Two year old cows were managed at pasture to calve in High (n = 10) or Low (n = 11) body condition score (BCS). Following calving all cows were managed as a single herd and offered a generous pasture allowance. Time of first ovulation was determined using weekly transrectal ultrasonography and measurement of milk progesterone concentrations thrice weekly. Liveweight (LW) change after calving, dry matter intake, milk yield and composition were determined at 20 and 40 days postpartum and energy balance calculated Pulsatile release of luteinising hormone (LH), concentrations of glucose, insulin, β-hydroxy butyrate (BHB) and non-esterified fatty acids were measured at 11, 25 and 46 days postpartum. Cows in the High group were in significantly greater positive energy balance, had lost more LW at 20 days postpartum than cows in the Low group, but did not differ in postpartum interval (PPI). There were significant correlations between PPI and BCS at calving and with concentrations of LH, glucose and BHB measured 11 days postpartum, but not with energy balance or change in LW after calving.

The last decade of the 20th century has seen unprecedented change within the UK dairy industry, with consolidated of production quotas, payment schemes based on milk solids, deregulation of the milk market with the demise of the Milk Marketing Board and more recently a major downward pressure on milk prices. All of these have contributed to further intensification of a large part of the industry. However the largest change has been the introduction of improved genetics, with the accompanying increase in Holsteins (now the preferred breed) at the expense of Friesians. As a consequence individual cow yields have increased dramatically and such cows are becoming an increasing problem in respect of nutritional management, as it relates to optimisation of milk solids content and yield and maintenance of reproductive competence. This paper has attempted to examine some of these issues by first describing the changes in more detail and then from a theoretical consideration of the biochemical principles associated with milk solids production to provide information which may permit better decisions to be made in relation to the feeding of such cows. In this respect the importance of glucose is established as well as the potentially competing processes of energy use for body maintenance or milk fat synthesis. Two experimental studies from this laboratory with high genetic merit cows are used to illustrate how the overall utilisation of energy for lactation and body tissue metabolism may be affected, especially when energy intake in early lactation fails to meet whole body energy demands. Subsequently, the nutrient costs of pregnancy, an important but often forgotten part of the ‘annual’ life cycle of dairy cows, are also considered and it is concluded that increased emphasis needs to be put on the management of cows for lifetime performance rather than simply for lactational output. Finally, whilst recognising that high yielding cows represent a major challenge to all management systems, the paper concludes that existing feeding systems which rely solely on nutritional entities such as metabolisable energy will need to be abandoned in favour of those which recognise specific nutrients, if the full potential of high genetic merit cows is ever to be achieved

Increased genetic potential for milk production has been associated with a decline in fertility of lactating cows. Following parturition the nutritional requirements increase rapidly with milk production and result in negative energy balance (NEBAL). NEBAL delays the time of first ovulation thereby affecting ovarian cycles before and during the subsequent breeding period The effects of NEBAL on reinitiation of ovulation are manifested through inhibition of LH pulse frequency and low levels of glucose, insulin and IGF-I in blood that collectively restrain oestrogen production by dominant follicles. Upregulation of LH pulses and peripheral IGF-I in association with the NEBAL nadir increases the likelihood that emerging dominant follicles will ovulate. The legacy of NEBAL is reduced fertility after insemination in conjunction with reduced serum progesterone concentrations. Diets high in crude protein support high milk yield, but may be detrimental to reproductive performance. Depending upon protein quantity and composition, serum concentrations of progesterone may be lower and the uterine luminal environment is altered. High protein intake is correlated with plasma urea concentrations that are inversely related to uterine pH and fertility. The direct effects of high dietary protein and plasma urea on embryo quality and development in cattle are inconsistent. In conclusion, the poor fertility of high producing dairy cows reflects the combined effects of a uterine environment that is dependent on progesterone, but has been rendered suboptimal for embryo development by antecedent effects of negative energy balance and may be further compromised by the effects of urea resultingfrom intake of high dietary protein.

