School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemSocial and sexual factors affecting reproduction of the domestic boar( 1978)This thesis describes the results of work in three general areas; (i) the influence 0f the social environment during rearing and after puberty on the sexual behaviour of the boar, (ii) the role of the courting behaviour of the boar in his reproductive performance, and (iii) the influence of sexual stimulation on the spermatozoan output of the boar. The major portion of the work relates to the importance of the social environment. Boars reared from three weeks of age in the absence of visual and physical contact with pigs had markedly lower copulatory performance and level of courting activity compared with those reared from three weeks in either an all-male or mixed-sex group. Lack of physical contact with pigs during rearing was responsible for approximately 70% of this depression in copulatory performance. The copulatory performance of boars reared from 12 weeks of age in the absence of visual and physical contact with pigs was considerably less than that of boars reared in an all-male group, but the difference was not significant. Boars reared from three or 12 weeks- of age individually in adjacent wire-mesh pens had similar copulatory performance to that of group-reared boars. However, an apparently important feature of courting behaviour, 'nosing activity', was significantly reduced for those boars reared from three weeks individually in adjacent wire-mesh pens. These data were interpreted as evidence of the importance of social contact, particularly physical contact with boars or gilts, during rearing on the level of sexual behaviour of the boar. In addition to the social environment during rearing, the social environment after puberty was demonstrated to have a dramatic influence on the level of sexual behaviour of the boar. Isolation of post-pubertal boars from female pigs for six weeks significantly reduced their copulatory performance and level of courting activity. The stimuli received from the presence of sexually receptive or sexually non-receptive female pigs were equally capable of maintaining the level of sexual behaviour of the boar. Two experiments conducted at a commercial piggery revealed the importance of the courting behaviour of the boar on reproductive performance. A significant positive correlation was found between the observed level of nosing activity during courting and the conception rate record of the boar. The proposal that the nosing activity of the boar may stimulate one or more physiological mechanisms leading to fertilisation in the sow is supported by the results of the second experiment. Brief courting of sows by a boar prior to artificial insemination significantly increased the farrowing rate and litter size of group-housed sows. Finally, sexual stimulation of boars prior to semen collection significantly increased the number of spermatozoa . in the sperm-rich fraction of the ejaculate. The technique of sexual stimulation of the boar involved either allowing the boar a false mount and then briefly restraining him or allowing the boar to observe a semen collection. The former technique failed to maintain a significant increase in the yield of spermatozoa from the sperm-rich fraction over a six-week collection period. Prostaglandin F2a, which may be involved in the mechanism by which sexual stimulation increases the number of spermatozoa in the short term, was also studied. Administration of 20 mg of PGF2a 30 minutes before collection significantly increased the number of spermatozoa in the sperm-rich fraction of the ejaculate. The experiments of this thesis demonstrate that sexual and social factors have a major influence on the reproduction of the domestic boar.
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ItemGenetic studies of reproduction in four species of domestic animals( 1975)The objectives of this study are to determine the most effective technique of assessing reproductive performance, to analyse factors influencing the performance and the problems involved in analysing data from records of performance from commercial farms, and to make recommendations for improving the reproductive performance, in meat-producing species of farm animals using genetics methods. The study consists of a review of the present status of knowledge on the genetics of reproduction, a series of analyses of reproductive performance using records of performance of three meat-producing species of farm animals (beef cattle, pigs and sheep) and one species of laboratory mammal (mice) and concludes with some suggestions of steps that may be taken for recording performance and improving breeding programs in meat producing farm animals to raise reproductive performance. The review discusses the concepts of reproduction and growth, identification and measurement of reproductive traits, factors affecting reproductive performance and alternative strategies for improving the level of reproduction in domestic animals. Reproduction is considered to be a function of fertility, survival and growth and the success of reproduction depends on the total contribution made by the sire, the dam and the progeny resulting from specific matings. Studies on the reproductive performance in beef cattle cover two chapters. The first (Chapter 2) describes an analysis of reproductive performance of beef cattle from 13 properties in Victoria. The overall data gave a mean age at first calving of 863.5 days, mean calving interval of 361 days, calving rate of 88.7%, mortality rate at birth of the calves of 4.3%, preweaning mortality of 1.5%, postweaning mortality