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Measurements of physical strength and their relationship to the chemical composition of four species of Brachiaria
Animal Feed Science and Technology 92 (2001) 149±158
Measurements of physical strength and their relationship to the chemical composition of four species of Brachiaria M. Herreroa,b,*, C.B. do Vallec, N.R.G. Hughesa, V. de O. Sabatelc, N.S. Jessopa a
Institute of Ecology and Resource Management, School of Agriculture Building, University of Edinburgh, Kings Buildings, West Mains Road, Edinburgh EH9 3JG, Scotland, UK b International Livestock Research Institute (ILRI), P.O. Box 30709, Nairobi, Kenya c Centro Nacional de Pesquisa de Gado de Corte da Empresa Brasileira de Pesquisa AgropecuaÂria (EMBRAPA Gado de Corte), Campo Grande, MS, Brazil Received 20 August 1999; received in revised form 23 May 2001; accepted 23 May 2001
Abstract Shear strength and grinding resistance of leaves of four species of Brachiaria collected at 4 and 6 weeks of re-growth were determined using modi®cations of techniques described in the literature. Physical attributes of strength were correlated with analyses of plant cell wall constituents and in vitro digestibility (IVDMD). Methodologies were compared to determine which was best able to describe the physical strength of the samples. Both the grinding resistance and the shear strength technique were able to detect differences between Brachiaria species and ages of re-growth. However, the shear strength technique was more sensitive for identifying physical strength differences at the species level. Both techniques identi®ed the same species (Brachiaria ruziziensis) as the softest, but subsequent ranking of species by the shear strength technique depended on the leaf morphological characteristic used to express the results. Shear strength measurements were correlated to the cell wall components and IVDMD of the samples. The highest correlations were obtained for acid detergent ®bre (ADF), cellulose and lignin with shear strength measurements expressed per unit of leaf width (kg/cm), per unit of linear density (kg/g cm) and for the raw shear strength data (kg). Grinding resistance was not correlated to the chemical composition and IVDMD of the samples. Preferential use of the shear strength technique is suggested since it provides a sensitive measure of the physical strength of forage leaf tissue and is a suitable indicator for identifying nutritive quality differences between Brachiaria species. # 2001 Published by Elsevier Science B.V. Keywords: Brachiaria brizantha; B. decumbens; B. ruziziensis; Grinding resistance; Shearing strength; Chemical composition *
Corresponding author. Tel.: 44-131-535-4384; fax: 44-131-667-2601. E-mail address: [email protected] (M. Herrero). 0377-8401/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 3 7 7 - 8 4 0 1 ( 0 1 ) 0 0 2 6 1 - 9
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1. Introduction Processes of feed particle-size reduction are closely associated with intake and passage through the gastro-intestinal tract of ruminants (see Kennedy and Doyle (1993) for a review). Four of the major physical processes related to particle-size reduction in the ruminant animal are mastication, rumination, microbial degradation, and detrition (Reid et al., 1977). The rate of breakdown of feed particles, whether through mechanical mastication and rumination or as a result of microbial digestion, will ultimately in¯uence the clearance rate of ingesta from the rumen. Therefore, forages with a lower resistance to breakdown can potentially be consumed in larger quantities by ruminants enabling increased animal production from grazed pastures. A number of techniques have been used to study physical strength of grasses. For example, the measurement of grinding resistance, has been used to relate physical strength with chemical composition and digestibility of forages (Mir et al., 1990). Grinding energy has been correlated to rate of intake in forages with similar chemical characteristics (Weston, 1985; Wales et al., 1990). However, one of the criticisms of grinding-based techniques is that dried and ground plant material does not represent what the animal consumes. A different approach was that of Wilson (1965), who found a positive correlation between shear strength of intact leaves of perennial ryegrass and cellulose content. He explained differences in animal weight gains by means of relating a chemical and physical measure of forage quality to animal performance. Wilson (1997) emphasised the importance of these studies in the tropics, where pastures play a major role in animal production and where, due to their physiology, forages tend to have higher ®bre concentrations and lower nutritive value compared to temperate forages. Nevertheless, a review of available literature failed to reveal information comparing techniques to measure physical strength of tropical grass species. For such methods to be used as a determinant of forage quality, the rankings of materials of differing quality need to be similar, regardless of the technique used. This study had three objectives: (1) to determine whether the techniques which use either fresh or ground and dried plant material are able to identify differences in physical strength (i.e. resistance to grinding or shear strength) between four different species of Brachiaria, (2) to determine if forages of different nutritional quality were ranked equally by both the grinding and shear strength techniques, and (3) to study relationships between measurements of physical strength and chemical composition. 2. Materials and methods 2.1. Samples collection Twelve accessions of Brachiaria, three for each of four species, were chosen from genetic selection plots at the EMBRAPA beef cattle (Campo Grande, Brazil). The species included Brachiaria brizantha (accession numbers B30, B140 and B144), B. decumbens (D40, D62 and D70), B. humidicola (H13, H17 and H112) and B. ruziziensis (R50, R128 and R134). Three accessions within each of the species were used as replicates, thereby,
