 mg n s 4 all administered by intramuscular injection in 2 ml of corn
Ž . Ž
. Ž .
Ž .
oil, 7 5 mg n s 4 , and 8 10 mg n s 5 ODB in powder form administered by gelatine capsule attached to the PRID. The heifers were kept indoors for the duration of
the experiment and fed 2 kg of concentrates per day with ad libitum access to grass silage.
2.2. Blood sampling and hormone assays Blood samples were collected by jugular venipuncture into 10 ml vacutainer tubes
containing EDTA K
3
. The first sample was taken just before treatment administration, and then at 6-h intervals for the first 48 h after ODB administration, at 12-h intervals for
Ž .
the second 48 h and twice daily 0900 and 1500 h for a further 6 days, which covered the period up to 3 days after PRID removal. Blood samples were placed in ‘iced’ water
and then centrifuged immediately at 1000 = g at 48C for 15 min. Following separation, the plasma was stored at y208C until assayed for E and FSH.
2
Ž .
A previously validated radioimmunoassay Prendiville et al., 1995 was used to measure E . The sensitivity of the E
assay was 0.2 pg ml
y1
and the inter- and
2 2
Ž .
Ž .
intra-assay coefficients of variation were 10.5 n s 9 and 8.7 n s 4 , respectively. Ž
. The FSH assay used USDA bFSH B1 as the standard preparation Crowe et al., 1997 .
The sensitivity of the assay was 1 ng ml
y1
and the inter- and intra-assay coefficients of Ž
. Ž
. variation were 14.7 N s 5 and 13.7 n s 6 , respectively.
2.3. Statistical analyses One heifer, receiving 10.0 mg ODB by intramuscular injection, failed to show any
increase in plasma concentrations of E until 4 days following treatment and her
2
hormonal data was excluded from analyses. The E , and FSH data were subjected to
2
Ž .
repeated measures analyses of variance PROC GLM, SAS, 1989 followed by regres- Ž
. sion analyses SAS, 1989 . Because of the between animal variability, concentrations of
FSH were analysed as changes from pre-treatment concentrations.
3. Results
