A meta-analysis of effects of chemical composition of incubated diet and bioactive compounds on in vitro ruminal fermentation☆
Introduction
Improving the efficiency of energy and protein utilization is a major goal in ruminant nutrition because energy and ammonia losses impact animal production and the environment. However this goal can be achieved by shifting the volatile fatty acid (VFA) profile towards propionate, without reducing total VFA production, while reducing protein degradation and CH4 yields in the rumen. In this context, ruminant nutritionists have dedicated their research to identify effective compounds and dietary formulations to modify ruminal fermentation. As ruminal fermentation is due to microbes (i.e., bacteria, archaea, fungi, ciliate protozoa), substances affecting activity of microbes, such as antibiotic ionophores (McGuffey et al., 2001), could modify ruminal fermentation. However, because the use of antibiotics has been banned in the European Union since 2006, there has been interest in exploiting natural feed additives, such as naturally occurring bioactive compounds, as modifiers of ruminal fermentation.
Bioactive compounds (BC) are defined as essential and non-essential compounds which occur in the nature and are a part of the feed-food chain (Biesalski et al., 2009). Despite their often very low quantities, BC possess several health-promoting properties (Biesalski et al., 2009), often antimicrobial activity, with some of them used as traditional medicines (Reichling, 2009). Since 2004 there have been a number of in vitro studies investigating effects of various BC, including essential oils, on ruminal microbial fermentation and CH4 mitigation (e.g., Cardozo et al., 2004, Castillejos et al., 2006, Benchaar et al., 2007). However, the effectiveness of BC on modifying ruminal fermentation has been inconsistent and inconclusive. The discrepancy of results could be attributed to different chemical structures and types as well as the different doses of BC used (Busquet et al., 2006, Benchaar et al., 2007). In addition, it appears that effects of some metabolites in BC could depend on ruminal pH (Cardozo et al., 2005) and type of diet used for in vitro incubation (Calsamiglia et al., 2007).
It is clear that there are several factors and, potentially, interactions among factors that modify the expected responses of BC relative to ruminal fermentation. This leads to conflicting results among studies, while evaluation of multiple relationships can generally not be accomplished in a single study. A meta-review of the literature may be used to resolve such discrepancies (St-Pierre, 2001). Results of a meta-analysis of BC in vitro may explain responses in vivo.
Our objective was to examine effects of BC on in vitro ruminal fermentation profiles and nutrient degradation using a meta-analysis approach. We focused particularly on effects associated with the dose of BC supplemented and the chemical composition of the incubated diets. Measurements of interest included the contents of neutral detergent fibre (NDF) and crude protein (CP) in the incubated diet as well as in vitro disappearance of dry matter (DM) and NDF, concentrations and proportions of the major VFA (i.e., acetate, propionate, butyrate), ammonia, and CH4 formation.
Section snippets
Data search
A literature search was conducted using public data search generators, such as Pubmed, Google scholar, Sciencedirect, and Scopus, as well as contact with researchers in the field to identify published articles on effects of BC on rumen fermentation characteristics in vitro. The search strategy aimed to identify articles which contained specific data on experiments examining effects of supplementing different types of BC on in vitro rumen fermentation characteristics, particularly on total VFA
Effects of BC supplementation
Associations between BC supplementation and in vitro fermentation parameters are in Table 3. Increasing the level of BC supplementation linearly decreased total VFA concentrations (P<0.05) and affected the proportions of individual VFA. Thus, acetate was lower with BC supplementation compared to control (P<0.05). No effects of BC dosage occurred on butyrate formation and the acetate:propionate ratio. Ammonia concentrations decreased with BC supplementation (P<0.05). In particular, high dosages
Discussion
This study summarizes information from 20 studies published during the last 7 years to examine the role that supplementation dose of BC and the chemical composition of the substrate play on in vitro ruminal fermentation. A meta-analysis approach was used to weigh the sample size used in each experiment, and to account for the random effect of each experiment and the unequal variance among studies, due to their experimental differences. By screening a broad range in the supplementation dosage of
Conclusions
Overall, our evaluation indicates that supplementation dose of ruminally bioactive compounds (BC) and the chemical composition of the incubated diet are factors determining responses of BC on ruminal fermentation and degradation. However, as BC are highly variable and differ in efficiency, responses also depend on BC composition, the results from our study do not apply to all BC. Nevertheless, the findings may have general implications for in vivo conditions, and more in vivo research is
Acknowledgment
A. Muro-Reyes is recipient of the postdoctoral fellowship of Consorcio de Universidades Mexicanas (CUMex), which is gratefully acknowledged.
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This paper is part of the special issue entitled: Plant Bioactive Compounds in Ruminant Agriculture – Impacts and Opportunities, Guest Edited by A.Z.M. Salem and S. López, and Editor for Animal Feed Science and Technology, P.H. Robinson.