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allowing interspecies comparison of data. Plant material was collected at two different stages of re-growth (i.e. 4 and 6 weeks) after a standardisation cut at 15 cm above ground. 2.2. Grinding resistance measurements Grinding resistance was measured as the proportion of dried and ground material that could be ground to a smaller size in a pre-determined period of time. Details of the technique used to measure grinding resistance are described by Mir et al. (1990). However, modi®cations to the technique were made in an attempt to increase reproducibility between replicates of the same sample and to adjust to the increased shear strength of the tropical forage species used in this study. The modi®cations made are described below. Material initially ground through a 5 mm mesh was sieved through a 1 mm hand sieve commonly used for sieving soil samples. This served to remove all material smaller than 1 mm. The proportion smaller than 1 mm was recorded, as this was assessed to be an important grinding characteristic of the forage. This ensured that material passing through the 1 mm sieve during the grinding test was a direct result of the grinding time to which it was subjected. There was a shortage of vegetative DM on the plots from which the sample plants were collected. These plants were part of a large germplasm collection and were kept in very small experimental plots (i.e. 5 m2 per accession) within an ongoing selection program for new promising varieties of Brachiaria. Therefore, it was necessary to reduce the amount of material to be used in the grinding test. All samples were in the weight range of 20±20.09 g and actual weights were corrected to a standard sample weight of 20 g. Preliminary grinding trials had been conducted to ensure that this reduction in test sample size was able to distinguish differences among accessions. Grinding time was increased to 25 s, allowing between 20 and 25% of the test material to be ground, as preliminary grinding trials had revealed that insuf®cient material was ground in the 10 s suggested by Mir et al. (1990). This was attributed to a higher shear strength of tropical versus temperate grasses, for which the technique had originally been developed. The arrangement of grinding tests were conducted to account for variability in: (a) mill behaviour between days by grinding a single replicate each day, (b) mill behaviour within any day by rotating sample test `blocks' of leaf and stem material, (c) mill behaviour according to the time that the mill had been running by alternating the order of sample grinding, (d) humidity to which the samples were exposed, and (e) the effect of power surges. 2.3. Shear strength measurements Shear strength was measured with the use of a Warner Bratzler meat shear. The purpose of the instrument, originally designed to determine the tenderness of meat, was modi®ed in order to measure the shear strength of grass leaf material as described by Hughes et al. (2000). All samples, consisting of ®ve bound leaves, were sheared at two horizons, being one-third and two-thirds of the leaf length and results averaged over these two horizons. There were ®ve replicates of groups of ®ve bound leaves. Replicates, and horizons within a replicate, were averaged for each accession. Additional measurements of leaf length, leaf
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width average of both horizons, leaf weight, and leaf area were taken from a sample group of 25 leaves. This information was used for the purpose of standardising shear strength parameters for leaf morphology, as there were large differences among species and accessions. 2.4. Chemical analysis Chemical components analysed in the samples were mainly those associated with structural carbohydrates although other components of nutritive value were examined. These were organic matter (OM; incineration at 4508C), nitrogen (N; Tedesco, 1982), neutral detergent ®bre (NDF; Van Soest et al., 1991), acid detergent ®bre (ADF), lignin (H2SO4 and KMnO4 methods) and silica (Robertson and van Soest, 1981) and in vitro digestibility (IVDMD; Tilley and Terry, 1963). Hemicellulose concentrations were calculated as NDF ADF, while cellulose concentrations were determined by subtracting the concentration of permanganate lignin from ADF. 2.5. Statistical analyses The GLM option of SAS (1996) was used to detect signi®cant differences between species, age of re-growth or their interactions on the variables of interest, and the relationships between chemical and physical constituents were analysed using linear correlations. Signi®cance was declared at the 0.05 level of probability. 3. Results and discussion 3.1. Morphological differences No signi®cant differences were found between the two ages of re-growth at 4 and 6 weeks, and there were no interactions between age and species for any of the morphological characters. Thus, data in Table 1 are presented by Brachiaria species only. Table 1 Morphological characteristics of leaves of four species of Brachiariaa Morphological attribute
Length (cm) Width (cm) Area (cm2) Weight (g) Area density (g/cm2) Linear density (g/cm) a b
S.E.D.b
Brachiaria species (n 6) Brizantha
Decumbens
Humidicola
Ruziziensis
39.0 a 1.6 a 43.7 a 1.35 a 0.026 a 0.029 a
16.4 b 1.1 b 13.7 b 0.27 b 0.020 b 0.016 b
16.9 b 0.6 c 7.5 b 0.22 b 0.028 a 0.011 b
16.7 b 1.4 ab 16.6 c 0.33 b 0.020 b 0.020 a
Means with different letters indicate significant differences (P < 0:05). S.E.D.: standard error of the difference between means.
