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G. P. Pearce and P. E. Hughes

Summary. Large White × (Large White × Landrace) prepubertal gilts, 165 days of age, were fitted with indwelling venous catheters and housed in modified metabolism crates. After a period of acclimatization, frequent blood samples were taken at regular intervals before, during and after the 7 gilts were exposed to various degrees of contact with male pigs. The plasma samples were assayed for cortisol concentration using a competitive protein-binding radioassay. Significantly elevated concentrations of plasma cortisol (P < 0·001) occurred only when full physical contact between the boar and the gilts was allowed. Boar exposure without full physical contact induced only minor changes in plasma cortisol concentrations of gilts. Plasma cortisol concentrations have been shown to constitute a reliable indicator of a stress response in pigs, and so the results of this study suggest that tactile stimulation from a male pig induces a stress response in the recipient prepubertal gilt. This stress response in the gilt may be involved in the stimulation of puberty onset by contact with a mature boar (i.e. the 'boar effect').

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G. P. Pearce and C. M. Oldham

Summary. In Exp. 1, 4 groups of 50 recently weaned ewes were exposed to various degrees of contact with rams for 65 days, followed by exposure to novel rams for 4 days. Ovarian activity in the ewes was determined by laparoscopy on Days 29, 65 and 69 of treatment. There were no treatment differences in the percentage of ewes ovulating on Day 4 whereas by Day 29 more ewes in clear fenceline and full ram contact were ovulating compared to controls (P < 0·05, P < 0·001). After 65 days ovarian activity was significant only in those ewes in full contact with rams (P < 0·001). Between 89 and 95% of ewes remaining anovulatory after 65 days ovulated after 4 days of full contact with novel rams.

In Exp. 2, 4 groups of about 30 anovulatory ewes were exposed to various degrees of contact with rams for 5 days. Ovarian activity was assessed before and after treatment by laparoscopy. After 5 days, more ewes were ovulating in response to full ram contact than in any other treatment (P < 0·05) and more ewes in fenceline contact with rams or with rams plus ewes were ovulating than in the isolated control treatment (P < 0·01).

In Exp. 3, 6 groups of about 40 anovulatory ewes were exposed to face masks with and without rams' wool and/or various degrees of contact with rams for 5 days. More ewes were ovulating after 5 days in the group in full physical contact with rams than in any other group (P < 0·01). Ram contact through a clear fence either with or without masks stimulated more ewes to ovulate than masks alone or isolation from rams (P < 0·01). The additional exposure to rams' wool did not increase the percentage of ewes ovulating in response to fenceline ram contact or masks alone but exposure to masks themselves with or without rams' wool did stimulate ovulation compared to isolated controls (P < 0·05, P < 0·01 respectively).

These results indicate that fenceline ram contact was effective in inducing ovulation in a high proportion of seasonally anovulatory ewes but less effective in lactationally anovulatory ewes. Furthermore, maximum stimulation of ovulation required full physical contact with rams in all cases. Visual and tactile/behavioural stimuli from the ram therefore appear most important in mediating the ram effect whereas olfactory cues from Merino rams' wool were ineffective either alone or in addition to fenceline ram exposure. Pre-isolation of ewes from rams was not required when novel rams were used to induce subsequent ovulation.

Keywords: ram stimuli; induced ovulation; ewes

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G. P. Pearce, A. M. Paterson and P. E. Hughes

Summary. Prepubertal gilts were fitted with jugular vein and carotid artery catheters at 148 days of age. At 160 days of age the 24 gilts were allocated to treatment in a 2 × 2 factorial design involving intra-carotid infusion of cortisol (10 mg in 40 ml saline) or saline alone with or without i.v. injection of 5 μg synthetic GnRH midway through the 1 h infusion. Plasma cortisol concentrations were elevated in gilts infused with cortisol (P < 0·05). The LH response to exogenous GnRH was reduced by cortisol infusion. Treated gilts released less LH (P < 0·001) and had a lower mean LH peak (P < 0·01) than did control gilts but the timing of the induced LH peak was not affected. In the absence of an exogenous GnRH challenge, cortisol infusion increased the endogenous secretion of LH (P < 0·01). These results suggest that acute elevations in plasma cortisol concentration may be involved in mediating changes in pituitary responsiveness and the secretion of LH in the peripubertal gilt.

Keywords: prepubertal gilts; elevated cortisol; LH secretion

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A. M. Paterson, G. B. Martin, A. Foldes, C. A. Maxwell and G. P. Pearce

Summary. Plasma melatonin concentrations were measured every 1–2 h over 24 h and plasma luteinizing hormone (LH) concentrations every 15 min over 12 h in domestic gilts reared under artificial light regimens that had previously been used to demonstrate photoperiodic effects on puberty. In Expt 1, the light regimens both commenced at 12 h light:12 h dark (12L:12D) and either increased (long-day) or decreased (short-day) by 15 min/week until the long-day gilts were receiving 16L:8D and the short-day gilts 8L:16D at sampling. In Expt 2, both light regimens commenced at 12L:12D and either increased (long-day) or decreased (short-day) by 10 or 15 min/week to a maximum of 14·5L:9·5D or a minimum of 9·5L:14·5D before being reversed. Sampling took place when daylength had returned to 14L:10D (long-day) or 10L:14D (short-day). In immature gilts housed at 12L:12D(Expt 1) and in postpubertal (Expt 1) and prepubertal (Expt 2) gilts reared under long-day or short-day light regimens, mean plasma melatonin concentrations were basal (3·6 pg/ml) when the lights were on and increased to peak concentrations > 15 pg/ml within 1–2 h after dark, before declining gradually to basal concentrations at or near the end of the dark phase. In prepubertal gilts bearing subcutaneous melatonin implants and reared under long-days (Expt 2), mean plasma melatonin concentration in the 6 h before dark was 91·9 ± 5·26 pg/ml and 125·0 ± 6·66 pg/ml 1 h after dark, but this increase was not statistically significant. In Expt 2, the short-day gilts had fewer LH pulses (2·6 ± 0·25 vs. 4·6 ± 0·24; P < 0·01) in the 12-h sampling period than the long-day gilts, but the amplitude of the pulses (2·28 ± 0·23 vs. 1·26 ± 0·16 ng/ml; P < 0·01) and the area under the LH curve (78·8 ± 5·60 vs. 47·3 ± 6·16;P < 0·01) was greater in the short-day gilts. In the short-day, but not in the long-day, gilts LH pulses were more frequent (2·0 ± 0·0 vs. 0·6 ± 0·25; P < 0·01), but had a smaller area (61·9 ± 7·2 vs. 120·2 ± 23·6; P < 0·05) in the 6 h of dark than in the 6 h of light, which together made up the 12-h sampling period. These data show that, in pigs, as in other species, the concentration of melatonin in plasma increases in the dark and the duration of the nocturnal increase depends on photoperiod. The implants provided high and variable concentrations of plasma melatonin, above which a nocturnal increase was not observed. The patterns of LH secretion were consistent with the short-day gilts being closer to puberty than the long-day gilts as a consequence of differing rates of sexual maturation due to the light regimens imposed during rearing.

Keywords: melatonin; LH; gilts; photoperiod; pig