Abstract

The effects of a plant extract (broadleaf plantain, peppermint) or monensin on dry matter intake (DMI), growth performance, ruminal fermentation products, as well as morphology and the expression of genes involved in acid-base regulation of the ruminal epithelium were determined. Thirty two male Afshari finishing lambs on a high-concentrate diet were subjected to four consecutive 14-d periods on one of four treatments with supplementation of (1) no additive (control, CON), (2) 2 g blend of plant extract/(d · lamb) (PE), or (3) 30 mg monensin/(d · lamb) (MON) over all four treatment periods. The fourth group received periodical inclusion of 30 mg monensin/(d · lamb) in periods 1 and 3 with no additive in periods 2 and 4 (MON-p). Despite no significant overall effects on DMI, ruminal pH, average daily gain (ADG), ADG:DMI ratio and plasma albumin, the interactions of treatment × period were significant for these measures (P<0.01). Compared with CON, periods with decreased DMI occurred selectively in groups MON (period 1 and 2) and MON-p (period 2 and 3), a period with decreased ruminal pH occurred in PE (period 1), and periods with decreased plasma albumin occurred in groups MON (period 1) and MON-p (period 2) (all P<0.05 or lower). In period 2, MON sheep had higher ADG than PE and MON-p (P<0.05) and a higher ADG:DMI ratio than all other groups (P<0.05). While total concentrations of ruminal short chain fatty acids were unchanged, increased propionate proportions and decreased acetate proportions were observed in PE (P<0.05) and, as a trend, in MON, resulting in decreased acetate:propionate ratios in PE and MON compared to CON (P<0.05). The proportion of butyrate was decreased by MON only (P<0.05). Compared with CON, the relative mRNA abundance of Na⁺/H⁺ exchanger isoforms NHE1 and NHE3 were lower and that of monocarboxylate transporter isoform MCT1 was greater for MON (P<0.05). In MON-p animals, the mRNA abundance of NHE1 was lower and that of MCT4 was greater compared to CON (P<0.05). Morphometry results did not differ between treatments. It is concluded that monensin and the used plant extract were equally effective to increase ruminal propionate fermentation. Moreover, the study elucidated for the first time that the ruminal epithelium itself is affected by the ionophore properties of monensin and needs to adapt epithelial acid-base transporter expression.