Effects of dietary addition of zinc and(or) monensin on performance, rumen fermentation and digesta kinetics in beef cattle

Two experiments (Exp1 and Exp2) were conducted to investigate the effect of dietary inclusion of Zn and(or) monensin on performance and rumen fermentation in beef cattle fed a barley grain, alfalfa hay and sunflower meal. In Exp1, 20 calves were assigned to one of the following treatments: CON = control; Z = 430 mg Zn kg(-1) of DM; M = 35 mg monensin kg(-1) of DM; and ZM=both Zn and monensin. Dry matter intake (DMI), DM digestibility (DMD), average daily gain (ADG), and feed to gain (F/G) ratio were determined. Blood analyses included hematocrit, glucose, urea, total protein, alkaline phosphatase and creatinine. In Exp2. the rumen fluid of four cannulated steers, in identical treatments. was studied for Zn concentration, pH, NH3-N, VFA. Rumen dilation rate. turnover time, and volume: in situ DM disappearance of barley and alfalfa were also determined. In Exp1, no treatment response was observed for DMI, ADG or DMD (P>0.10). Differences in F/G were not significant. despite a numeric F/G decrease in M (6.6 kg kg(-1)). No differences were detected in blood variables. In Exp2, an interaction of treatment x time (P = 0.0174) for Zn concentration was detected, where ZM, followed by Z, had highest mean values at all time intervals. Ruminal parameters, kinetics or DM degradability were not modified by treatments; pH reached the lowest value (6.1; P<0.05) 12 h after-supplement feeding. Overall, supplying more than 20 times the Zn requirement has not substantially affected performance or digestion. Additional key words: additives, blood parameters, intake, rumen kinetics. ruminants, weight gain.