Based on their practices, breeding stocks, and reproductive performance measures, such as farrowing rate, pigs born alive, and lactation length, the farm of this study is an average commercial farm in Japan. Using this typical commercial farm, our results indicate that females showing frequent vacuum chewing may produce fewer numbers of total pigs born than those with no vacuum chewing. Females in stall housing systems produce more piglets and show more active behaviors than tethered females (McGlone et al., 1994). The results of this study suggest that a subpopulation of females frequently showing vacuum chewing with fewer total pigs born exists on commercial farms. Our data suggest that approximately 10% of stalled females frequently (>36% of the time) show vacuum chewing.