Techniques to increase prolificacy in cattle have met with limited success; the aim of these studies was to investigate the potential of actively immunising cattle against certain gonadal hormones, and to examine the physiological basis of these treatments. Eight heifers were immunised against 8mg of a testosterone conjugate in Freund's Incomplete Adjuvant, & nine animals served as controls. These heifers were given one priming and two booster injections at four-month intervals. After the last booster injection, 7/8 animals had become anoestrous, and displayed significantly raised blood progesterone & mean LH concentrations,increased LH pulse frequency, & decreased mean FSH concentrations. Seven months after this treatment, 3/7 anoestrous heifers resumed ovarian cyclicity, with a mean ovulation rate of 2.7+0.7. To determine if the different ovarian responses observed above could be obtained by changes of ovarian steroid feedback seen during the oestrous cycle, groups of 6 heifers were implanted with large,medium or small sized oestradiol capsules during the luteal phase of the cycle. Five control heifers received empty implants. During the luteal phase of the cycle following implantation, all heifers were ovariectomised. The effect of the treatment on ovarian function and gonadotrophin secretion in the presence or absence of progesterone (PRID) was then determined. Increasing physiological concentrations of oestradiol reduced the number of large antral follicles and corpora lutea, but not the total number of antral follicles >lmm diameter. A combination of progesterone and oestradiol were fully effective in maintaining luteal-phase concentrations of LH and FSH, and follicular-phase concentrations of oestradiol alone were able to maintain LH and FSH concentrations within the physiological range. Thus changes of blood steroid levels similar to those seen during the oestrous cycle may interrupt ovarian function. Cattle were therefore immunised against a non-steroidal, partly purified fraction of ovine follicular fluid (PPFF) enriched ininhibin-like activity as measured in vivo and in vitro. Active immunisation against 0.4mg and 4mg ovine PPFF produced 1/5 & 3/5 heifers with multiple ovulations, respectively; this was not associated with changes of FSH secretion. To examine in more detail the endocrine responses to this treatment, and to investigate possible comparative aspects, cows were immunised against 4mg ovine, porcine or equine PPFF. No treatment increased ovulation rate, but the porcine-PPFF immunised heifers showed a 7-fold increase in mean LH secretion that could not be explained by alterations in pulsatile secretion or in steroid feedback. Collectively, these results suggest that the cow does not respond consistently to treatments so far designed to alter gonadotrophin secretion, that inhibin is not a major feedback hormone in this species, and that the heifer may possess an influential intra-ovarian control mechanism which ultimately determines ovulation rate.

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Call number: LD2668 .T4 1985 L825 / Master of Science

A series of experiments were conducted to investigate the regulatory mechanisms governing the absence of and the initiation of pulsatile pituitary LH secretion during the infantile and prepubertal periods of development in the bull calf. In the first experiment, patterns of hypothalamic GnRH secretion into hypophyseal portal vessels were measured in sixteen Holstein bull calves at 2, 5, 8 and 12 weeks of age. The results of this study correlated the attainment of an hourly rate of pulsatile GnRH release to the onset of prepubertal LH secretion. In the second experiment, nine Holstein bull calves were infused with exogenous GnRH (200 ng) on an hourly basis from 1 to 6 weeks of age. From this experiment, we were able to demonstrate that an hourly rate of pulsatile GnRH release stimulates the age-associated changes in the hypothalamic-pituitary axis necessary for initiating pituitary LH secretion during the transition from the infantile to prepubertal period of development.

Call number: LD2668 .T4 1986 S73 / Master of Science / Animal Sciences and Industry

Orientador: José Luiz Moraes Vasconcelos / Banca: Lindsay Unno Gimenes / Banca: Ocilon Gomes de Sá Filho / Banca: Ricardo Maria dos Santos / Banca: Ky Pohler / Abstract: Study 01- Our hypothesis was that fertility could be increased in a timed AI (TAI) protocol based on estradiol (E2) and progesterone (P4) by combining GnRH with E2-benzoate at the start of the protocol to increase circulating P4 concentration during preovulatory follicle development and by using two prostaglandin (PGF) treatments at the end to decrease circulating P4 concentration near TAI. Lactating Holstein cows (n = 1,808) were randomly assigned during the cool or hot seasons of the year to receive TAI (d0) following one of three treatments: I. Control: CIDR + 2 mg of E2-benzoate on d -11, PGF on d -4, CIDR withdrawal + 1.0 mg of E2-cypionate on d -2, and TAI on d 0; II. 2PGF: Identical to Control protocol with addition of a second PGF treatment on d -2; III. GnRH: Identical to 2PGF protocol with addition of 100 μg GnRH treatment on d -11. Pregnancy diagnoses were performed d32 and d60 after TAI. The GnRH protocol increased percentage of cows with CL (Control = 56.9%; 2PGF = 55.8%; GnRH = 70.5%) and circulating P4 concentration at PGF (Control = 3.28 ± 0.22; 2PGF = 3.35 ± 0.22; GnRH = 3.70 ± 0.21 ng/mL), compared to Control and 2PGF protocols. The positive effects of GnRH treatment on P/AI were only detected during the cool season (GnRH = 50.9%; 2PGF = 44.2%; Control = 41.0%) and not during the hot season. In addition, the effect of GnRH was only observed in cows with low circulating P4 concentration (<3ng/mL) at the start of the protocol and not in cows that began the protocol with high P4. Further, there was an interaction for presence of CL at PGF with follicle diameter such that cows with a CL at PGF had greater P/AI if they ovulated larger rather than smaller follicles near TAI.Season had major effects on many reproductive measures with cool season greater than hot season in percentage of cows with CL at PGF (62.9 vs. 56.2%), ovulatory follicle diameter (15.7 vs. 14.8 mm), expression of ... / Doutor

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