Dorset Horn Research Articles

A Radiographic Study of the Changes in Skeletal Growth and Development of the Foetus Caused by Poor Nutrition in the Pregnant Ewe

Observations were made and measurements taken from whole body radiographs of 64 foetuses taken from 24 Finnish Landrace × Polled Dorset Horn ewes mated to Suffolk rams. The ewes were fed a complete diet until the ninetieth day of gestation when eight ewes were changed to a low-plane diet of medium quality chopped hay. The remaining 16 ewes were given a high quality balanced diet. Eight of the high-plane ewes were slaughtered between 119 and 121 days gestation, the remaining eight high-plane and the eight low-plane ewes were slaughtered between 132 and 136 days gestation. At slaughter the foetuses from the low-plane ewes were in the same weight range as the foetuses from the 119 to 121-day high-plane ewes and about 30% lighter than those from the 132 to 136-day high-plane ewes. Measurements of the bones and observations on skeletal developmental of foetuses of the same age but different weight taken from the low and high-plane groups at 132 to 136 days showed that the low-plane foetuses had smaller bones and were less well developed. Compared with the 119 to 121-day high-plane foetuses of the same weight, but different ages, the low-plane foetuses had larger bones and were better developed. Some of the low-plane foetuses exhibited lines of arrested growth in a number of bones, most markedly in the scapula, os coxae and talus. The skeletal effects varied both between and within litters and it appeared that foetuses with fewer and lighter cotyledons were affected most.

A radiographic study of early skeletal development in foetal sheep

SUMMARYObservations were made on high resolution radiographs of foetal sheep, from ewes slaughtered at 7-day intervals from 34 to 55 days after mating, to examine the onset of ossification of some of the major components of the axial and appendicular skeleton. There were 83 foetuses measured, retrieved from 24 Finnish Landrace × Polled Dorset Horn ewes mated to Suffolk rams. The litters comprised five sets of twins, eight of triplets, seven of quadruplets, three of quintuplets and one set of sextuplets.Half of the ewes were subjected to a 50% reduction in food on the 28th day of gestation, but this was considered not to have affected skeletal growth and development.There were major changes in skeletal mineralization from one slaughter date to the next, particularly between 34 days, when no skeletal elements were detectable, and 41 days, when components of the skull, thorax, fore and hindlimbs were clearly seen. There were some differences in the number of primary centres present both within and between litters of the same gestational age. The mineralized shaft of the fibula was seen in 30/35 of the foetuses at 41 days, but only in 9/25 at 55 days; other transient centres were also observed.Changes in the linear dimensions of six limb bones were measured and the mean values are presented in tabular form.

The relative growth and development of the carcass tissues of Merino and crossbred rams and wethers

One hundred and twenty lambs were used to study the effects of breed (Merino or Dorset Horn x (Border Leicester x Merino)), sex (entire or castrate male) and liveweight (birth to 50 kg) on carcass conformation and composition. Merinos were lighter at birth and grew more slowly than crossbreds in both the pre- and post-weaning phases (P < 0.01). Rams grew significantly faster than wethers only after weaning (P < 0.01). Merinos exceeded crossbreds in height at withers and chest depth (P < 0.01), but the reverse was true for width at 'hooks' (P < 0.01) and length of pelvis (P < 0.05). Sex differences were confined to chest girth, which was greatest in wethers (P < 0.01). Eye muscle width and area were greater in crossbreds than Merinos (P < 0.01). Growth coefficients for bone and muscle calculated on the basis of both half-carcass weight and half-carcass muscle-plus-bone weight were comparable with reports in the literature. For fat, however, the coefficients were considerably higher than has been reported previously (1.938-2.010 and 2.364-2.563 for the half-carcass and the muscle-plus-bone bases respectively). When their values were averaged over the range of slaughter weights studied, crossbreds yielded more bone (P < 0.05), muscle and fat (P < 0.01) than Merinos. Rams yielded more bone (P < 0.05) and less fat (P < 0.01) than wethers. Variation in yields of muscle and fat were due largely to differences in tissue deposition in animals slaughtered at 40 and 50 kg liveweight. Partial correlation analysis indicated that changes in carcass composition were largely accounted for by differences in liveweight.

