Abstract

The objective of this work was to evaluate the effect of diets with cottonseed and vitamin E on the greenhouse gas (GHG) emissions of cattle feces analyzed in biodigesters. Animal feces were evaluated in 18 laboratory-scale anaerobic digesters through the following treatments: control, feces from cows fed with a control diet; CS, feces from cows fed with a cottonseed diet; and CSVitE, feces from cows fed with a cottonseed and vitamin E diet. The production of biogas and GHG was measured, and no differences were observed for the emissions of biogas, methane (CH₄), and nitrous oxide (N₂O). The partition of the gross energy of the anaerobic digestion process was similar among treatments. On average, 28% of gross energy fed was released as CH₄, 47% during digestion, and 25% as other gases and heat. The addition of cottonseeds to the cattle diet increases the concentration of CH₄ and reduces that of CO₂, but it does not affect the total production of CH₄, CO₂, and N₂O in the biodigesters. The inclusion of vitamin E in the diet has no effect on GHG production in the biodigesters.

Index terms:
anaerobic digestion; methane; nitrous oxide

Resumo

O objetivo deste trabalho foi avaliar o efeito de dietas com caroço de algodão e vitamina E nas emissões de gases de efeito estufa (GEE) de fezes de bovinos analisadas em biodigestores. As fezes dos animais foram avaliadas em 18 digestores anaeróbios de laboratório, por meio dos seguintes tratamentos: controle, fezes de vacas alimentadas com dieta controle; CS, fezes de vacas alimentadas com caroço de algodão; e CSVitE, fezes de vacas alimentadas com caroço de algodão e vitamina E. As produções de biogás e GEE foram medidas, e não foram observadas diferenças para as emissões de biogás, metano (CH₄) e óxido nitroso (N₂O). A partição da energia bruta do processo de digestão anaeróbica foi semelhante entre os tratamentos. Em média, 28% da energia bruta alimentada foi liberada como CH₄, 47% na digestão e 25% como outros gases e calor. A adição de caroços de algodão na dieta de bovinos aumenta a concentração de CH₄ e reduz a de CO₂, mas não afeta a produção total de CH₄, CO₂ e N₂O nos biodigestores. A inclusão de vitamina E na dieta não tem efeito na produção de GEE nos biodigestores.

Termos para indexação:
digestão anaeróbia; metano; óxido nitroso

Introduction

The manipulation of rumen fermentation by supplementing lipids in cattle diets (Wanapat et al., 2015WANAPAT, M.; CHERDTHONG, A.; PHESATCHA, K.; KANG, S. Dietary sources and their effects on animal production and environmental sustainability. Animal Nutrition, v.1, p.96-103, 2015. DOI: https://doi.org/10.1016/j.aninu.2015.07.004.
https://doi.org/10.1016/j.aninu.2015.07.... ) is a strategy to reduce methane (CH₄) emissions by ruminants. Lipids are directly related to fatty acids and, therefore, may be increased in diets through the addition of ingredients such as cottonseed (CS), a byproduct of the cotton fiber industry extensively used as a source of fiber, protein, or fat in cattle nutrition (Warner et al., 2020WARNER, A.L.; BECK, P.A.; FOOTE, A.P.; PIERCE, K.N.; ROBISON, C.A.; HUBBELL, D.S.; WILSON, B.K. Effects of utilizing cotton byproducts in a finishing diet on beef cattle performance, carcass traits, fecal characteristics, and plasma metabolites. Journal of Animal Science, v.98, p.1-9, 2020. DOI: https://doi.org/10.1093/jas/skaa038.
https://doi.org/10.1093/jas/skaa038... ), due to its high concentrations of unsaturated fatty acids (Paim et al., 2014PAIM, T. do P.; VIANA, P.; BRANDÃO, E.; AMADOR, S.; BARBOSA, T.; CARDOSO, C.; DANTAS, A.M.M.; SOUZA, J.R. de; MCMANUS, C.; ABDALLA, A.L.; LOUVANDINI, H. Carcass traits and fatty acid profile of meat from lambs fed different cottonseed by-products. Small Ruminant Research, v.116, p.71-77, 2014. DOI: https://doi.org/10.1016/j.smallrumres.2013.11.004.
https://doi.org/10.1016/j.smallrumres.20... ). Vitamin E has been used in animal feed to inhibit lipid oxidation (Juárez et al., 2012JUÁREZ, M.; DUGAN, M.E.R.; ALDAI, N.; BASARAB, J.A.; BARON, V.S.; MCALLISTERT, T.A.; AALHUS, J.L. Beef quality attributes as affected by increasing the intramuscular levels of vitamin E and omega-3 fatty acids. Meat Science, v.90, p.764-769, 2012. DOI: https://doi.org/10.1016/j.meatsci.2011.11.010.
https://doi.org/10.1016/j.meatsci.2011.1... ), relieving the effects caused by free radicals, being beneficial to rumen fermentation as it improves nutrient digestibility (Vázquez-Añón & Jenkins, 2007VÁZQUEZ-AÑÓN, M.; JENKINS, T. Effects of feeding oxidized fat with or without dietary antioxidants on nutrient digestibility, microbial nitrogen, and fatty acid metabolism. Journal of Dairy Science, v.90, p.4361-4367, 2007. DOI: https://doi.org/10.3168/jds.2006-858.
https://doi.org/10.3168/jds.2006-858... ).

