Effect of essential oils, monensin sodium, and calcium malate on in vitro gas production, in vivo nutrient digestibility, and growth performance of finishing lambs

Rumen modifiers play a crucial role in minimizing dietary energy loss for finishing lambs. This study endeavors to assess nutrient digestibility, in vitro gas production, productive performance, and carcass characteristics in finishing lambs through the incorporation of three rumen fermentation modifiers (monensin sodium, calcium malate, and essential oils). Thirty-five four-months-old Pelibuey lambs of 23.6 kg ± 3.2 were assigned to a completely randomized block design to evaluate five diets: control (CON, without rumen modifier), monensin sodium (MON, 25 g/t); calcium malate (MAL, 2.5 kg/t), essential oils (EO, 150 g/t); and EO (150 g/t) plus MON (25 g/t). Daily feed intake, average daily weight gain, feed conversion ratio, dorsal fat thickness, rib eye area, ruminal pH, and chewing time did not differ among the diets. Notably, feed efficiency trend to be superior (P = 0.07) in the EO lambs, showing a 15.31 % and 17.28 % increase versus CON and MON diets, respectively. Dry matter intake in g/kg^0.75 was highest (P < 0.05) in MAL lambs by 23 % higher than lambs fed on diets added with EO and MON. The control diet (i.e., CON) exhibited the lowest (P < 0.05) in vivo dry matter digestibility compared to all other diets. Additionally, there was a trend (P = 0.056) towards reduced crude protein digestibility in CON diet. The inclusion of EO led to a higher (P < 0.05) proportion of ruminal acetic acid and a decrease (P < 0.05) in propionic acid versus the CON diet. The observed effects can be attributed to the antimicrobial activity of EO, specifically their secondary metabolites, which demonstrate antimicrobial properties. This underscores their potential in addressing concerns related to antibiotic use. Compared to MON, dietary inclusion with EO improves feed efficiency, with no notable effects on average daily gain, final weight, or the investigated carcass characteristics. The EO supplementation emerges as a practical alternative to antibiotic ionophore monensin for enhancing feed efficiency in finishing feedlot lambs.