Plant extracts in ruminants: effects on fermentation in the rumen and on lipid quality in animal products

Abstract

Plant extracts are a complex group of substances whose properties can be used to manipulate the functioning of the rumen. Among them, condensed tannins, essential oils and saponins have been widely tested for their bactericidal/bacteriostatic effects. The variety of chemical structure of condensed tannins is probably at the origin of their difference of reactivity but also of a different activity on ruminal microorganisms. Generally speaking bacterial growth is more inhibited when the condensed tannins concentrations are raised. Condensed tannins inhibit the activity of methanogenic Archaea, and their effects on protozoa are different depending on the species. Generally, condensed tannins lead to a decrease of the concentration of ammoniacal nitrogen in the rumen but do not affect the concentration of total volatile fatty acids. At high concentrations, the effect is more pronounced in vitro than in vivo. But at a similar concentration, the amplitude of the response is different, depending on the source of condensed tannins. Their effect on methanogenesis, generally observed in vitro as well as in vivo, is a decrease in methane production, with a variation according to the incubation time, the total content of condensed tannins and the concentration. The main effects of the essential oils in the rumen are a reduction of degradation of protein and starch and an inhibition of the degradation of the amino acids, due to the selective action of certain rumen microorganisms. The bactericidal effect of essential oils on bacteria (pathogenic or ruminal) is related to the chemical structure of their aromatic molecules. At concentrations below 100 ppm, they inhibit the growth of most pure bacterial ruminal cultures. However, the adaptation of some bacterial species may explain the decrease or the lack of effect often observed in vivo. Some essential oils appear to have the potential to inhibit methane production in the rumen. However, the effects of saponins in the rumen are less clear, probably due to the development of microbial populations with a saponin degradation capacity, and/or adaptation of the concerned microorganisms to these plant extracts. Moreover, in the present state of knowledge, it is impossible to conclude on the effect of plant extracts on the lipid quality of animal products. Additional in vivo and long-term tests remains essential to verify the observed effects in more varied conditions. These tests should also make sure that no residues remains in the animal products and, if yes, that these residues will not induce any safety problem for consumers.