Some studies have investigated the addition of whole cottonseed and vitamin E in cattle diets (Polviset et al., 2015POLVISET, W.; SCHONEWILLE, J.T.; EVERTS, H.; WACHIRAPAKORN, YUANGKLANG, C.; CLAEYS, E.; DE SMET, S. Effect of whole cottonseed v. sunflower seed on the fatty acid profile of subcutaneous fat, longissimus dorsi and blood of Thai Native and Holstein bulls. Animal, v.9, p.2072-2080, 2015. DOI: https://doi.org/10.1017/S1751731115001627.
https://doi.org/10.1017/S175173111500162... ; Ferrinho et al., 2018FERRINHO, A.M.; NASSU, R.T.; ALDAI, N.; BRAVO-LAMAS, L.; FURLAN, M.L.N.; TODA, B.M.; UTEMBERGUE, B.L.; REZENDE, R.G.; MUELLER, L.F.; FURLAN, J.J.M.; ZANATA, M.; BALDI, F.; PEREIRA, A.S.C. Whole cottonseed, vitamin E and finishing period affect the fatty acid profile and sensory traits of meat products from Nellore cattle. Meat Science, v.138, p.15-22, 2018. DOI: https://doi.org/10.1016/j.meatsci.2017.12.002.
https://doi.org/10.1016/j.meatsci.2017.1... ; Nogueira et al., 2019NOGUEIRA, R.G.S.; PERNA JUNIOR, F.; PEREIRA, A.S.C.; RODRIGUES, P.H.M. Nutrient digestibility and changes in feeding behavior of cattle fed cottonseed and vitamin E. Scientia Agricola, v.76, p.112-122, 2019. DOI: https://doi.org/10.1590/1678-992X-2017-0334.
https://doi.org/10.1590/1678-992X-2017-0... , 2020NOGUEIRA, R.G.S.; PERNA JUNIOR, F.; PEREIRA, A.S.C.; CASSIANO, E.C.O.; CARVALHO, R.F.; RODRIGUES, P.H.M. Methane mitigation and ruminal fermentation changes in cows fed cottonseed and vitamin E. Scientia Agricola, v.77, e20180247, 2020. DOI: https://doi.org/10.1590/1678-992X-2018-0247.
https://doi.org/10.1590/1678-992X-2018-0... ). However, there are no known works associating these ingredients with the impact of feces on the environment or potential use in digesters. According to Møller et al. (2014)MØLLER, H.B.; MOSET, V.; BRASK, M.; WEISBJERG, R.; LUND, P. Feces composition and manure derived methane yield from dairy cows: influence of diet with focus on fat supplement and roughage type. Atmosphere Environment, v.94, p.36-43, 2014. DOI: https://doi.org/10.1016/j.atmosenv.2014.05.009.
https://doi.org/10.1016/j.atmosenv.2014.... , the effects of changes in cattle’s diets on biogas and GHG emissions from feces need to be further studied.

