Total gas production was closely related to the digestion of fermentation substrates, VFA production, and microbial activity and growth [
13]. Ruminal microbial activity was affected by the use of plant extracts and secondary plant metabolites [
14]. In the present study, EGP decreased in response to the increasing doses of
Rhus succedanea extracts. Similarly, E
DM linearly decreased with increasing
R. succedanea extract doses. However, the rate of the decrease in E
GP in response to the dose of
R. succedanea extracts tended to slow at 50 mg/L dosing of
R. succedanea extracts (quadratic effect: p = 0.069). Similarly, total VFA concentration was decreased following 48 and 72 h incubations (quadratic effect: p = 0.021; linear effect: p = 0.014). Busquet et al [
14] reported that at the highest concentrations of various plant extracts, most treatments showed decreased total VFA production, possibly reflecting decreased feed digestion. Plant secondary metabolites are particularly attractive as rumen modifiers that are generally accepted to be environmental friendly and safe to use in food production systems. Due to their potential to adversely affect feed intake and nutrient utilization, however, these should be administered at low concentrations to beneficially alter ruminal fermentation. The effect of
R. succedanea extract dose on ammonia-N concentration was quadratic (p = 0.003) following 24 h and 48 h incubations, and this effect was increased for 50 mg/L extracts and decreased for 70 mg/L extracts. These results suggest that changes observed to be caused by
R. succedanea extract on ruminal ammonia-N concentration may be contradictory depending on the dose used. The observed reduction in ammonia-N suggests that over 70 mg/L of
R. succedanea extracts reduced amino acid deamination. Inhibition of amino acid deamination has practical implications because it may increase ruminal use of dietary protein and improve the efficiency of N use in the rumen [
15].
Ruminal cellulolysis is conducted primarily by
Fibrobacter succinogenes,
Ruminococcus flavefaciens, and
Ruminococcus albus [
16], and their relative populations can potentially impact the ratios of VFA available to ruminants. In the present study, an increase in ruminal acetate concentration was observed to accompany the addition of 70 mg/L dose of
R. succedanea extracts, and this was consistent with the observed increase in abundance of the gram-negative bacteria
Fibrobacter succinogenes. This bacterium intensively degrades plant cell walls by an erosion-like mechanism, in which it burrows its way through the complex matrix of cellulose and hemicellulose in the cell wall resulting in the release of digestible and undigested cell wall fragments [
17].
Ruminococcus flavefaciens, a gram-positive bacteria, was also observed to increase with addition of 70 mg/L dose of
R. succedanea extract that corresponded with the increase of acetate concentration. Production of methane and propionate are negatively correlated because both these processes compete for hydrogen [
18]. However, the negative relationship between propionate concentration and methane output was not evident from our results. Formation of acetate and butyrate results in production of additional methanogenic substrates, which are formate and hydrogen, and propionate formation results in less hydrogen being available for methane production [
18]. In addition, low methane production might be related to reduced fiber digestibility, thus, also influencing the energy input to the animal. In the present study DM degradability and gas production linearly decreased, whereas
Fibrobacter succinogenes and
Ruminococcus flavefaciens, which are considered to be primarily responsible for plant cell wall biodegradation, increased after 24 h of incubation. Nevertheless, the decreasing methane production and abundance of methanogenic archaea observed with extract dosing were likely caused by the combined effects of decreased total VFA concentration and decreased DM fermentation.
R. succedanea produces a cytotoxic biflavonoid, and it has been reported that the methanol extract of
Rhus succedanea showed positive indications of the presence of phenols, steroids, alkaloids, flavonoids, tannins, glycosides, and carbohydrates [
19]. It has been documented that alkaloids, flavonoids, tannins, and phenols are plant secondary metabolites, which are well-known for their antimicrobial activity in the rumen. The extract of stem bark of
Rhus had antioxidant effects against hydroxyl radicals and antiproliferative activity against human cancer cell lines, and also augmented the activity of cell-associated detoxifying enzymes in hepatocytes [
20–
22]. The sap of the wax tree (
R. succedanea) is composed of urushiol, glycoprotein, flavonoids, a gummy substance that contains laccase, stellacyanin, polysaccharides, peroxidase, and water [
23]. Therefore, the observed antibacterial potency of methanol extracts from
R. succedanea can be attributed to the nature of its biologically active components, which might be enhanced in the rumen. However, further work is needed to clarify the relationship between fibrolytic microbes and methanogens, and although suppression of methanogenic archaea by
R. succedanea extracts was observed in this study, it must be noted that the long-term effects of the extracts might be different because adaptation of the rumen microbes might occur.