The Fermentative and Nutritional Effects of Limonene and a Cinnamaldehyde–Carvacrol Blend on Total Mixed Ration Silages

This study evaluated the effects of different doses of limonene essential oil (LEO) and a blend of cinnamaldehyde and carvacrol (BCC) on the fermentative quality and chemical–bromatological composition of total mixed ration (TMR) silages. Two independent trials were conducted, each focused on one additive, using a completely randomized design with four treatments (0, 200, 400, and 600 mg/kg of dry matter), replicated across two seasons (summer and autumn), with five replicates per treatment per season. The silages were assessed for their chemical composition, fermentation profile, aerobic stability (AS), and storage losses. In the LEO trial, the dry matter (DM) content increased significantly by 0.047% for each mg/kg added. Dry matter recovery (DMR) peaked at 97.9% at 473 mg/kg (p < 0.01), while lactic acid (LA) production reached 5.87% DM at 456 mg/kg. Ethanol concentrations decreased to 0.13% DM at 392 mg/kg (p = 0.04). The highest AS value (114 h) was observed at 203.7 mg/kg, but AS declined slightly at the highest LEO dose (600 mg/kg). No significant effects were observed for the pH, neutral detergent fiber (NDF), acid detergent fiber (ADF), crude protein (CP), or non-fiber carbohydrates (NFCs). In the BCC trial, DMR reached 98.2% at 548 mg/kg (p < 0.001), and effluent losses decreased by approximately 20 kg/ton DM. LA production peaked at 6.41% DM at 412 mg/kg (p < 0.001), and AS reached 131 h at 359 mg/kg. BCC increased NDF (from 23.27% to 27.73%) and ADF (from 35.13% to 41.20%) linearly, while NFCs and the total digestible nutrients (TDN) decreased by 0.0007% and 0.039% per mg of BCC, respectively. In conclusion, both additives improved the fermentation efficiency by increasing LA and reducing losses. LEO was more effective for DM retention and ethanol reduction, while BCC improved DMR and AS, with distinct effects on fiber and energy fractions.