Quantitative analysis of microbial fermentation under normal or high ruminal temperature in continuous cultures1

A dual-flow, continuous-culture system was used to investigate effects of ruminal temperature and forage-to-concentrate ratio in lactation dairy diets on in vitro fermentation characteristics. The experiment was performed in a 2 × 2 factorial design (n = 4). Diets were formulated to maintain different forage-to-concentrate ratios (60:40 or 40:60, DM basis) in the high-forage (HF) or the low-forage (LF) diet, respectively. Four treatments were tested: HF under normal ruminal temperature (NRT; 39°C), LF under NRT, HF under high ruminal temperature (HRT; 41°C), and LF under HRT. Each independent run lasted 11 d (8 d of treatment adaptation and 3 d of data and sample collection). The temperature of the HRT treatment was chosen to mimic ruminal fermentative environment when cows are under heat stress. The HRT increased (P < 0.01) culture pH from 5.73 to 5.82 on average. Total VFA concentration decreased (P = 0.05) in the HRT compared with the NRT; however, ruminal temperature did not affect molar proportion of VFA. Feeding the LF diet increased DM, OM, and NDF digestibilities regardless of fermentative temperature. Digestibilities of DM and NDF were not affected by ruminal temperature, whereas the HRT tended to decrease (P = 0.14) OM digestibility compared with the NRT (66.6 vs. 67.4%). However, the increases in DM and OM digestibilities due to feeding the LF diet were less in the HRT than in the NRT, indicated by the fermentative temperature and diet interactions (P < 0.04). The HRT increased (P < 0.01) methane production and ammonia-N concentration. The HRT treatment also increased the concentration of ruminal C18:0 (P = 0.02) but decreased that of C18:1 trans-11 (P = 0.02). Overall results in this study suggest that during HRT as experienced by cows under heat stress, nutrient digestion, energy utilization, and microbial protein synthesis are altered.