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Morphological characteristics, nutritive quality, and methane production of tropical grasses in Brazil

Características morfológicas, qualidade nutritiva e produção de metano de gramíneas tropicais no Brasil

Abstract:

The objective of this work was to evaluate three tropical forage species for their in vitro methane (CH4) production and organic matter degradability, in order to determine the relationships between forage grass nutritive quality and CH4 production. Guinea grass (Megathyrsus maximus), palisade grass (Urochloa brizantha), and signal grass (Urochloa decumbens) were evaluated. Palisade grass showed the highest organic matter, neutral detergent fiber, acid detergent fiber, lignin, and lower-crude protein content. Signal grass had the highest values for hemicellulose and neutral detergent fiber-nitrogen, and the lowest-cellulose content. Guinea grass and signal grass showed a higher-total gas production than palisade grass. Besides, Guinea grass showed an increased CH4 production, and palisade grass showed lower value for truly degraded organic matter, and reduced partitioning factor, in comparison to signal grass. An increased CH4 production was observed in cases of lower hemicellulose and paratitioning factor. The nutritive value and CH4 production of forages may be employed as parameters, aiming at the sustainability of ruminant production.

Index terms:
Brachiaria; Megathyrsus maximus; Urochloa; livestock; methane production

Resumo:

O objetivo deste trabalho foi avaliar três espécies de gramíneas tropicais quanto à produção de metano (CH4) in vitro e à degradabilidade da matéria orgânica, para determinar as relações entre a qualidade nutritiva das gramíneas e a produção de CH4. O capim-colonião (Megathyrsus maximus), o capim-braquiarão (Urochloa brizantha) e o capim-decumbens (Urochloa decumbens) foram avaliados. O capim-braquiarão apresentou os maiores teores de matéria orgânica, fibra em detergente neutro, fibra em detergente ácido e lignina, e o menor teor de proteína bruta. O capim-decumbens apresentou os maiores teores de hemicelulose e de nitrogênio em fibra de detergente neutro, além do menor teor de celulose. O capim-colonião e o capim-decumbens apresentaram maior produção total de gases do que o braquiarão. Além disso, o capim-colonião apresentou maior produção de CH4, e o capim-braquiarão, menor degradabilidade verdadeira da matéria orgânica e reduzido fator de partição, em comparação ao capim-decumbens. Maior produção de CH4 foi verificada em casos em que os valores de hemicelulose e fator de partição foram menores. A qualidade nutricional e a produção de CH4 das forragens avaliadas podem ser utilizadas como parâmetros a serem empregados, que visem a sustentabilidade da produção de ruminantes.

Termos para indexação:
Brachiaria; Megathyrsus maximus; Urochloa; pecuária; produção de metano

Introduction

One of the advantages of grassland-based livestock production is the ruminant ability to transform fibrous feed of relatively low-nutritional value into products for human consumption. Meanwhile, in this process, enteric methane (CH4) is produced under anaerobic conditions in the gastrointestinal tract of these animals, via methanogenic Archaea which use carbon-dioxide (CO2) and hydrogen (H2) to produce CH4 during the microbial degradation of feed (McAllister & Newbold, 2008MCALLISTER, T.A.; NEWBOLD, C.J. Redirecting rumen fermentation to reduce methanogenesis. Australian Journal of Experimental Agriculture, v.48, p.7-13, 2008. DOI: 10.1071/EA07218.
https://doi.org/10.1071/EA07218....
). This process is responsible for a significant loss of dietary energy (5-10%) that could be redirected potentially towards the production of milk and meat (Eckard et al., 2010ECKARD, R.J.; GRAINGER, C.; KLEIN, C.A.M. de. Options for the abatement of methane and nitrous oxide from ruminant production: a review. Livestock Science, v.130, p.47-56, 2010. DOI: 10.1016/j.livsci.2010.02.010.
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; Madsen et al., 2010MADSEN, J.; BJERG, B.S.; HVELPLUND, T.; WEISBJERG, M.R.; LUND, P. Methane and carbon dioxide ratio in excreted air for quantification of the methane production from ruminants. Livestock Science, v.129, p.223-227, 2010. DOI: 10.1016/j.livsci.2010.01.001.
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).