of 0.3% and twinning rate of 1.5%. At this level of performance the rate of reproduction of beef cattle from commercial properties in Victoria is better than that in northern Queensland and comparable if not better than those reported in the United States. The heritability estimates of reproductive traits calculated from the records of performance in many cases gave values beyond the theoretical limits (from 0 to 1). This was attributed to confounding effects of management practices. The next chapter (Chapter 3) consists of studies on the effects of crossbreeding between British breeds of beef cattle (Angus, Hereford and Shorthorn) and the crossing of these breeds with Friesian sires and Friesian-cross heifers. Traits analysed include growth rates from birth to yearling age. Factors affecting these traits are examined and their heritability is estimated. Calving performance of the dams and twinning are also studied. Crossbred calves generally showed superior preweaning growth rate than the purebred calves and even better performance occurred if one of the parents was a Friesian. There were no significant differences among the crossbred calves produced by the crossbred dams and the purebred dams when both were mated to Shorthorn sires indicating that in the material available maternal heterosis had little influence on growth compared to F1 heterosis as expressed in the crossbred progeny. Birthweight and postweaning growth rate generally were moderately heritable but preweaning growth rate generally gave negative estimates of heritability indicating greater influence of environmental variations on the latter trait. Data obtained from crossbred progeny tended to give lower estimates of heritability than those from the purebred progeny. It was not possible to analyse traits related to fertility and postnatal survival because of imcomplete recording of performance from the farm. The analysis of reproductive performance of Corriedale sheep (Chapter 4) gave an overall conception rate of 91%, lambing rate of 112%, twinning rate of 37%, mortality of lambs at birth of 5%, mean period between start of mating and lambing of 166 days and lambing interval of 369 days. This level of reproductive performance is slightly higher than generally reported by other workers for various breeds of sheep. Selection for wool and twinning made in the flock did not seem to have produced any effect on reproduction. Ewes born twins were lighter at birth, had high rate of mortality, lower rate of preweaning growth and wool production, had a lower rate of conception, rate of lambing and rate of twinning at first mating, and reached their peak level of reproductive performance at an earlier age than those born as singles. Females born co-twin to a male in the same litter showed effects of being at a disadvantage in their prenatal and preweaning development. Preweaning body growth had fairly high estimates of heritability while fertility of the dam had very low estimates of heritability. Measurement of overall reproductive performance gave the highest estimate of heritability suggesting greater accuracy of this measurement as a measure of reproductive performance of female ruminants. The analysis of reproductive performance in pigs gave an overall estimate of 78.8% success at mating, total litter-size at birth of 9.4 of which 6.5% were stillbirths, total litter-weight of 12.2 kg., average weight of piglet of 1.43 kg. and gestation length of 115 days. Except for the rather low conception rate, the level of reproductive performance is comparable to those reported for various breed in Britain, Canada and the United States. The reproductive performance was mainly affected by parity of sow while traits related to fertility of sows and survival of progeny were improved by using crossbred dams. Development of a new breed through selection for high litter-size did not result in significant improvement in reproductive performance. The analysis also indicated that there was little to be gained by having a litter-size of over 12. Most of the reproductive traits had low estimates of heritability except for average conception rate of the individual boar which was shown to be moderately heritable and should therefore be included in a selection program. The analysis of the effect of selection for high litter weight at 9 weeks of age in two lines of mice when compared with a control line indicated that there was very little direct or correlated response except perhaps for some initial improvement in the average bodyweight of the individuals selected. On the other hand there was an overall decline in production towards the end of the selection culminating in the extinction of both the selected lines. This was associated with a rapid rise in inbreeding coefficient in the small populations. Heritability estimates of traits related to litter production in the control line indicated that these traits have low heritability, particularly if measurement was based on litter-size rather than on litter-weight. Estimation of heritability using data from the selected lines produced incredibly high estimates. Possible genetic implications of the results are discussed. The difficulties involved in analysing data recorded in commercial farms were discussed, particularly, the insufficiency of information being recorded in the performance records. Genetic factors influencing the various components of reproduction and the overall reproductive performance in domestic animals based on the results of the various analyses were also examined. The feasibility of applying the different methods for improving the reproductive performance was considered and some recommendations were put forward for, the improvement of reproductive performance in meat-producing species of farm animals in southern Australia. The recommendations may be summarised as the encouragement of mating of crossbred females derived from breeds of superior maternal ability and efficiency of reproduction to males derived from breeds of superior growth rate and selected for good records of success at mating.