A comparison of carcass chemical fat content and muscle distribution in Merino and crossbred rams and wethers from birth to 50 kg liveweight

The chemical fat content of and distribution of 24 individual muscles in the carcasses of 120 ram and wether lambs of Merino and of Dorset Horn x (Border Leicester x Merino) breeding were studied between birth and 50 kg liveweight. Rams contained significantly less chemical fat than wethers (P < 0.01), this difference becoming more marked as slaughter weight increased. Crossbreds yielded significantly higher chemical fat than Merinos (P < 0.05) only at 50 kg liveweight. Nine of the 24 muscles studied showed significant differences in relative growth between genotypes and sexes, and in each case the muscles concerned were located in the pelvic and thoracic limbs. Changes in muscle-growth coefficients with increasing slaughter weight occurred mainly below 30 kg, and four of the 24 muscles studied significantly changed their growth pattern more than once and are classified as 'multiphasic'. We conclude that the genotypes and sexes studied do not differ in muscle distribution in an economically important way, but that they do show major variations in fat content and maturity type which fit them for different patterns of production.

The prediction of body composition in live ewes in early lactation from live weight and estimates of gut contents and total body water

SUMMARYThe efficacy of estimates of gut contents and total body water in increasing the precision with which the chemical composition of the body could be estimated in early lactation was evaluated in 36 Finnish Landrace × Dorset Horn ewes. The ewes were fed at two levels in pregnancy, and, in lactation, given diets of two metabolizable energy concentrations.The allometric relationships relating weight of chemical fat and protein to emptybody weight were not affected by treatment or stage of lactation. Inclusion of an index of gut contents, based on dry-matter intake, indigestibility and retention time of food residues, together with live weight in a regression equation predicting weight of body fat, only slightly increased the precision of estimate compared with equations using live weight alone.There was a close negative relationship between the proportions of water and fat in live weight. Inclusion of weight of body water with live weight in a regression equation predicting weight of body fat markedly increased the precision of estimate and the residual error (0·81 kg) was similar at different stages of lactation. However, when deuterium oxide space was used instead of body water there was only a small increase in precision of estimate and the residual error varied from 5·3 kg in early lactation to 2·1 kg in mid-lactation. The relationship between deuterium oxide space and body water was shown to be variable and altered by stage of lactation, and these differences were associated with differences in rate of water turnover in the animal's body.It is concluded that estimates of body water are unsuitable for estimating weight of body fat in early lactation.

Sire effect on fatty acid composition of ovine adipose tissue.

The fatty acid composition of perirenal adipose tissue from 255 purebred and crossbred sheep was examined to determine the genetic effects of sire on each of five fatty acids. The sheep, all rams, were the progeny of 30 Dorset Horn sires. The animals were grazed on pasture and slaughtered at an average age of 21 months, when their mean carcass weight was 30 kilograms. The fatty acids studied and their mean percentage compositions were: stearic, 38%; oleic, 31%; palmitic, 20%; palmitoleic, 2%, and linoleic, 2%. These amounted to over 90% by weight of the tissue sample fatty acids. In addition, numerical functions were constructed as ratios for two pairs of fatty acids, ratio 1 (palmitoleic to palmitic) and ratio 2 (oleic to stearic), for estimation of the desaturase enzyme activity in the tissue samples studied. A Softness Index, expressed as a ratio between monounsaturated and saturated acids, was also included in the analysis. Significant birth year and maternal breed effects were found for all the traits studied. This was in contrast to the regression on slaughter age, which with two exceptions was not a significant source of variation in the data. The sire effect, based on 27 degrees of freedom, was highly significant or significant for all traits except stearic or linoleic acid and ratio 2. These results are discussed with reference to utilization of genetic variation between sites in selective breeding programs to modify fatty acid composition of ovine adipose tissue.

Effects of body fatness at lambing and diet in lactation on body tissue loss, feed intake and milk yield of ewes in early lactation