Brazil has the largest commercial herd of ruminants in the world, and a great part of these animals is kept on tropical pastures (Bueno et al., 2015BUENO, I.C.S.; BRANDI, R.A.; FRANZOLIN, R.; BENETEL, G.; FAGUNDES, G.M.; ABDALLA, A.L.; LOUVANDINI, H.; MUIR, J.P. In vitro methane production and tolerance to condensed tannins in five ruminant species. Animal Feed Science and Technology, v.205, p.1-9, 2015. DOI: 10.1016/j.anifeedsci.2015.03.008.
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) which comprises approximately 170 million hectares of grasslands (Bustamante et al., 2012BUSTAMANTE, M.M.C.; NOBRE, C.A.; SMERALDI, R.; AGUIAR, A.P.D.; BARIONI, L.G.; FERREIRA, L.G.; LONGO, K.; MAY, P.; PINTO, A.S.; OMETTO, J.P.H.B. Estimating greenhouse gas emissions from cattle raising in Brazil. Climatic Change, v.115, p.559-577, 2012. DOI: 10.1007/s10584-012-0443-3.
https://doi.org/10.1007/s10584-012-0443-...
). The genus Urochloa (Syn. Brachiaria) is the most dominant, representing 50% of pastures of the country because of its remarkable adaptability and productivity in tropical climate conditions (Cabral et al., 2014CABRAL, C.H.A.; PAULINO, M.F.; DETMANN, E.; VALADARES FILHO, S. de C.; BARROS, L.V. de; VALENTE, É.E.L.; OLIVEIRA BAUER, M. de; CABRAL, C.E.A. Levels of supplementation for grazing beef heifers. Asian-Australasian Journal of Animal Sciences, v.27, p.806-817, 2014. DOI: 10.5713/ajas.2013.13542.
https://doi.org/10.5713/ajas.2013.13542....
; Demarchi et al., 2016DEMARCHI, J.J.A.A.; MANELLA, M.Q.; PRIMAVESI, O.; FRIGHETTO, R.T.S.; ROMERO, L.A.; BERNDT, A.; LIMA, M.A. Effect of seasons on enteric methane emissions from cattle grazing Urochloa brizantha. Journal of Agricultural Science, v.8, p.106-115, 2016. DOI: 10.5539/jas.v8n4p106.
https://doi.org/10.5539/jas.v8n4p106....
). However, despite being more challenging and demanding for soil fertility, the use of Megathyrsus maximus is increasing as an option to forage grass due to its high-leaf production and nutritional value, factors that may allow the achievement of a superior animal performance (Fernandes et al., 2014FERNANDES, F.D.; RAMOS, A.K.B.; JANK, L.; CARVALHO, M.A.; MARTHA JR, G.B.; BRAGA, G.J. Forage yield and nutritive value of Panicum maximum genotypes in the Brazilian savannah. Scientia Agricola, v.71, p.23-29, 2014. DOI: 10.1590/S0103-90162014000100003.
https://doi.org/10.1590/S0103-9016201400...
).

However, around 70% of the Brazilian pasture areas show some degree of degradation (Dias-Filho, 2014DIAS-FILHO, M.B. Diagnóstico das pastagens no Brasil. Belém, PA: Embrapa Amazônia Oriental, 2014. 36p. (Embrapa Amazônia Oriental. Documentos, 402). ; Cherubin et al., 2016CHERUBIN, M.R.; KARLEN, D.L.; FRANCO, A.L.C.; TORMENA, C.A.; CERRI, C.E.P.; DAVIES, C.A.; CERRI, C.C. Soil physical quality response to sugarcane expansion in Brazil. Geoderma, v.267, p.156-168, 2016. DOI: 10.1016/j.geoderma.2016.01.004.
https://doi.org/10.1016/j.geoderma.2016....
), which affects their nutritive value and contributes negatively to the efficiency of the system, impairing animal performance, and increasing the CH4 emission per unit of generated products. In addition to its impacts on the system productivity, CH4 emission by ruminants is also important for the environmental impact of livestock production, since this activity contributes 33% of the anthropogenic emissions of this greenhouse gas, which has a global warming potential 28 times as higher as that of CO2 (Eckard et al., 2010ECKARD, R.J.; GRAINGER, C.; KLEIN, C.A.M. de. Options for the abatement of methane and nitrous oxide from ruminant production: a review. Livestock Science, v.130, p.47-56, 2010. DOI: 10.1016/j.livsci.2010.02.010.
https://doi.org/10.1016/j.livsci.2010.02...
; Hünerberg et al., 2015HÜNERBERG, M.; MCGINN, S.M.; BEAUCHEMIN, K.A.; ENTZ, T.; OKINE, E.K.; HARSTAD, O.M.; MCALLISTER, T.A. Impact of ruminal pH on enteric methane emissions. Journal of Animal Science, v.93, p.1760-1766, 2015. DOI: 10.2527/jas.2014-8469.
https://doi.org/10.2527/jas.2014-8469....
).

