456 Kouamo: Current concepts for estrus synchronization in bovine INTRODUCTION Reliable methods for the artifcial regulation of the estrus cycle have long been sought for the purpose of increas- ing productivity and decreasing costs in food-producing animals (Looney et al., 2005). The widespread application of artifcial insemination to bovine has prompted inter- est in developing methods for the artifcial regulation of estrus cycles in order to inseminate maximum number of animals on a single day (Baruselli et al., 2007). Methods that were frst developed often resulted in an acceptable synchronization of estrus. In oestrus synchronization the manipulation of the bovine oestrus cycle results in standing oestrus in majority of animals, within a short period of time (Islam, 2011). If the oestrus synchroni- zation protocol needs to be successful then it requires to synchronize the follicular waves and/or luteal regression (Vikrama Chakravarthi and Sri Balaji, 2010). Techniques are now available to predictably alter the time of estrus within the breeding season (estrus synchronization) and to induce estrus during periods of anoestrus (estrus induction), while maintaining a high level of fertility. Protocols difer in hormones used, method of hormone administration (injection, vaginal insert, or consumption through feed), number of injections, number of times cattle or bufaloes must be handled, timing of injections, and heat detection requirements. Three basic approaches are used for synchronization of estrus in bovine classifed to pharmacological methods by the use of progestins (MGA, MAP, FGA, PRID et CIDR-B) to keep animals out of heat and extend the estrus cycle, prostaglandins (PG) to bring females into heat and shorten the estrus cycle, and GnRH to cause ovulation or start development of a new follicular wave (Kharche et al., 2010). ESTRUS SYNCHRONIZATION BY USING PROGESTINS One of the frst methods used to synchronize oestrus in cattle was the long-term feeding of melengestrol acetate (MGA; Zimbelman and Smith 1966). MGA is a synthetic progestin that suppresses estrus when fed at the rate of Current concepts for estrus synchronization in bovine J. KOUAMO 1 (Reçu le 14/03/2017; Accepté le 15/06/2017) Abstract In animal breeding, synchronization has major role to play especially to bring the animals to estrus at a desired time by using ex- ogenous hormones. Over the past years, a number of substances have been introduced to induce estrus in farm animals. Ovulation may be synchronized with progestins and estrogen combination treatment, a 2-dose prostaglandins (PG) regimen, or a GnRH and PG combination. Progesterone may be considered as a drug of choice, because it is easily applied at low cost. Melengestrol Acetate (MGA), Progesterone Release Intra-vaginal Device (PRID) and Controlled Internal Drug Release devices (CIDR) are progestins approved for use in cows. The fve systems for fxed-time insemination with GnRH-PG combinations are Ovsynch, Cosynch, Select Synch, Hybrid Synch and Heatsynch. Hybrid Synch has a lower cost and less handling compared with Ovsynch and CO-Synch but more than Select-Synch. The use of Estradiol cypionate within the Ovsynch protocol may ofer dairy producers an alternative. In sub Saharan Africa, the educational level of the farmers, the potential of the available resources (cultivated or natural pastures, rain distribution, water availability, ecological environment, etc.) and the cost of the technology are factors that explain partially why the use of modern reproductive techniques (estrus synchronization and artifcial insemination) are not applied on a commercial scale. Keywords: Bovine, fertility, GnRH, estrus, progestins, prostaglandins 1 School of Veterinary Medicine and Sciences. PO BOX: 454. Ngaoundere-Cameroon. Email: justinkouamo@yahoo.fr Résumé En reproduction animale, la synchronisation joue un rôle majeur en provoquant l’œstrus d’animaux à un moment souhaité en utilisant des hormones exogènes. Au cours des dernières années, un certain nombre de substances ont été introduites pour induire l’œstrus chez les animaux d’élevage. L’ovulation peut être synchronisée avec des progestatifs et un traitement par association d’œstrogènes, un régime de 2 doses de prostaglandines (PG), ou une combinaison de GnRH et de PG. La progestérone peut être considérée comme un produit de choix, car elle est facilement appliquée à faible coût. L’acétate de melengestrol (MGA) et les dispositifs PRID et CIDR sont les progestatifs approuvés pour l’usage chez les vaches. Les cinq systèmes d’insémination à temps fxe avec la combinaison GnRH-PG sont Ovsynch, Cosynch, Select Synch, Hybrid Synch et Heatsynch. Hybrid Synch est moins coûteux et facile à manipuler par rapport à Ovsynch et Co-Synch mais plus que Select-Synch. L’utilisation du cypionate d’Estradiol dans le cadre du protocole Ovsynch peut ofrir une alternative aux producteurs laitiers. En Afrique subsaharienne, le niveau d’éducation des éleveurs, le faible potentiel des ressources disponibles (pâturages cultivés ou naturels, répartition des pluies, disponibilité de l’eau, environnement écologique, etc.) et le coût de la technologie sont des facteurs expliquant en partie pourquoi l’utilisation des techniques modernes de reproduction (synchronisation de l’œstrus, insémination artifcielle…) ne sont pas appliquées à l’échelle commerciale. Mots clés: Bovins, fertilité, GnRH, oestrus, progestagènes, prostaglandines Concepts actuels pour la synchronisation de l’oestrus chez les bovins