Silent Heat Research Articles

Oestrus and Pregnancy Diagnosis by Milk Progesterone Assay in the Mare

The milk progesterone profiles of four mares were followed daily for four to five weeks after foaling. Progesterone was determined by direct radioimmunoassay using iodinated progesterone as the labelled antigen. The milk progesterone concentration varied from 1 to 5 nmol/1 (0·3 to 1·6 μg/1) during the first 10 days after foaling. The first ovulation took place at about the tenth day. During the luteal phase milk progesterone levels rose dramatically, reaching a maximum level of 30 to 45 nmol/1 (9·4 to 14·2 μg/1) within five to 10 days, and then fell to low levels unless pregnancy followed, in which case milk progesterone remained high. Examples of the use of milk progesterone determinations in the diagnosis of silent heat and ovarian dysfunction are given.

Post weaning oestrus and luteal function in primiparous and pluriparous sows

The ovarian activity in 33 primiparous and 52 pluriparous post weaning sows was studied by progesterone determination. The progesterone profiles revealed that 90.4 per cent of the pluriparous and 66.7 per cent of the primiparous sows had resumed ovarian activity within 10 days. Ovulation within 10 days with overt oestrus occurred in 45.5 per cent of the primiparous and in 53.8 per cent of the pluriparous sows. Ovulation without detectable heat ('silent heat') occurred in 31 per cent of all sows during the first 10 days after weaning. The ability to resume ovarian activity was better in sows weaned in the period January to June than in sows weaned from July to December. Irregularities of luteal function were seldom observed in unmated sows. In mated sows which later proved to be non-pregnant, high progesterone values were common three weeks after mating.

Total Thyroxine and Free Thyroxine Index in Plasma of Dairy Cows in Relation To Strength of Heat

Total thyroxine (TT4) and free thyroxine index (FT4I) were measured in peripheral plasma of cows. The samples were collected at the time of insemination from 66 cows showing pronounced signs of the heat and from 56 cows showing weak or silent heat. Neither TT4 or FT4I in plasma differed significantly between the two categories of oestrous cows.

The use of heat mount detectors in a large Iranian dairy herd

Heat mount detectors were applied in a large Iranian dairy herd in an attempt to appraise their value for fertility improvement. A total of 100 cows with a recorded anoestrous period of at least 60 days were selected, 69 of them suffering from suboestrus or silent heat and 31 from true anoestrus due to inactive ovaries. The results indicate that inaccurate observation of heat is a major limiting factor in the reproductive performance of this herd and that heat detectors could be a valuable tool for improvement of oestrus detection and consequent reduction of the interval from calving to conception.

Cloprostenol for treatment of silent heat

(1977). Cloprostenol for treatment of silent heat. New Zealand Veterinary Journal: Vol. 25, No. 12, pp. 402-402.

The biology and evolution of the reproductive cycle of Miniopterus schreibersii and M. australis (Chiroptera: Vespertilionidae)

The reproductive biology of Miniopterus schreibersii and M. australis was studied at latitude 28·5°S, in eastern Australia. Comparison was made with material from 23·5°S to 37°S.Both species are monoestrous and monotocous. Ovulation is from the left ovary and implantation is in the right cornu. At latitude 28·5°S, a “silent heat” precedes behavioural oestrus and fertilization by two months in M. schreibersii and three months in M. australis. In M. schreibersii copulation and fertilization occur in late May and early June, immediately after a relatively short period of spermatogenesis and about one month after the release of sperm to the epididymides. During winter, implantation of the blastocyst stage is delayed. The male accessory glands involute but a store of epididymal sperm is retained. Implantation occurs in August and birth is in December. M. australis conceives in mid‐August, after an extended period of spermatogenesis which ceases four to six weeks prior to copulation. An epididymal sperm store is present some 21/2 months prior to insemination. Implantation has occurred by mid‐September and parturition takes place in December.Comparative data from other latitudes in eastern Australia show no differences in the timing of a reproductive events of M. australis between 23·5°S and 31°S. In M. schreibersii in South Australia (37°S) mating occurs in April and May. Plasticity in the timing of parturition (October to December) and other anomalies of this population are discussed.The data suggest that evolutionary hypotheses involving extrapolation from one species to the other are questionable and that a more complex and varied adaptive response to climate may exist than has hitherto been supposed.