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ItemStudies of the preovulatory LH surge and ovulation in the ewe( 1989)The studies presented in this thesis addressed the hypothesis that the induction of the preovulatory LH surge and ovulation in the ewe involves changes in the pattern of GnRH and gonadotrophin secretion, which can be compromised by the effects of glucocorticoids from the adrenal gland. A series of experiments tested the hypothesis that administration of exogenous GnRH or gonadotrophins could induce the growth of follicles to the point of ovulation in ewes deficient in endogenous gonadotrophins. A number of regimes were tested in hypophysectomized ewes where the timing, dose and type of exogenous gonadotrophin varied, but none of these could elicit ovulations consistently. When one of these regimes was administered to hypothalamic-pituitary disconnected (HPD) ewes, a greater response was obtained (33% of ewes ovulating versus 4% in hypophysectomized ewes). Administration of exogenous pulses of GnRH was able to elicit ovulations in HPD ewes consistently (88% of ewes ovulated). These findings imply that a pituitary factor besides the gonadotrophins may be important for the normal growth and ovulation of follicles. In another group of experiments, exogenous regimes of GnRH were administered to ovariectomized HPD ewes to test whether modifications in GnRH input could effect changes in LH secretion during an oestrogeninduced LH surge. It was found that `signal' pulses of GnRH, as either a single large pulse or a rapid series of smaller pulses, were required to initiate an LH surge in the presence of oestrogen. These `signal' pulses of GnRH were unable to cause a similar surge of LH in the absence of oestrogen. A continuous infusion of 250 ng/hour GnRH decreased the magnitude of the oestrogen-induced LH surge compared to 250 ng pulses of GnRH, but elicited a significantly greater response than if GnRH input was abolished, suggesting that the basal secretion of GnRH as well as the pulsatile format was important in eliciting an LH surge. When the continuous infusion was doubled to 500 ng/hour, this treatment was as effective as the 250 ng pulses of GnRH, whereas decreasing the GnRH pulse amplitude from 250 ng to 125 ng had no effect. These findings suggest that an increased baseline level of GnRH secretion can overcome the lack of pulsatile input, and that within the range tested, the amplitude of the GnRH pulses is not critical during the oestrogeninduced LH surge. Removal of GnRH inputs following the initiation of the oestrogen-induced LH surge significantly decreased in the amount of LH secreted compared to when GnRH pulses were maintained, showing that GnRH input to the pituitary gland is still required once the LH surge has begun. To investigate the hypothesis that gonadotrophin subunit mRNA levels are dynamic during an oestrogen-induced LH surge, ovariectomized and ovariectomized HPD ewes were treated with oestrogen and sacrificed at various times during the short-term negative and positive feedback events. In the ovariectomized ewes, all gonadotrophin mRNA levels decreased progressively, with LH/3 mRNA levels being significantly less than control values by the onset of the LH surge, whereas FSH/3 and a subunit mRNA levels declined significantly during the LH surge. These findings are at variance with those reported for ovary-intact ewes, suggesting that the mechanisms responsible for the LH surge in ovariectomized ewes treated with oestrogen may be different from those involved in the preovulatory LH surge. In the ovariectomized HPD ewes, the levels of a subunit mRNA levels diminished after treatment with oestrogen, whereas the levels of LHJ3 and FSH/3 mRNA levels were unchanged, implying that the a and f3 gonadotrophin subunits may be differentially regulated. Another series of experiments investigated the hypothesis that the synthetic glucocorticoid, dexamethasone, modified reproductive function. Chronic administration of dexamethasone at rates of up to 2 mg/day had little or no effect on gonadotrophin secretion, the incidence of behavioural oestrus or ovulation rate in either the breeding or non-breeding seasons. Based on these findings, it was revealed that in the ewe, the induction of the preovulatory LH surge involves changes in the pattern of GnRH and gonadotrophin secretion, but that glucocorticoids from the adrenal gland have, at most, only a minor role in modulating these processes.
