Effects of Garlic Oil and Cinnamaldehyde on Sheep Rumen Fermentation and Microbial Populations in Rusitec Fermenters in Two Different Sampling Periods

Garlic oil (GO) and cinnamaldehyde (CIN) have shown potential to modify rumen fermentation. The aim of this study was to assess the effects of GO and CIN on rumen fermentation, microbial protein synthesis (MPS), and microbial populations in Rusitec fermenters fed a mixed diet (50:50 forage/concentrate), as well as whether these effects were maintained over time. Six fermenters were used in two 15-day incubation runs. Within each run, two fermenters received no additive, 180 mg/L of GO, or 180 mg/L of CIN. Rumen fermentation parameters were assessed in two periods (P1 and P2), and microbial populations were studied after each of these periods. Garlic oil reduced the acetate/propionate ratio and methane production (p < 0.001) in P1 and P2 and decreased protozoal DNA concentration and the relative abundance of fungi and archaea after P1 (p < 0.05). Cinnamaldehyde increased bacterial diversity (p < 0.01) and modified the structure of bacterial communities after P1, decreased bacterial DNA concentration after P2 (p < 0.05), and increased MPS (p < 0.001). The results of this study indicate that 180 mg/L of GO and CIN promoted a more efficient rumen fermentation and increased the protein supply to the animal, respectively, although an apparent adaptive response of microbial populations to GO was observed.

Sigla storax (Liquidambar orientalis) mitigates in vitro methane production without disturbances in rumen microbiota and nutrient fermentation in comparison to monensin

AIM

The aim of this study was to investigate the in vitro dose-dependent effects of sigla storax (Styrax liquidus) on rumen microbiota and rumen microbial fermentation in comparison to monensin as a positive control.

METHODS AND RESULTS

This study was carried out using a rumen simulation model (Rusitec). Treatments consisted of no additive (control), 10 mg l-1 of monensin sodium salt, 100 mg l-1 (Low-Sigla) and 500 mg l-1 (High-Sigla) of sigla storax (n = 6/treatment). In addition to rumen fermentation characteristics, rumen microbial composition was investigated using 16S rRNA sequencing. The methane variables and the acetate to propionate ratio decreased in the both High-Sigla and monensin groups (P < 0.05). High-Sigla had no effect on ammonia, total SCFA and nutrition degradation, while monensin decreased these parameters (P < 0.05). Unlike monensin, the sigla storax treatments did not affect the alpha or beta diversity indexes of the microbiota. The relative abundance of Methanomethylophilaceae and Ruminococcaceae decreased with High-Sigla and monensin (P < 0.05), and Atopobiaceae and Eggerthellaceae decreased with the both doses of sigla storax as well as monensin treatments (P < 0.05). Syntrophococcus, DNF00809 and Kandleria were among the genera that most decreased with High-Sigla and monensin (Q < 0.07) and were strongly positively correlated with methane production (r = 0.52-0.56).

CONCLUSIONS

The high dose of sigla storax (500 mg l-1) decreased methane in the rumen ecosystem without adverse effects on nutrient degradation and SCFA production, and without dramatically impacting the microbial composition. Sigla storax might be a novel feed additive to mitigate methane in cattle.

Replacing Forage by Crude Olive Cake in a Dairy Sheep Diet: Effects on Ruminal Fermentation and Microbial Populations in Rusitec Fermenters