A device for detection of heat in heat-silent buffaloes.

The chloride content of buffalo cervical mucus reached a maximum level of 632.4 mg/100 ml at the time of oestrus. A test was developed for the detection of oestrus in buffaloes. It was based upon changes in the chloride content of cervical mucus. Since it was simple and rapid, it could be done by attendants in the field. It was recommended as a routine test on a farm where silent heat was predominant among buffalo cows.

The seasonal nature of reproductive phenomena in the sheep. I. Variation in sensitivity to oestrogen.

Summary. Three groups, each of twelve spayed crossbred ewes, were maintained for 12 months on three different nutritional regimens. Twice each month each group was treated in a standardized fashion with progesterone followed by three doses (10\m=.\0,15\m=.\6, 24\m=.\4 \g=m\g) of oestradiol benzoate, and oestrous behaviour and vaginal oestrus were recorded. There was a seasonal pattern of variation in sensitivity to oestrogen which was modified by nutritional treatment. During late winterearly spring (August to October), loss of live weight was associated with decrease in sensitivity and maintenance or increase in live weight with increase in sensitivity. No such relationship was shown at other times of the year. Sensitivity was uniformly low, regardless ofnutritional regimen, in late spring-early summer (November to January). It is concluded that nutritional status during the August to October period when entire ewes are entering into anoestrus could have an important bearing on the incidence of ovulation without oestrus ('silent heat'). During the remainder of the year, nutritional status appears relatively unimportant.

A Review of the Causes of Reproductive Failure in Swine

SUMMARY Environmental and hereditary factors which are known to affect reproduction in swine are summarized. Losses in terms of eliminated breeding stock are considerable. It is estimated that 20–25 per cent of gilts and 12–14 per cent of sows may be eliminated due to infertility (non-pregnant, anoestrous, genital defects). Piglet mortality from parturition to weaning amounts to no less than 1:5–25 per cent. The average litter size is influenced by parity, age of sperms, ovulated eggs and maternal environmental factors. Conception rates are highly influenced by the number of natural matings or artificial inseminations per oestrous period. Anoestrus or silent heat is one of the foremost single restraining factors for the full utilization of the reproductive capacity of pigs. Prolonged anoestrous periods occur particularly in sows after their first farrowing. Continuous high plane feeding (protein as well as energy) after the lactation period and until mating must be considered of paramount importance for heat manifestation in breeding sows. The “flushing” technique seems to show correlation between the feeding level and the ovulation rate in pigs. Certain minerals and vitamins, particularly manganese and vitamin A, are essential for normal reproduction in sows and gilts. Hormonal control of oestrus with gonadotrophins is used extensively in some countries but hormonal treatments of highly selected breeding stock is not advisable. At present synchronization of heat in pig populations needs further elucidation before this technique can be recommended as a routine procedure in the field. Strictly venereal diseases are not known to occur in pigs. Some infections (Brucella suis and certain Leptospira species) are widespread causes of abortion and stillbirth. Also viral agents (hog cholera, pseudorabies, SMEDI viruses) are known to cause foetal death in pigs. Different bacteria may cause sporadic abortion, stillbirth and less severe cases of endometritis (streptococci, Staphylococcus aureus, Salmonella sp., Escherichia coli a.o.). Malformations of the genital organs occur rather frequently in gilts and sows. Between normal breeding boars some degree of variation exists within their fecundation rate (approximately 10 per cent). Nearly 40–50 per cent of breeding boars are eliminated at an early age due to infertility (impotentia coeundi, poor semen quality). The productive efficiency of boars used for A.I. is three times that of boars used for natural matings. The total production costs of piglets produced by A.I. is about one-half the costs for production of piglets by natural mating. Therefore expansion of A.I. may be expected in the future and underlines the importance of controlling inherited abnormalities (malformations, chromosomal abnormalities, etc.). Some defects (atresia ani, congenital spinal cord and vertebral anomalies, myoclonia congenita and dermatosis vegetans) may occur with a frequency which necessitates immediate replacement of the boar.

