Mechanisms mediating the stimulatory effects of the boar on gilt reproduction.

The major stimulatory effects of boar presence on female pig reproduction are well documented. The boar can stimulate the early appearance of first oestrus in the prepubertal gilt, the lactating sow and the weaned sow. Additionally, boar presence may influence the exhibition and detection of oestrus, together with the sow's receptivity (see Hemsworth & Barnett, 1990). Boar involvement in other aspects of sow reproduction such as the maintenance of cyclicity and pregnancy have also been postulated, although few research data are currently available. In this review the prepubertal gilt is used as the model for response of female pigs to boar stimuli, as most of the research data is to be found in this area. Since the major boar effects have been known for some time it may appear surprising thanhe mechanism controlling them is not well understood. This may be partly due to the variation observed between studies in female response to boar contact, probably reflecting confounding factors such as genotype, climatic and housing environment, nutritional status and the age of the gilt. There is also evidence that the manner of boar exposure will influence the response of the female, since variations in the length and frequency of the exposure period and, possibly, the size of the exposure pen all affect the level of stimulation perceived by the female. These interactions clearly influence the response of gilts to boar exposure. They therefore reeeive early attention in this review. The roles of individual components of the boar in gilt stimulation are then considered. Finally endocrine responses of gilts to boar exposure, or exposure to boar-component stimuli, are evaluated and a tentative mechanism of action of the boar effect is postulated.