Olive oil extraction generates large amounts of a highly pollutant by-product called olive cake (OC), and its use in ruminant feeding could be an alternative. This study was designed to evaluate the effects of partially replacing forage by crude OC (COC) in a mixed dairy diet on rumen fermentation and microbial populations in Rusitec fermenters. The COC replaced 33% of the forage (66% maize silage and 33% barley straw) and was included at 16.6% of the total diet. Four fermenters were used in a cross-over design with two 13-day incubation periods. Experimental diets had a 50:50 forage-to-concentrate ratio and were formulated to contain the same protein (16.0%) and neutral detergent fiber (32.5%) levels. Compared with control fermenters, those fed the COC diet showed greater (p ≤ 0.02) pH (6.07 vs. 6.22), diet disappearance (0.709 vs. 0.748), and butyrate proportions (18.0 vs. 19.4), but there were no differences in volatile fatty acids and ammonia production. Microbial growth, bacterial diversity, protozoal abundance, and relative abundance of fungi and archaea were unaffected by diet, although the solid phase of COC-fed fermenters showed greater (p = 0.01) bacterial abundance than control ones. Results indicate that COC could replace 33% of the forage in a mixed dairy diet.

Effects of Replacing Extruded Maize by Dried Citrus Pulp in a Mixed Diet on Ruminal Fermentation, Methane Production, and Microbial Populations in Rusitec Fermenters

Citrus pulp is a highly abundant by-product of the citrus industry. The aim of this study was to assess the effects of replacing extruded maize (EM; 20% of total diet) by dried citrus pulp (DCP; 20%) in a mixed diet on rumen fermentation and microbial populations in Rusitec fermenters. The two diets contained 50% alfalfa hay and 50% concentrate, and the same protein level. Four Rusitec fermenters were used in a cross-over design with two 13-d incubation runs. After 7-d of diet adaptation, diet disappearance, fermentation parameters, microbial growth, and microbial populations were assessed. Fermenters receiving the DCP showed greater pH values and fiber disappearance (p < 0.001) and lower methane production (p = 0.03) than those fed EM. Replacing EM by DCP caused an increase in the proportions of propionate and butyrate (p < 0.001) and a decrease in acetate (p = 0.04). Microbial growth, bacterial diversity, and the quantity of bacteria and protozoa DNA were not affected by the diet, but the relative abundances of fungi and archaea were greater (p < 0.03) in solid and liquid phases of DCP fermenters, respectively. Results indicate that DCP can substitute EM, promoting a more efficient ruminal fermentation.

Effect of combinations of feed-grade urea and slow-release urea in a finishing beef diet on fermentation in an artificial rumen system

This study evaluated the effect of combinations of feed-grade urea and slow-release urea (SRU) on fermentation and microbial protein synthesis within two artificial rumens (Rusitec) fed a finishing concentrate diet. The experiment was a completely randomized, dose-response design with SRU substituted at levels of 0% (control), 0.5%, 1%, or 1.75% of dry matter (DM) in place of feed-grade urea, with four replicate fermenters per dosage. The diet consisted of 90% concentrate and 10% forage (DM basis). The experiment was conducted over 15 d, with 8 d of adaptation and 7 d of sampling. Dry matter and organic matter disappearances were determined after 48 h of incubation from day 9 to 12, and daily ammonia (NH₃) and volatile fatty acid (VFA) production were measured from day 9 to 12. Microbial protein synthesis was determined on days 13-15. Increasing the level of SRU quadratically affected total VFA (Q, P = 0.031) and ammonia (Q, P = 0.034), with a linear increment in acetate (L, P = 0.01) and isovalerate (L, P = 0.05) and reduction in butyrate (L, P = 0.05). Disappearance of neutral detergent fiber (NDF) and acid detergent fiber (ADF) was quadratically affected by levels of SRU, plateauing at 1% SRU. Inclusion of 1% SRU resulted in the highest amount of microbial nitrogen associated with feed particles (Q, P = 0.037). Responses in the efficiency of microbial protein synthesis fluctuated (L, P = 0.002; Q, P = 0.001) and were the highest for 1% SRU. In general, the result of this study showed that 1% SRU in combination with 0.6% urea increased NDF and ADF digestibility and total volatile fatty acid (TVFA) production.