SUMMARYThirty-six mature Finnish Landrace × Dorset Horn ewes, each suckling two lambs, were used in a comparative slaughter experiment to measure changes in body tissues during early lactation. Two levels of body fatness at lambing were established by giving ewes a complete diet containing 10 MJ metabolizable energy (ME) and 139 g crude protein (CP)/kg d.m. either close to requirements orad libitumduring the second half of pregnancy. In lactation half the ewes in each group were given a complete diet containing either 90 (diet A) or 60 (diet B) % milled hayad libitum. These diets contained 7·9 and 9·2 MJ ME and 121 and 132 g CP/kg d.m. respectively.Ewes fed at the two levels in pregnancy contained 8·4 and 19·6 kg chemically determined fat 5 days after lambing but had similar amounts of body protein, ash and water. Over 6 weeks of lactation ewes given diet A lost 60 and 69% of these weights of fat respectively, while ewes given diet B gained 5% and lost 30% respectively. Up to 26 g of body protein was lost daily from ewes given diet A but none from ewes on diet B. During early lactation the weight of the empty digestive tract increased while the weights of most other body components, particularly the carcass, decreased. The ratio of body energy change to live-weight change varied from 24 to 90 MJ/kg. Thus live-weight change did not accurately reflect relative or absolute changes in body energy.Voluntary food intake was greater for ewes given the high-energy diet (B) than for those given diet A and was depressed in the fatter ewes. Differences in intake could be explained by the effects of body fatness and diet on the weight of gut contents. Milk yield was not significantly affected by body fat reserves but was higher on diet B than A. Fat content of milk was higher and protein content lower for ewes with the higher fat reserves at lambing.As the contribution of fat loss to energy available for milk synthesis increased there appeared to be a reduction in the energetic efficiency of milk synthesis. A number of possible reasons for this are discussed.

Pathogenicity for the Sheep Foetus of Bovine Virus Diarrhoea-Mucosal Disease Virus of Bovine Origin

Thirty pregnant, BVD antibody-free Dorset Horn ewes were divided into three equal groups and infected experimentally with a mixture of 10 strains of BVD virus isolated from cattle in Britain. Ewes of groups I and II were inoculated intramuscularly at day 25 to 40 of gestation with a high and low dose of virus respectively. Ewes of group III received inoculum partially inactivated by heat. Ten additional pregnant ewes served as uninoculated controls. None of the inoculated ewes showed any evidence of malaise, but all had seroconverted within 42 days. Of the 30 infected ewes 13 aborted and 12 were found ‘empty’ when killed 133 or 170 days after the presumed dates of conception. Four ewes produced five full-term lambs of which only two were bom alive, and both showed clinical signs characteristic of Border disease (‘hairy shakers’). One ewe was carrying two foetuses, one mummified and one alive when killed at 131 days of gestation. There was no significant difference in terms of foetal morbidity between the three infected groups. AH foetuses and lambs from infected dams showed moderate to severe growth retardation; in two foetuses and five lambs there were lesions of hypomyelinogenesis congenita; and one lamb showed cerebellar hypoplasia. It is concluded that some British strains of BVD (a) are highly pathogenic for the sheep foetus, (b) interfere with normal development of the foetus leading to growth retardation and (c) are capable of producing Border disease (hypomyelinogenesis congenita) and cerebellar hypoplasia. Possible practical implications of the findings are discussed.

Evidence for a decreased susceptibility to acute radiation lethality in young lambs.

The survival of 2- to 4-day old Romney-lambs was studied following bilateral 60Co irradiation at about 3.5 R/min (the exposure rate in air at the mid-line of the animal). Probit analysis of the data yielded an LD50/60 of 900 R with 95% confidence limits of 700–1150 R. Mature sheep irradiated under similar conditions are known to have an LD50/60 in the region of 250–350 R. These data indicate that very young lambs were less susceptible to radiation-induced hemopoietic failure than adults. Dorset Horn lambs and Romneys born by caesarian section also exhibited low susceptibility when irradiated at 2–4 days of age. There are few data available on LD50 values for very young, large mammals (as opposed to rodents). Consideration must be given to the possibility that large mammals may be less sensitive to radiation-induced lethality shortly after birth than they are at maturity. Further work on the radiation response as a function of age after birth seems warranted and suggestions for some of the parameters which require investigation are made.

Phenotypic and genetic parameters for lamb weights in a synthetic line of sheep

ABSTRACTData on 2120 lambs produced over 7 years in the Animal Breeding Research Organization synthetic Dam Line (49% Finn, 22% East Friesian, 16 % Border Leicester and 13 % Dorset Horn) were analysed by least squares. The effects of sex, age of dam, year of birth, birthrearing class, and age of lamb on weights at birth and at 4, 8, 12 and 16 weeks of age were all appreciable. However, pooled within-year correction factors should be adequate in adjusting records for selection. Heritability estimates for weights at the five ages ranged from 0·17 to 0·24 by the sire component method and from 0·18 to 0·28 by regression of offspring on dam. Previous reports of differences in heritability in singles and in twins were not confirmed. Estimates of the genetic correlations among the various lamb weights were high (0·62 to 1·04) indicating that selection for weight at one age should result in increased weights at all ages.

