Research Article

 

Effects of Saccharomyces cerevisiae and Aspergillus oryzae Supplementation in Swamp Roughage Haylage-Based Rations on in vitro Rumen Fermentation Characteristics and Methane Gas Emission

 

Riswandi1*, Asep Indra M Ali1. Afnur Imsya1, Sofia Sandi1, Basuni Hamzah2, Agus Supriadi3

1Department of Animal Science, 2Department of Agricultural Product Technology, 3Department of Fishery Product Technology, Faculty of Agriculture, Universitas Sriwijaya. Jl. Raya Palembang-Prabumulih KM. 32, Indralaya, South Sumatra, Indonesia, 30662.

 

Received | April 08, 2021; Accepted | April 30, 2021; Published | July 01, 2021

*Correspondence | Riswandi, Department of Animal Science, Universitas Sriwijaya. Jl. Raya Palembang-Prabumulih KM. 32, Indralaya, South Sumatra, Indonesia, 30662; Email: riswandi_dya@yahoo.com

Citation | Riswandi Riswandi, Ali AIM, Imsya A (2021). Effects of saccharomyces cerevisiae and aspergillus oryzae supplementation in swamp roughage haylage-based rations on in vitro rumen fermentation characteristics and methane gas emission. Adv. Anim. Vet. Sci. 9(8): 1143-1149.

DOI | http://dx.doi.org/10.17582/journal.aavs/2021/9.8.1143.1149

ISSN (Online) | 2307-8316; ISSN (Print) | 2309-3331

Copyright © 2021 Riswandi et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

 

INTRODUCTION

 

The productivity of farm ruminant animals in tropical regions is mainly influenced by the feed quality and quantity that seasonally fluctuate. Feeding of the animals for optimal production should contain balanced nutrients to meet the requirements of rumen microbes and host animals (McDonald et al., 2010; Lazzarini et al., 2016).

 

One of the strategies in the ruminant management in lowland areas of South Sumatra is the feeding of potential aquatic forage of swamp areas. Swamp rice grass or Bento Rayap grass (Leersia hexandra) and water mimosa or kemon air (Neptunia oleracea Lour) are two examples of grass and legume growing in swamp areas. The seasonal roughage availability in the region results in an oversupply in rainy season and an undersupply in dry season. The feeding of low quality forage (Ali et al., 2013) could reduce nutrients digestibility and increase methane production since the high fiber fractions content (Philippe and Nicks, 2015). Preserving swamp roughages by haylage in the rainy season is one of promising solution to overcome the scarcity of feed supply in the dry season. The anaerobic process in a mixture of grass and legume haylage could preserve nutrients of the roughage. (McDonald et al., 2010; Schroeder, 2013).

 

Utilization local probiotics serve an alternative way to fulfill the nutrient requirements of ruminant animals particularly in a feed shortage period. However, studies on Saccharomyces cerevisiae (SC) and Aspergillus oryzae (AO) supplementation in swamp roughage haylage-based rations is limited. The SC and AO supplementation in swamp roughage haylage-based rations can improve nutrients availability for rumen microbes and host animals. The main reason of probiotic supplementation in ruminant animals is the capability of probiotics to modify rumen ecosystem, microbe population, feed digestion, and rumen fermentation processes and so reduce methane gas production (Seo et al., 2010; Khan et al., 2016). Probiotic supplementation of SC has been shown to increase digestibility rate of fiber fractions, crude protein degradation, rumen fermentability, microbe efficiency, and ruminant animal performance (Ali et al., 2015; Riswandi et al., 2020). This study was aimed at assessing the effects of SC and AO supplementation in swamp roughage haylage-based rations on rumen fermentation characteristics and methane production.

 

MATERIALS AND METHODS

 

The study was conducted at Animal Nutrition and Feed Laboratory, Faculty of Agriculture, Universitas Sriwijaya.