Biogas is a mixture of CH₄ and carbon dioxide (CO₂) with some trace gases (Mata-Alvarez et al., 2014MATA-ALVAREZ, J.; DOSTA, J.; ROMERO-GÜIZA, M.S.; FONOLL, X.; PECES, M.; ASTALS, S. A critical review on anaerobic co-digestion achievements between 2010 and 2013. Renewable and Sustainable Energy Reviews, v.36, p.412-427, 2014. DOI: https://doi.org/10.1016/j.rser.2014.04.039.
https://doi.org/10.1016/j.rser.2014.04.0... ), resulting from the conversion of organic substrates during the biological process of anaerobic digestion (Zhang et al., 2016ZHANG, Q.; HU, J.; LEE, D.-J. Biogas from anaerobic digestion processes: Research updates. Renewable Energy, v.98, p.108-119, 2016. DOI: https://doi.org/10.1016/j.renene.2016.02.029.
https://doi.org/10.1016/j.renene.2016.02... ). Biogas and CH₄ production from organic matter, defined as volatile solids, have been evaluated during the testing of anaerobic digestion under controlled or monitored conditions in laboratory batch on a small scale (Kunz et al., 2022KUNZ, A.; STEINMETZ, R.L.R.; AMARAL, A.C. do. (Ed.). Fundamentals of anaerobic digestion, biogas purification, use and treatment of digestate. Concórdia: Sbera: Embrapa Suínos e Aves, 2022. 201p.). Nogueira et al. (2020)NOGUEIRA, R.G.S.; PERNA JUNIOR, F.; PEREIRA, A.S.C.; CASSIANO, E.C.O.; CARVALHO, R.F.; RODRIGUES, P.H.M. Methane mitigation and ruminal fermentation changes in cows fed cottonseed and vitamin E. Scientia Agricola, v.77, e20180247, 2020. DOI: https://doi.org/10.1590/1678-992X-2018-0247.
https://doi.org/10.1590/1678-992X-2018-0... , for instance, observed that the inclusion of cottonseed in cattle’s diet improved rumen fermentation and reduced rumen CH₄ production.

However, although different feeding strategies can reduce enteric CH₄ emission from stored manure, or improve ruminal conditions (Benchaar & Hassanat, 2019BENCHAAR, C.; HASSANAT, F. Methane emissions of stored manure from dairy cows fed conventional or brown midrib corn silage. Journal of Dairy Science, v.102, p.10632-10638, 2019. DOI: https://doi.org/10.3168/jds.2019-16822.
https://doi.org/10.3168/jds.2019-16822... ), the subsequent effects on GHG emissions from feces still remain unclear.

The objective of this work was to evaluate the effect of diets with cottonseed and vitamin E on the GHG emissions of cattle feces analyzed in biodigesters.

Materials and Methods

The study was conducted in the summer of 2017 at Universidade de São Paulo, located in the municipality of Pirassununga, in the state of São Paulo, Brazil (21°59'45"S, 47°25'37"W, at 625 m above sea level). The climate of the season is classified as Aw according to the Köppen-Geiger, with an average temperature of 21.7°C and an average annual rainfall of 1,346 mm. The experiment was approved by the ethics committee on animal use of Universidade de São Paulo (application number 0092013).