Sustainable pasture-based ruminant production may be defined as a system that ensures the adequate generation of animal products and profitability of farming with minimal impact on the environment, making it possible to future generations the use of the land, besides counting with the best use of locally available resources and animal welfare as important aspects of the system (Wathes et al., 2013WATHES, C.M.; BULLER, H.; MAGGS, H.; CAMPBELL, M.L. Livestock production in the UK in the 21st century: a perfect storm averted? Animals, v.3, p.574-583, 2013. DOI: 10.3390/ani3030574.
https://doi.org/10.3390/ani3030574....
). By this definition, the amount and quality of forage fed to animals are considered crucial factors, especially regarding the mitigation of enteric CH4 emissions. Since the ruminant livestock sector is a major contributor to climate change (O’Mara, 2011O’MARA, F.P. The significance of livestock as a contributor to global greenhouse gas emissions today and in the near future. Animal Feed Science and Technology, v.166-167, p.7-15, 2011. DOI: 10.1016/j.anifeedsci.2011.04.074.
https://doi.org/10.1016/j.anifeedsci.201...
), it is essential to investigate and determine the most important factors which affect both directly and indirectly the greenhouse gas emissions in animal production.

The objective of this work was to evaluate three tropical forage for in vitro CH4 production and the organic matter degradability, in order to determine the relationships between forage grass nutritive quality and CH4 production.

Materials and Methods

The experiment was carried out in accordance with the Brazilian laws of ethics in animal experimentation, and it was approved by the Ethics Committee on Use of Animals of the Escola Superior de Agricultura Luiz de Queiroz (CEUA-Esalq/USP).

Samples of three forage species: the cultivar Áries of Guinea grass (Megathyrsus maximus (Jacq.) B.K.Simon & Jacobs), palisade grass (Urochloa brizantha, Syn. Brachiaria brizantha), and signal grass (Urochloa decumbens, Syn. Brachiaria decumbens) were collected from plots located at the experimental station Água Limpa, of Universidade de Brasília, Brasília, DF, Brazil, at 1,014 m altitude. According to the Köppen-Geiger’s classification, the climate is an Aw type, with hot rainy summers, and cold dry winters (Nimer, 1989NIMER, E. Climatologia do Brasil. Rio de Janeiro: IBGE, Departamento de Recursos Naturais e Estudos Ambientais, 1989. 422p.). The predominant soil in the area is classified as an Oxisol, which is acidic showing high-aluminum and low-calcium and magnesium contents (Libano & Felfili, 2006LIBANO, A.M.; FELFILI, J.M. Mudanças temporais na composição florística e na diversidade de um cerrado sensu stricto do Brasil Central em um período de 18 anos (1985-2003). Acta Botanica Brasilica, v.20, p.927-936, 2006. DOI: 10.1590/S0102-33062006000400016.
https://doi.org/10.1590/S0102-3306200600...
).

In December 2007, Guinea grass (15 kg seed ha-1), palisade grass, and signal grass were sown (40 kg seed ha-1) in three different paddocks (0.375 ha each), combined with P and K fertilization (20 kg ha-1 P2O5, and 60 kg ha-1 K2O), followed by N fertilization (70 kg ha-1 N) thirty days later. Each paddock (0.375 ha) contained only a single grass species, and was divided by electric fences into six parcels of 0.0625 ha. Seventy days after sowing, thirty-six Santa Inês lambs aged four months [21.8±4.14 kg body weight (BW)] were used for grazing in each paddock (12 animals per paddock) for seven days, totaling 35 days of experimental period. From December 2007 to May 2008, the environmental conditions were 21.2ºC of mean temperature (27.5ºC maximum, and 14.9ºC minimum), 83% mean relative air humidity, 163.9 W m-2 mean solar radiation and 1,241.8 mm of accumulated rainfall. Four samples of 0.25 m2 were collected in random spots in each parcel, leaving plant residues with 20 cm height (grazing portion of the forage) prior (pre-grazing) to the entrance of animals in each experimental parcels, which was carried out respecting a 7-day interval.

The collected material was weighed and, then, dried at 60ºC, for 72 hours in air-forced oven. After that, samples were weighed for determining grass morphological characteristics (kg ha-1) such as available biomass, leaf, stem, and senescent material. Grass nutritive value was assessed by chemical analysis and in vitro gas-production assay, at the laboratory of animal nutrition of the Centro de Energia Nuclear na Agriculture (Lana-Cena/USP).

Dry matter (DM), organic matter (OM), and crude protein (CP) concentrations were determined according to Horwitz et al. (2011)HORWITZ, W.; LATIMER JR., G.W. (Ed.). Official methods of analysis of AOAC International. 18th ed. Gaithersburg: AOAC International, 2011. 2590p. . Neutral detergent fiber (NDF) was analyzed according to Mertens (2002)MERTENS, D.R. Gravimetric determination of amylase-treated neutral detergent fiber in feeds with refluxing in beakers or crucibles: collaborative study. Journal of AOAC International, v.85, p.1217-1240, 2002., and acid detergent fiber (ADF) and lignin (ADL) were determined sequentially, following the methodology of Van Soest et al. (1991)VAN 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: 10.3168/jds.S0022-0302(91)78551-2.
https://doi.org/10.3168/jds.S0022-0302(9...
. Hemicellulose (HEMI) and cellulose (CEL) were calculated by the differences between NDF, ADF, and ADL. Also, nitrogen bound to NDF (N-NDF) was determined according to Sniffen et al. (1992)SNIFFEN, C.J.; O’CONNOR, J.D.; VAN SOEST, P.J.; FOX, D.G.; RUSSELL, J.B. A net carbohydrate and protein system for evaluating cattle diets: II. Carbohydrate and protein availability. Journal of Animal Science, v.70, p.3562-3577, 1992. DOI: 10.2527/1992.70113562x.
https://doi.org/10.2527/1992.70113562x....
.