Two studies (expt. 1, 185 cows in 1996/97; expt. 2, 168 cows in 1997/98) were conducted with Prim ‘Holstein dairy cows in the Mayenne region (France) to investigate suboestrus. Cows which had not been observed in oestrus since calving were allocated alternately to treatment groups between 60 and 90 days post partum. Expt. 1. Group 1: GnRH (Day 0, 100 μg im), PGF2α (Day 7, 25 mg im), GnRH (Day 9, 100 μg im) with artificial insemination (AI) Day 10. Group 2 : PGF2α (Day 0, 25 mg im), AI at oestrus or if oestrus not observed a second PGF2α injection was given (Day 13) with AI on Day 16 and Day 17. Expt. 2. Group 1 : GnRH (Day 0, 100 /μg im), PGF2α (Day 7, 25 mg im), GnRH (Day 9, 100 μg im) with AI at observed oestrus after Day 0 or at Day 10 if oestrus not observed. Group 2 : PGF2α (Day 0, 25 mg im), AI at oestrus, or if oestrus was not observed a second PGF2α injection was given (Day 13) and AI at observed oestrus. Progesterone was measured in serum at Day 0 and in milk at AI. Pregnancy diagnosis was performed by measuring bovine Pregnancy Specific Protein B (bPSPB) (Day 50 + 3) and confirmed by ultrasonography when the result was doubtful.

In expt. 1, farmers observed 47/101 (46.9 %) of group 1 cows in oestrus, 33/91 on Day 10 and 10 before Day 10. The progesterone concentrations were compatible with oestrus in 69/86 (80%) of cows on Day 10. In group 2, 36/83 (43.4 %) of cows were inseminated after the first PGF2α injection. After the second PGF2 α, only 29/43 (67 %) of cows had a low progesterone concentration at AI. The pregnancy rates were 36.1% and 32.5 % for groups 1 and 2, respectively. In expt. 2, oestrus was observed in 31/93 (33.7 %) of group 1 cows. In group 2, 51/75 (66 %) of cows were inseminated after the first injection of PGF2α, 13/75 (17.3 %) after the second and 11/75 (14.7 %) were not seen in oestrus. Pregnancy rates were 53.7% and 53.3% in groups 1 and 2, respectively. In conclusion, it is recommended that suboestrus be treated with PGF2α followed by AI at observed oestrus when oestrus detection is good while, the use of GnRH + PGF2α + GnRH is recommended when oestrus detection is poor.

In Ireland, surveys in the 1970's have shown calving rates to first service of 60-69%. Since then genetic merit and milk yield per cow have increased significantly. The objectives of this study were to determine calving rates in Irish dairy herds for the period 1991-1996 and if these had declined over time. Breeding records for 58 Spring calving commercial dairy herds maintained on the computerised DAIRYMIS System were analysed. Inseminations were categorised as either DIY AI (DIY), commercial AI (COM) or Natural Service (NAT). Between 1991 and 1996 there was a significant trend towards DIY (P<0.01) and away from NAT (P<0.01) with no consistent trend in COM usage. In 1991, roughly one third of services were to each of the three service methods but by 1996 DIY accounted for nearly 50% of all services. A subset of 34 herds, which had been present for at least five of the six years between 1991 and 1996, was used in the calving rate analysis. Calving rates to first service for DIY (48.4%) and NAT (46.0%) were not significantly different although both were significantly lower (P<0.001) than those for COM (54.5%). For small (<65), medium (65-128) and large herds (˃128) the respective proportion of services in each category were 22%, 44% and 34%. The respective calving rates of 55.2%, 51.6% and 45.7% declined significantly as herd size increased For all service types, there was a significant (P<0.001) decline in first service calving rate over time. The increased usage of DIY, combined with its lower fertility rates, would appear to be one factor responsible for the reduction in calving rate for 1991-1996 period.

