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To date there is no satisfactory estimate for the ewe, of the loss of potential offspring from ovulation to parturition. Various studies have thrown some light on the subject, but they have generally been confined to a particular phase of gestation, or a single part of the breeding season. Hammond (1921) presented data on 80 ewes at various stages of pregnancy which showed normal foetuses equal to 87 % of the corpora lutea present. Henning (1939) used abattoir material of unknown origin to estimate the incidence of foetal mortality from the discrepancy between the number of corpora lutea in the ovaries and the number of live foetuses in the uteri. Because of the nature of their material, neither Henning nor Hammond could detect early embryonic death which had been followed by complete resorption. In Henning's data 16% of corpora lutea (total 773) were not represented by live foetuses. Brambell (1948) has pointed out that this type of data could greatly underestimate the actual loss.In a study involving 180 yearling cross-bred ewes, early in the breeding season, in which half the group was slaughtered to establish fertilization rate, Dutt (1954) found an embryonic death-rate of 32-7 %. Late in the breeding season, Laffey & Hart (1959) found that in 60 middle-aged half-bred ewes, 40 % of fertilized ova were abnormal. Hulet, Voigtlander, Pope & Casida (1956) reported that in 86 Hampshire and Shropshire ewes, there was an estimated embryonic loss from fertilization to lambing of 28-6 % early in the breeding season and 9-4% during a later period.Hasnain (1964) with small numbers of ewes (breed not stated) found that in mature nonlactating ewes during the breeding season, 70-9 % of corpora lutea were represented by embryos at 18 days post-mating. However, 22-8% of these embryos were classified on developmental criteria as ' abnormal' and therefore unlikely to survive.These apparently normal losses in early pregnancy have on occasion been increased by high temperatures, exogenous hormones (both reviewed 19

Injections of colchicine into the lumen of the uterus of the ewe have been employed to cause timed embryonic deaths between Day 5 and Day 19 after ovulation. The length of oestrous cycles following embryonic death has then been studied. Control ewes had a mean cycle length of 17\m=.\44 \ m=+-\ 1 \ m =. \ 0 6 days, and the cycle length of ewes treated on Days 5, 7, 9 and 11 did not differ significantly from the controls. Ewes treated on Day 13 had a mean cycle length of 19\m=.\75\ m= +-\ 0\m=.\90 days, a highly significant increase over the controls. Embryonic death on Days 15 and 19 caused mean extensions of the cycle length of approximately 8 and 18 days respectively, but with wide individual variation. The results are discussed in relation to the maintenance and regression of the corpus luteum in the presence of embryonic substance. The findings indicate that in the field, embryonic death occurring on or before Day 11 is unlikely to be distinguished from fertilization failure.

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Observations were made on mating behaviour and reproductive performance during the puberal period in 61 well-grown Perendale (Cheviot x Romney) ewe lambs. In four animals, the first oestrus (as indicated by raddle marks) was not accompanied by ovulation, and three other lambs, though marked by the rams, were not inseminated.The behavioural responses to a ram, of ewe lambs experiencing oestrus for the first and second times, showed many signs of inadequacy by comparison with those of mature ewes. Individual ewe lambs and mature ewes in oestrus were exposed to a ram in a pen for 3 h, and their responses were ranked on a scale of 1 (full adult response) to 5 (weak response). Of mature ewes 95% scored 1, compared with only 27 % of ewe lambs. Across the lambs 59 % scored 3 or more, indicating failure of some aspects of the courtship sequences which are required to sustain and reinforce the ram's interest. In a paddock-mating system many lambs showed a strong tendency to disassociate themselves from the rams after each courtship and mating sequence. Consequently there was little evidence of harem formation. Fifty-six per cent of services came in the first 2 h of oestrus, only 14 % after the 6th hour and none after the 11th hour.Duration of the first oestrus in lambs was 18-4 h compared with 29-3 h in mature ewes. Mating the 61 ewe lambs over the first three cycles after puberty resulted in 38 of them being pregnant with 46 foetuses at autopsy in mid to late pregnancy. The success rate did not differ between cycles. c a t e d t h a t m a h i S h P r°P°rtio n °f studies only 6-2