The CH4 production, the truly degraded organic matter (TDOM), and the in vitro gas production assays were conducted according to the methodology described by Theodorou et al. (1994)THEODOROU, M.K.; WILLIAMS, B.A.; DHANOA, M.S.; MCALLAN, A.B.; FRANCE, J. A simple gas production method using a pressure transducer to determine the fermentation kinetics of ruminant feeds. Animal Feed Science and Technology, v.48, p.185-197, 1994. DOI: 10.1016/0377-8401(94)90171-6.
https://doi.org/10.1016/0377-8401(94)901...
and Mauricio et al. (1999MAURICIO, R.M.; MOULD, F.L.; DHANOA, M.S.; OWEN, E.; CHANNA, K.S.; THEODOROU, M.K. A semi-automated in vitro gas production technique for ruminant feedstuff evaluation. Animal Feed Science and Technology, v.79, p.321-330, 1999. DOI: 10.1016/S0377-8401(99)00033-4.
https://doi.org/10.1016/S0377-8401(99)00...
), adapted to a semiautomatic system by Bueno et al. (2005)BUENO, I.C.S.; CABRAL FILHO, S.L.S.; GOBBO, S.P.; LOUVANDINI, H.; VITTI, D.M.S.S.; ABDALLA, A.L. Influence of inoculum source in a gas production method. Animal Feed Science and Technology, v.123-124, p.95-105, 2005. DOI: 10.1016/j.anifeedsci.2005.05.003.
https://doi.org/10.1016/j.anifeedsci.200...
.

Eight adults from 6.3 to 1.7-year-old rumen-cannulated Santa Inês male sheep (60±2.5 kg of BW) were used as inocula donors. The animals fed tropical grass pasture, a concentrate mixture (0.7 kg 100 kg-1 BW; 70% ground corn, and 30% soybean meal), and had ad libitum access to mineral supplement and water. Before morning feeding, ruminal liquid and solid fractions were collected from each animal, and kept at 39ºC under anaerobic conditions until inoculation was carried out. Four inocula were prepared (two animals per each inoculum) adopting a 50:50 solid:liquid ratio (on a volume basis) (Bueno et al., 2005BUENO, I.C.S.; CABRAL FILHO, S.L.S.; GOBBO, S.P.; LOUVANDINI, H.; VITTI, D.M.S.S.; ABDALLA, A.L. Influence of inoculum source in a gas production method. Animal Feed Science and Technology, v.123-124, p.95-105, 2005. DOI: 10.1016/j.anifeedsci.2005.05.003.
https://doi.org/10.1016/j.anifeedsci.200...
).

Half gram of each dried ground grass sample was weighed in filter bags (Ankom F57), and incubated in 160 mL bottles with 50 mL of incubation medium (Menke’s buffered medium), and 25 mL of rumen inocula, to complete 75 mL of final volume and 85 mL of head space (Longo et al., 2006LONGO, C.; BUENO, I.C.S.; NOZELLA, E.F.; GODDOY, P.B.; CABRAL FILHO, S.L.S.; ABDALLA, A.L. The influence of head-space and inoculum dilution on in vitro ruminal methane measurements. International Congress Series, v.1293, p.62-65, 2006. DOI: 10.1016/j.ics.2006.03.017.
https://doi.org/10.1016/j.ics.2006.03.01...
; Abdalla et al., 2012ABDALLA, A.L.; LOUVANDINI, H.; SALLAM, S.M.A.H.; BUENO, I.C. da S.; TSAI, S.M.; FIGUEIRA, A.V. de O. In vitro evaluation, in vivo quantification, and microbial diversity studies of nutritional strategies for reducing enteric methane production. Tropical Animal Health and Production, v.44, p.953-964, 2012. DOI: 10.1007/s11250-011-9992-0.
https://doi.org/10.1007/s11250-011-9992-...
). The bottles were immediately sealed with 20 mm butyl septum stoppers (Bellco Glass Inc, Vineland, NJ, USA), then manually mixed and incubated at 39oC in a forced-air oven (Marconi MA35, Piracicaba, SP, Brazil) for 24 hours. Two bottles (experimental units) were incubated per grass sample with each one of the four inocula. The same scheme was used for the internal standard samples ['Tifton' hay (Cynodon dactylon)] and blank (bottles without substrate, containing rumen inoculum + medium), to correct the gas production (GP) from the inoculum. Gas pressure was measured using a pressure transducer and data logger (Pressure Press Data 800) at 4:00, 8:00, 12:00 and 24:00 h, in order to determine the GP, using the equation V = 7.365 × p, in which: V is the gas volume (mL); and p is the measured pressure (psi) (Araujo et al., 2011ARAUJO, R.C.; PIRES, A.V.; MOURÃO, G.B.; ABDALLA, A.L.; SALLAM, S.M.A. Use of blanks to determine in vitro net gas and methane production when using rumen fermentation modifiers. Animal Feed Science and Technology, v.166-167, p.155-162, 2011. DOI: 10.1016/j.anifeedsci.2011.04.009.
https://doi.org/10.1016/j.anifeedsci.201...
).

