The effect of sulfur in cattle diets with varying cation-anion balances upon feedlot performance, blood chemistry and body fluid compartments.

Feeding higher levels of corn distillers grains with solubles (DGS) increases sulfur intake by animals. Excess sulfur in beef finishing diets has been associated with reduced dry matter intake (DMI), body weight (BW) gain, and a negative dietary cation-anion difference (DCAD). The objectives of this work were to determine if adding cations to cattle finishing diets high in sulfur could improve performance and to measure physiological responses in cattle to high sulfur intake with a negative or positive DCAD. The potential of supplemental cations to increase DCAD and improve performance of cattle consuming finishing diets containing modified distillers with solubles (MDGS) was investigated. In a completely randomized block design, 186 crossbred steers (342 kg ±4.80) were blocked by weight, allotted to 28 pens, and assigned to one of seven treatments with 3 levels of MDGS (25, 50, 75% of DM) supplemented or not with added cations. A Control diet contained no MDGS. Steers were fed to appetite twice daily (0800 and 1500 hr). On d 56, blood samples were collected prior to the morning feeding from each steer and analyzed for components. Steers were harvested at d 170 of the trial. Increasing DCAD with added cations reduced the metabolic acid load of steers as evidenced by reduced blood Cl and increased bicarbonate concentrations, but had no affect upon DMI, BW gain, or carcass characteristics. Steers fed MDGS did not differ compared with Control steers in ADG, DMI or G:F. Increasing the level of MDGS in diets resulted in a linear decrease in ADG above 25%, a quadratic decrease in DMI, and a trend towards a quadratic increase in F:G. Supplementation of finishing diets containing MDGS with sodium and potassium to increase DCAD is not an economical management tool to improve performance. Cattle fed diets with moderately negative DCAD caused by sulfur in MDGS are able to buffer the added acid load without loss of performance. Reductions in ADG and DMI with increasing level of MDGS are not due to reductions in DCAD. In a second trial, the effects of feeding negative DCAD and high S diets to steers upon plasma 3-methylhistidine (3MH) and body fluid compartments were measured. In a replicated 4 X 4 Latin Square design, 8 steers were blocked into two weight groups and randomly assigned to one of four diets that consisted of 1) a Control diet (58 mEq*kg-1 of DM) 2) High Cl diet (-108 mEq*kg-1 of DM) with added NH4Cl 3) High S diet (-130 mEq*kg-1 of DM) with added (NH4)2SO4 and 4) Pos DCAD High S diet (27 mEq*kg-1 of DM) with added (NH4)2SO4 plus NaHCO3 and K2CO3. On the final day of each period, d 10, a solution of markers (urea, sodium thiosulfate, and Evans Blue dye) was injected into each steer for determination of body fluid compartments. Steers fed negative DCAD diets had greater plasma 3MH compared with Control steers, indicating they had greater fractional protein catabolism. Steers fed negative DCAD with High Cl, but not those fed High S had reduced intracellular fluid volume compared with Control. Adding supplemental cations to High S diets to create positive DCAD did not mitigate the increase in 3MH (protein catabolism), suggesting another mechanism by which high diet S effects protein turnover in growing steers