Six non-pregnant and non-lactating Holstein dairy cows, with an average body weight of 876±16.1 kg, were arranged in individual pens with free access to water and sand bedding. The cows were fed ad libitum twice a day at 8:00 a.m. and 4:00 p.m with a high-energy ration containing 87% of concentrates, using sugarcane bagasse as roughage. Vitamin E (Lutavit E 50, BASF Australia Limited, Victoria, Australia), with 50% alpha-tocopheryl acetate, was included at an amount of 500 IU per day. Three dietary treatments were tested: control, control diet; CS, control diet with 30% cottonseed replacing ground corn grain; and CSVitE, CS diet supplemented with vitamin E. The ingredients and chemical compositions of the diets are presented in Table 1.

For feeding and feces collection, cows were arranged in a 3x3 Latin-square design with three experimental periods, each lasting 21 days. The first 14 days of each period were used for diet adaptation. Feces were collected during the next 15 to 21 days, twice a day, after the cows were fed. Representative samples – feces of each animal in each period – were pooled together for testing. The substrates (feces and water) were prepared to ensure an estimation of 5% of total solids. The feces were loaded into 18 laboratory-scale digesters, consisting of a 75 mm reactor, a 100 mm gasometer, and a 150 mm digester made with three PVC pipes, adapted from Sunada et al. (2018)SUNADA, N. da S.; ORRICO, A.C.A.; ORRICO JUNIOR, M.A.P.; LUCAS JUNIOR, J. de; LOPES, W.R.T.; SCHWINGEL, A.W. Anaerobic co-digestion of animal manure at different waste cooking oil concentrations. Ciência Rural, v.48, e20170517, 2018. DOI: https://doi.org/10.1590/0103-8478cr20170517.
https://doi.org/10.1590/0103-8478cr20170... .

In a chamber with controlled temperature, the anaerobic digestion test was carried out under mesophilic conditions, ideal for digestion kinetics (Metcalf & Eddy, 2014METCALF, L.; EDDY, H. Wastewater Engineering: treatment and resource recovery. 5th ed. New York: McGraw-Hill Education, 2014.). The digesters were arranged in a completely randomized design with three feces treatments (control, CS, and CSVitE) and six replicates. The digesters were started up using the feces with no inoculum. Every 15 days, biogas volume was calculated, through the vertical displacement of the gasometer, in centimeters, and the obtained value was standardized for the conditions of 1.0 atm and 20ºC.

Every time biogas volume was measured, biogas samples were collected with a syringe connected to the gas log on top of the gasometer. The concentrations of CH₄, CO₂ and N₂O were determined using the Trace 1300 gas chromatograph (Thermo Fisher Scientific, Milan, Italy), equipped with a flame ionization detector at 280°C, with a 3.5 m column packed with Porapak N matrix (Merck, Darmstadt, Germany), according to Kamiński et al. (2003)KAMIŃSKI, M.; KARTANOWICS, R.; JASTRZEBSKI, D.; KAMIŃSKI, M.M. Determination of carbon monoxide, methane and carbon dioxide in refinery hydrogen gases and air by gas chromatography. Journal of Chromatography A, v.989, p.277-283, 2003. DOI: https://doi.org/10.1016/S0021-9673(03)00032-3.
https://doi.org/10.1016/S0021-9673(03)00... . The volumes of CH₄, CO₂, and N₂O were calculated by multiplying biogas volume by its respective concentration. Specific gas yield (per gram of volatile solids fed or destroyed) was calculated by dividing the total gas production by the amount of volatile solids fed (before anaerobic digestion) or destroyed (difference between volatile solids fed and eliminated). The test was considered finished when biogas production ceased. The nutrients fed (NF) and eliminated (NE) were then weighed to calculate dry matter (DM) content in grams.