 

Haylage Production

Prior to haylage production, swamp roughages consisting of bento rayape grass and kemon air legume were chopped into 3 cm cuts and wilted for 24 hours to reduce their water content up to 40% (Jaelani et al., 2014). These wilted roughages were mixed with the ratio of 70% bento grass and 30% kemon air legume (as fed). Molasses was then added into the mixture by 5% as suggested by Riswandi (2014). The mixed roughages were moved to plastic bags silo and then compacted to ensure that anaerobic condition in the plastic bags was achieved. The bags were placed in a room at 26-28oC and incubated for 21 days.

 

Sample Preparation and Experimental Design

In the end of the incubation period, the haylage was taken out from the bags and dried in an oven at 60oC for 24 hours and then finely ground (1 mm). In vitro substrates consisted of the haylage (70%) and a concentrate mixture (30%) (Table 1). Nutrient contents of sample feed was determined by using an AOAC method (2005) in a proximate analysis. The amount of SC and AO probiotic supplementation was in accordance to each treatment. Nutrient composition of feeds and the ration are shown in Tables 2 and Table 3, respectively.

 

Table 1: Concentrate composition used in the rations

 

Feedstuff	Amount (kg) As Fed based
Rice bran	80
Ground corn	8
Tofu by-product	10
Ultra-mineral	0,5
Salt	0,75
Urea	0,75
Total	100

 

Table 2: Nutrient composition of haylage and concentrate (% dry matter)

 

Nutrient	Haylage	Concentrate
Dry matter	32.05	76.26
Organic matter	93.14	89.79
Crude protein	15.48	17.82
Crude fiber	22.37	17.78
Ether extract	3.82	7.88
Nitrogen Free Extract	56.13	55.28
TDN	66.41	73.54
Neutral detergent fiber	45.91	31.62
Acid detergent fiber	32.45	20.59
Hemicellulose	13.43	11.03
Cellulose	24,71	15.58
Lignin	5,79	3.26

 

The design used was a completely randomized design with 4 treatments and 4 replications. Treatments consisted of rations containing 70% haylage + 30% concentrate + 0 g probiotic (control), control + 0.05 % Saccharomyces cerevisiae (SC), control + 0.05% Aspergillus oryzae (AO), and control + 0.025 % Saccharomyces cerevisiae (SC) + 0.025 % Aspergillus oryzae (AO).

 

In Vitro Fermentation and Chemical Analyzes

The in vitro study was conducted based on the method of Theodorou dan Brooks (1990). Substrates (1 g) were put into each incubation bottle which already contained incubation medium of 90 ml buffer (pH 7.9) and 10 ml buffalo rumen fluid. Rumen fluid was taken from a 2.5-year old male swamp buffalo prior to feeding time in the morning. The buffalo was fed swamp grass (bento rayape grass) and concentrate (70:30). The rumen fluid was taken with a stomach tube and a vacuum pump. The collected rumen fluid was taken to the laboratory where it was filtered by using a 100-μm nylon sieve and added to a solution consisting of 100 ml rumen fluid and 900 ml buffer solution.

 

Table 3: Ingredients and nutrients composition of the experimental ration.

 

Ingredients (as fed)	Ration
Haylage (%)	70
Concentrate (%)	30
TOTAL	100
Nutrient composition (% dry matter)
Dry matter	45.32
Organic matter	91.44
Crude protein	16.18
Crude fiber	20.99
Ether extract	5.034
Free Nitrogen Extract	55.87
TDN	68.55
Neutral detergent fiber	41.63
Acid detergent fiber	28.90
Hemicellulose	12.71
Cellulose	20.61
Lignin	5.03

Source: Nutrition and Animal Feed Laboratory, Faculty of Agriculture, University of Sriwijaya, 2019.