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ItemThe nutrition of multiple-bearing ewes in late pregnancy( 1989)The nutrition of multiple-bearing ewes in late pregnancy and relationships with lamb survival are reviewed (Chapter 1). The frequent reductions in voluntary intake in late pregnancy, the reliance on maternal reserves and the key role of glucose and protein are highlighted. The sensitivity to nutritional manipulation of traits associated with lamb survival and their variation due to litter size are discussed. The traits include lamb birth weight, lamb vigour, ewe mammary growth and colostrum production, maternal behaviour and gestation length. A hypothesis 'That short-term supplementation of multiple-bearing ewes late in pregnancy will improve responses associated with lamb survival' was developed. Supplementing prolific ewes with lupin grain for the final 10 days of pregnancy has previously significantly increased lamb survival rates (Chapter 2). Experiments were designed to test the hypothesis. Treatments included supplements of different grains, nitrogen or rumen undegradable protein and direct infusions of glucose. Responses measured included production traits associated with lamb survival, glucose, urea and fatty acid production rates, some hormone concentrations, voluntary feed intake and mobilisation of maternal reserves. The variation in responses was compared at various litter sizes. Treatments were selected so that results could be modified and then applied directly to grazing sheep in southern Australia. In the first experiment (Chapter 3) a lupin grain supplement fed to Booroola (prolific) ewes in the final 17 days of pregnancy reduced condition score losses compared to no supplement or an oat grain supplement. -Live weight gains were highest with the lupin supplement and with no supplement. There was a trend for milk production at day two post-partum to be higher with the lupin supplement. Some of the multiple-born lambs died because of low colostrum intake, as indicated by low serum immunoglobulin concentrations. The possible importance of colostrum and initial milk production was demonstrated. Colostrum production and milk production on day one and nine were similar from single and multiple-bearing ewes fed a ration containing 100 g protein/10 MJ metabolizable energy and this ration supplemented with formaldehyde-protected casein or urea (Chapter 4). The ration was fed at 90% of the average estimated requirements of all ewes in the final five weeks of pregnancy. Gestation length was two days shorter with the casein supplemented diet which resulted in twin-born lambs from this diet being about 14% lighter than lambs born to ewes fed the basal ration. Glucose production on day 121 was 32% higher with multiple than single-bearing ewes even though intakes were comparable. Many ewes had low voluntary intakes on the grain/roughage ration and this resulted in large energy deficits. On a roughage/ oat grain ration fed for the final seven weeks of gestation, single and multiple-bearing ewes had consistently low and equivalent intakes (Chapter 5). Nonesterified fatty acid production rates on day 115 and 136 averaged 65 % higher for multiple than single-bearing ewes and rates were similar on both days. Glucose production increased by 32 % between these days and was 17 % greater for multiple than single-bearing ewes. The correlation coefficient of fatty acid and glucose production rates was about 0.7 on both days of measurement. The provision of a large glucose source late in pregnancy may be beneficial to multiple-bearing ewes when their voluntary intake potential seems low. Multiple-bearing ewes were infused at the abomasum from day 119 to 145 of gestation with nil, 106 or 207 g glucose /day (Chapter 6). The glucose was estimated to provide an additional 20 or 40 % energy compared to the basal ration of lucerne chaff. The ewes which were infused with glucose gained more weight during the treatment period (90, 159 and 267 g/d for basal, +20% and +40% energy respectively). Litter weight, colostrum yield and early milk production were insensitive to additional energy, as glucose, in the last 30 days of pregnancy in the circumstances where ewes were in low condition and fed a restricted roughage ration containing a high percentage of protein. The insensitivity seemed to lie with increased insulin levels leading to peripheral tissue accretion. Again high variation occurred in colostrum yields, although amounts were higher than in previous experiments. There were significant positive correlations between litter weight and both progesterone and plasma ovine placental lactogen levels in late pregnancy. Colostrum production was negatively correlated to progesterone concentrations measured in late pregnancy. Intake declined close to parturition even with the glucose infused ewes. When ewes were fed a medium quality roughage ad libitum, a lupin supplement in the final 10 days of pregnancy increased colostrum yield by 37% and milk production on day 1 by 28% when averaged over all ewes (Chapter 9). The largest response occurred with the triplet-bearing ewes, which had much lower production than single and twin-bearing ewes. Lambs born to lupin supplemented ewes had faster growth rates to day 1 and 3 post-partum. Benefits occurred either through additional substrates and /or by changing hormonal status, specifically progesterone. Colostrum levels at birth and total milk production to 24 hours were significantly improved when multiple-bearing ewes were supplied with supplements of glucose, lupin grain or undegradable rumen protein in the final 10 days of pregnancy. The basal ration was a medium quality roughage fed at a restricted level. The results provided strong evidence that additional protein which escapes rumen degradation will increase colostrum production near birth and advance the timing of copious milk production. The colostrum and milk yields were negatively correlated to progesterone concentrations near parturition. Prolactin and ovine placental lactogen concentrations measured before and after parturition were poorly correlated with the lactation results. Triplet bearing ewes had lower milk production than twin bearing ewes. There were no effects of the treatments on birth weight, but large effects on ewe live weight change and gestation length. The mechanisms could be due to extra fat mobilisation in late pregnancy with additional protein