Ingestate and digestate nutrients were obtained using the following equation (Tseu et al., 2021TSEU, R.J.; PERNA JUNIOR, F.; CARVALHO, R.F.; SENE, G.A.; PERES, A.H.; TROPALDI, C.B.; DOS ANJOS, F.; RODRIGUES, P.H.M. Gas emission from waste of cows fed monensin and Acacia mearnsii Tannins . Iranian Journal of Applied Animal Science, v.11, p.443-455, 2021.):

Nutrients (g) = (NF or NE – % × DM fed or eliminated – g) × 100

In addition, nutrient removal (NR) was determined by the equation:

Gross energy release, expressed as CH₄, when expressed as megajoule, was calculated using total CH₄ production in liters and considering the following information about CH₄ molecule: molar volume: 26.22 mol L^-1; molar mass: 16.04 g mol^-1; heat power: 13.16 kcal g^-1.

Other gases and heat released, expressed in megajoule, were determined as follows: gross energy fed minus energy released as CH₄ minus gross energy eliminated in the digestate. The percentage of gross energy released, when expressed as percentage of gross energy fed, was calculated by division between gross energy of CH₄ or other gas and heat or digestate and gross energy fed and multiplied by 100.

Individual feed and feces samples, before and after anaerobic digestion, were collected and composited in representative samples on an equal-weight basis. The samples were dried in a forced-air oven, at 60°C, for 48 hours, and then ground to pass a 1.0 mm Wiley mill screen and analyzed. DM content was determined by method 930.15 of Association of Official Analytical Chemists (AOAC) (Cunniff, 1995CUNNIFF, P. (Ed). Official Methods of Analysis of AOAC International. 16th ed. Arlington: AOAC International, 1995. Official Methods 920.39; 930.15 and 973.18.) in the forced-air oven at 105°C for 2 hours, followed by cold weighing. Nitrogen content was obtained by the micro Kjeldahl method, being multiplied by 6.25 to calculate crude protein (Cunniff, 1995CUNNIFF, P. (Ed). Official Methods of Analysis of AOAC International. 16th ed. Arlington: AOAC International, 1995. Official Methods 920.39; 930.15 and 973.18.). Ether extract was obtained using light petroleum ether in the Soxhlet extraction apparatus, as described in method 920.39 (Cunniff, 1995CUNNIFF, P. (Ed). Official Methods of Analysis of AOAC International. 16th ed. Arlington: AOAC International, 1995. Official Methods 920.39; 930.15 and 973.18.). Neutral detergent fiber, acid detergent fiber, and lignin were determined by the methods described in the literature (Van Soest et al., 1991VAN SOEST, P.J.; ROBERTSON, J.B.; LEWIS, B.A. Methods for dietary fiber, neutral detergent fiber, and nonstarch polysaccharides in relation to animal nutrition. Journal of Dairy Science, v.74, p.3583-3597, 1991. DOI: https://doi.org/10.3168/jds.S0022-0302(91)78551-2.
https://doi.org/10.3168/jds.S0022-0302(9... ), using the Filter Bag Technology (Ankom Technology, Macedon, NY, USA) and heat-stable α-amylase as in method 973.18 (Cunniff, 1995CUNNIFF, P. (Ed). Official Methods of Analysis of AOAC International. 16th ed. Arlington: AOAC International, 1995. Official Methods 920.39; 930.15 and 973.18.). The levels of total solid and volatile solids were measured according to American Public Health Association (APHA) (Rice et al., 2012RICE, E.W.; BAIRD, R.B.; EATON, A.D.; CLESCERI, L.E. (Ed). Standard methods for the examination of water and wastewater. 22nd ed. Washington: APHA, 2012.).

The experimental design was completely randomized, with 18 experimental units (digesters) for the three treatments (control, CS, and CSVitE) and six replicates. Data were tested for residual normality using the Shapiro-Wilk test. Statistical analyses were performed with the SAS, version 9.3, software (SAS Institute Inc., Cary, NC, USA) using mixed model, with the fixed effect of treatment. The analyses included descriptive statistics, in which mean values and standard errors of the mean were calculated. The variables were analyzed using the following model:

where Y_ijkl is the dependent response variable, μ is the overall mean, T is treatment effect, and e_ij is the residual error.