 

These anaerobic media were then put into incubation bottles (100 ml medium per bottle). The CO2 gas was added for an optimal aerobic condition. The filled bottles were incubated in water bath incubators (39 o C) for 48 hours. In the end of the incubation time, 2 drops of HgCl2 was added into each bottle. Then, samples were centrifuged at 4000 rpm for 10 minutes until the supernatant was separated from the residue. The supernatant was kept and analyzed for N-NH3, and volatile fatty acids (VFA) concentrations, methane production, and total bacterial count. N-NH3 concentration was determined by using modified Conway procedures (General Laboratory Procedures, 1966). VFA content was determined in a gas liquid chromatography (GLC Bruker Scion 436-GC, Bruker Daltonik GmbH, Bremen, Germany) by using a BR-Wax fame column (mmlD 0.32, 0.25 lm df) and FID detector. Supernatant preparation obtained from in vitro incubation was added with 3 mg sufosalicylic acid dihydrate and centrifuged at 12,000 rpm 7 o C for 10 minutes before it was injected into GLC column. Quantification of individual VFA was conducted by comparing it with an external standard and expressed in μmol/ml or mM. Total VFA content was measured as the sum of all individual VFA contents (AOAC, 2005). Samples of 3 mL gas resulted from fermentation were taken by using a syringe glass, stored in venoject tubes. Methane gas concentration was measured after 48-hour incubation by using a gas chromatography device (Hitachi Model 26-50). The device was equipped with an FID detector and an active carbon column of 1 m length and 3 mm diameter. The temperatures in injector, detector, and column was 190, 190, and 170oC, respectively. Gas flow rates were set at 50 ml/minute for N2 gas, 1 kg/cm2 for H2, and 1 kg/cm for air (Menke and Steingass procedures, 1988). Total bacterial count was determined by a roll tube method (Ogimoto and Imai procedures, 1988). Analysis of nutrients content was conducted on the residue of incubation which were added with 50 mL pepsin-HCl 0.20% and incubated for 48 hours. The solution was filtered by using Whatman 41 filter paper and the residue was then dried at 60oC for 48 hours.

 

Dry matter and organic matter digestibility was measured after 48-hour incubation. Sinterglasses were oven-dried at 105oC for 24 hours before they were weighed. Dry matter digestibility was determined by drying the incubation residue in an oven at 105oC for 24 hours. Organic matter digestibility was determined by ashing the incubation residue in a furnace at 550oC for 6 hours.

 

Data Analysis

Data of dry matter digestibility (DDM), organic matter digestibility (OMD), rumen pH, N ammonia (N-NH3) concentration, total volatile fatty acid (TVFA) concentration, partial VFA concentration, methane gas concentration, and total bacterial count were subjected to an analysis of variance and a Duncan multiple range test (SPSS 13.0 program).

 

RESULTS AND DISCUSSION

 

Dry Matter and Organic Matter Digestibility

Results showed that the probiotics supplementation in the rations improved (P<0.05) DMD and OMD. DMD and OMD were found to be the highest in SC+AO and those in SC, AO, and control were also different (Table 4).

 

The SC+AO supplementation in ration basal gave the highest DMD and OMD values. This might result from the synergy between the two kinds of probiotics used. S. Cereviseae was able to produce amylase enzyme to digest starch and AO produced cellulose and hemicellulose enzymes digesting fibers (cellulose and hemicellulose) (Alshaikh et al., 2002). In addition, SC is a growth factor for cellulolytic bacteria as it provides nutrients including vitamins, minerals, and amino acids for rumen bacterial growth (Khan et al., 2016). Increased population of cellulolytic bacteria improves cellulolytic activity resulting in improved fiber digestion. This later leads to increased DM and OM digestibility. Compared to combined probiotic supplementation, individual probiotic supplementation resulted in lower DMD and OMD. The values of DMD and OMD in SC were higher than that in AO. This might be attributed to the role of yeast (SC) in providing substrates in the form of organic acids and vitamins which promotes the growth of lactic acid bacteria (LAB) (Khan et al., 2016). By stabilizing rumen pH, increased LAB population improves rumen metabolism and cellulolytic bacteria population, reduces lactic acid accumulation in digestive tract, reduces oxygen concentration through glycolysis process, improves ration digestibility and rumen fermentability, and affects animal performance (Kumar et al., 2013; Pinloche et al., 2013).