For the measurement of CH4 using a gas chromatograph system (Shimadzu GC 2014, Chiyoda-ku, Tokyo, Japan), during each one of the pressure measurement events, 2.5 mL gas samples were collected from the bottles, using 5 mL syringes, and stored in 10 mL vacuum tubes, as described by Soltan et al. (2012)SOLTAN, Y.A.; MORSY, A.S.; SALLAM, S.M.A.; LOUVANDINI, H.; ABDALLA, A.L. Comparative in vitro evaluation of forage legumes (prosopis, acacia, atriplex, and leucaena) on ruminal fermentation and methanogenesis. Journal of Animal and Feed Sciences, v.21, p.759-772, 2012. DOI: 10.22358/jafs/66148/2012.
https://doi.org/10.22358/jafs/66148/2012...
. Gas production was expressed as (mL DM-1), and CH4 was expressed as a percentage of GP (%), as well as per unit of truly degraded organic matter (mL g-1 TDOM).

The TDOM was determined by neutral detergent solution (NDS) treatment according to Van Soest et al. (1991)VAN 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: 10.3168/jds.S0022-0302(91)78551-2.
https://doi.org/10.3168/jds.S0022-0302(9...
, but with some modifications. At the end of the 24-hour incubation, all the Ankom F57 (Ankom Technology, Macedon, NY, USA) bags (with the nondegraded samples) were removed from the bottles, and immediately immersed in cold water (-4ºC), so that the microbial fermentation process was interrupted. All the bags were treated with NDS for one hour at 90°C, washed with hot water and, then, acetone. The DM and ash of the residual were determined. The difference between the incubated OM sample and the nondegraded residual OM was the TDOM. The partitioning factor (PF) was calculated by the ratio between TDOM (mg) and gas volume (mL) at 24-hour incubation (Blümmel et al., 1997BLÜMMEL, M.; MAKKAR, H.P.S.; BECKER, K. In vitro gas production: a technique revisited. Journal of Animal Physiology and Animal Nutrition, v.77, p.24-34, 1997. DOI: 10.1111/j.1439-0396.1997.tb00734.x.
https://doi.org/10.1111/j.1439-0396.1997...
).

The statistical analysis was performed using the software SAS v.9.4 (SAS Institute Inc., Cary NC, USA). The obtained data were subjected to analysis of variance, using the Proc GLM procedure, at 5% probability. Correlation (Proc Corr) and factor analyses (Proc Factor) were performed to verify the relationships between the plant chemical composition and the in vitro gas production variables. Stepwise (Proc Stepdisc) and canonical (Candisc) discriminant analyses were carried out, to verify which characteristics were important in discriminating between the grass species.

Results and Discussion

In addition to the results of the present study, besides considering grasses grown in the same soil type, temperature, photoperiod, and rainfall conditions, it is important to take into account that using the same grazing management for different grass species may affect their qualitative parameters, such as morphological (Table 1) and chemical characteristics (Table 2). The correlations between grass characteristics and gas production variables are present in Table 3. To minimize possible impairment of these characteristics, it is important to consider the concept of the critical leaf area index (LAI), to determine the moment of grazing or cutting for each grass species, a condition in which the grass canopy intercepts 95% of the photosynthetically radiation and is near its maximum growth rate, without shading itself (Rhodes, 1971RHODES, I. The relationship between productivity and some components of canopy structure in ryegrass (Lolium spp.): II. Yield, canopy structure and light interception. The Journal of Agriculture Science, v.77, p.283-292, 1971. DOI: 10.1017/S0021859600024436.
https://doi.org/10.1017/S002185960002443...
; Parsons et al., 1983PARSONS, A.J.; LEAFE, E.L.; COLLET, B.; STILES, W. The physiology of grass production under grazing. 1. Characteristics of leaf and canopy photosynthesis of continuously grazed swards. Journal of Applied Ecology, v.20, p.117-126, 1983. DOI: 10.2307/2403380.
https://doi.org/10.2307/2403380....
). After this moment, the senescence and stem growth rate increase, decreasing the grass nutritive value and leaf production in many grass species (Silva et al., 2015SILVA, S.C. da; SBRISSIA, A.F.; PEREIRA, L.E.T. Ecophysiology of C4 forage grasses - understanding plant growth for optimizing their use and management. Agriculture, v.5, p.598-625, 2015. DOI: 10.3390/agriculture5030598.
https://doi.org/10.3390/agriculture50305...
).