Contrast statements, at p≤0.05, were used to evaluate differences between the following treatments: control vs. CS and CSVitE; and CS vs. CSVitE.

Methane yield curve parameters were estimated from the methane yield of individual digesters using the Gompertz model (Kafle & Chen, 2016KAFLE, G.K.; CHEN, L. Comparison on batch anaerobic digestion of five different livestock manures and prediction of biochemical methane potential (BMP) using different statistical models. Waste Management, v.48, p.492-502, 2016. DOI: https://doi.org/10.1016/j.wasman.2015.10.021.
https://doi.org/10.1016/j.wasman.2015.10... ), specifically through the following three equations:

where y1 is methane yield at anaerobic digestion days, A is asymptotic methane yield, B is the interaction constant, k is the yield constant rate, and exp is the base of natural logarithm 2.7183.

where t₁ is the point of inflection, ln is the logarithmic, B is the interaction constant, and k is yield constant rate.

where y₁ is methane yield at inflection point, A is asymptotic methane yield, and exp is the base of natural logarithm 2.7183.

Results and Discussion

Feces composition differed in accordance with the diet (Table 2). Cottonseed treatments had a higher concentration of total solids (TS), volatile solids (VS), neutral detergent fiber (NDF), acid detergent fiber (ADF), and lignin when compared to the control. This result is indicative that the digesters were not able to destroy the highest content of TS and VS present in the feces from cows fed with the diets with cottonseed, as confirmed by the similar values of TS and VS destroyed. Consequently, there was a higher elimination of TS and VS in the cottonseed treatments.

No significant differences were observed between TS destroyed and VS destroyed, whose mean values were 49.6% and 53.7%, respectively. Volatile solids destroyed are commonly used to measure the performance of processes, being a direct indicator of the metabolic activity of a microorganism community (Møller et al., 2004MØLLER, H.B.; SOMMER, S.G.; AHRING, B.K. Methane productivity of manure, straw and solid fractions of manure. Biomass and Bioenergy, v.26, p.485-495, 2004. DOI: https://doi.org/10.1016/j.biombioe.2003.08.008.
https://doi.org/10.1016/j.biombioe.2003.... ). The destruction of VS in cattle manure in the anaerobic digestion process is typically in the range of 30–45% (Davidsson et al., 2008DAVIDSSON, A.; LOVSTEDT, C.; LA COUR JANSEN, J.; GRUVBERGER, C.; ASPEGREN, H. Co-digestion of grease trap sludge and sewage sludge. Waste Management, v.28, p.986-992, 2008. DOI: https://doi.org/10.1016/j.wasman.2007.03.024.
https://doi.org/10.1016/j.wasman.2007.03... ). In the present study, regardless of the treatment, the destruction of TS and VS was over 45% and was indicative of a sludge stabilization process and good reactor performance in the batch digesters. As a result, feces from cattle fed with cottonseed, with or without vitamin E, did not affect biogas production.

