Comparing the effects of β1-4 galacto-oligosaccharides and l-cysteine to monensin on energy and nitrogen utilization in steers fed a very high concentrate diet

Effects of including monensin, β1-4 galacto-oligosaccharides (GOS) or l-cysteine on nutrient digestibility, methanogenesis, rumen fermentation, blood metabolites, purine derivatives as well as energy and nitrogen utilization were investigated in steers fed a very high concentrate diet. Monensin was incorporated at 30 mg/kg concentrates, whereas l-cysteine was incorporated as l-cysteine hydrochloride at 1.56 g/kg of concentrate. Four Holstein steers were assigned in a 4 × 4 Latin square design in individual metabolism stalls. Dietary treatments were (1) basal diet consisting of 200 g/kg DM mixed hay and 800 g/kg DM concentrates; (2) same as basal diet but concentrates blended with monensin; (3) concentrates blended with l-cysteine; (4) GOS to replace 200 g of concentrate. Steers were fed ad libitum twice daily to allow 100 g/kg refusals. Steers fed l-cysteine blended diets had higher (P < 0.01) acid detergent fiber (ADF) and cellulose digestibility than steers fed control diets. Ruminal pH was not affected by any treatment, although the oxidation-reduction potential (ORP) was higher (P < 0.01) in steers fed GOS containing diets. Ruminal ammonia N was lower (P < 0.05) in steers fed monensin diets versus steers fed control diets. Total volatile fatty acid (VFA) concentrations were higher (P < 0.05) in steers fed l-cysteine containing diets versus steers fed control diets due to higher (P < 0.01) acetate concentrations. Molar proportions of propionate were higher (P < 0.05) in steers fed monensin-containing diet versus steers fed the control diet. However, microbial N supply, and the efficiency of microbial N synthesis (EMNS) were lower in steers fed monensin or l-cysteine containing diets versus steers fed the control diet. Plasma glucose concentrations were higher (P < 0.05) in steers fed the monensin containing diet versus those fed the control diet. Energy retention per gross energy intake was higher (P < 0.01) in steers fed the l-cysteine containing diet. Steers fed on supplemented diets had higher (P < 0.05) retained N per N digested versus steers fed on control diets. Supplementation of l-cysteine and GOS has comparable effects on some physiological parameters, as does monensin in steers fed very high concentrate diets.