Table 1.
Morphological characteristics of the three experimental grass species(1).
Table 2.
Chemical composition (g kg-1 DM), gas production (GP), methane production (CH4), truly degraded organic matter (TDOM), and partitioning factor (PF) of the experimental grass species(1).
Table 3.
Pearson correlations coefficients between grass characteristics and gas production.

Overall, the specific grazing management for each grass can increase the use efficiency of forage by resulting in better canopy structure and nutritive value (Pereira et al., 2015PEREIRA, L.E.T.; PAIVA, A.J.; GEREMIA, E.V.; SILVA, S.C. da. Regrowth patterns of elephant grass (Pennisetum purpureum Schum) subjected to strategies of intermittent stocking management. Grass and Forage Science, v.70, p.195-204, 2015. DOI: 10.1111/gfs.12103.
https://doi.org/10.1111/gfs.12103....
; Anjos et al., 2016ANJOS, A.J. dos; GOMIDE, C.A. de M.; RIBEIRO, K.G.; MADEIRO, A.S.; MORENZ, M.J.F.; PACIULLO, D.S.C. Forage mass and morphological composition of Marandu palisade grass pasture under rest periods. Ciência e Agrotecnologia, v.40, p.79-86, 2016. DOI: 10.1590/S1413-70542016000100007.
https://doi.org/10.1590/S1413-7054201600...
), and should be taken into account when discussing the suitability of each of these grass species for the sustainable livestock production in Brazil.

As to the chemical composition, Guinea grass and signal grass showed better nutritive value due to their high-protein and low-fiber contents (Table 2), which leads to improvement of productivity of ruminant livestock in tropical regions. The superior quality characterized by the higher-protein and low-cellulose contents for signal grass was associated with a decrease of their CH4 production (Table 2).

Rumen microbiota requires a minimum crude protein of 70-80 (g kg-1 DM) to optimize the breakdown of cell wall components, otherwise, the use of diets with lower-protein levels than these, may result in a reduced feed intake (Van Soest, 1994VAN SOEST, P.J. Nutritional ecology of the ruminant. Ithaca: Cornell University Press, 1994. 476p.). Therefore, in a ruminant production system based on grass species, with crude protein content lower than 70 mg kg-1 DM (as Palisade grass, Table 2), nitrogen supplementation is an alternative that can be used, in order to improve feed intake and digestion by the animals (Sampaio et al., 2010SAMPAIO, C.B.; DETMANN, E.; PAULINO, M.F.; VALADARES FILHO, S.C.; SOUZA, M.A. de; LAZZARINI, I.; PAULINO, P.V.; QUEIROZ, A.C. de. Intake and digestibility in cattle fed low-quality tropical forage and supplemented with nitrogenous compounds. Tropical Animal Health and Production, v.42, p.1471-1479, 2010. DOI: 10.1007/s11250-010-9581-7.
https://doi.org/10.1007/s11250-010-9581-...
).

