477192/2007-6) and by a Student Fellowship from Funda??o de Amparo �� Pesquisa do Estado do Rio Grande do Sul (FAPERGS) to an undergraduate student (CR).
Litter size commonly increases with the parity number in sows [1]. However, in some sows the number of piglets in the second litter is lower or similar to that of the first litter, selleckchem phenomenon known as second-litter syndrome [2, 3]. The second-litter syndrome negatively affects the pregnancy rate of second parity sows and sows productive lifetime in the farm, since reproductive failure is one of the main reasons for culling young sows [4, 5]. Therefore, the estimation of and the identification of risk factors associated with the second litter-syndrome may be of great help in planning better management strategies to improve the second-parity reproductive performance.

Litter size at first parity is one of the main risk factors associated with this phenomenon, because the odds of the second-litter syndrome increase with litter size at parity 1 [6]. The decrease of litter size or farrowing rate in the second parity sows is often related to an excessive weight loss during first lactation [7, 8]. Other factors such as herd size, season of farrowing, and weaning to service interval have been reported as risk factors for the second-litter syndrome [6]. The pig industry is a very important activity in southern Mexico. To our knowledge, there are no reports of the incidence of and factors related to the second-litter syndrome in sows under tropical conditions.

Therefore, the objectives of this study were to estimate the incidence of sows with the occurrence of the second-litter syndrome and to determine the effect of some factors, in three farms in the south eastern of Mexico.2. Material and MethodsData from three commercial farms of the state of Yucatan, Mexico, were used. Yucatan is localized at latitude 19��30�� and 21��35��N and longitude 90��24��W. The climate of the region is subhumid tropical, with an average temperature of 26.6��C, an average rainfall of 1,100mm, and a relative humidity of 78% [9]. Pig production is the second more important livestock activity in Yucatan, being carried out under intensive conditions. There are approximately 105 farms with capacity for 28 to 3,500 sows. Farms 1, 2, and 4 were full cycle farms with 3,900, 1,200 and 550 sows, respectively.

Farm 3 was a two-site-type farm (breeding and production) with 320 sows. The four farms produced their own replacements and practiced the quarantine of the gilts. Sows were fed commercial feed according to the productive stages. Young sows were given approximately 2.6kg/day of a feed with 3,000kcalEM/kg, 16% Batimastat crude protein and 0.8% lysine, whereas the sows received 3.2kg/day of feed. In all farms, breeding was carried out mainly by artificial insemination. After estrous detection using a boar, sows were inseminated three times every 12 hours.