The microorganisms present in the control degraded more protein and organic matter than those in CS and CSVitE (Table 2). These results suggest that the microorganisms in the control had to hydrolyze higher quantities of crude protein and organic matter to obtain nutrients, as reported by Coelho et al. (2022)COELHO, L. de M.; BRITO, L. de F.; MESSANA, J.D.; CARDOSO, A. da S.; CARVALHO, G.M.; TORRES, R. de N.S.; CARLOS, R.S.; MALHEIROS, E.B.; CRUZ, M.C.P. da; BERCHIELLI, T.T. Effects of rumen undegradable protein sources on nitrous oxide, methane and ammonia emission from the manure of feedlot-finished cattle. Scientific Reports, v.12 , art.9166, 2022. DOI: https://doi.org/10.1038/s41598-022-13100-9.
https://doi.org/10.1038/s41598-022-13100... . By contrast, the microorganisms in the CS and CSVitE had to hydrolyze higher quantities of protein from the NDF provided by the cottonseeds included for nutrients. This means that both treatments showed a similar destruction of VS and TS, in addition to CH₄ yield. Although the CS and CSVitE have a higher amount of TS available for the anaerobic digestion, the nutrients consisted of fibrous contents, i.e., of NDF, ADF, and lignin. Orrico Junior et al. (2012)ORRICO JUNIOR, M.A.P.; ORRICO, A.C.A.; LUCAS JUNIOR, J. de; SAMPAIO, A.A.M.; FERNANDES, A.R.M.; OLIVEIRA, E.A. de. Biodigestão anaeróbia dos dejetos da bovinocultura de corte: influência do período, do genótipo e da dieta. Revista Brasileira de Zootecnia, v.41, p.1533-1538, 2012. DOI: https://doi.org/10.1590/S1516-35982012000600030.
https://doi.org/10.1590/S1516-3598201200... concluded that a higher concentration of fiber in the diet may favor a lower reduction in TS and VS, whereas Kafle & Chen (2016)KAFLE, G.K.; CHEN, L. Comparison on batch anaerobic digestion of five different livestock manures and prediction of biochemical methane potential (BMP) using different statistical models. Waste Management, v.48, p.492-502, 2016. DOI: https://doi.org/10.1016/j.wasman.2015.10.021.
https://doi.org/10.1016/j.wasman.2015.10... observed a strong negative relationship, with a coefficient of determination (R²) of 0.85, between lignin concentration in manure and CH₄ emissions. Therefore, although there was an increase in lignin content in the feces in the cottonseed treatments, there was no reduction in CH₄ production.

The crude protein content of a diet is the major factor determining the percentage of excreted N (Külling et al., 2002KÜLLING, D.R.; DOHME, F.; MENZI, H.; SUTTER, F.; LISCHER, P.; KREUZER, M. Methane emissions of differently fed dairy cows and corresponding methane and nitrogen emissions from their manure during storage. Environmental Monitoring and Assessment, v.79, p.129-150, 2002. DOI: https://doi.org/10.1023/A:1020248700255.
https://doi.org/10.1023/A:1020248700255... ). In the present study, a similar crude protein content was obtained for the diet and substracts used to load the digesters (Table 2). Since the crude protein content in the substracts is a N source for the process of N₂O production, a similar crude protein resulted in similar N₂O emissions. The same authors reported similar N losses and N₂O emissions for dairy manure under anaerobic conditions.

Regarding biogas, methane concentration was higher in the cottonseed treatments when compared to the control (Table 3). However, the diets with cottonseeds had a lower CO₂ concentration, CO₂ yield per gram of VS fed, and CO₂ yield per gram of VS destroyed, with no significant differences for the N₂O concentration in the biogas, total yield, yield, and Gompertz parameters. Mathot et al. (2012)MATHOT, M.; DECRUYENAERE, V.; STILMANT, D.; LAMBERT, R. Effect of cattle diet and manure storage conditions on carbon dioxide, methane and nitrous oxide emissions from tie-stall barns and stored solid manure. Agriculture, Ecosystems & Environment, v.148, p.134-144, 2012. DOI: https://doi.org/10.1016/j.agee.2011.11.012.
https://doi.org/10.1016/j.agee.2011.11.0... only found alterations in feces characteristics but not in GHG emissions, as observed here.