A note on the growth performance and carcass composition of Texel- and Suffolk-sired lambs in an intensive feeding system

ABSTRACTSixteen male and 16 female lambs were used to compare the performance and carcass composition of Texel- and Suffolk-sired lambs out of Finnish Landrace × Dorset Horn ewes. Lambs were weaned at 6 weeks of age and individually fed an all-concentrate diet ad libitum to slaughter at 30 kg (females) and 36 kg (males). At slaughter, offal weights were recorded and carcass left sides jointed and dissected.Breed types did not differ in weight at weaning, growth iate, age at slaughter, daily feed intake or feed conversion rate. Texels had heavier carcasses (17·2 v. 16·4 kg: P <0·01) and higher dressing percentages (50·7 v. 49·2: P <0·05) than Suffolks. Texel-cross carcasses had 2·9% more lean (P <0·05), but 3·8% less total fat CP <0·01).The results indicate the suitability of Texel-sired lambs for intensive lamb production.

Studies on reproduction in prolific ewes

SummaryEstimates of the energy contents of the foetuses, placentae, foetal fluids, empty uteri and maternal empty bodies at 88 and at 144 days of pregnancy were obtained from a comparative slaughter experiment on Finnish Landrace × Dorset Horn ewes. Over this period the mean daily intakes of metabolizable energy (ME) for ewes with twin, triplet or quadruplet foetuses were 11·2, 12·0 and 12·2 MJ respectively (low plane of feeding) or 14·4, 15·2 and 15·3 MJ (high plane of feeding), total numbers of ewes being 15, 23 and 7 respectively. The diet contained 8·2 MJ of ME and 153 g crude protein per kg dry matter. All six categories of ewe lost energy from maternal empty-body tissues between 88 and 144 days at mean daily rates of 2·7, 4·1 and 4·9 MJ (low plane) and 1·4, 2·7 and 3·6 MJ (high plane), according to litter size. Rates of energy deposition in the gravid uterus were 0·72, 0·93 and 1·07 MJ/day respectively, and were not affected by plane of nutrition. Estimates of heat production from these balance data were confirmed by direct measurements made on about half of the ewes.Estimates of the gross efficiency of utilization for foetal growth of the combined energy from maternal tissue and from ME intake, after allowing for maternal main–tenance, were similar for each litter size but were 0·14 and 0·11 on the low and high planes of feeding respectively. Consideration of these together with other published observations suggested that efficiency of utilization of ME in pregnancy increases linearly with the ME concentration of the diet and at concentrations below 10 MJ/kg dry matter is less than the efficiency of utilization of energy derived from maternal tissue loss.

Reproductive potential and endocrinological responses of sheep kept under controlled lighting. III. Prolactin and pituitary responsiveness to gonadotrophin releasing hormone in the post partum period

Five ewes of each of four breed types (pure Merino and crosses with Dorset Horn, South Suffolk and Border Leicester) were kept on a 6-monthly light-cycle in each of two light controlled rooms in an attempt to breed them twice in one calendar year. Four and 17 days following parturition they were treated with gonadotrophin releasing hormone (GnRH) (75, 150, 300 μg) and the pattern of release of luteinising hormone (LH) was studied and compared with that obtained in ‘normal’ cyclic ewes. Weekly plasma prolactin levels were determined at weeks 1–5. Overall, mean and peak levels of LH in the post-partum ewes were only one half those of cyclic ewes. The mean level ran from 8.4 ng/ml at day 4 to 18.2 ng/ml at day 17. This compared with 25.3 ng/ml for cyclic ewes. There was a significant effect of breed type, Merino and Dorset Horn × ewes releasing more LH than did South Suffolk × and Border Leicester × ewes. There were no significant effects of breed or week of sampling on plasma prolactin. None of the observed differences in pattern of release of LH could be related to the observed breed-type differences in fertility and it appears that factors other than endocrinological - e.g. uterine involution - may be involved in the ability to conceive of ewes in which oestrus and ovulation have been induced shortly after parturition.

