Managing reproductive seasonality in small ruminants
Most species and breeds of small ruminants, especially sheep and goats, have seasonal reproduction, meaning that females cycle for part of the year and remain in anestrous during the remaining part, while males increase and decrease in reproductive activity in sync with females. This is regulated primarily by variations in the length of daylight. During darkness, the hormone melatonin is secreted and synchronizes physiological processes with environmental conditions. Limits are thus placed on the months of the year in which these animals can reproduce and, therefore, the availability of lambs or kids. This review briefly introduces the principal techniques used to modify the seasonal pattern in sheep and goats, and for several of these, recent technical information is included. Hormonal methods, basically involve pharmacological estrous cycle creation in a female (progesterone + gonadotropin) or placement of subcutaneous implants that release melatonin. In the male use of these implants is also effective, and recent information suggests the potential use of equine chorionic gonadotrophin (eCG) to improve reproductive performance outside the normal season. Information is presented about stimuli that alter the physiology of the animal, such as manipulating the number of hours of light or darkness that an animal is exposed to, thus, allowing reversal of the seasonal effects in both males and females. Managing the social environment of the animals can also be effective in modifying seasonality. The male effect consists of suddenly exposing previously isolated anestrous females to rams and bucks, which induces neuroendocrinal changes that can lead to the expression of estrus accompanied by ovulation. The stimulus caused by the presence of other cycling females is also effective to induce ovulation in anestrus females, especially goat does, while in the case of ewes, this occurs after close physical contact with the cycling animals. Conversely, sudden contact of rams and bucks with females in estrus induces substantial increases in the secretion of gonadotropins and androgens and, if the stimulus is sufficiently maintained, it is possible to modify the reproductive status of these males. Lastly, the advantages and disadvantages of each of these technologies is discussed.