MPA impregnated intravaginal sponges for the induction of oestrus in anoestrous Border Leicester × Merino ewes. 2. Maiden ewes

In the spring of 1965 flocks of 18-month-old (flock 1) and 12-month-old (flock 2) anoestrous maiden ewes were treated for 13 days with intravaginal sponges impregnated with 17a - acetoxy 6a - methylpregn - 4 - ene - 3,20 - dione (MAP). Each ewe was given an injection of 1000 i.u. Pregnant Mare Serum Gonadotrophin (PMSG) at the time of sponge removal with half the ewes of each flock receiving a further injection of 1000 i.u. PMSG 16 days later. In the period 2-8 days after sponge removal, 45.9 and 57.1 per cent of the ewes in flocks 1 and 2 respectively were served, and of these 5.9 and 17.9 per cent respectively lambed. The second injection of PMSG produced a second oestrous period, 17-25 days after sponge removal, in 89.5 and 70.8 per cent of the ewes of flocks 1 and 2 respectively, and of these, 64.7 and 29.4 per cent, respectively, lambed. The additional administration of PMSG significantly increased the number of lambs born in flock 1 (P<0.01) but not in flock 2. Where a single injection of PMSG was used, failure to obtain a satisfactory lambing response was associated with a high incidence of 'silent heat' at the first induced ovulation, and a failure to promote regular sexual activity. No lambs were born in the untreated control groups until eight weeks after those of the treated groups.

The induction of pregnancy in the anoestrous ewe by hormonal therapy I. Progesterone-pregnant mare's serum therapy during the breeding season

Four hundred and twenty-two ewes, of a number of different breeds and ages (flocks 1–9) were used in experiments with hormones in the normal breeding season. Two main trials were conducted, the first making use of 377 animals, the second involving 45.Trial 1Three hundred and seventy-seven ewes were observed, in eight farm flocks, in a study of response to different treatments involving the administration of progesterone (in oil solution) and p.m.s. The trial sought to determine the effect of various dosage levels of progesterone and intervals of injection on the normal breeding performance of the ewe. Hormone treatment was applied at such time in the breeding season as to make it most improbable that conception would be limited by adverse animal or environmental factors. The aim was to determine the effect of the hormone technique per se on both immediate and long-term reproductive performance. Injection procedures and methods of flock management were arranged so that they might later be repeated in applications during the period of anoestrus.Two hundred and thirty-three ewes were injected intramuscularly with doses of progesterone in oil solution over a 7-day period. For eighty-nine ewes, four doses of 40 mg. were administered at 2-day intervals; for 144, 3 doses of 50 mg. were given at 3-day intervals. A single injection of 750 i.u. of p.m.s. was given subcutaneously to each ewe 2 days after the final dose of progesterone. One hundred and forty-four ewes remained untreated and served as a control.Progesterone therapy was effective in inhibiting oestrus and ovulation in the majority of ewes. Most injected animals came in oestrus 1–5 days following the injection of p.m.s.; 10% showed evidence of ‘silent heat’. 70% of ewes receiving progesterone at 2-day intervals and 68·2% of those receiving it at 3-day intervals conceived at the ‘controlled’ oestrus. 75% of the controls conceived at first mating. Treatment did not adversely affect the conception percentage to any appreciable extent.In animals conceiving at first oestrus, the average number of lambs born per ewe was 1·78 for injected and 1·44 for controls. In each flock there was an increase in lamb-crop, which would seem to be the result of the p.m.s. injection. Information on the effect of therapy on subsequent ewe fertility shows that 6·4% of ewes failed to conceive; this figure was the same for both treated and control animals. An attempt to use A.I. in conjunction with progesterone- p.m.s. therapy gave a conception percentage of 24, as compared with 68 in normally bred sheep.The results are discussed in relation to factors such as breed and age of ewe, and according to the particular mating procedure employed. There is no evidence that the forms of progesterone-p.m.s. therapy employed in this work would constitute a limiting factor to conception in out-of-season breeding.Trial 2The forty-five mature Welsh Mountain ewes used were in one flock. Each animal was injected on the 13th day of the oestrous cycle with a single dose of either 50 or 100 mg. of a micro-crystal suspension of progesterone. Two forms of suspension were used, the difference between the two being in the size of the individual crystals. Ewes came in oestrus 7–13 days following injection, the range for 50 mg. dosage being 7–13, and for 100 mg. 10–13 days. 25% of animals showed ‘silent heat’. Results suggest that injection of suspension adversely affects the normal expression of oestrus.

