Causes of variation amongst cattle within a herd in their ability to initiate and maintain pregnancy are largely unknown. An experimental animal resource has recently been established to understand the biology of early reproductive performance. This paper summarises the results achieved during the establishment phase and from several experiments aimed at determining the physiological basis of the difference between sub-herds of contrasting pregnancy rates on Day 60. Each of 155 contemporary yearling heifers received 2 in vitro-produced embryos on 6 separate occasions during a 26-month period. Sixty days after transfer, pregnancy and twinning rates were determined ultrasonically, pregnancies terminated and the process repeated. The interval between successive transfers was greater than 100 days. Heifers were ranked on their aggregate pregnancy rate performance after 6 rounds of transfer, and the highest (High) and lowest (Low) 25 were retained. Differences in reproductive performance during the establishment phase of the herd are reported. In addition, several subsequent experiments examined ovarian follicle turnover and progesterone levels during an oestrous cycle, early embryo development after either AI or embryo transfer, and protein, interferon tau and ubiquitin-cross-reactive protein levels in uterine luminal flushings.Pregnancy rates were 7-folder higher in the High sub-herd (76 vs. 11%), with much of this difference apparent by Day 25. The proportion of heifers observed in standing oestrus prior to embryo transfer and the interval from the end of synchronisation treatment to the onset of oestrus were similar in the sub-herds. Oestrous cycle length, ovarian follicular dynamics and progesterone profiles during the oestrous cycle were also similar. More conceptuses had elongated by Day 14 in the High sub-herd (67 vs. 14%, P<0.05), which also tended to have a higher pregnancy rate after artificial insemination (52 vs. 29, P<0.1). Total protein in flushings from the uterus was similar in the sub-herds on Day 14 and Day 17. Conceptuses in the High sub-herd were longer on Day 17 following embryo transfer (6.5 vs. 4.8, P<0.05). Interferon-tau levels were higher in the High sub-herd (25.9 vs. 16.1, P<0.01), although ubiquitin cross-reactive protein levels were also higher in the High sub-herd, but this difference just failed to reach significance. We conclude that: 1. Most of the difference in sub-herd pregnancy rate occurs within 3 weeks of ET; 2. Ovarian factors are unlikely to contribute to the difference; 3. Major differences occur after blastocyst hatching and probably depend upon a differing endometrial environment before Day 14; 4. Differences in the ability of the uterine milieu to stimulate the expression of interferon-tau may be responsible for the differences in pregnancy rate; 5. The two sub-herds are a unique experimental resource for understanding early pregnancy in cattle following either AI or ET.

In the cow, the embryo during the first three weeks of pregnancy is free living in the uterine lumen and is dependent on the maternal glandular secretions for its nutritional support. If the environment is appropriate, the embryo will develop sufficiently to prevent luteolysis. The aim of this study was to investigate the regulation of factors involved in embryonic-endometrial interactions during early pregnancy. Uterine horn sections were collected from 17 pregnant (PREG), 9 inseminated but no embryo present and 10 uninseminated cyclic control cows on days 12, 14, 16 and 18 after natural oestrus. The latter two groups were combined to form a single non-pregnant (NP) group. Trophoblast sections were also collected from the day 14, 16 and 18 embryos. The mRNA for interferon tau (IFNτ), oxytocin receptor (OTR), oestrogen receptor a (ER), prostaglandin G/H synthase -2 (PGHS-2), insulin-like growth factor (IGF) -I and IGF binding protein -1 (IGFBP-1) was determined by in situ hybridisation using 45 mer oligonucleotide probes end-labelled with35 S. The optical density (OD) readings were measured from the resulting autoradiographs. The expression of IFNτ mRNA in the trophodectoderm did not vary with embryo age. The expression of OTR mRNA in the luminal epithelium was first detectable on day 14 in 2 out of 5 NP cows and increased thereafter. Conversely, OTR mRNA was undetectable in all PREG cows except for one day 18 cow. In the NP cows, the first significant increase in ER mRNA concentrations in the luminal epithelium was observed on day 16. The pregnancy had no effect on ER mRNA concentrations in the luminal epithelium on days 12 and 14, but was significantly reduced on day 16 and was undetectable by day 18. On day 18, PGHS-2 mRNA was detectable in the luminal epithelium of all cows, but was unaffected by pregnancy status. The expression of IGF-I mRNA in the subepithelial stroma was maintained from days 12 to 18, but was reduced in the day 18 NP cows. IGFBP-1 mRNA concentrations in the luminal epithelium peaked on day 14 in both NP and PREG cows. Thereafter, concentrations declined in the NP group but were maintained in the PREG animals. In conclusion, the suppression of OTR mRNA expression by the embryo does not appear to require the prior suppression of ER mRNA. The continued expression of IGF-I and IGFBP-1 mRNA is likely to play an important role in the establishment of early pregnancy in the cow.