3.99 0.21 7.11 0.213 0.0026 0.0035
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B. brizantha had the longest, widest and heaviest leaves and consequently the largest leaf area of all species. Leaf length was similar between the other Brachiaria species. However, leaf width differed, with B. humidicola having a smaller leaf width and consequently the lowest leaf area. In addition, leaves of B. humidicola were observed to be more rolled than the other species. Measures of density also illustrate differences between Brachiaria species. Area density (g/cm2) was highest for B. humidicola and B. brizantha and lowest for both the B. decumbens and the B. ruziziensis accessions. The ranking for linear density (g/cm leaf length) differed substantially, with B. brizantha and B. ruziziensis being the most linearly dense. 3.2. Chemical analysis No differences between ages of re-growth occurred for chemical parameters, and there were no interactions between age and species. Thus, data in Table 2 are presented by Brachiaria species only. The CP concentrations varied between species with the highest concentration in B. ruziziensis followed by B. decumbens and B. brizantha, with B. humidicola containing the lowest CP level. The highest concentration of NDF was in B. brizantha and B. humidicola and the lowest level in B. ruziziensis. B. decumbens was in intermediate. Sulphuric acid lignin values did not suggest differences among species, whereas permanganate lignin outlined clear differences and two groups. The IVDMD were re¯ective of the NDF and permanganate lignin levels (r 0:77 and 0.78, respectively) with B. ruziziensis being the most digestible species, followed by B. decumbens, while B. brizantha and B. humidicola had lower digestibilities.
Table 2 Chemical composition of leaves of four species of Brachiariaa S.E.D.b
Chemical composition (g/kg DM)
Brachiaria species (n 6) Brizantha
Decumbens
Humidicola
Ruziziensis
Organic matter Crude protein NDF ADF Lignin (KMnO4) Lignin (H2SO4) Silica Cellulose IVDMDd
935 a 122 a 743 a 358 a 77 a 31 13 331 a 0.55 a
920 b 129 a 670 b 290 b 62 b 28 17 263 b 0.63 b
930 a 102 b 741 a 370 c 76 a 31 15 340 c 0.55 a
917 b 146 c 639 c 277 d 63 b 29 14 249 d 0.66 c
a
Means with different letters indicate significant differences (P < 0:05). S.E.D.: standard error of the difference between means. c NS: not significant. d IVDMD: proportion of DM digested. b
6.0 7.8 19.2 8.9 2.8 NSc NSc 7.5 0.019
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Parameter
Shear strength (kg) Shear strength/weight (kg/g) Shear strength/area (kg/cm2) Shear strength/length (kg/cm) Shear strength/area density (kg/g cm2) Shear strength/width (kg/cm) Shear strength/linear density (kg/g cm) Grinding resistance test (g per 25 s) Less than 1 mm ground particles (%) a
S.E.D.b
Brachiaria species (n 6) Brizantha
Decumbens
Humidicola
Ruziziensis
2.75 a 3.13 a 0.08 a 0.081 a 106.0 a 1.85 a 93.7 a 4.2 a 38.3
1.40 c 5.41 b 0.10 a 0.087 a 71.0 b 1.25 b 85.7 b 3.9 a 32.2
1.93 b 9.83 c 0.26 b 0.116 b 70.3 b 3.06 c 160.2 c 3.8 a 38.9
1.23 c 3.80 a 0.07 a 0.074 a 62.4 c 0.89 b 63.2 d 5.1 b 41.2
Means with different letters indicate significant differences (P < 0:05). S.E.D.: standard error of the difference between means. c NS: not significant. b
0.171 1.05 0.015 0.0087 5.46 0.206 11.79 0.33 NSc
S.E.D.b
Age of re-growth 4 weeks (n 12)
6 weeks (n 12)
1.71 a 5.00 a 0.12 a 0.083 a 68.8 a 1.59 a 76.5 a 5.1 a 33.9 a
1.95 b 6.12 b 0.14 b 0.096 b 83.5 b 1.94 b 112.3 b 3.5 b 41.4 b
0.121 0.742 0.011 0.0062 3.86 0.146 8.33 0.23 3.3
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Table 3 Effects of species and age of re-growth on physical strength parameters of Brachiaria species leavesa
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3.3. Strength parameters Differences were detected among species and between ages of re-growth for strength parameters, although there were no interactions. Thus, the main effects means are presented in Table 3. The absolute measure of shear strength was re-calculated on the basis of morphological characteristics of the leaves being tested. It is important to express results relative to these morphological traits so that strength differences among samples can be assessed (Hughes et al., 2000). Shear strength alone (kg) was highest for B. brizantha, intermediate for B. decumbens and B. humidicola, and lowest for B. ruziziensis. However, when shear strength results were expressed per unit weight, area or leaf length basis, B. humidicola was tougher (Table 3). No