The use of serum gonadotropin (P.M.S.) in the induction of twin-pregnancy in the cow

The use of serum gonadotropin (P.M.S.) in the induction of twin-pregnancy in the cow

THE INDUCTION OF BREEDING ACTIVITY IN LACTATING EWES DURING ANOESTRUS

: ( ) during normal anoestrus (Control), (2) after breeding activity had been induced by a series of three intra-muscular injections of 50 mg of progesterone (in oil) at 3-day intervals, followed after 48 hr by a subcutaneous injection of 750 i.u. of PMS, (3) after breeding activity had been induced, in response to the animals' endogenous endocrine secretions, by artificially manipulating the diurnal light : dark ratio. Untreated control ewes showed no evidence of recent ovulations but a degree of ovarian activity was present as measured by the number of medium- and large-sized ovarian follicles. Injections of progesterone and PMS induced ovulation in nineteen of the twenty ewes treated and oestrus accompanied ovulation in twelve ewes. However, in ewes which mated there was a complete failure of fertilization which could not be attributed to poor semen quality. Manipulation of the diurnal light : dark ratio induced ovulation in all ten treated ewes and oestrus was observed in nine ewes. Oestrus was preceded by a 'silent heat'. The response to this form of treatment was slow but uniform. Of the ova recovered 41.2 % were cleaving. A semi-quantitative estimate of the potency of total gonadotrophin in the anterior pituitary gland indicated a significantly higher level of gonadotrophin in untreated control ewes than in ewes which ovulated, suggesting that both forms of treatment induced a release of gonado¬ trophin from the anterior pituitary gland. The results are briefly discussed.

Nutrition in relation to reproduction in sows

Gilts of the ‘Palouse’ breed were divided into two groups; one group was full fed from weaning to 1501b. and the other group was fed 70% of full feeding. After six generations, one-half of the pigs in each group were shifted to the other plane of nutrition, thus forming four groups: high-high (HH), low-high (LH), low-low (LL) and high-low (HL). F10 gilts were slaughtered 38 or 100 days post-coitum to study reproductive and prenatal phenomena.1. Ten successive generations of controlled feeding had no significant effect on: length of Fallopian tubes, weight of corpus luteum, percentage of implantation, number of viable foetuses, foetus weight, weight of placental membranes, volume of placental fluids or embryonic mortality.2. There were significant differences between the high and low planes of nutrition in: weight of pituitary, thyroid and adrenals, weaning weight, puberty weight, puberty weight to birth weight, puberty weight to weaning weight and puberty age. The effect of six generations of controlled feeding was not maintained when the plane of nutrition was changed from high to low or low to high.3. The frequency of silent heat and the number of services per conception was lower in the low planes than in the high planes.4. Ova migration occurred in both directions in 41% of the gilts; one to four ova migrated. The foetal sex ratio ranged from 4:0 to 1:7 with an average of 1:1. The sexes were unevenly distributed in the horns.5. Foetus weight at 38 or 100 days post-coitum was not affected by plane of nutrition or sequence in the uterine horn.6. Within-litter variations were highest in the volume of allantoic fluid followed in order by variations in weight of placental membranes and then the amniotic fluid and foetus.7. Foetal mortality was not related to sequence in the uterine horn or the number of implantations.8. There seemed to be two peaks of early prenatal mortality, one at implantation and another at the onset of organo-genesis.