 

Table 4: Effect of Saccharomyces cereviceae (SC) and Aspergillus oryzae (AO) supplementations of mixed haylage (70%) and concentrate (30%) ration on Digestibility of Dry Matter (DM) and Organic Matter (OM)

 

Treatment	Digestibility (%)
	DM	OM
Control	51.28±1.46a	60.83±1.46a
SC	57.56±0.54c	67.77±1.76c
AO	55.97±0.47b	64.64±0.94b
SC+AO	59.89±0.45d	69.86±0.71d

Note: Different superscripts in the same column indicate significant differences (P<0.05

 

Table 5: Effect of Saccharomyces cereviceae (SC) and Aspergillus oryzae (AO) supplementations of mixed haylage (70%) and concentrate (30%) ration on Fermentation Characteristics (pH, N-NH3, and Total VFA Concentration)

 

Treatment	pH	N-NH3 (mM)	Total VFA (mM)
Control	6.59±0.04a	11.95±1.57c	67.72±3.28a
SC	6.70±0.05b	7.61±1.01ab	104.61±5.92c
AO	6.69±0.06b	8.18±1.01b	85.63±2.97b
SC+AO	6.72±0.03b	7.18±0.38a	122.98±3.42d

Note: Different superscripts in the same column indicate significant differences (P<0.05).

 

Characteristics of rumen fermentation

Compared to that in control group, rumen pH in the probiotic supplementation groups was significantly (P<0.05) higher. Yet, no different rumen pH was found in treatment groups (Table 5). All treatments contributed to increases in rumen TVFA (P<0.05) with the highest found in SC + AO and the lowest in control (Table 5). Compared to that in control group, rumen N-NH3 contents in the probiotic supplementation groups were lowered. The lowest rumen N-NH3 content was found in SC + AO. However, no different rumen N-NH3 content was found in SC, AO, and SC + AO (Table 5).

 

Rumen pH

Rumen pH was elevated as the effect of the probiotics supplementation. This was indicated by rumen pH of 6.61 to 6.75 which was still in the normal range (6.2 to 7.00) (Barber et al., 2010) so that no disturbance in rumen microbial growth was found. Furthermore, in addition to providing nutrients and important cofactors stimulating rumen microbial activity, SC probiotic also plays a role in controlling rumen environment to be more anaerobic. This condition stimulates rumen microbial growth followed by improved ammonia and lactic acid utilization to stabilize rumen pH. The inclusion of SC in rations with high energy content was found to stabilize rumen pH and avoid the occurrence of acidosis (Pantaya et al., 2010). It was also reported that balanced fermentation products (VFA and N-NH3) could maintain rumen pH (Mc Donald et al., 2010).

TVFA

The content of TVFA in this study was still within normal range for the optimal activity and growth of rumen microbes. The optimal VFA content required for rumen microbial growth was 80–160 mM (Mc Donald et al., 2010). The highest VFA production which might be caused by high ration digestibility was found in SC+AO supplementation. The increased DMD was found to improve ration fermentation to produce volatile fatty acids (VFA) (Mc Donald et al., 2010). The effects of individual probiotic supplementation on TVFA content were in line with those on ration digestibility. The composition of VFA produced in rumen was affected by a fermented substrate, microbe population, and rumen ecology (Bannink et al., 2008).

 

N-NH3

It was reported that probiotic supplementation lowered rumen N-NH3 content (Mohammed et al., 2018) that was in line with the result of the present study. The concentration of N-NH3 resulted from protein degradation by rumen microbes are used as a nitrogen source for microbial protein synthesis (Mc Donald et al., 2010). N-NH3 concentration was found to be the highest in control indicating less N-NH3 utilization for rumen microbial growth. The N-NH3 concentration in SC and AO was not different but there were higher than that in SC+AO. This indicated that, compared to combined probiotic (SC+AO) supplementation, individual probiotic supplementation (SC and AO) resulted in lower N-NH3 utilization by rumen microbes for microbial protein synthesis. Higher N-NH3 utilization for rumen microbial protein synthesis by rumen microbes in SC+AO was reflected in higher total rumen bacterial count (Table 4). Rumen ammonia concentration found in this study was higher than the optimal N-NH3 requirement for microbial protein synthesis. Rumen microbial growth required an optimal ammonia concentration of about 4 to 21 mM (Yuan, 2010).