The reduced fertility that is becoming more evident in high yielding dairy cows may be related to many factors including changes in milk production, food intake and fluctuations in body condition. Metabolic and production markers have been studied as a way of predicting success to a particular artificial insemination. Successful conception to a particular service was not associated with milk production, body condition or plasma concentrations of several indicators of metabolic state around the time if insemination. This highlights the importance of time of information collection in fertility management programmes. Increased food intake may reduce systemic progesterone concentrations. This is more evident in sheep than cattle, but a positive relationship between systemic progesterone early post mating and establishment of pregnancy in cattle has been reported. However, progesterone concentrations in the ovarian vein and endometrium are not strongly correlated with systemic progesterone. Thus, the significance of modest changes in systemic progesterone in affecting oocyte and embryo development must be questioned. Blood urea concentrations can be altered by diet, and reduced pregnancy rates have been reported in cows with high urea concentrations. However, in other recent studies, no difference was reported in serum urea in cows that conceived and those that failed to conceive. Pregnancy rate was equally high in heifers when in-vitro produced embryos were transferred to heifers on high and low urea diets. When embryos were produced in sheep on high and low dietary urea, the effects on embryo development appear to occur early in the developmental process, suggesting a substantial effect on the development of the oocyte. The developmental capacity of oocytes and quality of embryos is reduced in cattle maintained on extremely high dietary intakes. Oocyte developmental capacity is reduced in cows of higher genetic merit and embryo quality can be substantially reduced in the early postpartum period. Collectively, these results suggest that high dietary intake or high metabolic load is deleterious to normal oocyte development and establishment of pregnancy. This highlights the importance of further studies on the effect of dietary intake on metabolic state and follicle, oocyte and embryo development. In a practical context, these results highlight the importance of nutritional management and avoiding changes in the amount or type of diet around the time of mating in high-production dairy cows.

The objective of the study was to investigate the influence of different levels of β-carotene supplements on the β-carotene concentration in the corpus luteum and on hormone concentrations in the plasma of heifers. 32 heifers (average body weight: 371 kg) were fed a low carotene diet (< 1 mg per kg DM) for 120 days. The heifers were divided into four groups according to body weight and age and supplied with 0, 100, 200 or 300 mg β-carotene per animal and day. Heifers were artifically inseminated after day 60 of the experiment and were slaughtered after day 120 of the experiment. Carotene concentration in the corpus luteum (2.3, 27, 50 and 81 μg/g for 0, 100, 200 and 300 mg β-carotene per animal per day), in plasma and in ovary was significantly influenced with increased carotene supplements. LH-concentration of plasma decreased and β-oestradiol-concentration increased with carotene supplementation. High concentrations of β-carotene in the corpus luteum and ovaries of cattle seem to act as a depot which is available when high vitamin A requirements during ovulation have to be met.

To further identify factors which influence pregnancy rates, three experiments were conducted to determine the effect of insemination time on sperm transport, fertilization rate, and embryo quality. All cows were continuously monitored for behavioural oestrus by HeatWatch®, and received AI at heat onset (0 h after the first standing event), 12 h after onset, or received natural service at 0 hfrom one of three bulls (Exp. 1). In Exp. 2, cows received AI at 0 h, 12 h, or 24 h after the first standing event. On d 6 after insemination 115 embryos(ova) (Exp. 1) and 117 embryos(ova) (Exp. 2) were recovered from single-ovulating cows. For Exp. 1, median accessory sperm values were: 1 (0 h), 10 (12 h), 27 (natural service O h) (P < 0.05). For Exp. 2, median accessory sperm values were: 1 (0 h), 2 (12 h), 4 (24 h) (P < 0.05). Fertilization rates were: 67% (0 h), 79% (12 h), 98% (natural service O h) (P < 0.05)(Exp. 1); and did not differ in Exp. 2. Embryo quality was not different in Exp. 1. In Exp. 2, percentages of excellent and good fair and poor, and degenerate embryos were: 77, 15, 8 (0 h), 52, 38, 10 (12 h), 47, 19, 34 (24 h) (P < 0.05). In Exp. 3, 30 cows were superovulated and were inseminated once at either 0 h, 12 h, or 24 h after the first standing event. On d 6 after insemination, 529 embryos(ova) were recovered. Fertilization rates were: 29% (0 h); 60% (12 h); 81% (24 h)(P < 0.01). Percentages of embryos with accessory sperm were: 5 (0 h); 8 (12 h); and 41(24 h) (P < 0.01). Embryo quality was not affected by time of AI. We conclude that the time of insemination affects: 1) sperm transport as measured by median accessory sperm number (Exp. 1 and 2) and the percentage of embryos with accessory sperm (Exp. 3); 2) fertilization rate (Exp. 1 and 3); and embryo quality (Exp. 2).

