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W. R. CARR

For studies on the serum lh concentrations of Zebu cattle during the oestrous cycle (Carr, 1971), sequential blood samples were collected from one pregnant cow. A solid phase radioimmunoassay for lh (Goding, Catt, Brown, Kaltenbach, Cumming & Mole, 1969) was carried out on the separated sera and several were found to provide elevated values. As levels of blood lh in pregnant sheep are reported to be consistently low (Geschwind & Dewey, 1968; Niswender, Roche, Foster & Midgley, 1968; Goding et al., 1969), it was decided to collect regular samples for assay until parturition.

The subject was a 5-year-old Angoni (short-horned Zebu) cow in good condition which had calved 9 months previously. Rectal palpation indicated that the animal was 3 to 4 months pregnant at the time that the first blood samples were collected. Blood

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W. R. CARR

Summary.

A solid-phase radioimmunoassay was used to measure lh in the serum of a Zebu cow and heifer during the oestrous cycle. Shortly after the onset of oestrus, serum levels of lh rose sharply to 20 to 120 ng/ml before dropping to baseline levels of about 2 ng/ml after 8 to 10 hr. Elevated lh values were also detected in mid-cycle, although considerable variation in activity over several days occurred with the heifer. Peak values of lh were considerably higher in the serum of the cow than the heifer.

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R. B. LAND and W. R. CARR

Summary.

The mean testis diameter of 20- to 25-week-old Blackface, Finn and Merino rams was ranked in the same order as the ovulation rates of females of their breeds. The removal of one testis at 12 or 16 weeks of age resulted in hypertrophy of the remaining testis. The relative increase in testis growth following hemicastration was greatest in the Merino rams (72%), least in the Finns (42%) and intermediate in the Blackfaces (57%), so that it was inversely related to their breed ovulation rates. This hypertrophy was associated with increases in the concentration of plasma LH in all breed types.

The results indicate that differences in testis growth rate are associated with differences in gonadotrophic stimulation rather than intrinsic growth potential, and it is postulated that these may arise from breed differences in sensitivity to negative feed-back from the testes.

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W. R. CARR and R. B. LAND

Summary.

Plasma LH levels were examined at 1½-hr intervals in ram lambs of the Finnish Landrace, Border Leicester and Cheviot breeds on three occasions at 3-week intervals by radioimmunoassay. On each occasion, the mean plasma LH concentration was found to be significantly higher in the Finnish Landrace than the other two breeds, although big variations occurred between individual samples in the same animal. This form of hormonal pulsatile release was most marked in the Finnish Landrace lambs. Testis growth was also measured and was found to be correlated, independently of body weight, with LH concentration on the final sampling occasion.

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W. R. Carr and R. B. Land

Summary. Finnish Landrace, Scottish Blackface and Tasmanian Merino rams were castrated and bled twice weekly from March to September. The breeds have widely different timing of seasonal variation in reproductive activity, but the plasma prolactin concentrations rose and fell at the same time in all 3 breeds, i.e. at the end of March and in September respectively. It is suggested that, even if major seasonal variations on plasma prolactin reflect primary changes in hypothalamic activity, these changes are modified by other factors to control the seasonal variation which occurs in the reproduction of the sheep.

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R. B. Land, W. R. Carr and R. Thompson

Summary. The influence of breed and season on the sensitivity of the pituitary gland of sheep to LH-RH was assessed. Ovariectomized ewes of 3 breeds (Finnish Landrace, Scottish Blackface and Tasmanian Merino) with differing normal breeding seasons and with differing ovulation rates were injected (i.v.) with 3 doses of LH-RH (1·56, 6·25 or 25·0 μg) at 3 different times of the year covering the anoestrous and the breeding seasons of intact ewes; 9 ewes of each breed (3 per sub-class) were examined on the first and third occasions, 6 (2 per sub-class) on the second. The response was measured in terms of the concentration of LH in peripheral plasma 20, 40, 60 and 80 min after injection. Time of year, but not the breed of sheep, affected the magnitude of the response; the data indicated that the duration of LH secretion was greater during the breeding season than during anoestrus. It was concluded that changes in the spontaneous activity of the hypothalamus/hypophysis could contribute to seasonal changes in LH secretion independently of the modifying effects of gonadal steroids. Such variation in unmodulated activity apparently does not contribute to the differences in ovulation rate among the 3 breeds.