 

Partial VFA and C2/C3 Ratio

With regard to the effects of treatments on partial VFA concentration, it was found that C2 proportion was 67.99, 58.85, 58.79, and 58.67% and C3 proportion was 20.05, 25.16, 26.24, and 26.98% in control, SC, AO, SC+AO, respectively (Table 6). The means of rumen C2/C3 ratio were reduced with the probiotic supplementations with the lowest in SC+AO and the highest in control (Table 4).

Partial VFA concentration is the result of structural and non-structural carbohydrate fermentation by rumen microorganisms. Partial VFA is also produced from microbial and feed protein fermentation (McDonald et al., 2010). In this study, probiotic supplementation resulted in C2 and C3 in a proportion closed to that presented by Arora (1995), namely about 50 - 65% C2 and 18 - 24% C3.

 

Table 6: Effect of Saccharomyces cereviceae (SC) and Aspergillus oryzae (AO) supplementations of mixed haylage (70%) and concentrate (30%) ration on partial VFA concentration and ratio of C2/C3 production in rumen

 

Treatment	C2 (mM)	C3 (mM)	C4 (mM)	Ratio C2/C3
Control	46.04± 2.54a	13.58± 0.99a	8.10± 0.47a	3.40± 0.18c
SC	61.50± 4.72c	27.45± 1.41c	15.66± 1.47c	2.24± 0.13ab
AO	50.40± 1.72b	21.55± 0.93b	13.69± 0.88b	2.34± 0.11b
SC+AO	72.15± 5.28d	33.18± 1.01d	17.65± 1.21d	2.18± 0.22a

Note: Different superscripts in the same column indicate significant differences (P<0.05).

C2: Acetate , C3: Propionate, C4; Butyrate

 

The C2/C3 ratios in SC+AO and AO were different, while in AO and SC were not different. The low C2/C3 ratio in SC+AO was caused by the notion that probiotic supplementation stimulated starch-degrading bacteria to produce propionic acid in higher concentration than acetic acid. The SC supplementation was found to decrease C2 and increase C3 (Inal et al., 2010). Meanwhile, AO supplementation was found to reduce C2 and C4 production and improve C3 production, DMD, and VFA concentration (Sosa et al., 2011). The C2/C3 ratio is an important parameter in ruminology as low C2/C3 ratio stimulates fattening and body fat formation. Propionic acid is glycogenic and the main precursor of blood glucose formation (Vlaeminck et al., 2006). High C2/C3 ratio found in control indicated that microbial fermentation pattern was directed to acetic acid formation.

 

Methane gas concentration

Methane gas concentration was reduced with probiotic supplementation (P<0.05) as depicted in Figure 1. Methane gas concentration in control was significantly higher than those in the SC, AO, and SC+AO groups. However, methane gas concentration among the probiotics groups were not different. The SC, AO, and SC+AO supplementation were found to increase total bacterial counts as depicted in Figure 2.

 

 

 

The formation of methane gas produced by enteric fermentation of ruminants is determined by the type of VFA produced, the formation of propionate requires H2, while the formation of acetate and butyrate produces H2. This indicates that acetate and butyrate formation triggered the formation of H2 so that it will be used by methanogenic bacteria to be converted into CH4, while high propionate production requires H2 so that the formation of CH4 will decrease (Martin et al., 2009). The C2/C3 ratio has a closed relation with methane gas emission. Low C2/C3 ratio means low methane gas production (Mitsumori and Sun. 2008; Martin et al., 2009). It was shown in this study that C2/C3 ratio was lower in supplementation groups than in control group. Improved rumen fermentability and lower C2/C3 ratio were shown with supplementation of SC and rumen microbe isolates (Riyanti et al., 2016). Increased hydrogen supply for methanogenesis was indicated as the cause of increased C2/C3 ratio and methane gas production (Martin et al., 2009; Wallace et al., 2017). Yeast supplementation was able to stimulate acetogens to compete with methanogen in utilizing hydrogen which made methane gas emission reduced (Mwenya et al., 2004). Lowered methane gas production in rumen improved feed energy availability, which subsequently gave positive effects on animal productivity