supplies, changes in hormone status, or possibly the supply of extra amino acids compared to the non-protein treatments. Variability in the colostrum and milk responses on the glucose treatment made it unclear what the role of glucose was for colostrum production. Milk production of multiple-bearing ewes is likely to be below that required by their litter in a cold and wet environment in the first 24 hours when the ewes are fed a medium quality roughage diet (Chapters 7 and 10). Lambs had a potential colostrum intake in the first two hours of an average 110 g and 230 g/kg birth weight during the first day (Chapter 8). This amount was usually above that available to all lambs in the litter from the dam. The potential intake was also above that required in most environments. Limited variation in intake between lambs within a litter may be crucial to survival of the complete litter. It is proposed that protein has a specific effect on the endocrinology of the ewe in late pregnancy. Thus a possible hypothesis consistent with these data is that increased amino acid supply at the tissue level results in a faster clearance of progesterone allowing lactogenic hormones and hormones involved with the initiation of parturition to act. These experiments have thus demonstrated that short-term supplementation of ewes can influence some traits associated with lamb survival, including colostrum production at birth and milk production during the following 24 hours. Multiplebearing ewes will often have lower amounts of colostrum and early milk production than single bearing ewes. Supplementing ewes on medium quality protein/energy pastures with feed sources which provide a high protein yield at the small intestine should increase the initial lactation output and possibly survival rates of multiple-born lambs.
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ItemPlacental size and foetal growth in relation to maternal undernutrition during mid-pregnancy in sheep( 1989)Examination of the evidence published in the literature revealed a variety of responses in birth weight of the lamb to maternal undernutrition during mid pregnancy. Explanations of the various responses are commonly made in terms of what has been named, in this thesis, as the Size Hypothesis. This hypothesis states that placental size is limiting the transfer of nutrients across the placenta during late pregnancy, and therefore determining foetal growth. The two main assumptions of this hypothesis are that (i) placental size is an index of its maximum functional capacity, and.(ii) the placenta is functioning at its maximum capacity during late pregnancy. In this thesis the validity of both of these assumptions is questioned, and, an alternative hypothesis proposed. Chapter One reviews the factors which are involved in the movement of metabolic substrate from the dam, across the placental barrier and to the site(s) of deposition in the foetus. From the limited amount of information available in the literature, it is apparent that placental size is not necessarily an accurate indicator of the maximum functional capacity of the placenta. Therefore the Size Hypothesis cannot be functionally correct. Consideration of the evidence used to support the Size Hypothesis questioned the causal relationship between placental size, and growth of the foetus during late pregnancy. Consequently experiments were designed to: (i)investigate the effect of maternal undernutrition during mid pregnancy on placental size. (ii)determine the functional relationship between placental size and foetal growth. Maternal undernutrition during mid pregnancy was chosen as the means of manipulation of placental size because of its direct relevance to the extensive grazing system of sheep management in Southern Australia. In the first experiment (Chapter Two) maternal undernutrition imposed between 75 and 100 days post coitus tended (P=0.08) to reduce both placental size, measured at 100 days post coitus, and weight of the lamb at birth. The relationship between placental size and growth of the foetus remained unresolved. Therefore a second experiment was designed. In this experiment (Chapter Three) single bearing pregnant ewes were exposed to undernutrition during one of three periods during mid pregnancy; 30 to 96 days (Group RA), 50 to 96 days (Group RB) and 75 to 96 days (Group RC) post coitus. Placental growth was retarded in ewes exposed to maternal undernutrition between 30 and 96 days post coitus, while the shorter periods of undernutrition had no significant effect on placental growth. Despite a smaller maximum placental size being achieved for group RA compared to control (well fed) ewes, and all ewes being fed on a high plane of nutrition during late pregnancy, foetal growth during late pregnancy was not significantly different between group RA and control (well fed) This evidence supports the hypothesis that placental size per se is not the prime determinant of foetal growth. The effect of altering placental size, by maternal undernutrition between the time of conception and 70 days post coitus (Chapter Four), or the number of foetuses sharing one uterus (Chapter Four, Chapter Five), on birth weight of the lamb was investigated. One experiment in Chapter Four (Experiment One) twin lambs at birth were not significantly lighter in weight, despite being associated with a smaller placenta, when compared to the single lambs. This is further evidence to suggest that placental size per se was not limiting growth of the foetus during late pregnancy. The final two experimental chapters in this thesis quantify the relationship between placental size and some functional aspects of