In vitro effects of Monensin and Tween 80 on ruminal fermentation of barley grain:barley silage-based diets for beef cattle

Abstract

Two in vitro incubations utilizing batch rumen culture procedures were conducted to study the effects of Monensin and Tween 80, alone and in combination, on ruminal fermentation. Dried, ground backgrounding and finishing diets for feedlot cattle were used as substrates in experiments 1 and 2, respectively. The diets, formulated as total mixed rations, contained (DM basis) barley grain and barley silage in ratios of 58:42 (backgrounding diet, BKGD) or 93:7 (finishing diet, FNSH). Inoculum comprised ruminal fluid collected from cattle fed BKGD or FNSH diluted with two volumes of mineral buffer, and amended with Monensin (0, 2.5 or 15 μg/ml) and Tween 80 (0, 0.1 or 0.5 μl/ml) in a 3 × 3 factorial arrangement of treatments. Incubations were conducted anaerobically for 4, 12 or 24 h at 39 °C ( n = 3). 15N-labelled ammonium sulphate was included to enable measurement of microbial N production (MN). Interactive effects ( P < 0.001) of Monensin and Tween 80 on acetate:propionate (A:P) ratios at 4 and 12 h of incubation, on MN at 4 and 24 h, and on accumulation of reducing sugars (RS) at 4, 12 and 24 h were observed with BKGD, but not with FNSH. Tween 80 applied at 0.5 μl/ml increased ( P < 0.05) in vitro true dry matter disappearance (TDMD) and RS accumulation at 4 h and MN at 24 h, but reduced ( P < 0.05) A:P ratio at 12 h of incubation with BKGD. Similar effects of Tween 80 on RS and A:P ratio were observed with FNSH. In contrast, throughout 24 h of incubation with either substrate, Monensin reduced gas production (GP), TDMD, MN and the A:P ratio, and increased RS accumulation (BKGD, P < 0.01; FNSH, P < 0.001). These effects on A:P and RS were enhanced ( P < 0.001) if Monensin and Tween 80 were supplemented together. Amylase, carboxymethylcellulase and β-glucanase activities in the incubation liquid were increased ( P < 0.01) by Monensin and Tween 80 supplemented in combination with both diets; xylanase activity was increased ( P < 0.001) with BKGD only. Effects on enzyme activity when Monensin and Tween 80 were supplemented individually were diet-associated, but interactive effects of Monensin × Tween 80 on each of the four activities were observed both with BKGD ( P < 0.05–0.001) and with FNSH ( P < 0.01). Supplementing Tween 80 with Monensin synergistically enhanced the Monensin-mediated increase in RS concentration and reduction of A:P ratio in the incubation mixtures. This effect may have potential to improve energy efficiency in feedlot cattle.