Studies on the treatment of the silent heat in the cow with progesterone and estrogen

Studies on the treatment of the silent heat in the cow with progesterone and estrogen

The induction of coincident superovulation and oestrus in the anoestrous ewe

Eight Merino and eight Dormer ewes, none of which had been on heat for 2–3 months prior to treatment, were given six intra-muscular injections of 30 mg. progesterone at 3-day intervals. The ewes were divided into four groups, each consisting of two Merinos and two Dormers. On the second day after the last progesterone injection, 1000 i.u. P.M.S. were administered to each of the ewes in group I. The remaining ewes each received an injection of 500 i.u. P.M.S. on either the first and second days (group II), the second and third days (group III), or the first and third days after the end of the progesterone treatment (group IV).Five of the Dormer ewes, but not one of the Merino ewes, were served after the treatment. Mean ovulation rates for the four groups were 2·50, 2·75, 2·00 and 2·50, respectively, and the mean numbers of large follicles (i.e. those which were observed at laparotomy, approximately a week after treatment, to have a diameter of not less than 4 mm.) were 3·00, 2·75, 1·00 and 2·00, respectively. Contrary to expectation, it was observed that only one of the ewes (a Merino) showed evidence of a silent ovulation shortly before treatment.It is suggested that (i) the amount of oestrogen produced by the ovarian follicles of the Merinos may have been exceptionally low, thus accounting for the non-occurrence of oestrus in these ewes, (ii) ovulation rates might be increased by greater reductions in the time between the last progesterone injection and P.M.S. treatment, and (iii) if silent heat in the Merino is rare, it may be simpler to induce coincident heat and superovulation in ewes of this breed by two injections of P.M.S. at 16-day intervals.

The effect of the presence of the ram on the ovarian activity of the ewe.

Observations were made on the effect of the presence of the ram when introduced to Merino ewes in the transition from the "non-breeding" to the "breeding" season in the Roseworthy environment. These observations showed that the primary effect was to stimulate ovulation without oestrus ("silent heat") in the majority of those ewes which had not already commenced cyclic breeding activity. This effect occurred within 6 days of joining the rams with the ewes. It is concluded that this mechanism explains the "lag and peak" incidence of oestrus previously observed under these conditions (Underwood, Shier, and Davenport 1944; Thompson and Schinckel 1952; Schinckel 1954).