 

Total Bacterial Population

The SC, AO, and SC+AO supplementation were found to increase total bacterial counts as depicted in Figure 2. Probiotic supplementation in rations increased the total rumen bacterial population as indicated by high DMD and NH3 and TVFA concentrations within the optimal limit. This condition was an advantage for the increment of microbial protein synthesis. Improved digestibility was found to be in line with enhanced microbial protein synthesis (Nurhaita et al., 2010). This occurred because of a higher supply of carbon frames from TVFA and nitrogen from N-NH3 for substrates to be used by rumen microbes for their growth (McDonald et al., 2010). Zain et al. (2011) showed that the inclusion of probiotic in ruminant rations consisting of roughage and concentrate improved bacterial population growth. This contributed to the availability of microbial protein sources supplying about 70-90% amino acid requirement of host animal (Russell et al., 2009).

 

CONCLUSIONS

 

It was concluded that combination 0.025 % Saccharomyces cerevisiae and 0.025 % Aspergillus oryzae in the swamp roughage haylage-based rations gave the best increase in the ration digestibility, total rumen bacterial count, rumen fermentation characteristics, and reduced methane emissions. This present study recommend that the probiotic combination should be tested in an in vivo study to determine the effect on the performance of ruminants.

 

CONFLICT OF INTEREST

 

We declare that in this research there is no conflict of interests.

 

ACKNOWLEDGEMENT

 

Research fund assistance from University of Sriwijaya through a featured competitive research scheme Number 0015/UN9/SK.LP2M.PT/2019 was acknowledged.

 

AUTHOR’S CONTRIBUTION

 

Riswandi: original concept, study design and conducted the experiment. Asep Indra M. Ali: involvement in conduct of the experiment. Basuni Hamzah and Agus Supriadi: responsible for statistical analysis. Sofia Sandi and Afnur Imsya: significant contribution to discussion and writing, critical revisions of the article. All authors have read and agree with the final version of the manuscript.

 

REFERENCES

 

• Alshaikh, MA, AY Alsiadi, SM Zahran, HH Mugawer, TA Aalshowime (2002). Effect of feeding yeast culture from different sources on the perfor- mance of lactating holstein cows in saudi arabia. Asian-Australia. J. Anim. Sci. 15(3): 352-355. https://doi.org/10.5713/ajas.2002.352

• Ali AIM, Sandi S, Muhakka, Riswandi, Budianta D (2013). The Grazing of Pampangan Buffaloes at Non Tidal Swamp in South Sumatra of Indonesia. APCBEE Procedia ICAAA 2013: July 27-28, Moscow, Russia. https://www.researchgate.net › publication › 280.

• Ali AIM, Sandi S, Riswandi, Imsya A, Prabowo A, Rofiq N (2015). Evaluation of yeast supplementation with urea-molasses in rice straw-based diets on in vitro ruminant fermentation. Pakistan J. Nutrit. 14(2):988-993. https://scialert.net/abstract/?doi=pjn.2015.988.993

• AOAC (2005). Offical Methods of Analysis. 18th ed. Association of Official Analytical Chemists, USA.

• Arora, SP (1995). Pencernaan Mikroba pada Ruminansia. Edisi Indonesia. Penerbit Gajah Mada University Press. Yogyakarta

• Bannink A, France J, López S, Gerrits WJJ, Kebreab E, Tamminga S, Dijkstra J (2008). Modelling the implications of feeding strategy on rumen fermentation and functioning of the rumen wall. Anim. Feed Sci. Technol. 143:3–26.

• Barber D, Anstis A, Posada V (2010). Managing for Healthy Rumen Function. Departement of Employment, Economic, and Innovation. Queensland Government.

• General Laboratory Prosedures (1966). Department of Dairy Science. University of Wisconsin, Madison.

