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Six 300-kg steers were each fed a 70% cracked corn plus corn silage diet at two levels of intake above maintenance with and without 3 mg monensin/kg.75 body weight (W.75). A changeover design was used. Dietary energy and nitrogen were partitioned by duplicate, 22-h indirect respiration calorimetry measurements of heat and methane production on each animal concurrent with 7-d total feces and urine collection trials. Feed and feces were analyzed for energy, N, starch and neutral detergent fiber (NDF). The partial efficiency and(or) heat increment of metabolizable energy (ME) used for gain and maintenance was partitioned by regression of energy storage vs intake. At equalized gross energy intakes (adjustments made by covariance), monensin improved (P less than .01) the apparent digestibilities of energy, from 71.8 to 74.8%; NDF, from 50.5 to 57.5%, and crude protein, from 61.6 to 65.8%. Methane production averaged 5.7% of the control gross energy (GE), but was reduced by 26% (P less than .01) by monensin additions. Metabolizable energy was increased (P less than .01) from 63.3 to 66.8% of gross energy intake by monensin, resulting in an increase (P less than .01) in retained energy (64.7 to 72.3 kcal/W.75). Heat production was increased (P less than .05) by monensin, but only in proportion to the increased ME. Daily energy retentions adjusted to equal ME intake were similar with or without monensin (69.9 vs 67.1 kcal/W.75). No significant effects of monensin on the heat increment of ME use for maintenance or gain were observed. Both NEm and NEg were improved by approximately 7% by monensin; however, the relative effect on NEm vs NEg depends on method of calculation. At equalized GE intakes, approximately one-third of the improved energy utilization was explained by the reduced methanogenesis caused by monensin and two-thirds by reduced fecal losses.

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