differences were detected among the other species for these parameters, with the exception of shear strength/weight where B. decumbens was greater than B. brizantha and B. ruziziensis. McClelland and Spielrein (1957) and Prince (1961) suggested that intrinsic differences in leaf length and weight between species could be accounted for by expressing shear strength measurements on the basis of linear density (g/cm). Shear strength per unit of linear density demonstrated that the toughest species was B. humidicola, with B. ruziziensis having the softest leaf tissue. Similar results were obtained when using area densities (g/cm2) as covariates of shearing strength. These results were expected based on qualitative assessments of the experimenters who appreciated clear differences in plucking resistance and texture when collecting samples in the ®eld. Resistance to plucking at leaf sheath level, with care so as not to damage the leaf itself, could be described as tensile strength, a measure describing breakage resulting when two forces are applied along the longitudinal axis of the leaf. However, shear strength and tensile strength are not necessarily positively correlated (Kennedy and Doyle, 1993). The implications of plant material having lower resistance to physical breakdown have already been outlined. Mackinnon et al. (1988) studied effects of reduced intake of pasture and found that the ®rst animal response to pastures with reduced shear strength was a faster rate of DM consumption. These authors also outlined the potential use of the shear strength technique for selecting grasses of higher nutritive value. Results in the current study support the ®ndings of Mackinnon et al. (1988), in that reduced shear strength was correlated with reduced levels of structural carbohydrates in plant tissues. It is common to misinterpret grinding results as a high value indicates reduced resistance to grinding (i.e. reduced strength). Overall grinding results differed from shearing results. There were effects of age of re-growth and species in grinding resistance. However, these differences appeared to be small for most, but not all, parameters (Table 3). Both grinding and shearing measures identi®ed older material to have increased strength with material collected at 4 weeks of re-growth having lower grinding resistance than material collected at 6 weeks of re-growth (Table 3). There were no correlations between any measures of shear strength and grinding resistance. This suggests that the two measures are not associated and raises the question as to which technique is preferable to obtain a measure of physical strength. Unlike measurements of shear strength, grinding is not amenable to standardisation to account for morphological characteristics because the method uses dried and pre-ground samples. Shear strength certainly has the advantage that it relies on fresh green material, thus,
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maintaining the anatomical structure and the bonds between cell wall constituents in the samples, factors recognised as important in determining the physical strength of forages (Wilson, 1997; Wilson and Hat®eld, 1997). Also, the Warner Blatzer apparatus used for measuring shear strength is very sensitive to changes in the force (kg) required to split groups of leaves. Therefore, it is able to detect the small physical strength changes associated with changes in the chemical composition of the leaf samples. Even when substantial modi®cations were made to the grinding resistance test for increasing its sensitivity to the tougher tissue from tropical grasses, it is possible that the magnitude of the differences in the physical strength and chemical composition between the species used was small for the test to detect them. These different sensitivities may explain the lack of relationships found between the two methods. 3.4. Hand sieving and separation of 1 mm grindings There was an increase in the proportion of small particles separated through hand sieving with increased age of re-growth (Table 3). It is likely that this effect is due to the fracture properties of the dried and ground samples used. This effect may be due to increased brittleness, as there were no changes in the levels of structural components between 4 and 6 weeks of re-growth. The effect of increased brittleness is likely to be occurring through changes in the patterns of anatomical association between structural components of the cell walls as forages mature (Wilson, 1997). 