The objective of this study was to estimate the effects of genetic merit for milk yield on energy balance (EB), dry matter intake (DMI), and fertility for cows managed on three different grass based feeding systems, and to estimate possible interactions between genetic merit and feeding system. Individual animal intake estimates were obtained at pasture on 11 occasions across three grazing seasons. The data set contained 96 first lactation animals in 1995, 96 second lactation animals in 1996, and 72 third lactation animals in 1997. Half of these animals were of high (HG), and half of medium genetic merit (MG) for milk solids production. Genetic effects for the traits of interest were estimated as the contrast between the two genetic groups, and by the genetic regression of phenotypic performance on the estimated breeding value for fat and protein yield, based on pedigree information alone (PI). Significant effects of feeding system were observed on yields, DMI and EB, whereas there was no effect on live weight, condition score or reproductive performance. The interaction between genetic merit and feeding system was not significantly different from zero for any of the traits. Yields, grass DMI, and total DMI were all higher for HG than for MG, and also positively correlated (P<0.001) with PI. Furthermore, condition score, conception to first and second service, and pregnancy rate were significantly negatively correlated with PI. While at pasture, EB was positively (P<0.01) correlated with PI, although the contrast between HG and MG was not significantly different from zero. Condition score changes during very early lactation, demonstrated that HG had a more negative EB than MG. The results clearly illustrate the production potential of HG cows on grass based systems, however the reduced reproductive performance questions their suitability for seasonal calving systems.

The National Dairy Herd Fertility Project (renamed the InCalf Project) consists of two large, prospective observational studies conducted in commercial dairy herds from 4 Australian states. The project aims to identify factors associated with variation in reproductive performance between cows and herds. The larger of the two studies included over 33,000 cows and 168 herds. The database has undergone extensive error checking and correction resulting in high quality data. Large variations in reproductive performance were observed between cows and herds, indicating that there are important risk factors for reproductive performance and that cows and herds are exposed to varying combinations of these risk factors. Models for several reproductive outcomes were constructed using multivariable stepwise logistic regression. No significant associations were detected between any measures of cow genetic merit and 6 week in-calf rate. Neither milk volume nor protein yield were significantly associated with 6 week in-calf rate. A weak negative association was detected between fat yield in the first 120 days of lactation and 6 week in-calf rate. However, differences in estimated 6-week in-calf rate between moderate and high producing groups (2 to 5 percentage points) were small when contrasted against the 63 percentage point range in performance observed between herds (23% to 86%).

The study compared the impact of feeding different energy supplements (barley, molassed sugar beet and fat) prior to calving and the effects of feeding supplemental fat post-partum, on subsequent production and reproductive efficiency of dairy cows. Forty-eight multiparous Holstein-Friesian dairy cows were assigned to one of two groups, six weeks prior to expected calving date and fed a grass based total mixed ration according to ME requirements for late gestation. Group 1 was supplemented with barley (B) or molassed sugar beet feed (SB) prior to calving and was then given a high starch lactation ration. Group 2 was supplemented with either fat (F) or no supplement (C) pre partum, and was then given a similar lactation ration as Group 1 but supplemented with fat. Lactation rations were fed through to week-20 post partum and the cows were monitored during this period. Milk yield (P<0.002) and milkfat (P<0.02) production were higher and milk protein concentration (P<0.001) was lower in Group 2. The number of days to first rise in progesterone following parturition was greater (P<0.01) in Group 2. Due to the design of the study, effects of prepartum supplementation were only evaluated within each lactation ration group. Conception rate to first service was higher (P<0.001) for B than SB supplemented cows in Group 1 and higher (P<0.02) for F than C supplemented cows in Group 2. Services per conception were lower (P=0.06) for B than SB supplemented cows in Group 1 and lower (P<0.05) for F than C supplemented cows in Group 2. Overall pregnancy rates and days open were not significantly different between the groups. The data shows that pre-partum nutrition had an important role in determining subsequent fertility. Despite having negative effects early post partum, supplementing with fat did not affect overall reproductive performance but it did improve milk production.