The first study was conducted to investigate the possible relationship between milk urea concentrations and pregnancy rate in dairy cows. A total of 173 dairy cows were used in the study. They had not experienced reproductive problems post partum, however they were treated for suboestrus between 60 to 90 d post partum with either PGF2α or GnRH, PGF2α and GnRH. Suboestrus cows were studied in this experiment in order to see whether, when ovulation is initiated, high milk urea concentrations were associated with a poorer response to treatment. Milk samples were collected on the day of AI, for the measurement of urea concentrations, and pregnancy diagnosis was performed by the assay of Pregnancy Specific Protein Binding (PSPB) 50 d post-AI. Peak milk production (P=0.06), the type of diet (P=0.05), the main forage type (P=0.07) and the concentrate distribution system (P=0.03) influenced milk urea concentrations. There was no difference in the concentrations of urea in the milk of pregnant and non-pregnant cows (P = 0.33, 0.25 g L-1 vs. 0.27 g L-1, respectively). Pregnancy rate was not influenced by the type of suboestrus treatment used. A second experiment showed that there was a linear relationship between milk urea concentrations measured by a dipstick method (Azotest) and a laboratory method (r=0.51, P<0.001, n=89)

The economic consequence of low heat detection rates is the main reason cattle reproduction research programmes continue to focus on developing practical controlled breeding systems for dairy cows. Three approaches can be taken to control the oestrous cycle in cattle: (i) Use of the luteolytic agent prostaglandin F2α alone or one of its potent analogues, (ii) Cycle regulation using short-term progestagen treatments and (Hi) Prior follicle wave synchrony followed by induced luteolysis. Administration of prostaglandin F2α (PGF2α) to cows after about Day 5 of the oestrous cycle causes immediate regression of the CL with progesterone concentrations declining rapidly to basal concentrations within 24 hours. Because PGF2α is only effective in animals between Days 5 and 17 of the oestrous cycle up to 40% of any group of randomly cyclic animals will not respond to a single administration. A number of PGF2α administration regimens have been developed for dairy cows. A two-injection regimen, with an interval of 9-13 days between successive administrations, elicits a higher response, as all animals possess a susceptible CL the time of the second injection. In general, cyclic heifers respond with good precision of heat onset to two PGF2α injections 11 days apart. Fixed-time AI at 72 and 96 hours after the second injection can result in acceptable pregnancy rates. In post partum dairy cows the slower turnover of follicle waves, contribute to the greater variation in oestrus onset, resulting in more variable and generally lower pregnancy rates to fixed-time AI at 72 and 96 hours. Consequently, intensive oestrous detection and AI on the basis of observed oestrus is more appropriate for lactating cows following PGF2α administration. It is generally accepted that fertility of the cyclic heifer after PGF2α treatment is not impaired or may even be higher when compared with untreated control animals inseminated at a natural oestrus. However, in lactating cows, conception rate following PGF2α treatment have been frequently lower (about l0%) than in cows bred following a natural heat. Recently, aprotocol to synchronise follicular development using GnRH and to induce luteal regression using PGF2α was developed In this protocol (Ovsynch), GnRH is administered to cows at random stages of their cycle 7 days before luteolysis is induced with PGF2α; a second GnRH injection at 36-48 hours later induces ovulation. All cows are inseminated once at 16-20 hours after the second GnRH injection. Ovsynch system is not recommended for use in heifers. The development of progestogen delivery devices, such as the PRID, CIDR and Crestar has facilitated the use ofexogenous progestogens for oestrous cycle synchronisation in cattle. Oestradiol is administered at the start of treatment to (i) shorten the life span of the corpus luteum (CL) and (ii) to induce the emergence a new follicle wave. Generally, a high proportion (up to 85%) of the cows that exhibit oestrus after removal of the progestogen do so between 36 and 60hours ofremoval. This allows for either one (54-56 hours) or two fixed-time AIs (48 & 72 hours) after progestogen withdrawal. Alternatively, cows can be observed and inseminated once at a detected oestrus. While fixed-time AI of all treated cows gives 100% submission rate conception rates tend to be lower than when AI is on the basis of observed heat. However, the overall number of cows becoming pregnant (submission x conception rate) is frequently the same, reflecting submission and conception rate differences for the two AI options. Choice of AI option will depend the herdsman 's ability to accurately detect heat, semen costs and labour availability.