3.5. Relationships between physical strength parameters and chemical composition Shear strength (kg) had the strongest correlations with structural constituents such as NDF, ADF, cellulose and KMnO4 lignin (r 0:65, 0.74, 0.76 and 0.68, respectively, Table 4). When measures of shear strength were expressed relative to the morphological characteristics of the leaves being tested, it was the measure of shear strength per width that had the strongest correlations with chemical parameters (r 0:73, 0.85, 0.85 and Table 4 Linear correlations between physical strength and chemical composition parameters of leaves of four species of Brachiaria NDF Shear strength (kg) Shear strength/area (kg/cm2) Shear strength/length (kg/cm) Shear strength/area density (kg/g cm2) Shear strength/width (kg/cm) Shear strength/linear density (kg/g cm) Grinding resistance test (g) a
Lignin: KMnO4 method. NS: not significant, P > 0:05. * P < 0:05. ** P < 0:01. b
**
0.65 0.45* 0.44* 0.50* 0.73** 0.61* NSb
ADF **
0.75 0.55** 0.51* 0.52* 0.85** 0.75** NSb
Cellulose IVDMD Lignina **
0.75 0.53** 0.50* 0.47* 0.86** 0.74** NSb
**
0.61 NSb NSb 0.43* 0.65** 0.54* NSb
**
0.68 NSb NSb 0.45* 0.70** 0.58* NSb
Lignin/cellulose NSb NSb 0.47* NSb 0.50* 0.45* 0.57**
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0.70 for chemical constituents of NDF, ADF, cellulose and lignin (KMnO4), respectively). This is not surprising, since shear strength is estimated by applying a perpendicular force to split groups of bound leaves, hence, it is partly a function of leaf width. This shows the importance of expressing shear strength results on the basis of leaf morphological characteristics for enabling comparisons between grasses with different leaf morphology. Several shear strength expressions were positively correlated with leaf morphological measurements (correlations of 0.67, 0.70 and 0.82 between shear strength (kg) and area, weight and length were found, respectively), thus, demonstrating the importance of standardising strength parameters for morphological traits. Linear densities (g/cm) were also positively correlated to shear strength (r 0:68). A similar relationship was observed by McClelland and Spielrein (1957) and Prince (1961). A measure of `linear density' can be used to relate measurements of strength with other quality parameters. Even though the IVDMD and grinding resistance methodologies both use dried and ground material, there was no correlation (r 0:18) between these two parameters nor any of the cell wall constituents. This lack of relationship may have been caused by the relatively small difference in most of the chemical composition parameters and the lack of sensitivity of the grinding test. However, there was a negative correlation (r 0:61) between shear strength per se and IVDMD. This relationship re¯ects a process common to the disruption of tissues that occurs during the application of a shearing force, and accessibility of tissues by microbes during digestion. Use of techniques to evaluate this relationship in green/fresh material more characteristic of consumed herbage, might reveal a closer relationship, specially if kinetic aspects of digestion are described by use of nylon bag or gas production techniques (Nagadi et al., 1998). 4. Conclusions 1. Both the grinding resistance and the shear strength technique were able to detect differences between Brachiaria species and ages of re-growth. However, the shear strength technique was more sensitive for identifying physical strength differences at the species level. 2. Both techniques identified the same species (B. ruziziensis) as the softest, but subsequent ranking of species by the shear strength technique depended on the leaf morphological characteristic used to express the results. Similar rankings were obtained when shear strength measurements were expressed on the basis of leaf width (kg/cm) or on the basis of linear density (kg/g cm). 3. Shear strength measurements were correlated to the cell wall components and IVDMD of the samples. The highest correlations were obtained for ADF, cellulose and lignin with shear strength measurements expressed per unit of leaf width (kg/cm), per unit of linear density (kg/g cm) or for the raw shear strength data (kg). Grinding resistance was not correlated to the chemical composition and IVDMD of the samples. 4. The shear strength technique provides a sensitive measure of the physical strength of forage leaf tissue and is a suitable indicator for identifying nutritive quality differences between Brachiaria species.