The amount of feed intake affects enteric fermentation and CH4 production, even when these are also moderated by feed digestibility and animal characteristics (Hegarty et al., 2007HEGARTY, R.S.; GOOPY, J.P.; HERD, R.M.; MCCORKELL, B. Cattle selected for lower residual feed intake have reduced daily methane production. Journal of Animal Science, v.85, p.1479-1486, 2007. DOI: 10.2527/jas.2006-236.
https://doi.org/10.2527/jas.2006-236....
; Liu et al., 2017LIU, Z.; LIU, Y.; SHI, X.; WANG, J.; MURPHY, J.P.; MAGHIRANG, R. Enteric methane conversion factor for dairy and beef cattle: effects of feed digestibility and intake level. Transactions of ASABE, v.60, p.459-464, 2017. DOI: 10.13031/trans.11744.
https://doi.org/10.13031/trans.11744....
). In general, increasing feed intake reduces CH4 per unit of consumed feed, which is usually attributed to a shorter time of digesta retention in the rumen, and, consequently, to less extensive fermentation (Pinares-Patiño et al., 2003PINARES-PATIÑO, C.S.; ULYATT, M.J.; LASSEY, K.R.; BARRY, T.N.; HOLMES, C.W. Persistence of differences between sheep in methane emission under generous grazing conditions. The Journal of Agricultural Science, v.140, p.227-233, 2003. DOI: 10.1017/S0021859603003071.
https://doi.org/10.1017/S002185960300307...
; Yan et al., 2010YAN, T.; MAYNE, C.S.; GORDON, F.G.; PORTER, M.G.; AGNEW, R.E.; PATTERSON, D.C.; FERRIS, C.P.; KILPATRICK, D.J. Mitigation of enteric methane emissions through improving efficiency of energy utilization and productivity in lactating dairy cows. Journal of Dairy Science, v.93, p.2630-2638, 2010. DOI: 10.3168/jds.2009-2929.
https://doi.org/10.3168/jds.2009-2929....
; Hammond et al., 2013HAMMOND, K.J.; BURKE, J.L.; KOOLAARD, J.P.; MUETZEL, S.; PINARES-PATIÑO, C.S.; WAGHORN, G.C. Effects of feed intake on enteric methane emissions from sheep fed fresh white clover (Trifolium repens) and perennial ryegrass (Lolium perenne) forages. Animal Feed Science and Technology, v.179, p.121-132, 2013. DOI: 10.1016/j.anifeedsci.2012.11.004.
https://doi.org/10.1016/j.anifeedsci.201...
). However, it is important to consider that variations of feed intake not always affect the whole tract digestibility (Molano & Clark, 2008MOLANO, G.; CLARK, H. The effect of level of intake and forage quality on methane production by sheep. Australian Journal of Experimental Agriculture, v.48, p.219-222, 2008. DOI: 10.1071/EA07253.
https://doi.org/10.1071/EA07253....
).

Furthermore, our results showed that grasses with higher biomass amount (Table 1) tend to increase the cell wall fraction, and to decrease the soluble contents of the plants, thus reducing the feed quality (Table 2). Improving forage quality, both through feeding with lower-fiber and higher-soluble carbohydrates can reduce CH4 production (Beauchemin et al., 2008BEAUCHEMIN, K.A.; KREUZER, M.; O’MARA, F.; MCALLISTER, T.A. Nutritional management for enteric methane abatement: a review. Australian Journal of Experimental Agriculture, v.48, p.21-27, 2008. DOI: 10.1071/EA07199.
https://doi.org/10.1071/EA07199....
; Hristov et al., 2013HRISTOV, A.N.; OH, J.; FIRKINS, J.L.; DIJKSTRA, J.; KEBREAB, E.; WAGHORN, G.; MAKKAR, H.P.; ADESOGAN, A.T.; YANG, W.; LEE, C.; GERBER, P.J.; HENDERSON, B.; TRICARICO, J.M. Mitigation of methane and nitrous oxide emissions from animal operations: I. A review of enteric methane mitigation options. Journal of Animal Science, v.91, p.5045-5069, 2013. DOI: 10.2527/jas.2013-6583.
https://doi.org/10.2527/jas.2013-6583....
). Rodríguez et al. (2007)RODRÍGUEZ, R.; SOSA, A.; RODRÍGUEZ, Y. Microbial protein synthesis in rumen and its importance to ruminants. Cuban Journal of Agricultural Science, v.41, p.287-294, 2007. reported that a proper diet synchronization of available nitrogen and energy for microbial utilization should lead to a decreased CH4 production. Enhancing forage quality (for instance, low-fiber and high-soluble carbohydrates) also tends to increase voluntary intake and to decrease retention time in the rumen, promoting energetically more efficient post-ruminal digestion, and reducing the proportion of dietary energy converted to CH4 (Eckard et al., 2010ECKARD, R.J.; GRAINGER, C.; KLEIN, C.A.M. de. Options for the abatement of methane and nitrous oxide from ruminant production: a review. Livestock Science, v.130, p.47-56, 2010. DOI: 10.1016/j.livsci.2010.02.010.
https://doi.org/10.1016/j.livsci.2010.02...
). Overall, the inverse relation between feed intake level and CH4 yield offers an opportunity to lower emissions while increasing animal productivity (Liu et al., 2017LIU, Z.; LIU, Y.; SHI, X.; WANG, J.; MURPHY, J.P.; MAGHIRANG, R. Enteric methane conversion factor for dairy and beef cattle: effects of feed digestibility and intake level. Transactions of ASABE, v.60, p.459-464, 2017. DOI: 10.13031/trans.11744.
https://doi.org/10.13031/trans.11744....
), contributing to the sustainability of livestock production.

