CONICET | Buscador de Institutos y Recursos Humanos

The objective was to investigate the effect of dietary inclusion of Zn and(or) monensin on animal performance and rumen fermentation in beef cattle in two experiments (Exp1 and Exp2). In Exp1, 20 calves were fed ad libitum twice a day cracked barley grain, chopped alfalfa hay, sunflower meal and a supplement with or without Zn and monensin (added to the morning meal). The treatments were: CON= control; Z= 430 mg Zn kg-1 of DM; M= 35 mg monensin kg-1 of DM and ZM= both Zn and monensin, in a completely randomized design. Dry matter intake (DMI), DM digestibility (DMD), average daily gain (ADG) and feed/gain ratio (F/G) over a 77-d period were determined. Blood analyses included hematocrit, glucose, urea, total protein, alkaline phosphatase and creatinine. In Exp2, four ruminally cannulated steers were assigned to the same treatments in a 4 x 4 Latin square. Ruminal measurements included: Zn concentration, pH, NH3-N, VFA, ruminal fluid dilution rate (FDR), turnover time (FTT), volume (RFV) and in situ DM disappearance from barley and alfalfa. In Exp1 no treatment response was observed for DMI, ADG and DMD (P> 0.10). Despite a numeric F/G decrease for M (6.6 kg kg-1) differences were no significant for F/G. No differences were detected in blood variables (P> 0.10). In Exp2, an interaction of treatment x time (P= 0.0174) was found for Zn concentration, with highest mean values for ZM followed by Z at all time intervals. Ruminal pH, NH3-N or VFA were not modified by treatments; pH reached the lowest value (6.1; P< 0.05) 12 h after the supplement was fed, which corresponded to the highest total VFA concentration. Treatments did not alter FDR, FTT and FRV (P> 0.10) and in situ disappearance differences were only found in the alfalfa potentially degradable fraction (P= 0.0038) but not barley (P= 0.0653). Overall, treatments with Zn addition at more than 20 times the animal requirement did not affect animal performance or digestion measurements.