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Acknowledgements This study was conducted as part of the joint research project no. 545MHY `The Relationships between the Physical Structure of Tropical Pastures and their Nutritive Value for Grazing Ruminants' carried out between IERM, University of Edinburgh, Scotland and EMBRAPA Gado de Corte, Brazil. The ®nancial assistance from both institutions as well as a scholarship from the Tropical Agricultural Award Fund are appreciated. References Hughes, N.R.G., do Valle, C.B., Sabatel, V. de O., Boock, J., Jessop, N.S., Herrero, M., 2000. Shearing strength as an additional selection criterion for quality in Brachiaria ecotypes. J. Agric. Sci. (Camb.) 135, 123±130. Kennedy, P.M., Doyle, P.T., 1993. Particle-size reduction by ruminants: effects of cell wall composition and structure. In: Jung, H.G., Buxton, D.R., Hatfield, R.D., Ralph, J. (Eds.), Forage Cell Wall Structure and Digestibility. ASA/CSSA/SSSA, Madison, pp. 499±534. Mackinnon, B.W., Easton, H.S., Barry, T.N., Sedcole, J.R., 1988. The effect of reduced leaf shear strength on the nutritive value of perennial ryegrass. J. Agric. Sci. (Camb.) 111, 469±474. McClelland, J.H., Spielrein, R.E., 1957. An investigation of the ultimate bending strength of some common pasture plants. J. Agric. Eng. Res. 2, 288±292. Mir, P.S., Mir, Z., Hall, J.W., 1990. Physical characteristics of feeds and their relation to nutrient components and dry matter disappearance in sacco. Anim. Feed Sci. Technol. 31, 17±27. Nagadi, S., Herrero, M., Jessop, N.S., 1998. A comparison of the gas production profiles of fresh and dry forage, Proc. Br. Soc. Anim. Sci., 62. Prince, R.A., 1961. Measurement of the ultimate strength of forage stalks. Trans. ASAE 4, 208±209. Reid, C.S.W., Ulyatt, M.J., Monro, A., 1977. The physical breakdown of feed during digestion in the rumen. Proc. New Zealand Soc. Anim. Prod. 37, 173. Robertson, J.B., van Soest, P.J., 1981. The detergent system of analysis. In: James, W.P.T., Theander, O. (Eds.), The Analysis of Dietary Fibre in Food. Marcel Dekker, NY, Chapter 9, pp. 123±158. SAS Institute 1996. SAS: Statistical Analysis System, Version 6.12. SAS Institute Inc., Cary, NC. Tedesco, M.J. 1982. ExtracËaÄo simultaÃnea de N, P, K, Ca e Mg em tecido de plantas por digestaÄo com H2O2± H2SO4. S.l., IPAGRO, 1982, 23 pp. (IPAGRO. Informativo Interno, 1±82). Tilley, J.M.A., Terry, R.A., 1963. A two-stage method technique for the in vitro digestion of forage crops. J. Br. Grassland Soc. 18, 104±111. Van Soest, P.J., Robertson, J.B., Lewis, B.A., 1991. Methods for dietary fiber, neutral detergent fiber, and nonstarch polysaccharides in relation to animal nutrition. J. Dairy Sci. 74, 3583±3597. Wales, W.J., Doyle, P.T., Pearce, G.R., 1990. The feeding value of straws. I. Wheat straws. Anim. Feed Sci. Technol. 29, 1±14. Weston, R.H., 1985. The regulation of feed intake of herbage fed ruminants. Proc. Nutr. Soc. Aust. 10, 55±62. Wilson, D., 1965. Nutritive value and the genetic relationships of cellulose content and leaf tensile strength in Lolium. J. Agric. Sci. (Camb.) 65, 285±292. Wilson, J.R., 1997. Structural and anatomical traits of forages influencing their nutritive value for ruminants. In: Gomide, J.A. (Ed.), SimpoÂsio Internacional sobre ProducËaÄo Animal em Pastejo. Universidade Federal de VicËosa, VicËosa, Brazil, pp. 173±208. Wilson, J.R., Hatfield, R.D., 1997. Structural and chemical changes of cell wall types during stem development: consequences for fibre degradation by rumen microflora. Aust. J. Agric. Res. 48, 165±180.