Poor reproductive performance is a major problem on dairy farms across Northern Ireland and the United Kingdom (UK) as a whole, and recent studies suggest that the problem is increasing. In order to identify the key factors influencing reproductive performance at farm level, a major research initiative was established in 1998 with the objective of collating a comprehensive database on reproductive performance from 20 herds, representing over 2000 dairy cows, across Northern Ireland. Preliminary results for five herds from the first year of the study indicate a high heat detection rate (84%) across all farms. The mean intervals to first observed heat and to first service for cows calved within 19 days of the start of the breeding season were similar between herds (42.8 ± 14.0 days and 50.0 ± 11.0 days respectively). In contrast, the interval to first service for all recorded services was 89.8 ± 66.3 days and varied considerably between farms (69.9 to 112.7 days). Investigation of progesterone profiles indicates that 62.3% of cows had resumed normal cyclic activity by 40 days post calving. Of the cows with atypical ovarian patterns, 19.4% were classified as prolonged post-partum anoestrus with a further 12% exhibiting at least one post-partum prolonged luteal phase.

Previous studies have shown that 35 — 40 % ofmodern dairy cows experience ovarian disturbances during early lactation. Even though negative energy balance (NEB) has been implicated as a regulator of ovarian function, the exact metabolite(s) or hormone(s), which mediate this effect is still not clear. In this study, we investigated the relationship between blood metabolites (NEFA, BHB and glucose) and plasma insulin like growth factor-I (IGF-I) and ovarian dysfunction. Thirty three Holstein-Friesian cows were fed a lactational ration ad libtium and were inseminated at observed oestrus starting from 56 days post calving. Three times weekly milk samples and weekly blood samples were collected from cows from calving until confirmed pregnant. Cows were placed into “NORMAL” or “ABNORMAL” categories of ovarian activity based on their milk progesterone profiles. The days to first service were 71 ± 2 in “NORMAL” and 78 ± 6 in “ABNORMAL” animals. The calving to conception interval (days open) was shorter in the “NORMAL “ than the “ABNORMAL” group (90 ± 8.7 vs 119 ± 15.2 days). IGF-I levels were significantly lower in the “ABNORMAL” group from 2 until 7 weeks after calving (P<0.02) and NEFA concentrations were higher in the “ABNORMAL” cows in the early postpartum period (P<0.03). There was no significant difference in either plasma BHB or glucose. This study confirms that elevated plasma NEFA concentrations are associated with “ABNORMAL” ovarian activity. The most significant difference was in plasma IGF-I concentrations, which stayed lower for nearly 2 months in the “ABNORMAL” animals.

The relationship between body condition at calving, uterine tone and discharge, time to first oestrus and changes in concentration of serum calcium and magnesium in early post partum was examined in 209 multiparous dairy cows located in four herds in the San Joaquin Valley of California. Body condition score (BCS) was graded from 0 to 5 and BCS at calving was estimated from BCS in the late dry period (LDBCS). Uterine tone and discharge were graded from l(best) to 3(worst) after rectal palpation at two weeks post partum. Concentrations of blood metabolites and levels of progesterone were determined from blood samples taken at selected stages of lactation from the late dry period to ten weeks post partum. Abnormal uterine discharges were strongly correlated with uterine tone. Uterine tone and discharge scores were significantly lower in cows that calved with body condition scores of 2.5 and 3.0 than in those which calved with body condition scores of < 2.5 and ≥ 3.5. Serum calcium and magnesium concentrations were higher at two weeks post partum in cows with scores of 2.5 and 3 than in cows with higher or lower scores. Serum calcium and magnesium concentrations appeared to be higher with decrease in grades of uterine discharge and tone but this was significant only in the case of serum calcium and uterine discharge. The number of days to first oestrus was significantly associated with uterine discharge and tone (P < 0.05) but not significantly associated with LDBCS. The number of days to second oestrus was not significantly associated with uterine discharge and tone but was significantly less in cows with LDBCS 3 (17.28 days) than cows with lower or higher LDBCS (P < 0.05). It is concluded that body condition at calving is associated with serum micromineral balance and subsequent uterine and ovarian health. Based on these measurable indicators, cows calving with body condition scores of 2.5 and 3 appeared to be in optimum condition for post partum reproductive performance.

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.

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 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.

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)

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.

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.

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.

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.