Palaeontological deductions from the fossil remnants of extinct dinosaurs tell us much about their classification into species as well as about their physiological and behavioural characteristics. Geological evidence indicates that dinosaurs became extinct at the boundary between the Cretaceous and Paleogene eras, about 66 million years ago, at a time when there was worldwide environmental change resulting from the impact of a large celestial object with the Earth and/or from vast volcanic eruptions. However, apart from the presumption that climate change and interference with food supply contributed to their extinction, no biological mechanism has been suggested to explain why such a diverse range of terrestrial vertebrates ceased to exist. One of perhaps several contributing mechanisms comes by extrapolating from the physiology of the avian descendants of dinosaurs. This raises the possibility that cholecalciferol (vitamin D3) deficiency of developing embryos in dinosaur eggs could have caused their death before hatching, thus extinguishing the entire family of dinosaurs through failure to reproduce.

Early embryo development following fertilization occurs in the oviduct. However, despite being the site of fertilization in cattle, it is possible to by-pass the oviduct by producing embryos in vitro and/or by transferring blastocysts recovered from one female into the uterus of another. While there is substantial evidence for the oviduct having an influence on the quality of the developing embryo, manifested in altered morphology, gene expression and cryotolerance, evidence for a two-way dialogue is weak. In contrast, successful growth and development of the post-hatching blastocyst and pregnancy establishment are a result of the two-way interaction between a competent embryo and a receptive uterine environment. Progesterone (P4) plays a key role in reproductive events associated with establishment and maintenance of pregnancy through its action on the uterine endometrium. Elevated concentrations of circulating P4 in the immediate post-conception period have been associated with an advancement of conceptus elongation, an increase in interferon-tau production and, in some studies, higher pregnancy rates in cattle. This review summarizes current knowledge on the communication between the developing embryo and the maternal reproductive tract.

High intakes of dietary protein, particularly rumen degradable protein (RDP), lead to elevations in systemic concentrations of ammonia and (or) urea and these may be increased further if associated with inadequate fermentable energy intake. High systemic concentrations of ammonia and urea have been associated with reduced reproductive performance in cattle. The objective of this study was to examine the effect of RDP and fermentable energy intake on a range of blood metabolites and on embryo survival in heifers. Oestrous synchronized, nulliparous beef heifers (no. = 162) were randomly assigned in a 2 ✕ 2 factorial designed experiment to two levels of RDP and two levels of fermentable energy. Grass silage-based diets were supplemented with either 0 (0U) or 240 (240U) g dietary urea (460 g/kg N) and these in turn with either 0 (0P) or 3 (3P) kg dry matter of molassed sugar-beet pulp pellets (MSBP) per day. The four treatments were, therefore, (1) 0U + 0P (no. = 43), (2) 0U + 3P (no. = 44), (3) 240U + 0P (no. = 40) (4) 240U + 3P (no. = 35), respectively. Systemic concentrations of ammonia, urea, insulin, glucose and progesterone were measured. Heifers were given artificial insemination (AI) and embryo survival measured by ultrasonography at 30 and again at 40 days after AI. Systemic ammonia and urea were elevated (P < 0·001) in the animals given the high RDP diets. Supplementation with MSBP reduced systemic urea in the heifers on both high and low RDP diets. Plasma ammonia concentrations were not affected by MSBP supplementation (P > 0·05). Plasma glucose was not affected by urea or MSBP treatment (P > 0·05) but was affected by day and time of sampling (P > 0·05). Plasma concentration of insulin was not affected by urea or MSBP supplementation or by day or time of sampling (P > 0·05). Plasma concentration of progesterone was not affected by diet or time of sampling (P > 0·05). The overall embryo survival rate was 62% and was not affected by dietary urea or fermentable carbohydrate or by systemic concentrations of ammonia, urea, glucose, insulin or progesterone (P > 0·05).

Seasonal milk production systems rely on heavy inputs of nitrogenous fertilizer, which typically generate pastures with a high crude protein (CP) and low fermentable energy concentration. High intake of CP, particularly in association with low rumen fermentable energy, increases systemic ammonia and urea and has been associated with reduced fertility in cattle. The objective of this study was to examine the relationship between pasture protein intake and fermentable energy supplementation on a range of blood metabolites and on embryo survival and development in cattle.

