This research was designed to study the long-term effects of feeding a high corn reduced-fat distillers grains (RFDG) on long-term performance, and nutrient excretion and utilization of dairy cattle and if the addition of monensin to a high-RFDG diet affected performance and nutrient excretion. To assess these objectives, 36 mid-lactation Holstein cows were fed either a control (CON), a 28.8% RFDG diet (DM basis) replacing SBM, soyhulls, and supplemental fat and calcium phosphate (DG), or the DG diet with monensin at 20 mg/kg of DM (DGMon). The experiment was conducted for 11 weeks during which dry matter intake (DMI); milk yield, composition, and fatty acid profile; fecal, and urinary excretion and milk secretion of nitrogen (N), phosphorus (P) and sulfur (S) were measured.
For the production portion of the experiment, RFDG diets (i.e., DG + DGMon) vs. CON did not affect milk yield (40.3 vs. 40.8 kg/d), but decreased DMI (24.9 vs. 26.4 kg/d), milk fat yield (1.12 vs.1.55 kg/d), milk protein yield (1.24 vs. 1.32 kg/d), and energy-corrected milk yield (37.7 vs. 43.5 kg/d). Adding monensin to the DG diet (i.e., DGMon vs. DG) did not affect DMI (24.4 vs. 25.4 kg/d) and milk yield (39.2 vs. 41.3 kg/d), but decreased milk fat yield (1.08 vs. 1.23 kg/d), milk protein yield (1.20 vs. 1.28 kg/d), and energy-corrected milk yield (36.0 vs. 39.4 kg/d). Additionally, difference in DMI between treatments increased as the experiment progressed and tended to be different by the conclusion of the trial (27.8, 25.4, and 23.4 kg/d for CON, DG and DGMon, respectively). Furthermore, milk fat yield and energy-corrected milk yield decreased for DG diets vs. CON as the experiment progressed. For milk fatty acid profile, feeding DG and DGMon compared to CON resulted in an increased concentration of trans-10, cis-12 18:2, trans-10 18:2, long-chain (> 16C), and polyunsaturated fatty acids, and a decrease in short-chain (<16C) and most odd- and branched-chain fatty acids compared with CON. Milk fatty acid profile for DGMon did not differ from DG.
For the nutrient excretion portion of the experiment, RDP (9.9 vs. 10.7%) decreased whereas dietary P (0.48 vs. 0.36%) and S content (0.41 vs. 0.21%; DM basis) increased in the DG diets compared to CON. This resulted in decreased urinary N excretion (255 vs. 283 g/d) and increased fecal P (66.7 vs. 46.9 g/d), urinary P (5.4 vs. 0.3 g/d), fecal S (25.7 vs. 22.7 g/d), and urinary S (57.5 vs. 16.1 g/d) excretion for DG diets vs. CON. Adding monensin to the DG diet (i.e., DGMon vs. DG) did not affect most N, P, and S utilization and excretion measurements except that N digestibility increased (74.0 vs. 72.3 %).
Feeding a high-RFDG negatively affected long-term performance of dairy cattle via decreased DMI, milk fat and milk protein yield. The addition of monensin to a high-RFDG diet decreased milk fat and protein yield and long-term DMI. As expected RFDG increased P and S excretion, however, feeding RFDG instead of SBM decreased urinary N excretion. Additionally, monensin did not improve nutrient utilization or decrease nutrient excretion. This discrepancy with expectation could be a consequence of the negative affect monensin had on performance.