Fermentation Pattern of Several Carbohydrate Sources Incubated in An in Vitro Semicontinuous System with Inocula From Ruminants Given Either Forage or Concentrate-Based Diets

Abstract

The fermentation pattern of several carbohydrate sources and their interaction with the nature of microbial inoculum was studied. Barley (B), maize (M), sorghum, (S), sugarbeet pulp (BP), citrus pulp (CP) and wheat bran (WB) were tested in an in vitro semicontinuous system maintaining poorly buffered conditions from 0 to 6 h, and being gradually buffered to 6.5 from 8 to 24 h to simulate the rumen pH pattern. Rumen fluid inoculum was obtained from lambs fed with either concentrate and barley straw (CI) or alfalfa hay (FI). The extent of fermentation was higher with CI than FI throughout the incubation (p < 0.05). Among the substrates, S, BP and M maintained the highest pH (p < 0.05), whereas CP recorded the lowest pH with both inocula. Similarly, CP recorded the highest gas volume throughout the incubation, followed by WB and B, and S recorded the lowest volume (p < 0.05). On average, the total volatile fatty acid (VFA), as well as lactic acid concentration, was higher with CP than in the other substrates (p < 0.05). The microbial structure was more affected by the animal donor of inoculum than by the substrate. The in vitro semicontinuous system allows for the study of the rumen environment acidification and substrate microbial fermentation under intensive feeding conditions.

Fitting of the In Vitro Gas Production Technique to the Study of High Concentrate Diets

“…As has been highlighted by most authors and reviews [ 4 , 23 ], the composition and type of diet given to the donor animals determines its fermentative capacity and the degree it fits to the substate to be studied. Thus, the rate and extent of fermentation of a fibrous or a concentrate feed highly depends on its evaluation with inoculum from an animal fed on either a forage- or a concentrate-type diet [ 24 ]. Therefore, it is recommended to feed donor animals with a diet similar to the substrate to be incubated in vitro, or to the in vivo feeding conditions that are intended to be studied [ 25 ].…”

“…In addition to medium pH, the source of rumen fluid has an important role on the pattern of in vitro fermentation [ 26 , 50 ] in terms of adapting nutrient evaluation of concentrate substrates to low pH rumen conditions. Rumen microbiota promoted by high forage/fibre diets is not well suited for fermenting non-fibrous carbohydrates, producing a lower extent of substrate digestion than that promoted by the microbiota from concentrate diets in terms of gas production, volatile fatty acids production and substrate disappearance [ 24 , 26 , 50 , 51 ]. Therefore, the choosing of donor animals’ diet is important when concentrate feeds are evaluated for ruminants.…”

“…Therefore, the choosing of donor animals’ diet is important when concentrate feeds are evaluated for ruminants. Despite that the pH of an inoculum from donor animals given a concentrate diet was lower than that from animals fed on forages [ 24 , 26 ], the microbial population promoted by a concentrate diet was more favourable for fermentation of non-fibrous carbohydrates than that induced by a fibrous diet. This has been previously stated in classic revisions [ 4 , 23 ] and has been attributed to the lack of adaptation of microbiota from a forage inoculum to the fermentation of starch and sugar substrates [ 26 , 52 ] and its higher pH is related to the lower extent of fermentation, with the help of the inherent buffering capacity of forage legumes when these are included in diet.…”

“…However, according to stoichiometric calculations, differences in gas production from a concentrate feed with rumen inocula from either a high concentrate or a forage feed should render similar gas volumes when referring to per unit of VFA. Thus, observed acetate:propionate:butyrate ratios of after 10 h incubation of barley meal with inocula from either a high concentrate or a forage diet were 61:25:14 and 68:23:09, respectively [ 24 ]; thus, estimated gas volumes should be 48.4 vs. 47.6 mL/mmol VFA, respectively, of which 22.8 and 22.0 mL are direct gas.…”

“…However, mimicking the daily rumen pH pattern in high concentrate feeding conditions requires an adaptation to a circadian rhythm, which consisted of a previous drop to pH 6.0-5.6 in the first 6-8 h after substrate availability, followed by a progressive recover to initial levels (pH 6.4-6.6) after 24 h [ 34 ]. Amanzougarene et al [ 24 ] also succeed in mimicking such pH pattern by inoculating incubation solution with a minimal bicarbonate ion concentration (0.006 M, adjusting for a medium pH of 5.5) for the first 6 h, allowing for a free drop of pH, and thereafter, an incubation solution with 0.058 M bicarbonate ion to allow pH to rise to 6.5 from 8 to 24 h ( Figure 3 ). Thus, with this simple semicontinuous approach, incubation conditions can be more closely adapted to high concentration feeding conditions, and the fermentation pattern can be monitored through the gas production technique.…”