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R. B. LAND

Summary.

The ovulation rate of the mouse was reduced by twice daily injections of small doses of pregnant mares' serum gonadotrophin (pmsg). A dose of 0·5 i.u. pmsg per injection had the greatest effect, and reduced the number of eggs shed at oestrus and in response to hcg by 3·5 eggs (26%) and 4·7 eggs (34%), respectively. This effect was greater than that of single doses.

The equilibrium level of pmsg which produced the greatest reduction in the ovulation rate was estimated to be 1·5 i.u.

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R. B. Land

Variation in ovulation rate among different sheep breeds or crosses has been found to be related to the characteristics of the preovulatory discharge of LH, but there is no marked association with the concentration of LH during the oestrous cycle (Land, Pelletier, Thimonier & Mauléon, 1973). Studies of the oestrogen-induced discharge of LH in ovariectomized ewes indicated that breeds with high ovulation rates may be less sensitive to the positive and negative feed-back effects of oestrogen than are those with low ovulation rates. It was postulated that a low sensitivity of gonadotrophin release to the inhibitory effects of gonadal steroids may enable ewes which normally shed large numbers of eggs to tolerate higher concentrations of plasma oestrogen before the release of gonadotrophins is reduced (Land, Wheeler & Carr, 1976). Breed differences in follicular development and ovulation rate would therefore be compatible with similar LH concentrations. Alternatively, animals of a high-ovulation breed might catabolize the steroid produced more rapidly, or might produce less per follicle, than those of a breed in which fewer follicles developed; Bindon, Blanc, Pelletier, Terqui & Thimonier (1975) found no differences in the peripheral plasma oestrogen concentrations of ewes of breeds with differing ovulation rates.

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R. B. LAND

Summary.

Pure-bred Blackface and Welsh sheep were found to have significantly more oocytes at birth (20 and 16 × 104 respectively) than had their Finnish Landrace (Finn) crosses (11 and 15 × 104 respectively). The proportion of oocytes in large follicles with an antrum was, however, significantly greater in Finn cross than in the pure-bred animals (0·58 and 0·18% respectively).

As both pure and cross-bred offspring had pure-bred mothers, it was concluded that the higher proportion of developing oocytes in the Finn crosses was due to an increased level of foetal follicular sensitivity or stimulation rather than maternal gonadotrophic activity.

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R. B. LAND

Summary.

Treatment with 10 mg chlorpromazine/kg twice daily for six different 3-day periods during the oestrous cycle (five ewes/group) indicated that the ovulation rate was not depressed by any of the treatments. Treatment during the luteal phase of the cycle, however, raised the subsequent mean ovulation rate slightly (2·70 versus 2·27 eggs).

Unilateral ovariectomy on Day 2, 8 or 14 (eleven or twelve ewes/group) did not affect the total number of eggs shed (measured as the number of CL) at the oestrus immediately following ovariectomy.

It was concluded that in the sheep, the number of eggs to be ovulated is finally determined within the 3 days immediately preceding the onset of oestrus.

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R. B. LAND

Summary.

The litter size of Finnish Landrace sheep was found to be positively correlated with the duration of oestrus at the time of mating. The within-year regression of litter size on duration of oestrus was 0·011 lambs/hr and the correlation between the two traits 0·18 (1% > P > 0·1%).

Data from Finnish Landrace × Blackface, Blackface and Merino × Blackface females, indicated that the correlation was mediated through ovulation rate, and that it was not limited to the Finnish Landrace.

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R. B. LAND

Summary.

The incidence of oestrus was observed in thirteen Finnish Landrace, twenty-five Dorset Horn and 123 Finn-Dorset females between 0 and 56 days after lambing in the spring of 1968, and in 133 Finn-Dorset females between 10 and 56 days after lambing in the spring of 1969.

In 1968, 100% of the Finnish Landrace, 68% of the Dorset Horn and 59% of the crossbred ewes showed an oestrus and of those mated, 77%, 29% and 40%, respectively, conceived. In 1969, 23% of the females showed an oestrus, of which 71% were fertile. Within the Finn-Dorsets, the mean incidence of oestrus was found to be positively correlated with the age of the females (r = 0·97).

In addition to the normal 17-day oestrous cycles, many of the sheep displayed oestrus around 7 days after the preceding oestrus.

The hypothesis is advanced that a high level of gonadotrophic stimulation is an important common factor underlying the various aspects of increased reproductive activity in Finnish Landrace sheep.

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R. B. Land

Summary. 'The extent of genetic and environmental variation' in the development of reproduction in sheep is illustrated by examples with particular reference to variation among breeds and to the effects of photoperiod. The interactions between genetic and environmental effects are introduced; these may be so great that genetic groups may reverse their ranking for rate of development in different environments. The 'physiology of puberty' is then discussed. The difficulty of separating puberty from seasonal variation is stressed, and a possible contrast is drawn between the physiological characteristics of genetic variation and those of environmental variation in reproductive development. Finally the physiological factors associated with sterility in young females are discussed; most studies, however, have been conducted during the time of year when adult females would also be expected to be sterile, so that conclusions are difficult and a 'missing link' cannot be identified.

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R. B. LAND and ANNE McLAREN

Summary.

A study was made of the response of female mice to 1, 6, 12, 18 and 24 days of treatment with 2 i.u. of human chorionic gonadotrophin daily. It was found that: (1) ovulation in response to the exogenous hormone ceased between the 6th and the 12th day; (2) between the 6th and the 18th day, the mice resumed normal oestrous cycles; (3) the quality of eggs shed at natural oestrus during and after treatment was unimpaired, as judged by pregnancy examinations; (4) the ability of the females to maintain pregnancy after treatment was similarly unimpaired; and (5) during the period in which the females were responding to the exogenous hormone, they did not display the `Whitten effect'. The results indicated that immunity had arisen to human chorionic gonadotrophin, but not to endogenous luteinizing hormone.

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R. J. Scaramuzzi and R. B. Land

Summary. The concentration of oestradiol was measured by radioimmunoassay in ovarian venous plasma collected from ewes of three breeds (Finnish Landrace, Scottish Blackface and Tasmanian Merino) on Day 9 of the oestrous cycle and in jugular venous plasma collected daily around oestrus in two of these breeds. The mean ± s.e.m. concentration in the ovarian venous plasma of the Merino (44·1 ± 7·6 pg/ml) was lower than that in Blackface (72·2 ± 10·2 pg/ml) and Finn ewes (66·8 ± 10·2 pg/ml) The overall fitted mean concentration in peripheral venous plasma was 1·7 pg oestradiol-17β/ml, with no difference between the Finn and Blackface ewes, in which the highest preovulatory values were 3·9 0·5 and 0·5 pg/ml respectively.

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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.