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R. B. Land, D. T. Baird and W. R. Carr

Summary. Passive immunization against oestrone and oestradiol increased the rate of testicular growth (P < 0·01) without a statistically significant change in the concentrations of LH and FSH at the time of measurement. Although the concentration of testosterone was higher in the group with the largest testes, the difference was not statistically significant. Within groups, the concentration of FSH was correlated with the oestrone antibody titre (r = 0·5), suggesting that oestrone may have a particular role to play in the feedback control of FSH release in the ram; in addition, the concentration of testosterone was correlated with that of LH (r = 0·65).

The data indicate a role of oestrogens in the control of testicular growth and are compatible with earlier reports that the testes of Merino lambs grow more quickly when gonadal feedback is reduced.

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Y. SHEVAH, W. J. M. BLACK, W. R. CARR and R. B. LAND

Summary.

Progesterone and LH concentrations were measured in the plasma of blood samples taken from forty-eight pregnant ewes on Days 100, 120 and 134 of gestation. The ewes, in two groups of twenty-four were maintained from Day 100 until parturition on two planes of nutrition which supplied daily energy and protein intakes of about 4·1 or 2·3 Mcal metabolizable energy and either 192 or 111 g digestible crude protein per ewe. Within the groups, the ewes carried one, two or three fetuses and the feed intake was adjusted according to litter size to produce a uniform nutritional state within the group.

On Day 100, litter size affected the concentration of plasma progesterone (P< 0·001), but had no effect on Days 120 or 134 when the ewes were fed according to litter size. The low feed intake however caused a significant increase in plasma progesterone concentrations. The LH concentrations showed no major changes during late pregnancy and no effect of nutrition or litter size on the plasma hormone concentration was observed.

It was concluded that the effect of litter size on plasma progesterone concentration recorded on Day 100 of gestation was partly mediated by level of nutrition.

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Y. SHEVAH, W. J. M. BLACK, W. R. CARR and R. B. LAND

Summary.

The resumption of cyclic reproductive activity after lambing was studied in a total of fifty Finn × Dorset ewes which lactated for 1, 24 or 56 days. The ewes lambed in December 1971 and were maintained on a high plane of nutrition throughout pregnancy. Eight of the seventeen ewes which suckled for 1 day showed spontaneous oestrus between 2 and 20 days after lambing compared to only one of the thirty-three lactating ewes. Following treatment with `progestagen' vaginal sponges for 12 days from 20 to 24 days post partum, all the ewes showed oestrus. The mean interval between sponge withdrawal and the onset of oestrus was 35·3±1·5 and 33·2±1·6 hr for ewes which suckled lambs for 1 and 24 days respectively, and 41·0±1·5 hr for the lactating ewes (P<0·01). Thirty-one of thirty-three ewes ovulated. No significant differences were found between the numbers of corpora lutea observed for the three treatments: 3·09±0·3, 2·4±0·3 and 3·09±0·3, respectively. A preovulatory LH release was detected in the peripheral plasma of thirty-four of the thirty-five ewes studied. The mean intervals from the onset of oestrus to the start of the LH discharge were 9·1±1·2 and 6·1±1·3 hr for the ewes which suckled lambs for 1 or 24 days and 12·6±1·3 hr for the lactating ewes (P<0·01). The magnitude and duration of the LH peak were not influenced by lactation.

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R. B. Land, W. R. Carr, A. S. McNeilly and R. D. Preece

Summary. The effects of pharmacological reduction of the high plasma prolactin concentration typical of seasonal anoestrus in sheep were assessed with respect to positive feedback of oestrogen on LH release, ovulation, and progesterone secretion. Treatment of 16 Scottish Blackface ewes with 1 mg bromocriptine, i.m. twice daily for 12 days, reduced prolactin concentrations in peripheral plasma from 64 ± 10 ng/ml before treatment to <4 ng/ml. This treatment had no effect on the proportion of ewes discharging LH and FSH in response to 12·5 μg oestradiol benzoate (3/8 before compared with 5/16 during treatment) or the proportion of ewes ovulating in response to oestrogen treatment. Plasma progesterone concentrations remained low even in ovulating ewes.

It is concluded that treatment with bromocriptine alone is unlikely to restore oestrous cycles to ewes in seasonal anoestrus.