Differently Pre-treated Alfalfa Silages Affect the in vitro Ruminal Microbiota Composition

Alfalfa (Medicago sativa L.) silage (AS) is an important feedstuff in ruminant nutrition. However, its high non-protein nitrogen content often leads to poor ruminal nitrogen retention. Various pre-ensiling treatments differing with respect to dry matter concentrations, wilting intensities and sucrose addition have been previously shown to improve the quality and true protein preservation of AS, and have substantial effects on in vitro ruminal fermentation of the resulting silages. However, it is unknown how these pre-ensiling treatments affect the ruminal microbiota composition, and whether alterations in the microbiota explain previously observed differences in ruminal fermentation. Therefore, during AS incubation in a rumen simulation system, liquid and solid phases were sampled 2 and 7 days after first incubating AS, representing an early (ET) and late (LT) time point, respectively. Subsequently, DNA was extracted and qPCR (bacteria, archaea, and anaerobic fungi) and prokaryotic 16S rRNA gene amplicon sequence analyses were performed. At the ET, high dry matter concentration and sucrose addition increased concentrations of archaea in the liquid phase (P = 0.001) and anaerobic fungi in the solid phase (P < 0.001). At the LT, only sucrose addition increased archaeal concentration in the liquid phase (P = 0.014) and anaerobic fungal concentration in the solid phase (P < 0.001). Bacterial concentrations were not affected by pre-ensiling treatments. The prokaryotic phylogenetic diversity index decreased in the liquid phase from ET to LT (P = 0.034), whereas the solid phase was not affected (P = 0.060). This is suggestive of a general adaption of the microbiota to the soluble metabolites released from the incubated AS, particularly regarding the sucrose-treated AS. Redundancy analysis of the sequence data at the genus level indicated that sucrose addition (P = 0.001), time point (P = 0.001), and their interaction (P = 0.001) affected microbial community composition in both phases. In summary, of the pre-ensiling treatments tested sucrose addition had the largest effect on the microbiota, and together with sampling time point affected microbiota composition in both phases of the rumen simulation system. Thus, microbiota composition analysis helped to understand the ruminal fermentation patterns, but could not fully explain them.

Scrophularia striata Extract Supports Rumen Fermentation and Improves Microbial Diversity in vitro Compared to Monensin

In the search for natural alternatives to antibiotic feed additives, we compared the efficacy of two doses of Scrophularia striata extract [S. striata-Low at 40 and S. striata-High at 80 mg g^-1 dry matter (DM)] with monensin (monensin) and a negative control in the modulation of rumen fermentation, methane production and microbial abundance in vitro. Microbes were investigated using qPCR and 16S rRNA targeted sequencing. Data showed that the addition of S. striata increased production of total short chain fatty acids (SCFA) in comparison to both monensin and control (P = 0.04). The addition of S. striata increased acetate production, and increased propionate at the higher dosage (P < 0.001). Supplementation of S. striata lowered methane production (P < 0.001) compared to control but with no effect compared to monensin. Ammonia concentration decreased by 52% (P < 0.001) with S. striata-High supplementation (4.14 mmol L^-1) compared to control, which was greater than that of monensin (36%). The diversity of rumen bacteria was reduced (P < 0.001) for monensin and S. striata for both the number of observed OTUs and the Chao1 index. Quantitative analysis of Protozoa showed a decrease in the monensin treatment (P = 0.05) compared to control. Archaea copy numbers decreased equally in both S. striata-High and monensin treatments compared to the control group. Supplementation with S. striata increased relative abundances of Fibrobacteres (P < 0.001) and Planctomycetes (P = 0.001) in comparison to both the control and monensin treatments. Significant negative correlations were observed between the abundances of Bacteroides, Fusobacterium, and Succinivibrio genera and methane (r > -0.71; P ≤ 0.001). The abundance of Fibrobacter genera and total SCFA (r = 0.86), acetate (r = 0.75), and valerate (r = -0.51; P < 0.001) correlated positively. These results suggest that S. striata supplementation at 80 mg g^-1 DM inclusion, similar to monensin, supports rumen fermentation, lowers methane and ammonia production. However, S. striata supported rumen fermentation toward higher total SCFA and propionate production, while unlike monensin still supported a diverse rumen microbiome and an increase in cellulolytic bacteria such as Fibrobacter.