Plasma urea concentrations have been used as a diagnostic tool in the investigation of reproductive performance in cattle. Data were compiled from three recent studies on bovine fertility and a retrospective comparison of plasma urea concentrations was made between those animals that conceived to an insemination or embryo transfer. In studies I and 2 plasma urea concentrations around the time of insemination were determined. Pregnancies were diagnosed using ultrasonography 35 days later. There was no significant difference between the mean plasma urea concentrations around the time of insemination in the cattle subsequently diagnosed pregnant or not pregnant. In study 3, in vitro produced good quality embryos were transferred into three groups of beef heifers. The three groups were allocated to diets of high energy / high urea, high energy / no urea and low energy / high urea. The plasma urea concentrations at the time of embryo transfer were different between the three groups. However, the pregnancy rates 28 days post transfer, were not significantly different between the three groups. This suggests that the previously reported effects of high protein diets on fertility are not solely due to disruptive effects on the uterine environment. The main effect of urea on fertility may be on oocyte development within the follicle. Overall, these results indicate that measurement of plasma urea concentrations in individual animals around the time of insemination or embryo transfer is not a useful predictor of subsequent pregnancy rate.

A range of hormonal therapies has been evaluated to potentially improve the reproductive performance of lactating dairy cows. Early lactation treatments with gonadotrophin releasing hormone (GnRH) or prostaglandin F2∝ (PGF) may reduce the interval to first insemination or increase the conception rate to first insemination, but mainly in cows which have had a difficult pueperium or which are in herds with low conception rates. These two hormones, as well as progesterone and oestradiol benzoate (ODB) are commonly used either singly, or in combination (GnRH + PGF; progesterone + ODB + PGF) to synchronise the oestrus preceding first inseminations. None of these synchrony treatments is associated with increased conception rates. Extensive series of trials have been completed to identify post-oestrous or post-insemination hormonal therapies which could increase conception rates to the preceding insemination. The wide variation in results has precluded any being commonly regarded as sufficiently reliable for routine use. Nonetheless, meta-analyses have shown that GnRH treatment at insemination or in late dioestrus (11 to 13 day post-first insemination) can significantly increase “the risk of pregnancy”. Insemination treatments have been most effective with repeat breeders (+22.5%), whereas late dioestrous treatments (10%) may be dose and analogue specific (10 μg buserelin). Although metoestrous supplementation with progesterone can stimulate early embryonic development, the associated reduction in oestrous cycle length also reduces conception rates in heifers. Late dioestrous use of GnRH can prevent both of these negative effects. Early dioestrous supplementation with progesterone may enhance production of interferon tau, but this potentially beneficial effect has not been able to be reliably translated into increased conception rates. Many of these hormonal therapies are associated with altered patterns of ovarian follicle development which are similar to those in some synchrony treatments preceding first insemination. Recent studies have indicated that OBD and progesterone can be used to synchronise returns to service and increase the submission rate for second inseminations made about 3 weeks after first inseminations. This can make the non-return rate a more accurate measure of the response to a hormonal treatment and potentially overcome confusing impressions created when oestrous detection rates may be around 50%. Even if effective hormonal therapies are successfully developed, the results may be compromised by environmental factors such as heat stress, energy balance or energy partitioning for lactation. These factors may reduce oocyte quality, fertilization rates or normal uterine secretion patterns. Reduced conception rates associated with high daily milk yields in early lactation may not be able to be remedied simply with hormonal supplementation or by altering patterns of ovarian follicle development. Under these circumstances, controlling the inter-service interval could reduce the impact of the lowered conception rates.