• Inal F, E Gurbuz, B Coskun, MS Alatas, OB Citil, ES Polat, E Seker dan C Ozcan (2010). The effect of live yeast culture (Saccharomyces cerevisiae) on rumen fermentation and nutrient degradability in Yearling Lambs. Kakfas Univ. Vet. Fak. Derg. 16(5): 799-804. https://www.researchgate.net/publication/281535435

• Jaelani A, Rostini T, Zakir MI dan Jonathan (2014). Pengaruh penggunaan hijauan rawa fermentasi terhadap penampilan kambing kacang (Capra hircus). J. Sains Peternakan. 12(2): 76-85. https://doi.org/10.20961/sainspet.v12i2.4770

• Khan RU, Shabana N, Kuldeep D, Karthik K, Ruchi T, Mutassim MA (2016). Direct-fed microbial: beneficial applications, modes of action and prospects as a safe tool for enhancing ruminant production and safeguarding health. Int. J. Pharm. 12:220-31. http://scialert.net/fulltext/?doi=ijp.

• Kumar DS, Srinivasa Prasad Ch, Prasad RMV (2013). Effect of yeast culture (Saccharomyces cerevisiae) on ruminal microbial population in buffalo bulls. Buffalo Bulletin. 32: 116-119. DOI: 10.14456/ku-bufbu.2013.18

• Lazzarini I, Detmann E, Sebastião de Campos VF, Paulino MF, Erick DB, Luana M de Almeida R, William LS dos R, Marcia de OF (2016). Nutritional performance of cattle grazing during rainy season with nitrogen and starch supplementation. Asian Australas. J. Anim. Sci. 29 (8): 1120-1128. https://doi.org/10.5713/ajas.15.0514

• Menke KH dan, H Steingass (1988). Estimation of the energetic feed value obtained from chemichal analysis and in vitro gas production using rumen fluid. Anim. Res. Develop. 28:7–55.

• Martin C, Morgavi DP, Doreau M (2009). Methane mitigation in ruminants: from microbe to the farm scale. Animal. 4:351–65. https://doi.org/10.1017/S1751731109990620.

• McDonald P, Edwards RA, Greenhalagh JFD, Morgan CA, Sinclair LA,Wilkinson RG (2010). Animal Nutrition. Seventh Ed. New York. C A Morgan, JFD Greenhalgh, LA Sinclair and RG Wilkinson, Inc.

• Mitsumori, M, W Sun (2008). Control of rumen microbial fermentation for mitigating methane emissions from the rumen. Asian-Aust. J. Anim. Sci. 21:144-154. https://doi.org/10.5713/ajas.2008.r01

• Muhammed A, Arowolo, Jianhua He (2018). Use of probiotics and botanical extracts to improve ruminant production in the tropics: A review. Anim. Nutrit. 4 (2018): 241-249. https://doi.org/10.1016/j.aninu.2018.04.010.

• Mwenya, B, B Santoso, C Sar, Y Gamo, T Kobayashi, I Arai, J Takahashi (2004). Effects of including 1,4-galacto- oligosaccarides, lactic acid bacteria rr yeast culture on methanogenesis as well as energy and nitrogen metabolism in sheep. Anim. Feed Sci. Technol., 115: 313-326. https://doi.org/10.1016/j.anifeedsci.2004.03.007

• Nurhaita, N Jamarun, L Warly, Mardiati Z (2010). Sintesis protein mikroba pada domba yang mendapat ransum daun sawit amoniasi yang disuplementasi mineral S, P dan daun ubi kayu. J. Penelitian Universitas Jambi Seri Sains. 12: 107-114.

• Ogimoto K dan Imai S (1988). Atlas of Ruminant Microbiology. Tokyo: Japan Sci. Press. Japan.