Oestrous synchronized, nulliparous beef heifers (no. = 175) were randomly assigned to one of four pasture-based dietary treatments in a 2 ✕ 2 factorial study carried out over 2 years. Animals were randomly allocated to either high (85 kg nitrogen (N) per ha; HN) or low (0·0 kg N per ha; LN) N fertilized pastures and within pasture treatment were randomly allocated to receive either zero or three (+3P) kg dry matter (DM) of molassed sugar-beet pulp (MSBP) per head per day as follows: (1) HN (no. = 44), (2) HN + 3P, (no. = 43), (3) LN (no. = 44), (4) LN + 3P (no. = 44). Blood samples were collected to measure systemic concentrations of ammonia, urea, insulin, glucose and progesterone. Heifers were artificially inseminated (AI) and pregnancy diagnosis was carried out by ultrasonography 30 days after AI. Subgroups of pregnant animals across treatments were slaughtered 40 days after AI to estimate conceptus development.

The HN pasture had a higher CP (P < 0·001) and lower water-soluble carbohydrate (P < 0·01) concentration. Plasma concentrations of ammonia (P < 0·05) and urea (P < 0·001) were higher in the animals on the HN pastures and were reduced (P < 0·05) by MSBP supplementation, but only in animals on the HN pastures. Embryo survival rate across treatments was high overall (71%) and not related to pasture CP concentration, fermentable energy supplementation or systemic concentrations of ammonia, urea, glucose or insulin. There was no relationship between dietary treatment or systemic metabolites and any of the estimates of conceptus development. Systemic insulin was not affected by pasture N treatment or MSBP supplementation (P > 0·05). Systemic concentrations of glucose were not affected by pasture N treatment (P > 0·05) but were increased by MSBP supplementation (P < 0·05). Systemic progesterone was not affected by pasture CP or MSBP supplementation (P > 0·05) but at day 7 after AI was positively related (P < 0·05) to embryo survival. Intake of high CP herbage elevated systemic ammonia and urea but there was no association with embryo survival rate or embryo development in heifers.

Data on ovulation rate, embryo survival and litter size have been analysed from the 1988-96 records collected at AgResearch's Woodlands Research Station on 2180 elite (highly prolific) ewes of three breeds: Romney, Coopworth and Perendale. The Romneys included a subgroup found to carry the sex-linked Inverdale high prolificacy gene. Foundation animals for all breeds were screened on high litter size (lambs born, alive or dead, per ewe lambing) from industry flocks. Prior to being transferred to Woodlands between 1979 and 1984, their average litter size was 2·66. Ovulation rates for descendants of the foundation ewes averaged 2·15 (Romney), 2·43 (Coopworth), 2·15 (Perendale) and 2·96 for the Romney subgroup carrying the Inverdale gene. Embryo survival averaged 0·84 except for a low value in Inverdales (0·76). Using a restricted maximum likelihood model, with full relationship matrix, the within-breed heritability of ovulation rate over all ages and breeds (excluding Inverdale) was 0·14 (s.e. 0·03), with repeatability 0·26 (s.e. 0·02). Litter size averaged 1-80 (Romney), 2·05 (Coopworth), 1·76 (Perendale) and 2·17 (Inverdale). The heritability estimate for litter size (excluding Inverdale) was 0·073 (s.e. 0·018), from data for all ages. The genetic correlation between ovulation rate and litter size was 0·91 (s.e. 0·08). The standard deviation for ovulation rate in the Inverdales was 1·75 times as great as the average of the other three breeds. Embryo survival to term for ewes conceiving to two, three or four ovulations averaged 0·89, 0·77 and 0·64, respectively, and was significantly different among breeds fP < 0·001). The greater ovulation rate in Inverdales than Romneys was sufficient to explain the Inverdale effect on reduced embryo survival. The highest embryo survival was observed in the Coopworth breed (P < 0·001).

Predictions of two models were compared. The models relate ovulation rate (OR) and prenatal survival (PS) to litter size (LS): the uterine capacity model (UCM), where maximum LS is limited by uterine capacity (UC), and the threshold model (TM) whereby PS is modelled as a binary threshold trait. Records were simulated according to both models using statistics from Trench Large White gilts. Both models were able to reproduce closely the observed curvilinear relationship between OR and LS, with LS reaching a plateau at high OR. Several genetic correlations (ρg) fulfilling the conditions h20R = 0·34 and h2LS = 0·12 (the residual maximum likelihood estimates in the population) were studied by means of stochastic computer simulation. The genetic correlation between OR and LS was very sensitive to changes in h2uc, whereas ρgLS,PS was always positive, and ρgOR,PS was always negative. The correlation between PS and UC was larger than 0·90, except for very small h2UC This suggests that genes affecting PS have a strong influence on UC and that PS can be a good indirect criterion to select for UC. Both models predicted that the advantage of an index combining OR and LS with respect to direct selection on LS diminishes in successive generations of selection and that the size of the experiment needed to detect significant differences is very large. Records were also simulated by halving the mean and variance of UC, so as to mimic unilateral hysterectomy-ovariectomy (UHO). If the UHO treatment results in halving UC, LS of UHO females should behave very much as half the UC of intact females.