There is concern that the substantial increases in production in dairy herds that can be achieved will bring greater health and fertility problems. Despite considerable advancements in the understanding of the biology of reproduction and its control, fertility apparently continues to decline. As many factors, including health and nutrition, influence reproductive performance it is generally agreed that a multidisciplinary approach is necessary if improvement in fertility is to be achieved Herd health schemes have evolved to improve health, fertility, productivity and profitability but the uptake of such schemes in the UK has been low. The veterinary input to many farms is at best routine fertility visits, confined to examination of the reproductive tract, and at worst occasional emergency calls. The Dairy Herd Health and Productivity Service (DHHPS) provides the opportunity for veterinary surgeons to lead a multidisciplinary team which can monitor health, fertility and production and can plan, when necessary, remedial action. Over a period of twenty years it has continued to identify infertility, mastitis and lameness as the main reasons for the involuntary disposal of dairy cows, although on some farms BSE has been a major factor. The trend in the last few years has been for an increase in culling for reasons of disease rather than for age or yield. The average disease rates have remained high with over 100 treatments per 100 cows each year for conditions which may directly or indirectly influence reproduction. Blood profiling and condition scoring demonstrated that at least a third of the cows sampled were mobilising excessive fat during transition from the dry period to early lactation. Improving both health and nutrition, before and after calving, would greatly improve reproductive performance in many herds. A team approach, with farmers, veterinarians, nutritionists and other advisors working together with well defined goals and objectives, is necessary if progress is to be made in improving reproductive performance. High yield can not always be the excuse for poor fertility.

The development of production systems, which allow increased nutrient intakes to be achieved, is a key issue in the management of high genetic merit dairy cows. Consequently, forty high genetic merit autumn calving dairy cows (PTA95 fat + protein = 38.2 kg) were managed on either a ‘high forage (HF)’ or ‘high concentrate (HC)’ based system of milk production for the first 305 days of lactation, with the study encompassing both the indoor winter and outdoor summer grazing periods. System HF involved a high feed value silage, a lax grazing regime, and a low concentrate input (842 kg DM), while system HC involved a medium feed value silage, a tighter grazing regime and a higher concentrate input (2456 kg DM). Total milk outputs with each of systems HF and HC were 7854 and 8640 kg respectively (P<0.01), illustrating that high genetic merit cows can perform satisfactorily on very different inputs over a single lactation. However animals on system HF experienced a more extreme and prolonged period of negative energy balance post partum than those on system HC, and completed the winter with a significantly lower condition score. Detailed fertility records were maintained for all animals on the study. Days to first observed heat were 51.2 and 59.3 with systems HF and HC respectively, while the respective conception rates to first service were 26 and 21%. The number of services/conception were 2.22 and 2.50, while the calving interval was 390 and 404 days for systems HF and HC respectively. Despite the greater degree of negative energy balance associated with system HF, none of the fertility measures was significantly affected by system of milk production (P>0.05), although fertility with both systems was poor. There were no obvious reasons for the poor fertility noted in this trial.

High intakes of crude protein (CP), in particular rumen degradable protein (RDP), have been associated with reduced fertility in cattle. This reduction in fertility has been attributed to putative toxic effects of elevated systemic concentrations of ammonia and, or urea, the by-products of protein metabolism, on gametes and embryos. The objective of the studies reported here was to examine the effect of CP and fermentable carbohydrate intake on various blood metabolites and on fertility, in beef heifers. In the first experiment, 40 heifers were randomly assigned to either low (no urea) or high (240 g urea) RDP, grass silage based diets, supplemented with 1.5 or 3.0 kg dry matter (DM) of either rolled barley (BAR) or molassed sugar beet pulp (MSBP) or no carbohydrate (control). The high RDP diet increased plasma ammonia and urea concentrations. MSBP at 3 kg DM/day was more effective than BAR in reducing ammonia (P<0.05) and urea (P<0.001). In animals fed no urea, MSBP at 3 kg DM/day was also more effective than BAR in reducing systemic ammonia (P<0.001) and urea (P < 0.05). In the second experiment, 83 beef cross-bred heifers were randomly assigned over two replicates to one of 4 treatment groups in a 2 x 2 factorial design. Animals consuming high (85 kg N/ha; HN) or low (0 kg N/ha; LN) nitrogen fertilised pastures were supplemented with either 0 or 3 kg dry matter (DM) MSBP daily as follows: 1) HN only (n=21); 2) HN + 3kg MSBP, (n=22); 3) LN only (n=21); 4) LN + 3kg MSBP (n=19). The HN pastures had higher (P<0.001) CP and lower (P<0.001) water soluble carbohydrate concentrations than LN pastures. Systemic ammonia (P<0.05) and urea (P<0.001) were higher in heifers on the HN pasture. Embryo survival rate was high overall (80%) and was not affected by systemic ammonia or urea concentrations or by supplementation with MSBP. Systemic progesterone, insulin and glucose were not affected by treatment. High CP intake coupled with low fermentable carbohydrate, lead to elevated systemic ammonia and urea concentrations in heifers. However, no adverse effect on fertility was observed.