Post-ruminal effects of rumen-protected methionine supplementation with low protein diet using long-term simulation and in vitro digestibility technique

Microbial degradation in the rumen and dietary availability of methionine amino acid have been reported as limiting in dairy ruminants. The aim of the present study was to examine the post-ruminal effects of feeding ruminants different concentrations of rumen-protected methionine (RPM) in low crude protein diets using the long-term rumen simulation method (Rusitec) followed by in vitro abomasum and ileum digestibility technique. The experiment contained four treatment groups: (1) high protein, without RPM supplementation (HP); (2) low protein, without RPM supplementation (LP); (3) low protein supplementation with low RPM (LPLM); and (4) low protein supplementation with high RPM (LPHM) mixed per 20 ± 0.04 g basal diet in every fermenter. The results showed that the LPLM and LPHM groups had significantly higher disappearance of crude protein and neutral detergent fiber in the abomasum and ileum than the HP treatment (P < 0.05) and were the same as the LP group (P > 0.05). The proportions of short-chain fatty acids and total volatile fatty acids in the abomasum and ileum were the same between the LPHM and HP groups (P > 0.05); however, the LPLM group was found to be significantly (P < 0.05) lower than the HP group and similar to the LP group (P > 0.05). Rusitec pH before or after changing feed bags and daily ammonia nitrogen production in the abomasum and ileum were non significantly (P > 0.05) different among all groups. In conclusion, RPM supplementation with low crude protein diets promoted post-ruminal digestibility and production of volatile fatty acids.

Graded replacement of maize grain with molassed sugar beet pulp modulated ruminal microbial community and fermentation profile in vitro

BACKGROUND

Molassed sugar beet pulp (Bp) is a viable alternative to grains in cattle nutrition for reducing human edible energy input. Yet little is known about the effects of high inclusion rates of Bp on rumen microbiota. This study used an in vitro approach and the quantitative polymerase chain reaction technique to establish the effects of a graded replacement of maize grain (MG) by Bp on the ruminal microbial community, fermentation profile and nutrient degradation.

RESULTS

Six different amounts of Bp (0-400 g kg^-1 ), which replaced MG in the diet, were tested using the in vitro semi-continuous rumen simulation technique. The increased inclusion of Bp resulted in greater dietary content and degradation of neutral detergent fibre (P < 0.01). Further, Bp feeding enhanced (P < 0.01) the abundance of genus Prevotella and shifted (P < 0.01) the short-chain fatty acid patterns in favour of acetate and propionate and at the expense of butyrate. A total replacement of MG with Bp resulted in an increased daily methane production (P < 0.01).

CONCLUSION

Results suggest positive effects of the replacement of MG by Bp especially in terms of stimulating ruminal acetate and propionate fermentation. However, high replacement rates of Bp resulted in lowered utilization of ammonia and higher ruminal methane production. © 2017 Society of Chemical Industry.

An Integrated Multi-Omics Approach Reveals the Effects of Supplementing Grass or Grass Hay with Vitamin E on the Rumen Microbiome and Its Function

