The objectives of this study were to evaluate the effects of sodium monensin supplied as a conventional form or as a controlled release capsule (bolus) on ruminal fermentation profile of bovine fed low nutritive value forage, supplemented or not with non-protein nitrogen. Twelve rumen-fistulated cows (736 kg of BW) were allocated in blocks, in function of body weight, using two successive 28-day periods (24 experimental units). The treatments were arranged as a factorial 3 x 2, where two forms of monensin administration (conventional or controlled release capsule), at doses of 300 mg/anim.day, were compared to the control, in combination with the supplementation or not with urea (20g/100kg BW). The low nutritive value hay (Tifton 85) was the only feed offered. Ruminal fluid was sampled on the 28th day at 0, 2, 4, 6 and 8 hours after first meal and the hay was incubated for 0, 6, 12, 24, 48, 72, and 96 hours, between the 24th and 27th day, for evaluation of in situ degradability. The only effect caused by urea administration was the increase in ruminal ammonia nitrogen concentration. Regardless on the urea supplementation, the conventional and bolus forms of monensin decreased respectively the dry matter intake in 25.4% and 27.8%, the effective degradability of hay NDF from 17.4 to 34.4% and from 11.9 to 12.7%, depending on the ruminal passage rate used, and increased the molar proportion of propionic acid in 60.2 and 42.7%, when compared to the control. However, they did not alter pH, total VFA or ammonia concentrations. Based on ruminal acetate/propionate ratio, bolus treatment showed an efficiency of 77%, when compared to conventional treatment, corresponding to liberation of 232 mg of sodium monensin/anim.day.
degradability; ionophores; ruminants; volatile fatty acids
