Reproductive performance in high-producing dairy cows

Milk yields >10,000 kg/year are common in modern dairy production, owing to improved nutrition, management and genetic gains through use of progeny-tested bulls. However, reproductive performance has decreased worldwide in many cows with a high genetic potential for milk production, particularly in the Holstein breed. Moreover, cow robustness and longevity is also threatened by increasing stress, udder health disturbances and of locomotion disorders. Genetic global misuse of a narrow base of AI sires -including those selected for high milk yield but not consequently for health and reproductive traits- has not only contributed to these undesirable effects on animal health and welfare but, together with sub-optimal management, jeopardized the ethical and economical sustainability of modern dairy farming. This review describes the state-of-the-art of this multifaceted problem and advises on how to ameliorate it, since it is not seen as an unsolvable problem. Use of high-fertility sires, of balanced breeding programs with adequate trait measurements, diet optimization, design of buildings and management systems that best support reproduction as well as cross-breeding; are among short- and medium-term strategies. In a longer perspective, holistic- and trait-orientated research on interrelations between gene regulation of nutrition, lactation and stress is needed; aiming at identifying reliable and cheap markers to be used on-line and on-farm as recorders of genetic traits. Awaiting the full application of juvenile genomic selection, a wider inclusion of functional traits (fertility, health and longevity) and of product quality are mandatory for breeding programs in order to secure acceptable fertility, sustained milk production and the best welfare of dairy cows. Such strategies have proven successful in the Nordic countries and are being increasingly adopted by others