Rumen function is generally suboptimal leading to losses in methane and nitrogen. Analysis of the rumen microbiome is thus important to understanding the underlying microbial activity under different feeding strategies. This study investigated the effect of forage conservation method and vitamin E supplementation on rumen function using a rumen simulation technique. Ryegrass (GRA) or ryegrass hay (HAY) was supplemented with 20% concentrate containing zero or 50 IU/d vitamin E, as α-tocopheryl acetate, according to a 2 × 2 factorial design. The forage conservation method did not substantially change the nutrient composition but had a profound impact on the structure and diversity of the rumen microbiome. HAY diets promoted a more complex bacterial community (+38 OTUs) dominated by Firmicutes. This bacterial adaptation, together with increased rumen protozoa levels and methanogen diversity, was associated with greater fiber disappearance (+12%) in HAY diets, but also with greater rumen true N degradability (+7%) than GRA diets. HAY diets also had a higher metabolic H recovery and methane production (+35%) suggesting more efficient inter-species H transfer between bacteria, protozoa and methanogens. Contrarily, GRA diets promoted more simplified methanogen and bacterial communities, which were dominated by Bacteroidetes and Lactobacillus, thus lactate formation may have acted as an alternative H sink in GRA diets. Moreover the structure of the bacterial community with GRA diets was highly correlated with N utilization, and GRA diets promoted greater bacterial growth and microbial protein synthesis (+16%), as well as a more efficient microbial protein synthesis (+22%). A dose-response experiment using batch cultures revealed that vitamin E supplementation increased rumen fermentation in terms of total VFA and gas production, with protozoal activity higher when supplying α-tocopheryl acetate vs. α-tocopherol. Moreover, α-tocopheryl acetate promoted a small increase in feed degradability (+8%), possibly as a result of its antioxidant properties which led to higher bacterial and protozoal levels. Vitamin E supplementation also modified the levels of some methanogen species indicating that they may be particularly sensitive to oxidative stresses. Our findings suggested that when possible, grass should be fed instead of grass hay, in order to improve rumen function and to decrease the environmental impact of livestock agriculture.

A Metagenomics Approach to Evaluate the Impact of Dietary Supplementation with Ascophyllum nodosum or Laminaria digitata on Rumen Function in Rusitec Fermenters

There is an increasing need to identify alternative feeds for livestock that do not compete with foods for humans. Seaweed might provide such a resource, but there is limited information available on its value as an animal feed. Here we use a multi-omics approach to investigate the value of two brown seaweeds, Ascophyllum nodosum (ASC) and Laminaria digitata (LAM), as alternative feeds for ruminants. These seaweeds were supplemented at 5% inclusion rate into a control diet (CON) in a rumen simulation fermenter. The seaweeds had no substantial effect on rumen fermentation, feed degradability or methane emissions. Concentrations of total bacteria, anaerobic fungi, biodiversity indices and abundances of the main bacterial and methanogen genera were also unaffected. However, species-specific effects of brown seaweed on the rumen function were noted: ASC promoted a substantial decrease in N degradability (−24%) due to its high phlorotannins content. Canonical correspondence analysis of the bacterial community revealed that low N availability led to a change in the structure of the bacterial community. ASC also decreased the concentration of Escherichia coli O157:H7 post-inoculation. In contrast, LAM which has a much lower phlorotannin content did not cause detrimental effects on N degradability nor modified the structure of the bacterial community in comparison to CON. This adaptation of the microbial community to LAM diets led to a greater microbial ability to digest xylan (+70%) and carboxy-methyl-cellulose (+41%). These differences among brown seaweeds resulted in greater microbial protein synthesis (+15%) and non-ammonia N flow (+11%) in LAM than in ASC diets and thus should led to a greater amino acid supply to the intestine of the animal. In conclusion, it was demonstrated that incorporation of brown seaweed into the diet can be considered as a suitable nutritional strategy for ruminants; however, special care must be taken with those seaweeds with high phlorotannin concentrations to prevent detrimental effects on N metabolism. This study highlights the value of combining fermentation and enzyme activity data with molecular characterization of the rumen microbiome in evaluating novel feeds for ruminants. Further experiments are required to determine the maximum seaweed inclusion rate tolerated by rumen microbes.

