105 Effects of Monensin on Intake, Growth Performance, and Health of Newly-Received Feedlot Calves

Abstract Monensin is an ionophore commonly fed to feedlot cattle to increase feed efficiency (gain:feed ratio; G:F) typically by decreasing dry matter intake (DMI) when fed at the commonly adopted dose of 33 mg∙kg DM-1. During the receiving period when intake is often suppressed due to management stressors, feeding monensin to light-weight feedlot calves may be detrimental to DMI and consequently to growth performance. The objective of this experiment was to evaluate the effects of feeding sodium monensin to newly-received feedlot calves. A total of 380 crossbred beef steers [initial body weight (BW) = 231 ± 25 kg and approximately 8 months of age] were sourced from local auctions in Delhi, LA and transported approximately 16 h to Clayton, NM. Upon arrival, steers were processed, blocked by off-truck shrunk BW, randomly assigned to 20 pens (19 animal∙pen-1), and pens were then randomly assigned to one of two dietary treatments (n = 10 pens/treatment): control (CON; no feed additive) or sodium monensin at 170 mg∙animal-1∙day-1 (MON; Rumensin 90; Elanco Animal Health, Greenfield, IN). The basal diet consisted of a complete starter feed composed predominantly of wet corn gluten feed (RAMP; Cargill Sweet Bran, Dalhart, TX). The experiment was 56 days in length. The amount of feed offered to each pen was adjusted based on the DMI of the previous day and bunks were managed to contain trace amounts of feed at 0600 h. Body weight was recorded on days 0, 14, 28, and 56 for average daily gain (ADG) and G:F calculation. Dietary net energy for maintenance (NEm) and gain (NEg) were estimated based on observed growth performance. Daily animal health evaluations were performed using a 4-point scale method based on depression, appetite, respiration, and temperature throughout the experiment. All data were analyzed as a randomized complete block design using SAS with pen serving as the experimental unit. Feeding MON did not affect feed intake (P = 0.58) compared with CON, yet ADG was increased (P < 0.01) by 12.5% in steers fed MON compared with CON. Thus, there was a 10.7% increase (P < 0.01) in G:F for MON, which corresponds with a greater (P = 0.02) observed NEm and NEg for MON compared with CON. Feeding MON tended (P = 0.07) to elicit a 2.9% increase in final BW compared with CON. Additionally, there was no treatment effect (P ≥ 0.57) on morbidity based on the number of therapeutic treatments among calves which were treated for indications of bovine respiratory disease. Moreover, treatments did not affect mortality rate. In summary, feeding monensin at 170 mg∙animal-1∙d-1 (26 mg∙kg DM-1) increases growth performance of light-weight, newly-received feedlot cattle without influencing DMI or animal health.