This study was aimed to test whether combination of methanogens inhibitor and a hydrogen sink may result in any complementary effects in vivo. Growing calves (15; average 7- month-old, and weighing 130 kg) were arranged into 3 groups in a randomized complete block design. Treatments were: Control (no additives), FA and AQ (control + 6.5 mM of FA and AQ at the level of 4 ppm) and MA and AQ (control + 6.5 mM of MA and 4 ppm of AQ). AQ in combination with FA or MA had no effect on final BW, ADG and daily feed intake. There was no effect of supplementation of additives on intestinal flow of microbial-N, apparent digestibility of DM, OM, CF, EE, NDF, and ADF but CP digestibility increased as a result of feeding AQ and organic acids. Nitrogen intake was similar in three groups but supplementation of diet with AQ and FA or MA decreased nitrogen losses through feces and urinary losses of nitrogen also slightly declined resulting in a nonsignificant improvement of nitrogen retention in treated groups than control. Feeding AQ and OAs resulted in a significant reduction of 9.5% methane per unit DMI in animals fed AQ and MA. It can be concluded that reduction of methane production caused by feed additives in the present study was not notable and could not improve animal performance and nutrients utilization which challenges previous in vitro findings observed in the use of organic acids and methane inhibitors in combination.

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