The hormone control of ovulation in the calf

1. Unlike many other mammals, the calf is capable of responding to pituitary injections almost from birth. Age has little effect on the ability of the follicles to develop during a series of a.p.h. injections, the follicular response generally being as good 3 weeks after birth as in calves 6–8 months old.2. Ovulation can occur spontaneously in the a.p.h. treated calf before the end of the first month of post-natal life.3. Ova obtained from such ovulations are capable of fertilization, although the percentage becoming fertilized is very low. This is thought to be due to the infantile state of the Fallopian tube and the difficulty in inseminating into the uterus, rather than faulty maturation of ova.4. Although many follicles (10–60) can develop during a series of a.p.h. injections, superovulation was not consistently obtained either by intravenous injections ofa.p.h., l.h.orp.u., or combinations of any of these three.5. Superovulation can generally be obtained, with the formation of many corpora lutea, if the a.p.h. treatment is repeated a second time after a period of luteal activity.6. The inhibitory effect of a corpus luteum on ovulation in cattle can be overridden in tho a.p.h. treated animal if suitable quantities ofa.p.h., l.h.orp.u.are intravenously injected when the newly formed follicles are approaching maturity.7. Ovulation in the calf at all ages up to 8 or 9 months of age, either in the presence or absence of luteal tissue, is unaccompanied by oestrus. Oestrus, when occurring during a series of a.p.h. injections, commenced after 90–100 hr. of treatment. In such cases slaughter has always revealed the presence of luteal tissue, indicating that ovulation during a ‘silent heat’ had occurred before the commencement of pituitary treatment.8. Great individual variation exists in the ability of the ovary to respond to pituitary injections. This insensitivity to gonadotropins is not confined to young calves and has been observed in calves 4–6 months of age.

Studies on the breeding season and reproduction of the ewe Part III. The breeding season and artificial light Part IV. Studies on the reproduction of the ewe Part V. Mating behaviour and pregnancy diagnosis

During the course of three breeding seasons, daily observations of oestrus were undertaken on the following groups of ewes, all of which were running with ochred rams. One hundred and twenty ewes (pure bred and first-cross) of different breeds and ages, running with vasectomized rams under natural conditions. Twenty-eight grade Suffolk ewes fed on a submaintenance diet. Eighteen grade Suffolk ewes running with a fertile ram. Twenty ewes of several breeds exposed to artificial light.The effects of heredity (breed and individual), environment (season, year and nutrition), age and artificial light on the breeding season and related phenomena have been investigated. The results and conclusions were as follows:1. (a) There are breed differences in the extent of the breeding season, cycle length, incidence of silent heat and duration of heat (Table 29). (b) The duration of the breeding season is related to the geographical origin (latitude and altitude) of the breed, (c) The duration of the breeding season of the first-cross is intermediate between that of the two parents, (d) Individual differences in the number of oestrous cycles per ewe per season were more marked in the mountain breeds.2. (a) Of the ewe lambs, 79% exhibited oestrus during the shortest days only of the first breeding season. Their breeding season is not spread evenly about the shortest day as it is with adults. (b) The occurrence and length of the breeding season in ewe lambs is associated with early birth dates or with higher growth rates.3. (a) Significant differences exist between breeds in the age at first oestrus. (b) Ewe lambs born early in the season showed their first oestrus at later age and heavier weight than those born late.4. Annual differences in the duration of the breeding season, cycle length, incidence of silent heat and occurrence of first oestrus were negligible.5. (a) Submaintenance diet had no effect on the onset of the breeding season, but it converted oestrus into silent heat. Conception occurred less frequently after periods of underfeeding. (b) At high latitudes nutrition has only a minor effect on the breeding season of the ewe.6. (a) A constant high ratio of artificial darkness (8 hr. light: 16 hr. darkness) hastened the onset of the breeding season some 57 and 27 days in the two experimental groups. (b) A constant high ratio of artificial light (16 hr. light: 8 hr. darkness) hastened the end of the breeding season some 104 days on an average (in one experimental group), (c) There were breed differences in the latency of initiation and of cessation of the induced breeding season, (d) In the induced breeding season cycles of ovulation preceded the first oestrus, (e) Two thresholds of pituitary activity are suggested, one for the onset of ovulation, and the other for the manifestation of oestrus.7. (a) A high frequency of cycles outside the normal range (14–19 days) was observed in the mountain breeds and in ewe lambs. (b) The shortest average cycle length coincided with the shortest days of the year.8. A high frequency of silent heats (during the breeding season) was observed in the mountain breeds, in ewe lambs and during the second half of the breeding season.9. Oestrus was of longer duration in adults and yearlings than in ewe lambs.10. (a) Post-partum heat occurred in 56% of Suffolk ewes with an average lactation anoestrus of 35 days. (b) There is a relationship between the early onset of the breeding season and the incidence of post-partum heat, (c) Conception post-partum is partly inhibited by lactation.11. Mating behaviour was observed in animals of several breeds and ages (three patterns were recorded for ewes and eight patterns for rams).12. Irregular columnar cells in the vaginal smear of the ewe characterized late pregnancy.In addition, the breeding season of wild and domestic sheep was reviewed in relation to the environment with special reference to the length of daylight.