the placenta. Placental size was not closely related to either the volume of blood in the placenta (Chapter Six), or the rate of movement of calcium (Chapter Seven) across the placenta. All experiments discuss the possible sites of limitation, or regulation, to the movement of nutrients to the foetus. These include the regulation of the intake of feed, the digestion, metabolism and partition of the metabolic substrates in the dam as they influence growth of the foetus. The experiments in this thesis demonstrated that growth of the placenta can be improved, retarded or remain unaffected by maternal undernutrition being imposed at various times during the first 100 days post coitus. Explanation of the potential effect which nutritional stress has in altering placental growth, is often made in terms of the degree to which the mobilization of maternal body reserves may buffer the conceptus from any nutrient limitation on growth of the foetus. In contrast to this, the experiment described in Chapter Seven reported that growth of the placenta, in ewes fed at a restricted level of nutrition, was greater when compared with their control (well fed) counterparts. The factors causing this stimulation in placenta growth are yet to be identified. Regression analysis of data from all experiments revealed that both placental size near parturition (P<0.001), and the change in liveweight during the period of nutritional restriction (P=0.06), were related to weight of the foetus near parturition. Conversely the liveweight and/or condition score of the dam at the time of joining was not related to placental size or foetal weight near parturition. This observation suggested that the level of body reserves at the time of joining does not significantly modify the effect which maternal undernutrition has. on placental and foetal growth. Maternal undernutrition during the first 100 days post coitus had little or no effect on growth of the foetus, measured at the end of the restriction period, despite a significant depletion in the body reserves of the dam. In addition, the large differences in the patterns of placental growth caused by maternal feed restriction were not reflected in weight of the lamb at birth. Differences in voluntary food intake and the level of body reserves available for mobilization/deposition, between the previously restricted and control (well fed) ewes, were not reflected in the whole-body metabolism of either glucose or calcium (Chapter Seven). Therefore the Size Hypothesis was not confirmed by the observations made in this thesis. Evidence used to support the Size Hypothesis includes the observation that the rate of foetal growth during late pregnancy does not continue to increase at an increasing rate, even when the ewe is well fed during late pregnancy. It has been proposed by other workers (e.g. Mellor, 1983) that this indicates that the functional capacity of the placenta is limiting foetal growth. The experiment reported in Chapter Three demonstrated that, despite a retardation in placental size of as much as 23 percent by 96 days post coitus, growth of the foetus during the final 12 days of pregnancy was similar for both the control (well fed) and previously under fed (restricted) ewes. It was therefore concluded that placental size per se cannot be limiting foetal growth during late pregnancy. In another experiment (Chapter Seven) maternal undernutrition during mid pregnancy resulted in an increase in placental growth of 21 percent, despite the ewes being severely under fed between 30 and 96 days post coitus. The rate of transport of calcium across the placenta, and its lack of any relationship to placental size, indicated that placental size was not an appropriate indicator of its movement. In contrast to the prediction of the Size Hypothesis, foetal growth during late pregnancy was not improved despite the ewes being fed on a high plane of nutrition during the period between 96 days post coitus and the time of parturition. The circumstances in which a relationship between "size" and "function" were strong, and those in which no close relationship existed, were examined in an attempt to define the physiological conditions regulating foetal growth. An alternative hypothesis, termed the Functional Reserve Hypothesis was proposed. It aims to incorporate all available observations made in the experimental work reported in this thesis, and from the relevant literature reported elsewhere. The concept central to Functional Reserve Hypothesis is that the ovine placenta is not in a state of functional saturation during late pregnancy, and hence does not act as a limitation to growth of the foetus. In addition, the Functional Reserve Hypothesis proposes that placental size is not an accurate indicator of the capacity of the placenta to transfer nutrients. It is hypothesized that the ovine placenta has same level of functional reserve, even in the situations where it is retarded in its growth. Additionally it is proposed that factors regulating both (i) the partition and supply of nutrients from the dam, and (ii) the potential for growth, and the demand for nutrients by the foetus, all interact to regulate the rate of' foetal growth during late pregnancy and therefore ultimately the birth weight of the lamb. Before successful manipulation of foetal growth during late pregnancy is possible, the key metabolic substrates, and the factors regulating their movement to the foetus, need to be identified. Finally, how the above results contribute to the development of strategic feeding recommendations for the Australian sheep flock are discussed. Two specific areas for further research are identified.