Relationships among uterine and placental factors in prolific ewes and their relevance to variations in foetal weight

ABSTRACTObservations were made on the distribution of ovulations between the two ovaries, the distribution of foetuses between the two uterine horns, the total numbers of cotyledonary burrs, and the weights and numbers of foetal cotyledons for each foetus in 80 Finnish Landrace × Dorset Horn ewes killed between 50 and 145 days of gestation. While ovulations were randomly distributed between right and left ovaries, viable foetuses were more evenly distributed between right and left uterine horns. Migration of embryos between the horns contributed to this relative balance but early foetal mortality, which increased with ovulation rate, tended to upset it. The mean number of foetal cotyledons per foetus depended on litter size and distribution, and at any fixed litter size was substantially reduced in those ewes with six or more ovulations. Foetal weights were also relatively low in these ewes. The total weight of cotyledons per foetus decreased by about 12% for each increase of one in number of foetuses, as compared with a 20% decrease in number of cotyledons but only a 7 to 11 % decrease in the weight of the individual foetuses. Comparisons between foetuses within the same litter led to similar trends in the ratios of cotyledon numbers, cotyledon weights and foetal weights. It is suggested that the presence of some very small lambs in large litters should not be attributed to maternal under-nutrition in late pregnancy.

Use of carcase measurements to predict percentage carcase composition in crossbred lambs

Half carcases from 108 lambs, from two sexes (wethers and ewes), six genotypes (Dorset Horn and Border Leicester rams crossed with Merino, Corriedale and Border Leicester x Merino first cross ewes) and three slaughter weights (34, 44 and 54 kg liveweight) were dissected into fat, muscle, bone and connective tissue. Measurements of carcase weight, fatness (1 2-13th and 8-9th rib fat depth), size (carcase length, chest depth and leg length) and shape (fleshing index) were recorded. Carcase weight and fat depth at the 12-13th and 8-9th rib sites were the three best single predictors of percentage fat (RSD's = 3.564,3.912 and 3.291 ), percentage muscle (RSD's = 3.1 64,2.970 and 2.948) and percentage bone (RSD's = 1.455, 1.848 and 1.467) in the carcase. The combination of carcase weight with a measure of fat depth at the 12-13th or 8-9th rib sites improved the prediction of percentage fat (RSD's = 2.845 and 2.955), percentage muscle (RSD's = 2.629 and 2.851) and percentage bone (RSD's = 1.425 and 1.342) over that of carcase weight or fat depth as single predictors (P< 0.05). The further addition of size or shape to a model already containing weight and fatness was not significant (P > 0.05). Interactions between carcase measurements and weight were significant for some measures of fatness and size in the prediction of percentage fat and bone. No interactions were significant in the prediction of percentage muscle. The effect of genotype on the prediction of percentage composition was significant for many of the measurements, whereas the effect of sex was only significant for prediction of percentage bone and not for prediction of percentage fat and muscle. It was concluded that carcase weight and fatness provided the most accurate prediction of percentage carcase composition and that additional measures of size or shape did not improve the accuracy. Using carcase weight and fatness, knowledge of genotype improved the prediction of percentage carcase composition, whereas sex was relatively unimportant.

The growth rates of weaned lambs grazing three legume-based pastures during summer

An experiment was conducted on the Central Western Slopes of New South Wales with Dorset Horn x Merino lambs to assess their liveweight gains during summer when weaned on to a lucernebased pasture under dryland conditions, and when weaned on to irrigated lucerne and red clover pastures. Lambs grazing irrigated lucerne, irrigated red clover and dryland lucerne had average growth rates of 155, 178 and 124 g day-1/head, respectively, throughout the 106 day experimental period. The poor growth rates of lambs on the dryland lucerne were apparently due mainly to the low digestibility of the dry feed on offer as a result of low rainfall, which also caused feed supply to be limiting during part of the experimental period. Lamb growth and intake data suggest that the diets selected from the red clover were of higher digestibility than those selected from the irrigated lucerne. It was concluded that irrigated red clover and irrigated lucerne provided useful diets for young lambs in this environment, although the growth rates achieved were less than reported elsewhere for these species in cooler climates. Growth rates achieved by lambs grazing dryland lucerne were less satisfactory for prime lamb production.

