A meta-analysis of variability in continuous-culture ruminal fermentation and digestibility data

Abstract

A meta-analysis was conducted to compare ruminal fermentation and digestibility data and variability between continuous-culture (CC) experiments and in vivo data. One hundred eighty CC studies representing 1,074 individual treatments, published in refereed journals between 1980 and 2010 were used in this analysis. Studies were classified into 2 groups based on the type of CC used: CC systems specified as rumen simulation techniques (RUSITEC) and non-RUSITEC CC systems (non-RUSITEC). The latter was a diverse group of systems, all of which were termed CC by the investigators. The CC data were compared with a data set of in vivo trials with ruminally cannulated lactating dairy cows (data from a total of 366 individual cows). The reported neutral detergent fiber (NDF) concentration of the diets fed in the 3 data sets was, on average (dry matter basis), 44, 34, and 32%, respectively. The average total volatile fatty acid (VFA) concentration for the RUSITEC and non-RUSITEC data sets was 67 and 80% (respectively) of the total VFA concentration in vivo. The average concentration of acetate was also lower for the CC data sets compared with in vivo and that of propionate was considerably lower for RUSITEC compared with in vivo, but butyrate concentrations were similar between the CC and in vivo data sets. Variability in the VFA data was generally the highest (higher coefficients of variation and variance) for the non-RUSITEC data set, followed by RUSITEC, and was the lowest for in vivo. Digestibilities of NDF and particularly organic matter were lower in the CC data sets compared with in vivo; the average NDF digestibility was 34.2, 45.5, and 53.0% for RUSITEC, non-RUSITEC, and in vivo, respectively. Variability in nutrient digestibility data followed the pattern of variability of the VFA data: highest variability for the non-RUSITEC data set, followed by RUSITEC, and the lowest for in vivo. This analysis showed that CC systems are generally characterized by lower total VFA and acetate concentrations, extremely low counts or lack of ruminal protozoa, and lower organic matter and NDF digestibilities than in vivo. Overall, variability was much greater for CC than for in vivo experimental data.

Design, implementation and interpretation of in vitro batch culture experiments to assess enteric methane mitigation in ruminants—a review

“…Similar differences in responses to Japanese horseradish oil have been reported between in vitro and in vivo studies (Mohammed et al, 2004). Much larger decreases in CH 4 production were observed in vitro (−89%) than in vivo (−18.7%, Table 2), findings that are in agreement a recent meta-analysis (Hristov et al, 2012). Such discrepancies in the effectiveness of test compounds when given in similar doses may be explained by a combination of several factors: (1) test compounds used are typically administered in 1-2 pulses via the ruminal cannula that often coincide with feeding times, and as a consequence might not be rapidly and well mixed with rumen contents; (2) differences in the degradation rate of the active compounds in vitro and in vivo; (3) decrease in microbial density and changes in bacterial community structure of rumen contents during processing as inoculum for in vitro studies associated with the exposure of microorganisms to oxygen and the removal of solids during filtration (Soto et al, 2012); (4) potential washout of these compounds from the rumen or absorption through the rumen wall and (5) adaptation of the rumen microbial ecosystem to the tested compound in vivo that is not emulated by inoculated microbiota in vitro.…”

An inhibitor persistently decreased enteric methane emission from dairy cows with no negative effect on milk production

“…Therefore, it is important to evaluate methane mitigation strategies in long-term experiments, which for livestock experimentation requires treatment periods considerably longer than the 21-28 d, common for crossover designs. In addition, due to a variety of constraints and confounding factors of batch or continuous culture in vitro systems (5,10), mitigation compounds, including methane inhibitors, have to be tested in vivo using animals with similar productivity to those on commercial farms. An example of the limitations of in vitro systems is a series of experiments with garlic oil.…”

Substitution of common concentrates with by-products modulated ruminal fermentation, nutrient degradation, and microbial community composition in vitro

“…High concentrate levels are known to decrease ruminal pH and create unfavorable environmental conditions in the rumen (Ametaj et al, 2010). However, due to a continuous infusion of artificial saliva, pH is usually maintained at a normal and constant level in Rusitec systems (Hristov et al, 2012). Although we observed a slight but significant reduction in pH for the 50% concentrate level, pH always remained within an optimal range and therefore it is unlikely that it impaired the fermentation or microbiota in any way.…”

“…For this, we chose a semicontinuous rumen simulation technique (Rusitec), with standardized similar rumen environmental conditions (i.e., temperature, pH, buffer flow). As it is only a simulation of the rumen, conditions in the Rusitec system differ from in vivo conditions (e.g., lower digestibility, lower total SCFA concentrations, lower presence or lack of protozoa and significant shifts in the microbial population; Prevot et al, 1994;Martínez et al, 2010;Hristov et al, 2012). Thus, results from Rusitec have to be interpreted carefully if applied to in vivo conditions.…”