Increasing dietary neutral detergent fiber concentration decreases ruminal hydrogen sulfide concentrations in steers fed high-sulfur diets based on ethanol coproducts.

Cattle feedlot diets commonly contain ethanol coproducts that are high in S. This dietary S is reduced in the rumen by sulfate reducing bacteria, resulting in an accumulation of hydrogen sulfide (H2S), increasing the risk for S toxicity. A negative correlation between H2S and ruminal pH has been observed previously. The objective of this study was to determine the effect of varying dietary NDF from chopped bromegrass hay (66% NDF) on performance, ruminal pH, and ruminal H2S gas concentration of steers fed a high-S finishing diet. One hundred fifty crossbred steers (359 ± 51 kg BW) were blocked by BW into pens of 5 steers and randomly assigned within block to 1 of 5 treatments (n = 6 pens per treatment) and fed for 84 d. Dietary treatments included 3.5, 5.7, 7.9, 10.1, or 11.4% roughage NDF (rNDF) from bromegrass hay and contained 0.46% dietary S from a combination of dried distillers grains with solubles and condensed corn distillers solubles. In all diets, hay was added at the expense of dry-rolled corn. Effective NDF increased linearly (P < 0.01) with increased inclusion of rNDF. Final BW was not affected by rNDF (P ≥ 0.12). The addition of roughage did not affect ADG (P ≥ 0.13) or gain efficiency (P ≥ 0.12). Dry matter intake increased linearly (P < 0.01) as rNDF concentration increased. There was a treatment × month interaction for S intake (P < 0.01), explained by steers fed 3.5 or 11.4% rNDF increasing S intake each month whereas the middle rNDF inclusions had similar S intake between months 1 and 2 and increased in month 3. Ruminal H2S concentrations and ruminal fluid pH were measured at 6 h postfeeding on d 7, 14, 21, 29, and 84. Ruminal pH increased linearly (P < 0.01; 5.48, 5.61, 5.71, 5.74, and 5.80 ± 0.041 for 3.5, 5.7, 7.9, 10.1, and 11.4% rNDF, respectively) and ruminal H2S concentrations decreased linearly (P < 0.01; 1.00, 0.86, 0.76, 0.70, and 0.62 ± 0.037 g/m(3) for 3.5, 5.7, 7.9, 10.1, and 11.4% rNDF, respectively) as rNDF inclusion increased. Using mixed model regression analysis, ruminal pH had a strong negative relationship with ruminal H2S concentrations (ß = -0.63; P < 0.01). Under conditions of this study, increasing roughage did not affect cattle gains but helped maintain greater ruminal pH and decreased H2S concentration, suggesting that this dietary strategy may lessen the risk of S toxicity in feedlot cattle.

Determining the influence of dietary roughage concentration and source on ruminal parameters related to sulfur toxicity.

Cattle feedlot diets often include ethanol coproducts that provide excess dietary sulfate, which is reduced to sulfide by ruminal bacteria and can be converted to hydrogen sulfide, which has been correlated to S toxicity. The objective of this study was to determine the impact of feeding varying concentrations of NDF from chopped cornstalks (CS) or chopped bromegrass hay (BH) on ruminal pH, ruminal H2S concentration, and DMI of steers fed a high-S finishing diet. Five ruminally fistulated steers (595 ± 87 kg BW) were used in a 6 × 6 Latin square with 14-d periods and fed diets containing 0.45% S from a mixture of dried distillers grains and condensed corn distillers solubles. The study was a 2 × 3 factorial arrangement of treatments with 2 roughage sources--CS or BH--and 3 concentrations of added roughage NDF (rNDF)--4, 7, or 10%. Steers had individual ad libitum access to feed and adapted to each diet for the first 7 d of each period. Effective NDF linearly increased (P < 0.01) as rNDF increased and did not differ between sources (P = 0.44). There was no effect of concentration or source of rNDF on DMI (P ≥ 0.69). Steer behavior was observed on d 13 of each period for 3 h postfeeding. Source of rNDF did not affect time at bunk, DMI during observation, or rate of DMI (P ≥ 0.42). Time at bunk linearly increased as rNDF increased (P = 0.01), while rate of DMI linearly decreased (P = 0.02). Area under the curve for ruminal pH of 5.4, 5.6, and 5.8, calculated using data from d 8 to 14 via an indwelling ruminal bolus, were linearly decreased (P ≤ 0.03) as rNDF increased. Manual ruminal pH taken 6 h postfeeding on d 14 of each period did not differ by source (P = 0.12) but linearly increased (P < 0.01) as rNDF increased. Ruminal H2S concentrations measured 6 h postfeeding on d 14 of each period did not differ by source (P = 0.47) but linearly decreased (P < 0.01) as rNDF increased (0.62, 0.35, 0.31 g/m(3) for 4, 7, and 10% rNDF, respectively). A segmented linear model was found to best fit the ruminal pH and H2S relationship data, suggesting that at or below a pH of 5.6 ± 0.08 with 95% confidence intervals of 5.4 and 5.8, pH is a strong predictor of H2S (P ≤ 0.05), while above this pH range H2S concentrations are not well correlated with ruminal pH (P > 0.50). In conclusion, adding at least 7% NDF from CS or BH to high-S feedlot cattle diets will increase ruminal pH and decrease H2S concentrations, thus decreasing potential for S toxicity.