Despite the similar gas production showed by Guinea grass and signal grass (Table 2), it is important to consider that CH4 (mL g-1 TDOM) is the most adequate parameter to evaluate the CH4 mitigation potential of a given substrate, when performing in vitro assays. In this context, Guinea grass did not show to be the most suitable option due to its increased cell wall content - which limits its consumption and nutritional value (Van Soest, 1994VAN SOEST, P.J. Nutritional ecology of the ruminant. Ithaca: Cornell University Press, 1994. 476p.) - and CH4 (mL g-1 TDOM) production, in comparison with signal grass (Table 2). Furthermore, the increased protein level observed in those grasses contributed to ruminal DM degradability, which consequently led to a higher-gas production (Sallam et al., 2010SALLAM, S.M.A.H.; BUENO, I.C. da S.; GODOY, P.B. de; NOZELLA, E.F.; VITTI, D.M.S.S.; ABDALLA, A.L. Ruminal fermentation and tannins bioactivity of some browses using a semi-automated gas production technique. Tropical and Subtropical Agroecosystems, v.12, p.1-10, 2010.). In such cases, the increased gas production, as a consequence of the increased degradability, may contribute towards the sustainability of the system as well, since higher-animal production indexes may be observed in those situations, leading to a higher generation of animal products.

The lower-protein content, as well as the high-cell wall and lignin found in palisade grass impaired the degradation of its organic matter possibly due to microbial inefficiency (Table 2). This fact led to a lower-gas production and CH4 (mL g-1 TDOM) than those of Guinea grass (Table 2). Therefore, it is important to emphasize that the crude protein content of palisade grass was lower than 70 g kg-1 DM (Table 2), which is not enough to sustain an optimal microbial activity to achieve an efficient ruminal fermentation (Van Soest, 1994VAN SOEST, P.J. Nutritional ecology of the ruminant. Ithaca: Cornell University Press, 1994. 476p.).

Forages with higher-crude protein and nitrogen content were capable of promoting high-degradation rates by ruminal microbiota for the reason that nitrogen stimulates the microbial growth and ruminal efficiency (Table 2). Methane is a by-product of the breakdown of carbohydrate molecules in the rumen, and characteristics of those (structural or nonstructural ones) are determining factors regarding CH4 production (Hristov et al., 2013HRISTOV, A.N.; OH, J.; FIRKINS, J.L.; DIJKSTRA, J.; KEBREAB, E.; WAGHORN, G.; MAKKAR, H.P.; ADESOGAN, A.T.; YANG, W.; LEE, C.; GERBER, P.J.; HENDERSON, B.; TRICARICO, J.M. Mitigation of methane and nitrous oxide emissions from animal operations: I. A review of enteric methane mitigation options. Journal of Animal Science, v.91, p.5045-5069, 2013. DOI: 10.2527/jas.2013-6583.
https://doi.org/10.2527/jas.2013-6583....
). Therefore, despite the similar crude protein levels showed by Guinea grass and signal grass, the increased cellulose content observed for the Megathyrsus species has led to an increased CH4 (Table 2), which explains the positive correlation observed for this plant with both CH4, as a percentage of total gas production, and the degraded organic matter (Figure 1).

Figure 1.
Two main factors analysis showing the relation between plant chemical composition and in vitro gas production variables. OM, organic matter; CP, crude protein; NDF, neutral detergent fiber; ADF, acid detergent fiber; ADL, acid detergent lignin; GP, gas production; N-NDF, nitrogen bound to NDF; TDOM, truly degraded organic matter; and PF, partitioning factor.

The most remarkable discriminatory variable in our study was CH4 (mL g-1 TDOM), which was increased in Guinea grass (Figure 2). Generaly, the Megathyrsus grass species shows a better nutritional quality (fiber and protein content) than the Urochloa (Syn. Brachiaria) grasses. Despite that, in the present study, Guinea grass showed a higher-ADF and cellulose content than signal grass (Table 2) which had a higher hemicelulose content (potentially more degradable fiber fraction), possibly due to forage management and soil fertility conditions employed here. However, an in vivo study is necessary to precisely characterize these grass species, considering the animal performance and its methanogenic potential, which are key factors concerning the sustainability of pasture-based ruminant production.

Figure 2.
Discriminant canonical (Can) analysis for chemical and gas production variables of in vitro gas production of tropical grasses grazed by sheep. OM, organic matter; CP, crude protein; NDF, neutral detergent fiber; ADF, acid detergent fiber; ADL, acid detergent lignin; GP, gas production; N-NDF, nitrogen bound to NDF; and TDOM, truly degraded organic matter.

Conclusions

  1. Chemical characteristics and nutritional quality show to be major aspects concerning CH4 production.

  2. Despite the similar organic matter degradatibility of Guinea grass with the other grass species, its poorest fiber quality led to an increased CH4 production.

Acknowledgments

To Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, process No. 303719/2013-3), for financial support; and to the technicians of the Laboratório de Nutrição Animal of the Centro de Energia Nuclear na Agricultura (Cena), for their support.

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Publication Dates

  • Publication in this collection
    Mar 2018

History

  • Received
    17 Mar 2017
  • Accepted
    29 June 2017
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