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Heat stress (HS) has been indicated to increase ruminal temperature, increase digestibility, and reduce ruminal pH of steers fed high concentrate diets. However, it is unclear if this effect is the result of greater fermentation rate, slower passage rate, or a combination. The effect of ruminal incubation temperature on substrate digestibility and rate of fermentation were evaluated. Four cannulated British-crossbreed steers (BW = 520 kg ± 30 kg) consuming an 87% (DM) concentrate diet were utilized as rumen fluid donors in a randomized complete block design with two incubation treatments (CON=39°C and HS=42°C). Within block, duplicate 125 mL serum bottles containing 200 mg of dietary substrate were inoculated with a 2:1 buffer:ruminal fluid mixture and incubated for 24 h to measure total gas production, fermentation rate, fermentation lag, pH, and ammonia-N concentration. In vitro organic matter digestibility (IVOMD) was measured separately in 100 mL centrifuge tubes. Data were analyzed using the MIXED procedure of SAS using the fixed effect of treatment and random effect of rumen fluid donor (block). There was no treatment effect on total gas production (P = 0.92) or fermentation rate (P = 0.11); however, HS began fermenting substrate sooner than CON (P < 0.005). There was a significant effect of treatment on IVOMD where HS was greater compared to CON (79.3 vs. 70.4%; P = 0.05). Final pH and relative pH change were not different (P ≥ 0.25) likely due to buffering capacity of the in vitro technique Ammonia-N concentration was greater for HS than CON (7.92 vs. 5.33 mM; P < 0.05) and may indicate a temperature effect on ruminal nitrogen availability.In conclusion, it does not appear that incubating at a greater temperature affected the fermentation rate but likely induced a change in fermentation kinetics, which may have contributed to the greater overall IVOMD

An experiment was performed to determine the effects of bismuth subsalicylate (BSS) and calcium-ammonium nitrate (CAN) on in vitro fermentation of a high-concentrate (87% concentrate, DM basis) substrate. Serum bottles containing 20 mL of a 2:1 buffer:ruminal fluid inoculum and 0.2 g of substrate were incubated for 24 h. Four ruminally cannulated steers (BW = 520 ± 30 kg) were used as ruminal fluid donors and each donor was considered a block. Treatments were arranged in a 2 × 2 factorial with the following factors: BSS (0 or 0.33%, DM basis) and CAN (0 or 2.22%, DM basis). Treatments were made isonitrogenous with urea. In vitro organic matter digestibility (IVOMD) was determined in separate 100-mL centrifuge tubes. Data were analyzed using the MIXED procedure of SAS with the fixed effect of BSS, CAN, BSS × CAN, and the random effect of donor. An interaction (P < 0.01) was observed for total gas production (TGP). When CAN was included, without BSS, TGP was increased (P < 0.01); however, the combination of CAN with BSS did not affect (P = 0.85) TGP when compared to the combination of urea and BSS. Ammonia-N tended (P = 0.10) to increase when CAN was used as N source rather than urea. In vitro OM digestibility (P > 0.23) and final pH (P > 0.66) of in vitro ruminal fermentation were not affected by treatments. A tendency (P = 0.06) for an interaction regarding the production of H2S was observed; however, there were no treatment mean differences (P > 0.28). The combination of CAN and BSS did not negatively affect in vitro fermentation parameters such as OM digestion and gas production; however, a reduction in H2S with the combination of BSS and CAN may indicate potential benefits of such feeding strategies for feedlot cattle