An investigation was conducted to compare late embryonic losses in ewe lambs and ewes and to determine the effect of inducing embryonic mortality on day 15 of pregnancy on reproductive function in ewe lambs. Ovulation rate and embryo survival were determined, at 15 and 30 days after mating, in a total of 83 ewe lambs and 67 ewes. Ovulation rate was lower, at spontaneous oestrus, in ewe lambs (1·07) than in ewes (1·25; P < 0·05). There was no difference in embryo survival at day 15, as 79% of ewe lambs and 80% of ewes contained expanded conceptuses. However, embryo survival was lower in ewe lambs (69%) than ewes (88%) at day 30 (P < 0·05). Embryonic mortality was induced in an additional 18 ewe lambs with an intrauterine injection of colchicine on day 15 post mating. Four of these animals had normal length oestrous cycles, six had inter-oestrus intervals ranging from 22 to 35 days and eight did not return to oestrus. Overall, the results of this study demonstrate that low ovulation rate and late embryonic losses make a significant contribution to subfertility in ewe lambs. Moreover, conceptus death is associated with luteal persistence, which may reduce reproductive performance by preventing remating.

An experiment zoas conducted to assess differing methods of twin pregnancy establishment in Hereford × British Friesian beef cows and heifers. The experiment was 2 × 2 × 2 × 2 factorial design in which the factors were (i) source of embryos (in vivo or in vitro produced); (ii) pregnancy status of recipient (inseminated or non-inseminated); (Hi) method of embryo transfer (surgical or cervical); and (iv) uterine location of a native and transferred embryo, or two transferred embryos (both located in the ipsilateral, or one in each of the ipsi and contralateral uterine horns). Pregnancy and twinning rates for 285 animals used for embryo transfer were initially diagnosed at day 56 after induced oestrus by transrectal ultrasonography. Subsequently, calving rate and birth weiglit at calving were recorded.

Pregnancy rates at day 56 after induced oestrus were similar for both surgical and cervical transfers (58·6% v. 55·2%), as was the case for twinning rate (36·2% v. 30·0%). Similarly, there were no differences between these two methods of transfer (50·0% v. 46·9%) and (26·1% v. 17·7%) for calving and twin calving rates respectively. Recipients which had two embryos located in the ipsilateral uterine horn had higher (P < 0·001) pregnancy rates (66·6% v. 47·3%) but similar twinning rates (32·6% v. 33·4%) at day 56 after induced oestrus to recipients which had one embryo located in each horn. A greater (P < 0·05) percentage of recipients with two embryos originally located in the ipsilateral horn calved (56·0% v. 41·0%) but fewer (P > 0·05) produced twins (17·8% v. 25·7%) than was the case for recipients which originally had one embryo located in each horn. In vivo produced embryos resulted in higher (P < 0·001) pregnancy rates (74·4% v. 39·7%) and twinning rates (48·3% v. 18·0%) at day 56, and higher (P < 0·001) calving rates (64·5% v. 32·7%) and twin calving rates (36·3% v. 7·6%) than did in vitro produced embryos. Inseminated (Al + ET) recipients had slightly greater (P>0·05) pregnancy rates (61·6% v. 51·6%) and twinning rates (36·9% v. 28·7%) than non-pregnant recipients which received two embryos. A greater (P<0·05) percentage of inseminated recipients (Al + ET) calved (54·3% v. 42·0%) than was the case for non-pregnant recipients which received two embryos. The percentage producing twins at calving were similar for these two methods of twin pregnancy establishment.

Embryo survival to day 56 after induced oestrus averaged 45·0% and was found to be non-independent of its co-twin. From day 56 to parturition foetal loss averaged 21·0% and foetal survival was found to be independent of the fate of its co-foetus. Twin foetuses located in the same uterine horn were lighter at birth than twin foetuses located in separate uterine horns (33·0 v. 35·2 kg; P < 0·05).

A total of 233 does were subjected to laparoscopy in order to study the relationships between litter size (LS) and its components, using linear and quadratic models. The relationships were essentially linear, but there was a quadratic relationship between ovulation rate (OR) and the number of live embryos at 12 days of gestation. OR explained a small proportion only, but prenatal survival a greater proportion of the variation in LS. Both embryo survival and foetal survival were of similar importance and appeared to operate independently.