REPRODUCTION IN THE EWE

Summary1. The relative low prolificacy of the ewe is a factor severely limiting the sheep's ability to compete with other forms of livestock production in intensive agriculture. This article is an attempt to examine critically all the available information on the reproductive physiology of the ewe, with particular emphasis on the factors controlling the various phenomena and the possibility of increasing prolificacy.2. All breeds exhibit, to a greater or less extent, a breeding season which is under light control. Available data on the extent of this season are tabulated.3. The ‘height’ of the breeding season corresponds roughly to the shortest day of the year, and the ‘depth’ of anoestrum to the longest day.4. This and other data suggest a seasonal fluctuation in pituitary gonadotrophic activity controlled by, and roughly inversely related to, the period of daylight relative to dark. It reaches a maximum when the days are shortest and a minimum when they are longest.The ability of some breeds to come into oestrus and conceive shortly after parturition depends on the breeding season being sufficiently long to permit ewes mated early in the season to lamb well before its end. Provided then that the lactation anoestrum, the length of which appears to vary according to the stage of the breeding season, does not run into the true anoestrum the ewes can mate and conceive again while lactating.6. The onset of the breeding season is preceded by an ovulation without heat–‘silent heat’. The evocation of heat with ovulation appears dependent on the presence of a waning corpus luteum.7. Seasonal variation in the reproductive capacity of the ram is evident but is not as marked as in the ewe.8. The literature on the dioestrous cycle, ovulation, fertilization, attachment and subsequent development of the embryo and foetus is reviewed. Very loose attachment to the cotyledons occurs at about 11 days after ovulation, which attachment remains loose up to the 30th day. Full placentation is not completed until the middle of pregnancy.9. Embryonic mortality within the first 21 days accounts for an appreciable number of the ova fertilized. While the number of ova shed is probably the main factor controlling the number of lambs born, this mortality must be considered.10. The use of gonadotropins in anoestrum to obtain early pregnancies is fully discussed and the literature reviewed. While, in general, results have been disappointing there appears to be a definite pattern of behaviour depending on the state of the ovaries at the time of injection.11. An interesting report on the use of pregnant mare's serum (PMS) in inducing fertile matings shortly after parturition within the breeding season is discussed. If this can be confirmed, it may be of value in obtaining two pregnancies within a year.12. The subcutaneous injection of 500 i.u. of PMS 12 days after the preceding oestrus will result in multiple ovulations at the ensuing heat period 4 or 5 days later. However, the individual response to any given level of PMS is extremely variable, from two to nine ovulations having been observed following 500 i.u. Mating at this oestrus results in a high proportion of fertilizations and attachments. Subsequent embryonic mortality within the first 21 days following ovulation results in a reduction in the number of embryos surviving to two or three, occasionally four and rarely five or six. Up to four live young have been obtained at Cambridge and five in Russia. The number of lambs born in a small group of Suffolk ewes treated at Cambridge was greater than in the control group. However, some were born dead and the number weaned was similar in both groups.13. There appear to be breed differences in the number of embryos surviving the first 3 weeks of pregnancy following such PMS treatment, probably due to differences in uterine environment.14. Overstimulation with gonadotropins results in accelerated tubal transport of the ova, with lack of fertilization and complete failure to attach.15. Taking a long‐term view, increase in prolificacy can probably be best obtained by selection for a long breeding season and for multiple births. In most breeds there is sufficient variation in these characteristics to offer considerable hope of success.