Sulfur (S) is a requisite part of the ruminant diet, but may sometimes be present in excess, leading to decreased animal performance and illness. In this experiment, 96 crossbred yearling steers (321 y 29 kg BW) were used to determine the effects of dietary S concentration of a supplement fed to cattle on pasture and of the total diet in the finishing period. Steers were blocked by weight and allocated to smooth bromegrass-based pastures and supplemented with either a low S dried distillers grains with solubles (DDGS; 0.3% total dietary S; LS; n = 4 plots) or LS DDGS with additional S (0.45% total dietary S; HS; n = 4 plots). After 36 d on pasture, steers moved to the feedlot where half remained on the previous S treatment while half switched treatments, resulting in 4 treatments (LS-LS; LS-HS; HS-LS; and HS-HS; n = 6 feedlot pens). Steers were transitioned to the final finishing diet using a series of step up diets. Once fully transitioned, steers were receiving either 0.3% dietary S (LS) or 0.6% dietary S (HS). Plasma and liver mineral concentrations of half of the steers were determined at beginning of the trial (d 0), the end of the pasture period (d 35), and near the end of study (d 155). Plasma Mg was decreased (P = 0.05) in response to increased dietary S during the pasture period, however steers were not deficient. Copper concentrations in the plasma and liver were decreased (P < 0.05) in steers which had received increased dietary S when measured at the end of study (d 155). Average daily gain of steers did not differ (P = 0.50) between LS and HS during the pasture period, but was lower (P = 0.01) in HS vs. LS steers during the finishing period. Decreased gains translated to a tendency (P = 0.06) for hot carcass weight to be decreased in HS steers vs. LS steers. Fatty acid analysis of longissimus dorsi showed increased stearic and heptadecanoic acid (P = 0.04 and 0.01, respectively) in steers receiving increased dietary S. Cattle exposed to higher S diets during the pasture period did not show differences (P = 0.40) in performance based on the previous pasture treatment. However, cattle fed high dietary S on pasture had greater carcass fat cover (P = 0.01), suggesting S may have influenced lipid metabolism.

Rumen H₂S concentration was determined 6 hr post-feeding at multiple time points throughout transition and finishing (d 36, 45, 52, 59, 66, 91,125, and 155). Concentrations of rumen H₂S did not differ while cattle were consuming a high forage diet (1733 and 1400 ppm for HS and LS, respectively; P = 0.54). During the feedlot period, rumen H₂S concentration was affected by day of sampling (P < 0.01). Rumen H₂S began to differ due to treatment when forage in the diet decreased to 15% of diet DM (P = 0.04). This increase in H₂S in HS cattle was sustained throughout the finishing period (P < 0.05) when forage was present at 8% of DM. Peak H₂S concentrations in HS steers were observed on d 91 when steers had been on the full finishing diet with 8% forage for 23 days (HS: 4813 ppm; LS: 1317 ppm; P < 0.01) and again near the end of trial (d 155; HS: 5275 ppm; LS 1248 ppm; P < 0.01). Rumen fluid was collected esophageally for determination of VFA and lactate proportions at the end of the pasture period (d 36), during transition (d 56) and near the end of study (d 155). Lactate proportion was not affected (P > 0.50) by dietary S concentration and minimal effects on VFA profiles were observed. Isovalerate was decreased (P = 0.002) in cattle receiving elevated dietary S on d 155. Rumen fluid samples collected near the end of study (d 155) from steers which represented treatments maintained on the same S treatment throughout the study (LS-LS and HS-HS, n = 6 pens per treatment) were analyzed for sulfate reducing bacteria (SRB) populations and methanogens using quantitative real time PCR. Steers receiving the HS diet demonstrated an increase (P = 0.03) in total SRB and an increase (P = 0.03) in a SRB group which represents the SRB most commonly found in the rumen, specifically Desulfovibrio desulfuricans . The simple correlation between SRB populations and H₂S concentrations (R = 0.60, P = 0.05) illustrates that a considerable portion of the variation in H₂S concentrations among cattle may be accounted for by the differences in SRB populations. These combined effects of dietary S concentration on rumen H₂S concentration, VFA proportions, and microbial populations illustrate that rumen metabolism is altered by increased dietary S. Furthermore, these effects on rumen metabolism may partially explain the decreases in performance, evidenced by reduced gains, and HCW, while part of the decrease in performance may also be explained by effects of increased dietary S on mineral status, especially Cu.