Sheep production on the north coast of New South Wales. 2. A feasibility study of prime lamb production

Lamb production was studied in an experiment that ran for four years, 1968 to 1972, at Grafton, New South Wales, in a region where no commercial sheep are grown. Merino and Dorset Horn x Merino ewes stocked at 15 ewes ha-1 were mated in spring to Border Leicester rams and the lambs sold for slaughter in October. The mean number of lambs born per 100 ewes mated was 81.7 for crossbred ewes and 48.4 for Merino ewes (P< 0.05) and the mean carcase weights of the lambs were 14.8 kg and 12.7 kg, respectively (P< 0.01). The mean weight of fleeces cut from Merino ewes (3.24 kg) was greater (P< 0.01) than that from crossbred ewes (2.58 kg). The ewes were treated with anthelmintics either every four weeks or according to climatic conditions. There were no significant differences between the anthelmintic treatments for lamb liveweights at birth, weaning or slaughter; carcase weight; ewe fleece weight; or mortality rate. Treatment every four weeks increased ewe live weight during only one short period from August 1971 to February 1972 (P< 0.05). Data from this experiment compared favourably with survey data for the high rainfall zone in Australia and it was concluded that prime lamb production on the north coast of New South Wales is biologically feasible.

Genotype as a source of variation in selenium concentration and glutathione peroxidase activity of whole blood from grazing sheep and cattle

Selenium and glutathione peroxidase activities in whole blood were measured in a herd of 213 cattle containing Bos indicus and Bos taurus heifers and their crosses grazed at Rockhampton, Qld, and in a herd of 125 pregnant cows containing Hereford, Friesian x Hereford, Brahman x Hereford, and Simmental x Hereford genotypes, which was grazed at Grafton, N.S.W. Similar measurements were made on a flock of 18 Suffolk and 24 Cheviot lambs, and on a flock of 358 ram and ewe lambs of the Dorset Horn, Merino and Corriedale breeds and their reciprocal crosses. All lambs grazed at Armidale, N.S.W. Genotype was a significant source of variation in blood composition in all herds and flocks; there were differences between Bos indicus and Bos taurus cattle, between Bus indicus x Bos taurus cattle and Bos taurus cattle, between breeds of sheep and between sire groups within breeds. Heterosis was identified in blood selenium concentrations and glutathione peroxidase activities of blood from Dorset-Merino lambs. Other significant sources of variation were nematode and tick infestation in cattle and single versus twin births in sheep.

Ovulation and embryo survival rates and plasma progesterone concentrations of prolific ewes treated with PMSG.

Finnish Landrace x Dorset Horn ewes of two flocks comprising 47 and 51 animals were mated to Suffolk rams at a synchronized oestrus on 9 December (Flock D) and 8 April (Flock A) after treatment with 500 i.u. PMSG. The ewes were slaughtered between 50 and 145 days of pregnancy. The mean +/- s.e.m. ovulation rates were 4.5 +/- 0.24 (Flock D) and 5.0 +/- 0.32 (Flock A) for ewes which conceived to the induced oestrus and the corresponding litter sizes were 2.8 +/- 0.15 and 2.8 +/- 0.18. Mean concentrations of progesterone in the peripheral plasma during pregnancy were higher for Flock D than for Flock A ewes both in early (P less than 0.001) and late (P less than 0.05) pregnancy. An increase in ovulation rate, up to 4 or 5, was associated with an increase in mean litter size. There was evidence that higher ovulation rates were counterproductive.

Effects of season, lactation and plane of nutrition on prolactin concentrations in ovine plasma and the role of prolactin in the control of ewe fertility.

Plasma prolactin concentrations during the first 2 months after lambing, at oestrus, and during early pregnancy were investigated in 2 experiments in which Finn x Dorset Horn ewes were mated at an induced oestrus approximately 9 weeks after lambing. Mean prolactin concentrations between lambing and mating were dependent on seasons, being greater than 260 ng/ml plasma in lactating ewes mated in July and less than 150 ng/ml in those mated in October. Within 8 days of weaning of the lambs at 50 days post partum values declined to 122 and 30 ng/ml respectively. Plane of nutrition had little effect on prolactin levels. Higher prolactin values were recorded during oestrus in ewes mated in March or July, the normal period of anoestrus, than in December, the normal breeding season, mean values being approximately 200 ng/ml and 35 ng/ml respectively. The mean increases in the concentrations of prolactin during oestrus were smaller in lactating than non-lactating ewes. It is suggested that these differences in prolactin levels may be responsible for the effects of season and lactation on ewe fertility.