Ruminal Degradation Kinetics and Effective Degradability of Concentrate Feed with Added Thymol- and Carvacrol-Rich Essential Oils

Essential oils (EOs) have been extensively investigated as feed additives; however, their effects on ruminal degradation kinetics remain inconsistent, particularly for thymol- and carvacrol-rich EOs. This study evaluated the effects of concentrate feed with added thymol- and carvacrol-rich EOs, providing different levels of thymol and carvacrol, on ruminal degradation kinetics and effective degradability of dry matter (DM), organic matter (OM), and crude protein (CP) using an in situ nylon bag technique. Treatments included the concentrate feed (control); the concentrate feed with added carvacrol-rich EO to provide 300 or 100 mg carvacrol/kg concentrate (C300 and C100); the concentrate feed with added thymol-rich EO to provide 300 or 100 mg thymol/kg concentrate (T300 and T100); and the concentrate feed with both EOs added to provide 150 mg carvacrol + 150 mg thymol/kg concentrate (CT300) or 50 mg carvacrol + 50 mg thymol/kg concentrate (CT100). The carvacrol treatments generally maintained DM and OM degradability at levels comparable to the control, whereas the thymol and combined EO treatments showed lower degradability, particularly during the early incubation periods (p < 0.05). Thymol treatments also showed lower CP degradability than the control and carvacrol treatments at several incubation times (p < 0.05). Planned contrasts showed significant differences in DM and OM degradation kinetics between the carvacrol and thymol treatments, with thymol treatments showing lower effective degradability of DM, OM, and CP at all evaluated passage rates (p < 0.05), whereas no differences in CP kinetic parameters were detected. Degradation kinetics and effective degradability were similar between the individual EO treatments providing 300 and 100 mg carvacrol or thymol/kg concentrate. Combined EO treatments showed lower effective degradability of DM and OM than the individual EO treatments (p < 0.05), whereas CP effective degradability did not differ between the combined and individual EO treatments. Overall, these findings highlight the importance of considering EO type and nutrient-specific responses when selecting EOs for targeted use in ruminant nutrition.