Postweaning diarrhea is a common issue in pig production which is currently controlled by feed supplementation with zinc oxide. However, new alternatives are being sought due to an expected ban on zinc oxide in feed supplementation from 2022 in the EU. One possible alternative is to use novel types of probiotics consisting of microbiota characteristic for healthy weaned piglets. In this study, we therefore collected rectal swabs of piglets 3 days before weaning and 4 days after weaning in a commercial farm considering all risks of field trial like the use of antibiotics, classified the piglets as predisposed, healthy or sick and using 16S rRNA sequencing, we determined and compared the microbiota composition. Increased Actinobacteria before weaning was a marker of piglets predisposed for diarrhea. Increased Chlamydia or Helicobacter before weaning was surprisingly a marker of healthy and resistant piglets after weaning. After weaning, unclassified Clostridiales, Deltaproteobacteria, Selenomonadales, Fusobacterium, Akkermansia or Anaerovibrio increased in microbiota of piglets with postweaning diarrhea while an increase in Prevotella and Faecalibacterium was characteristic for healthy, weaned piglets. Both changes in individual microbiota members and also correct timing of microbiota reshaping around weaning and the increase of mainly Prevotella species just after weaning are equally important for resistance to postweaning diarrhea in piglets under field conditions.