A Meta-Analytical Approach to Evaluate the Relative Effectiveness of Virginiamycin for Veal Calf Performance Is Maximized after Ten Weeks of Administration

The objective of this analysis was to evaluate the growth performance and feed conversion ratio (FCR) of veal calves receiving 80 mg/kg DM of virginiamycin (Vmax) alone or in combination with 50 mg/kg DM of furazolidone for up to 18 wks of life. Eight studies that were conducted in the Netherlands (five studies, n = 901 calves), Mexico (one study, n = 47 calves), and Czech Republic (two studies, n = 30 calves) evaluated the impact of different timing and length of administration of treatments (TRT). The statistical model had TRT as the fixed effect and the period of administration of the TRT (PHASE: initial, final, whole) within studies as the random effect. The number of calves in the pens was used as a weight variable in the PROC GLIMMIX of SAS. For multiple comparisons, the LSMEANS and PDIFF statements were used, and different subscript letters indicate statistical difference at P < 0.05. The unadjusted ADG for Vmax plus furazolidone, and Vmax alone were 40 and 32.8% greater than the control group (1.084, 1.028, and 0.774 kg/d, respectively). When ADG was adjusted for the effect of the random variable phase within the study, Vmax plus furazolidone and Vmax alone had greater ADG than the control group (1.045^a, 1.026^a, 0.918^b; respectively, P < 0.05). When we analyzed the ADG adjustment for phase within study, relative to the average ADG, the relative adjustments for ADG during the early period of medication (initial phase; less than 10 wks) were negative (down to -0.4 kg/d) whereas for later period of medication (final phase; greater than 10 wks) they were positive (up to 0.3 kg/d). When we regressed the relative adjustment of ADG for phase within study on length of medication, using the lm of R, the quadratic response (R² = 0.5; N = 16; P = 0.0198) indicated the maximum relative adjustment of ADG occurred at 14 wks of medication with Vmax alone, though greater ADG adjustments were observed between 10 and 12.5 wks of medication. There was no TRT effect on FCR (P = 0.3736), and veal calves that received Vmax had greater feed intake than control ones (1.56 versus 1.46 kg/d; respectively; P < 0.001). Vmax alone or in combination with furazolidone enhanced ADG of veal calves. Longer administration of the medication (at least 10 wks) seems to be the beneficial, with a maximum increase on ADG at 14 wks of medication.