Selection of dairy cattle for increased milk production has decreased some indices of reproductive efficiency. For example, days open are increased by one day for every 100 kg of increased milk yield per lactation. Some of the change in days open can be explained by delayed onset of oestrous cyclicity and lower conception rate to artificial insemination in cows with greater milk production. Despite these negative associations between milk production and reproduction, reproduction in herds of high producing dairy cattle is not necessarily compromised relative to reproduction in herds of low producing dairy cattle. This is because there is a large environmental effect on dairy reproduction. High producing herds generally have better management and better oestrous detection. Therefore, high producing dairy herds may partially overcome the antagonistic relationship between milk production and reproduction. Physiological mechanisms that lead to poorer reproduction in high producing cows are partially defined. Negative energy balance that occurs in high producing dairy cows can be associated with a delay in the initiation of ovarian cycles and the interval to first breeding. Many of the effects of negative energy balance on postpartum reproduction can be explained by decreased serum luteinizing hormone (LH) that is associated with negative energy balance. Serum LH increases as cows move toward positive energy balance and greater LH stimulates growth and ovulation of ovarian follicles. We have initiated studies to address physiological differences in high and low index dairy cows. The reproductive endocrinology of cows from a control line (5,900 kg milk/lactation) and a select line (10,900 kg milk/lactation) of dairy cows at the University of Minnesota was studied over a two-year period. Cows in Year 1 were similar for serum concentrations of LH, follicle stimulating hormone (FSH), and oestradiol (preovulatory period). In both years, serum concentrations of progesterone during luteal phases, however, were decreased in select cows. The Year 2 cows also had a delay in the return to oestrous cyclicity that was associated with reduced LH. The possibility that decreased progesterone causes infertility in dairy cows will require further study. Collectively, these data suggest that changes in blood progesterone concentrations may explain, partially, lower fertility in high index dairy cows.

Early embryo loss is the major cause of reproductive failure in cattle. The objective of this study was to investigate the effect of changes in feed intake around the time of insemination on systemic progesterone and on embryo development and survival and in cattle. Nutrition treatments were designed to provide 0.8 times (Low, L) or twice (High, H) maintenance energy requirements. Oestrus was synchronised in heifers (n=314) using two injections of prostaglandin (PG; 50 μg Cloprostenol, Estrumate) administered 10 days apart. On the day of oestrus following the first PG injection, heifers were randomly allocated to L or H (pre-AI). At the oestrus following the second PG injection, heifers were inseminated with semen from the same Limousin bull and on the following day were randomly allocated to either L or H until embryo recovery at day 8 (n=25), 14 (n=57) or 16 (n=71) or pregnancy diagnosis at day 30 (n=161). Hence there were four nutrition treatments; L-L, L-H, H-H and H-L. There was no significant difference in embryo survival rate determined at day 14, 16 or 30 and so the data were combined for analysis. Overall embryo survival rate was significantly lower (P<0.001) in H-L (0.37) than L-L (0.70), L-H (0.71; P<0.001) or H-H (0.68; P<0.01) treatment groups. Nutrition treatment did not affect systemic progesterone measured on days 4, 5, 6 or 7 of the pre- or post-AI oestrous cycle. The total cell number of 8-day-old blastocysts was not significantly affected by nutrition treatment neither was the length, diameter or estimated total protein content of 14-day-old embryos. Reducing energy intake immediately after AI reduced embryo survival rate but there was no evidence that this effect was mediated through changes in systemic progesterone.

