BACKGROUND: There is a growing interest in the development of novel and innovative vehicles for controlled release of urea into the rumen, aiming to provide ammonia-N for the biosynthesis of proteins of bacterial origin and to prevent urea intoxication by direct feeding to livestock. Urea microencapsulation is a system that can control the release of urea to be slow and steady. RESULTS:The amount of encapsulated urea was 69% of CSU (calcium silicate + urea + Eudragit RS100 ® + dichloromethane) and 71% of ACU (activated charcoal + urea + Eudragit RS100 ® + dichloromethane) groups (p > 0.05) The buoyancy of the microcapsules was over 50% after 12 h of agitation in both groups (CSU and ACU), producing significant differences in the volume of the organic phase factor, which was 20 mL at the lowest value (p = 0.0005). The morphology of the microcapsules produced with CSU and ACU showed no significant differences in microcapsule morphology (p > 0.05). The lower temperature (35 versus 40 ∘ C, p = 0.035) retained better morphology of the microcapsules. Regarding the in vitro ammonia-N release kinetics, unprotected urea reached a maximal peak after 6 h, while CSU and ACU took more than 24 h to reach ammonia-N released concentration. CONCLUSIONS:We stabilized the physical factors in the microencapsulation of urea that can allow slow release of rumen fluid. RESULTS AND DISCUSSIONPhase 1 Urea encapsulation Urea encapsulation was ca 65% for the APV. Urea is hygroscopic and is very soluble. 8 The proportion of encapsulated urea was J Sci Food Agric 2019; 99: 2541-2547