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.
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.
R. B. LAND
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.
R. B. LAND
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.
R. B. LAND
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.
R. B. LAND
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.
R. B. LAND
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.
R. B. LAND and ANNE McLAREN
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.
W. R. CARR and R. B. LAND
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.
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.