Efecto del uso de aditivos sobre el metabolismo ruminal y la utilización digestiva de alimentos utilizados para producción de leche

In milk production systems the use of additives, mainly monensin, is common. Currently, its use is questioned due to the emergence of resistant bacterial strains and the possibility of residues in products for human consumption. Research has been developed in search of alternatives on stabled and in vitro systems, but not in grazing systems as in Uruguay. The objective of this work was to determine the effect of plant-derived bioactive compounds on the rumen utilization of nutrients from different foods (A): pastures and mixed diets, in relation to monensin and a control. Nine Holando-American cows, with 618 ± 13.5 kg of LW, with rumen cannula; were randomly assigned to one of three treatments (T): Control (TC; no additive), Monensin (Tmon; 300 mg/cow/day) and Bioactive extracts derived from plants (TexV; 50 x103 mg/cow/day). In the morning, they grazed alfalfa (AA) with an allocation of 30kg DM/a/d and in the afternoon they received a partial mixed diet (PMD) that contained or did not the additive in the concentrate. Between 50-60 days of lactation, using the in situ technique proposed by Orskov et al. (1980), the degradation kinetics of dry matter (DM), organic matter (OM) and neutral detergent fiber (NDF) of alfalfa hay (HAAE) and the digestibility of HAAE, AA, Chicory (Ach), Fescue (Fest), Oats (AV), Total mixed diet (TMD) were determined, PMD and PMD+AA. For 2 consecutive days, rumen fluid sampling was carried out before access to feeding and at 4, 8, 16, 24 h post-feeding to measure pH, NH₃, VFA and protozoa. Kinetic and digestibility data were analyzed with a randomized design using the MIXED procedure, the model included the T, A effect and their interaction as fixed effects and cow as random effect. For rumen parameters; a repeated measures model including T, h and their interactions as fixed effects and cow as random effect was used. Regarding the degradation kinetics of DM and OM, additives do not affect the degradation rate of alfalfa hay (p>0.05), Tmon has a higher soluble fraction, but a potentially degradable insoluble fraction lower than TexV and TC (p<0.05). The potential degradability of neutral detergent fiber (NDF) is not affected by additives, but reduced the rate of degradation of NDF with respect to TC (p<0.05). The in situ digestibility of food at 48 hours was not affected by the treatments, but treatment x food interaction was detected (p<0.0001), only PMD reduces its digestibility before the use of additives (p>0.05). Tmon and TexV reduce the concentration of NH₃ and protozoa (p<0.05). The reduction of protozoa and the decrease in the rate of fiber utilization, caused by additives, could be enhanced in DTMs that use more fibrous foods. Lower ammonium concentration, better acetic:propionic ratio and stability of the rumen environment are encouraging results regarding better efficiency of feed utilization in the rumen.