Effect of pH on in vitro microbial fermentation and nutrient flow in diets containing barley straw or non-forage fiber sources

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Abstract Roughage in beef diets could be reduced or removed if replaced with less costly non-forage fiber sources, but there is little information about their effects on microbial fermentation and nutrient flow in different ruminal conditions. In the present experiment, eight dual-flow continuous culture fermenters (1320 ml) were used in three periods (5 days adaptation, 3 days sampling) to study the effect of pH and different sources of non-forage fiber in beef diets on rumen microbial fermentation. In each period, fermenters were randomly assigned to treatments. Temperature (39 °C), solid (0.05 h−1) and liquid (0.10 h−1) dilution rates were maintained constant. Fermenters were fed 97 g of DM/day in three equal portions. A factorial arrangement of treatments was used, the factors being the culture pH (5.5 and 6.2) and the main fiber source in the diet: barley straw (BS; 100 g/kg DM), beet pulp (BP; 170 g/kg DM), soybean hulls (SH; 170 g/kg DM) and whole cottonseed (WCS; 160 g/kg DM). Diets were formulated to have similar crude protein (150 g/kg DM), metabolizable energy (2.95 Mcal/kg of DM) and NFC level (550 g/kg DM). Low pH (5.5) reduced apparent DM and OM digestibility, total and branched chained volatile fatty acid concentrations, and acetate proportion, and increased propionate proportions, as expected. Replacement of BS with BP and SH increased OM, aNDF and ADF digestibility. An interaction diet×pH was detected for aNDF and ADF digestibility because the level of reduction in fiber digestibility at low pH was different among diets. Total volatile fatty acid concentration was lower in BS than in BP, SH and WCS and acetate proportion was higher in BP compared with WCS (50.3 vs 44.4 mol/100 mol). Low pH decreased NH3-N concentration (2.24 vs 4.10 mg/100 ml) and increased non-ammonia N flow (2.77 vs 2.70 g/day) compared with high pH. There were no differences among treatments in the flow of bacterial N and the efficiency of microbial protein synthesis. These results suggest that the effect of low pH on fiber and true OM digestibility was different among fiber sources. Replacing straw with non-forage fiber had no detrimental effects on in vitro fermentation, suggesting that these sources could be potentially useful in beef diets. However, many important differences exist between in vitro and in vivo experiments, which can influence the digestion and fermentation processes, so data need to be interpreted with the understanding of the nature and limitations of the technique used.