This study was carried out to find out the changes on serum concentrations of estradiol-17β, progesterone in primiparous Duroc, Landrace and Yorkshire sows weaned at 7 or 21 days. Also, we compared the litter size at birth and weaning among the breeds weaned after lactation for 7 or 21 days. The estradiol-17β concentrations among the breeds were 6.9∼8.8 pg/ml and 6.4∼8.8 pg/ml after lactation for 7 or 21 days, respectively. The progesterone concentrations ranged from 0.3 ng/ml to 1.6 ng/ml. Duroc sow showed higher progesterone concentration compared with Landrace and Yorkshire sows weaned after lactation for 7 or 21 days. Also, we found out that litter size at birth and weaning, respectively, did not show any differences between day 7 and day 21 of lactation. From the facts mentioned above, it was suggested that very early weaning systems could work with no apparent adverse effect on prolificacy.

• Kim, J. S(Livestock Experiment Station, Chungnam Livestock Sanitation Research Institute)
• H. K. Kim(Livestock Experiment Station, Chungnam Livestock Sanitation Research Institute)
• C. B. Yang(National Livestock Research Institute, RDA)
• D. S. Son(National Livestock Research Institute, RDA)
• S. H. Lee(College of Visual Image ＆ Health, Kongju National University)
• Y. J. Yi(Division of Animal Science and Resources, Research Center for Transgenic Cloned Pigs, Chungnam National University)
• C. S. Park(Division of Animal Science and Resources, Research Center for Transgenic Cloned Pigs, Chungnam National University)