• Pantaya D, Wiryawan KM, Amirroenas DE., dan Suryahadi (2016). Detoksifikasi mikotoksin melalui optimalisasi fungsi rumen dengan pemberian ragi. J. Vet. 17(1): 143-154. https://ojs.unud.ac.id/index.php/jvet/article/view/19178>

• Philippe FX dan B Nicks (2015). Review on greenhouse gas emissions from pig houses: Production of carbon dioxide, methane and nitrous oxide by animals and manure. Agri. Eco. Env. 199 10-25. https://doi.org/10.1016/j.agee.2014.08.015

• Pinloche E, McEwan N, Marden J-P, Bayourthe C, Auclair E, Newbold C-J (2013). The effects of a probiotic yeast on the bacterial diversity and population structure in the rumen of cattle. PLoS. 8(7): e67824. https://doi.org/10.1371/journal.pone.0067824

• Riswandi (2014). Evaluasi Kecernaan Silase Rumput Kumpai (Hymenachne acutigluma) dengan Penambahan Legum Turi Mini (Sesbania rostrata). J. Peternakan Sriwijaya. 3(2):43-52. DOI: https://doi.org/10.33230/JPS.3.2.2014.1761

• Riswandi, Hamzah B, Wijaya A, Abrar A (2020). Bali heifers performance on cassava leaves, palm oil sludge and yeast supplementation in a ration based on Kumpai grass (Hymenachne amplexicaulis (Rudge) Nees). Adv. Anim. Vet. Sci. 8(8): 813-818. https://doi.org/10.17582/journal.aavs/2020/8.8.813.818

• Riyanti L, Suryahadi and Evvyernie D (2016). In vitro fermentation characteristics and rumen microbial population of diet supplemented with saccharomyces cerevisiae and rumen microbe probiotics. J. Media Peternakan. 39(1):40-45. https://doi.org/10.5398/medpet.2016.39.1.40

• Russell JB, Muck RE, Weimer PJ (2009). Quantitative analysis of cellulose degradation and growth of cellulolytic bacteria in the rumen. FEMS Microbiol. Ecol. 67:183-197. https://doi.org/10.1111/j.1574-6941.2008.00633.x

• Schroeder JW (2013). Silage Fermentation and Preservation. Extension Dairy Speciaslist. AS-1254. http://www.ext.nodak.edu/.

• Seo JK, SW Kim, MH Kim, SD Upadhaya, DK Kam, JK Ha (2010). Direct-fed microbials for ruminant animals. Asian-Aust. J. Anim. Sci. 23:1657-1667. DOI: https://doi.org/10.5713/ajas.2010.r.08

• Sosa A, J Galindo, ML Tejido, A Diaz, ME Martinez, C Saro, MD Carro, MJ Ranilla (2011). Effects of Aspergillus oryzae on in vitro ruminal fermentation. Options Mediterraneennes: Série A. Séminaires Mediterraneens; n. 99. http://om.ciheam.org/article.php?IDPDF=801553

• Theodorou MK, Brooks AE (1990). Evaluation of A New Laboratory Procedure for Estimating the Fermentation Kinetics of Tropical Feeds. Annual Report. Meidenhead (GB): AFRC Inst.

• Vlaeminck B, V Fievez, S Tamminga, RJ Dewhurst, A Van Vuuren, D De rabander, D Demeyer (2006). Milk odd-and branched-chain fatty acids in relation to the rumen fermentation pattern. J. Dairy Sci. 89(10):3954–3964. https://doi.org/10.3168/jds.S0022-0302(06)72437-7

• Wallace RJ, Snelling TJ, McCartney CA, Tapio I, Strozzi F (2017). Application of metaomics techniques to understand greenhouse gas emissions originating from ruminal metabolism. Genet. Select. Evol. 49: 9. https://doi.org/10.1186/s12711-017-0285-6

• Yuan ZQ (2010). Effects of dietary supplementation with alkyl polyglycoside, a non ionic surfactant, on nutrient digestion and ruminal fermentation in goats. J. Anim. Sci. 88:3984-3991. https://doi.org/10.2527/jas.2009-2397

• Zain M, Arnim RWS, Ningrat, Herawati (2011). Effect of yeast (Saccharomyces cerevisiae) on fermentability, microbial population and digestibility of low quality roughage (in vitro). Arch Zootecnica. 14:4-11. https://www.researchgate.net/publication/224860252