In recent years there has been considerable genetic progress in milk production. Yet, increases in yield have been accompanied by an apparent lengthening of calving intervals, days open, days to first heat and a decline in conception rates, which appears to be both at the genetic and phenotypic level. Fertility has a high relative economic value compared to production traits such as protein, making it attractive to include in a breeding programme. To do this there needs to be genetic variance in fertility. Measures of fertility calculated from service dates have a small genetic compared to phenotypic variance, hence heritability estimates are small, typically less than 5%, although coefficients of genetic variance are comparable to those of production traits. Heritabilities of commencement of luteal activity determined using progesterone profiles are generally higher, and have been reported as being from 0.16 to 0.28, which could be because of a more precise quantification of genetic variance, as management influences such as delaying insemination and heat detection rates are excluded. However, it might not be the use of progesterone profiles alone, as days to first heat observed by farm staff has a heritability of 0.15. The most efficient way to breed for improved fertility is to construct a selection index using the genetic and phenotypic parameter estimates of all traits of interest in addition to their respective economic values. Index traits for fertility could include measures such as calving interval, days open, days to first service, or days to first heat but there may also be alternative measures. Examples include traits related to energy balance, such as live weight and condition score (change), both of which have higher heritabilities than fertility measures and have genetic correlations of sufficient magnitude to make genetic progress by using them feasible. To redress the balance between fertility and production, some countries already publish genetic evaluations of fertility including: Denmark, Finland, France, Germany, Israel, The Netherlands, Norway and Sweden.

Female fertility affects the culling rate as well as the direct reproduction costs and influences the calving interval and the calving season. It is therefore obvious that female fertility should be included in a Total Merit Index (TMI), which directs the genetic changes in dairy cattle selection. When the first TMI was introduced in Sweden in 1975 female fertility was included. A basic prerequisite for inclusion of fertility in the Scandinavian breeding programme was that milk recording, pedigree and AI-data were integrated in the same databases.

The paper deals exclusively with the additive genetic effects on female fertility. The heritabilities of the measures used for female fertility are low, usually below 0.05. Despite this, the additive genetic variance is high as demonstrated by large differences between progeny groups. Thus, selection must be based on progeny testing of bulls on daughter groups of 100-150 daughters or more, which is the case in all the Scandinavian countries. Different procedures for calculation of daughter fertility indices are practised in Scandinavia.

Research shows the importance of including measures of both the time interval from calving to insemination and a direct fertility trait such as number of inseminations or non-return rate (NR). It has also been shown that reproductive measures of different stages of life, i.e. heifer and lactating cow periods, must be considered. The genetic correlations between daughter fertility and different yield measures are negative (-0.2 to -0.4). Selection for yield without consideration of daughter fertility results in a deterioration of daughter fertility. A comparison of the flat genetic and phenotypic trends for daughter fertility along with strongly increased productivity in the SRB-breed, where Scandinavian bulls have dominated as sires of sons, and the negative trends in the SLB-breed where almost all sires of sons during a decade and a half have been imported from North America, illustrates the consequences of selection with and without consideration of daughter fertility in the sire-son path.

Daughter fertility will probably remain an important trait in the future, not only in countries with seasonal milk production connected to the pasture period. As dairy cattle breeding is quite international, calculation of international breeding values for daughter fertility in the same way as is practised by INTERBULL for production and conformation seems to be an important challenge for the future.

The relationships between body condition score (BCS), metabolic measurements and pregnancy rates were studied from 214 cows belonging to 6 herds located in Reunion Island between January and December 1998. BCS (0 to 5 scale) and biochemical blood parameters were measured before calving, 0-30, 60-90 and 120-150 days post-partum. Pregnancy status was also checked precisely after 171 AI's (n=104 cows) by EIA progesterone determination in milk 23 to 24 days after AI and pregnancy specific protein B (PSPB) measurements 30 to 45 days after AI and was further confirmed by rectal palpation by 60 days post AI. A preliminary analysis led to determine 3 groups of herds according to their pregnancy rates (40%, 26% and 14.5% respectively for group 1, 2 and 3 cows). Significant differences between the 3 groups were found for BCS, glucose and urea (p < 0.0001) and a significant main effect of the post-partum stage was observed as well (p < 0.0001). Mean BCS were not different in the 3 groups before calving but were higher in group 1 cows than in group 2 and 3 cows at all other stages studied. A similar trend was found for urea. Glucose concentrations were severely depressed in group 3 cows when compared to group 1 and 2 cows whatever the stage studied. These results sugggest that variables illustrating negative energy balance (BCS decrease and low glucose concentrations) or deficit in nitrogen supply (urea concentrations) on a herd basis are related to poor pregnancy rates.