202 Dietary supplementation with a blend of essential oils and a proprietary cobalt source along monensin may be antagonistic to growth performance of Holstein yearling bulls

Abstract Our previous work has indicated that feeding a blend of essential oils and a proprietary cobalt source (EOC) in combination with monensin (MON) may not be mutually beneficial to dairy calf growth performance. To evaluate this observation further, a 240 d long-term growth experiment was conducted using 12 young growing Holstein bulls using a 2 × 2 factorial treatment arrangement. Main factors were EOC (Rum-A-Fresh, Ralco, Inc., Marshall, MN) and monensin (MON; Norvikon Biotechnology Co., Hunan, China) arranged in 4 treatments: 1) Control: ration fed without EOC or MON; 2) EOC: EOC fed at 26 mg/kg DM; 3) MON: MON fed at 25 mg/kg DM; and 4) EOC+MON: EOC and MON fed in combination. Holstein bulls were 70 d of age and similar in BW (93.3 ± 4.54 kg) and individually fed for 280 d. Body weights, frame measurements, and blood samples were collected monthly. Interactions of EOC by MON were detected for BW, BW gain, ADG, and a trend for feed conversion. Bulls fed EOC or MON demonstrated greater final BW (367.7, 385.2, 381.2, and 357.5 kg for Control, EOC, MON, and EOC+MON, respectively), while BW gains (277.2, 291.5, 284.8, and 265.2 kg) and ADG (1.16, 1.22, 1.19, 1.11 kg/d) were greatest for bulls fed EOC or MON compared with bulls fed EOC+MON, with bulls fed Control being intermediate and similar to bulls fed MON. Intake of DM was greater for bulls fed EOC (6.55, 6.99, 6.60, and 6.42 kg/d) compared with bulls fed remaining treatments. Feeding EOC or MON separately can improve growth performance of growing Holstein bulls. Why the combination of EOC and MON is antagonistic to growth performance of Holstein bulls is unknown.