Reproductive challenges facing the cattle industry at the beginning of the 21st century.

The aim of this review is to pinpoint the areas that require further research for greatest impact to improve the efficiency of dairy and beef production. Increased knowledge about the principal causes of reduced fertility is essential. Increases in milk yield have been at the expense of reduced fertility in dairy cows and although diet has a major impact, the precise interaction between nutrition and reproduction still needs to be characterized in both beef and dairy cows. Furthermore, during periods of inadequate nutrition or stress, the intensity of oestrus is reduced by inadequate exposure to oestradiol. However, it is still unclear which pheromones are involved at oestrus, how synthesis is controlled and how pheromones are detected in herd-mates. GnRH may be involved in behaviour but the brain centres that translate hormonal messages are unknown. Attempts to overcome poor oestrous detection include measurements of milk progesterone, telemetric pressure detectors and devices that record the extra activity at oestrus. Substitution of heat detection by 'hormone treatment remedies' has met strong consumer resistance in Europe. The creation of larger cattle units and increased movements world-wide render herds more susceptible to infectious agents, such as Neospora, Leptospira, Trypanosoma and Bovine Viral Diarrhoea virus (BVDv), but it is unclear how other clinical conditions, such as lameness or endometritis, also interfere with ovarian function. The future of dominant follicles selected within 14 days after parturition is crucial--normal ovulation, prolonged persistence or atresia. Calving carries the greatest risk in the reproductive life of a cow and yet little work has focused on reducing the frequency of this event. For dairy cows, a greater understanding about induced or extended lactation is required. For beef animals, precise induction of twinning and nutritional adjustments could produce two offspring per pregnancy. At the start of pregnancy, the trophoblast produces interferon to prevent luteolysis, but the immunological implications are unknown and it is not clear how the rest of pregnancy is maintained. Profiles of pregnancy specific protein B (PSPB) have increased understanding of embryonic death. However, 25% of cows in abattoirs are pregnant, even though 30% of involuntary cullings are 'for failing to conceive'. Clearly, this is an area of wastage that requires urgent resolution. It is unknown why undernutrition at the time of insemination or in early pregnancy leads to delayed births, low fetal weights and later adverse health. At the end of pregnancy, the fetus controls the onset of parturition, but very little is known about the biochemical control of cervical dilation and placental separation. On the male side, bulls are selected for optimal freezability of semen; however, there is as yet no reliable predictor for semen fertility. Methods for accurately pre-determining the sex of both semen and embryos will revolutionize the dairy and beef industries.