To calculate the CH₄ emissions of the manure of dairy cows, the Intergovernmental Panel on Climate Change (IPCC) (Dong et al., 2006DONG, H.; MANGINO, J.; MCALLISTER, T.A.; HATFIELD, J.L.; JOHNSON, D.E.; LASSEY, K.R.; LIMA, M.A. de; ROMANOVSKAYA, A. Emissions from Livestock and Manure Management. In: EGGLESTON, S.; BUENDIA, K.M.; MIWA, K.; NGARA, T.; TANABE, K. (Ed). 2006 IPCC Guidelines for National Greenhouse Gas Inventories: v.4: Agriculture, forestry and other land use. Kanagawa: iGES, 2006. p.1-87.) suggests a default value of 0.240±36 L g^-1 of VS as the maximum amount of CH₄ producing capacity of manure. In the present work, the specific CH₄ productivity, measured in terms of VS destroyed, was, on average, 0.303 L g^-1 of VS destroyed, which is indicative of the complete degradation of the organic components of the feces. The ultimate CH₄ yield, obtained in terms of VS fed (Dong et al., 2006DONG, H.; MANGINO, J.; MCALLISTER, T.A.; HATFIELD, J.L.; JOHNSON, D.E.; LASSEY, K.R.; LIMA, M.A. de; ROMANOVSKAYA, A. Emissions from Livestock and Manure Management. In: EGGLESTON, S.; BUENDIA, K.M.; MIWA, K.; NGARA, T.; TANABE, K. (Ed). 2006 IPCC Guidelines for National Greenhouse Gas Inventories: v.4: Agriculture, forestry and other land use. Kanagawa: iGES, 2006. p.1-87.), was 54.0% of the theoretical CH₄ yield, confirming the reduction of 53.7% in VS. The ultimate CH₄ yield will always be lower than the theoretical yield because a fraction of the feces is used to synthesize bacterial mass; therefore, the fraction of the organic material lost in the effluent and lignin-containing compounds will only be degraded to a limited degree (Franco et al., 2007FRANCO, A.; MOSQUERA-CORRAL, A.; CAMPOS, J.L.; ROCA, E. Learning to operate anaerobic bioreactors. In: MÉNDEZ-VILAS, A. (Ed.). Communicating Current Research and Educational Topics and Trends in Applied Microbiology. Santiago de Compostela: Formatex, 2007. p.618-627.).

The cumulative CH₄ yield was estimated by the Gompertz curve (Figure 1), which explains most of the variability in the response data, confirmed by the higher R² values obtained. A CH₄ yield difference of 3.4% was observed between the measured and predicted values, which is in alignment with Kafle & Chen (2016)KAFLE, G.K.; CHEN, L. Comparison on batch anaerobic digestion of five different livestock manures and prediction of biochemical methane potential (BMP) using different statistical models. Waste Management, v.48, p.492-502, 2016. DOI: https://doi.org/10.1016/j.wasman.2015.10.021.
https://doi.org/10.1016/j.wasman.2015.10... , who concluded that the Gompertz model was the one that better predicted CH₄ yield. Inflection point was reached, on average, on day 38 when cumulative CH₄ represented 38% of total CH₄ yield, which increased up to day 38, after which it decreased until day 220, when yield ceased.

Figure 1
Cumulative CH₄ production in relation to anaerobic digestion time observed and adjusted by the Gompertz curve. The inflection point represents the day when the maximum point of CH₄ production occurred. Control, control diet; CS, control diet with 30% cottonseed replacing ground corn grain; and CSVitE, CS diet supplemented with vitamin E.

The cottonseed treatments had higher gross energy fed and released in the digestate when compared to the control (Table 4). However, no differences were observed for gross energy released when expressed as a percentage of gross energy fed. On average, 47% of the gross energy fed was released during digestion, 25% as other gas and heat, and 28% as CH₄ that can be used for electricity or heat generation.

Conclusions

1. The addition of cottonseeds to cattle diets increases the concentration of CH₄ and reduces that of CO₂, but does not affect the total yield of CH₄, CO₂ and N₂O in biodigesters.

2. The inclusion of vitamin E in cattle diets has no effect on greenhouse gas production in the biodigesters.

Acknowledgment

To Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), for financial support (grant number 201311983-5).

References

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