Treatment of grain with organic acids at 2 different dietary phosphorus levels modulates ruminal microbial community structure and fermentation patterns in vitro

Recent data indicate positive effects of treating grain with citric (CAc) or lactic acid (LAc) on the hydrolysis of phytate phosphorus (P) and fermentation products of the grain. This study used a semicontinuous rumen simulation technique to evaluate the effects of processing of barley with 50.25 g/L (wt/vol) CAc or 76.25 g/L LAc on microbial composition, metabolic fermentation profile, and nutrient degradation at low or high dietary P supply. The low P diet [3.1g of P per kg of dry matter (DM) of dietary P sources only] was not supplemented with inorganic P, whereas the high P diet was supplemented with 0.5 g of inorganic P per kg of DM through mineral premix and 870 mg of inorganic P/d per incubation fermenter via artificial saliva. Target microbes were determined using quantitative PCR. Data showed depression of total bacteria but not of total protozoa or short-chain fatty acid (SCFA) concentration with the low P diet. In addition, the low P diet lowered the relative abundance of Ruminococcus albus and decreased neutral detergent fiber (NDF) degradation and acetate proportion, but increased the abundance of several predominantly noncellulolytic bacterial species and anaerobic fungi. Treatment of grain with LAc increased the abundance of total bacteria in the low P diet only, and this effect was associated with a greater concentration of SCFA in the ruminal fluid. Interestingly, in the low P diet, CAc treatment of barley increased the most prevalent bacterial group, the genus Prevotella, in ruminal fluid and increased NDF degradation to the same extent as did inorganic P supplementation in the high P diet. Treatment with either CAc or LAc lowered the abundance of Megasphaera elsdenii but only in the low P diet. On the other hand, CAc treatment increased the proportion of acetate in the low P diet, whereas LAc treatment decreased this variable at both dietary P levels. The propionate proportion was significantly increased by LAc at both P levels, whereas butyrate increased only with the low P diet. Treatments with CAc or LAc reduced the degradation of CP and ammonia concentration compared with the control diet at both P levels. In conclusion, the beneficial effects of CAc and LAc treatment on specific ruminal microbes, fermentation profile, and fiber degradation in the low P diet suggest the potential for the treatment to compensate for the lack of inorganic P supplementation in vitro. Further research is warranted to determine the extent to which the treatment can alleviate the shortage of inorganic P supplementation under in vivo conditions.

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

A rumen simulation technique was used to evaluate the effects of the complete substitution of a common concentrate mixture (CON) with a mixture consisting solely of by-products from the food industry (BP) at 2 different forage-to-concentrate ratios on ruminal fermentation profile, nutrient degradation, and abundance of rumen microbiota. The experiment was a 2×2 factorial arrangement with 2 concentrate types (CON and BP) and 2 concentrate levels (25 and 50% of diet dry matter). The experiment consisted of 2 experimental runs with 12 fermentation vessels each (n=6 per treatment). Each run lasted for 10d, with data collection on the last 5d. The BP diets had lower starch, but higher neutral detergent fiber (NDF) and fat contents compared with CON. Degradation of crude protein was decreased, but NDF and nonfiber carbohydrate degradation were higher for the BP diets. At the 50% concentrate level, organic matter degradation tended to be lower for BP and CH4 formation per unit of NDF degraded was also lower for BP. The BP mixture led to a higher concentration of propionate and a lower acetate-to-propionate ratio, whereas concentrations of butyrate and caproate decreased. Concentrate type did not affect microbial community composition, except that the abundance of bacteria of the genus Prevotella was higher for BP. Increasing the concentrate level resulted in higher degradation of organic matter and crude protein. At the higher concentrate level, total short-chain fatty acid formation increased and concentrations of isobutyrate and valerate decreased. In addition, at the 50% concentrate level, numbers of protozoa increased, whereas numbers of methanogens, anaerobic fungi, and fibrolytic bacteria decreased. No interaction was noted between the 2 dietary factors on most variables, except that at the higher concentrate level the effects of BP on CH4 and CO2 formation per unit of NDF degraded, crude protein degradation, and the abundance of Prevotella were more prominent. In conclusion, the results of this study suggest that BP in the diet can adequately substitute CON with regard to ruminal fermentation profile and microbiota, showing even favorable fermentation patterns when fed at 50% inclusion rate.

