Effects of l-cysteine supplementation on growth performance and ruminal fermentation in growing steers in comparison to monensin

A study was conducted to examine the effects of l-cysteine on growth and ruminal fermentation in growing steers. Nine Holstein steers of 7 months old were allotted to three treatments, and were fed a concentrate corresponding to Control (C), Control supplemented with l-cysteine (CYS) or supplemented with monensin (M), respectively. Cows were allowed free access to hay. Feed intake was measured weekly. Cows were weighed and bled monthly. Rumen fluid was collected at the 10th and 18th month of age. All cows were slaughtered at the end of the study for carcass evaluation. Average daily gain during the experimental period was not significantly different among treatments, but numerically higher for CYS and M than for C (1.18, 1.27 and 1.27 kg/day for C, CYS and M, respectively). Feed conversion rate did not differ significantly but tended to be lower for CYS and M than for C (7.84, 7.40 and 7.46 for C, CYS and M, respectively). There was no significant difference in carcass characteristics. CYS showed significantly higher acetate and lower propionate concentration in the rumen than C at 18th month of age while the effect was in reverse in M. In conclusion, neither CYS nor M had significant effects on growth performance in this trial. However, both additives modified ruminal fermentation characteristics, though the mechanism seemed to be different for each additive.