Effect of fibrolytic enzymes added to a Andropogon gayanus grass silage-concentrate diet on rumen fermentation in batch cultures and the artificial rumen (Rusitec)

In vitro batch cultures were used to screen four fibrolytic enzyme mixtures at two dosages added to a 60 : 40 silage : concentrate diet containing the C(4) tropical grass Andropogon gayanus grass ensiled at two maturities - vegetative stage (VS) and flowering stage (FS). Based on these studies, one enzyme mixture was selected to treat the same diets and evaluate its impact on fermentation using an artificial rumen (Rusitec). In vitro batch cultures were conducted as a completely randomized design with two runs, four replicates per run and 12 treatments in a factorial arrangement (four enzyme mixtures×three doses). Enzyme additives (E1, E2, E3 and E4) were commercial products and contained a range of endoglucanase, exoglucanase and xylanase activities. Enzymes were added to the complete diet 2 h before incubation at 0, 2 and 4 μl/g of dry matter (DM). Gas production (GP) was measured after 3, 6, 12, 24 and 48 h of incubation. Disappearance of DM (DMD), NDF (NDFD) and ADF (ADFD) were determined after 24 and 48 h. For all four enzyme mixtures, a dosage effect (P>0.05) DM, N, NDF or ADF disappearance after 48 h of incubation nor daily ammonia-N, volatile fatty acids or CH(4) production. However, enzyme application increased (P<0.05) microbial N production in feed particle-associated (loosely-associated) and silage feed particle-bound (firmly associated) fractions. With A. gayanus silage diets, degradation may not be limited by microbial colonization, but rather by the ability of fibrolytic enzymes to degrade plant cell walls within this recalcitrant forage.

Effects of replacing dietary starch with neutral detergent-soluble fibre on ruminal fermentation, microbial synthesis and populations of ruminal cellulolytic bacteria using the rumen simulation technique (RUSITEC)

A rumen simulation technique (RUSITEC) apparatus with eight 800 ml fermenters was used to investigate the effects of replacing dietary starch with neutral detergent-soluble fibre (NDSF) by inclusion of sugar beet pulp in diets on ruminal fermentation, microbial synthesis and populations of ruminal cellulolytic bacteria. Experimental diets contained 12.7, 16.4, 20.1 or 23.8% NDSF substituted for starch on a dry matter basis. The experiment was conducted over two independent 15-day incubation periods with the last 8 days used for data collection. There was a tendency that 16.4% NDSF in the diet increased the apparent disappearance of organic matter (OM) and neutral detergent fibre (NDF). Increasing dietary NDSF level increased carboxymethylcellulase and xylanase activity in the solid fraction and apparent disappearance of acid detergent fibre (ADF) but reduced the 16S rDNA copy numbers of Ruminococcus albus in both liquid and solid fractions and R. flavefaciens in the solid fraction. The apparent disappearance of dietary nitrogen (N) was reduced by 29.6% with increased dietary NDSF. Substituting NDSF for starch appeared to increase the ratios of acetate/propionate and methane/volatile fatty acids (VFA) (mol/mol). Replacing dietary starch with NDSF reduced the daily production of ammonia-N and increased the growth of the solid-associated microbial pellets (SAM). Total microbial N flow and efficiency of microbial synthesis (EMS), expressed as g microbial N/kg OM fermented, tended to increase with increased dietary NDSF, but the numerical increase did not continue as dietary NDSF exceeded 20.1% of diet DM. Results suggested that substituting NDSF for starch up to 16.4% of diet DM increased digestion of nutrients (except for N) and microbial synthesis, and further increases (from 16.4% to 23.8%) in dietary NDSF did not repress microbial synthesis but did significantly reduce digestion of dietary N.