Measurements of physical strength and their relationship to the chemical composition of four species of Brachiaria M. Herreroa,b,*, C.B. do Vallec, N.R.G. Hughesa, V. de O. Sabatelc, N.S. Jessopa a
Institute of Ecology and Resource Management, School of Agriculture Building, University of Edinburgh, Kings Buildings, West Mains Road, Edinburgh EH9 3JG, Scotland, UK b International Livestock Research Institute (ILRI), P.O. Box 30709, Nairobi, Kenya c Centro Nacional de Pesquisa de Gado de Corte da Empresa Brasileira de Pesquisa AgropecuaÂria (EMBRAPA Gado de Corte), Campo Grande, MS, Brazil Received 20 August 1999; received in revised form 23 May 2001; accepted 23 May 2001
Abstract Shear strength and grinding resistance of leaves of four species of Brachiaria collected at 4 and 6 weeks of re-growth were determined using modi®cations of techniques described in the literature. Physical attributes of strength were correlated with analyses of plant cell wall constituents and in vitro digestibility (IVDMD). Methodologies were compared to determine which was best able to describe the physical strength of the samples. Both the grinding resistance and the shear strength technique were able to detect differences between Brachiaria species and ages of re-growth. However, the shear strength technique was more sensitive for identifying physical strength differences at the species level. Both techniques identi®ed the same species (Brachiaria ruziziensis) as the softest, but subsequent ranking of species by the shear strength technique depended on the leaf morphological characteristic used to express the results. Shear strength measurements were correlated to the cell wall components and IVDMD of the samples. The highest correlations were obtained for acid detergent ®bre (ADF), cellulose and lignin with shear strength measurements expressed per unit of leaf width (kg/cm), per unit of linear density (kg/g cm) and for the raw shear strength data (kg). Grinding resistance was not correlated to the chemical composition and IVDMD of the samples. Preferential use of the shear strength technique is suggested since it provides a sensitive measure of the physical strength of forage leaf tissue and is a suitable indicator for identifying nutritive quality differences between Brachiaria species. # 2001 Published by Elsevier Science B.V. Keywords: Brachiaria brizantha; B. decumbens; B. ruziziensis; Grinding resistance; Shearing strength; Chemical composition *
Corresponding author. Tel.: 44-131-535-4384; fax: 44-131-667-2601. E-mail address: [email protected] (M. Herrero). 0377-8401/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 3 7 7 - 8 4 0 1 ( 0 1 ) 0 0 2 6 1 - 9
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1. Introduction Processes of feed particle-size reduction are closely associated with intake and passage through the gastro-intestinal tract of ruminants (see Kennedy and Doyle (1993) for a review). Four of the major physical processes related to particle-size reduction in the ruminant animal are mastication, rumination, microbial degradation, and detrition (Reid et al., 1977). The rate of breakdown of feed particles, whether through mechanical mastication and rumination or as a result of microbial digestion, will ultimately in¯uence the clearance rate of ingesta from the rumen. Therefore, forages with a lower resistance to breakdown can potentially be consumed in larger quantities by ruminants enabling increased animal production from grazed pastures. A number of techniques have been used to study physical strength of grasses. For example, the measurement of grinding resistance, has been used to relate physical strength with chemical composition and digestibility of forages (Mir et al., 1990). Grinding energy has been correlated to rate of intake in forages with similar chemical characteristics (Weston, 1985; Wales et al., 1990). However, one of the criticisms of grinding-based techniques is that dried and ground plant material does not represent what the animal consumes. A different approach was that of Wilson (1965), who found a positive correlation between shear strength of intact leaves of perennial ryegrass and cellulose content. He explained differences in animal weight gains by means of relating a chemical and physical measure of forage quality to animal performance. Wilson (1997) emphasised the importance of these studies in the tropics, where pastures play a major role in animal production and where, due to their physiology, forages tend to have higher ®bre concentrations and lower nutritive value compared to temperate forages. Nevertheless, a review of available literature failed to reveal information comparing techniques to measure physical strength of tropical grass species. For such methods to be used as a determinant of forage quality, the rankings of materials of differing quality need to be similar, regardless of the technique used. This study had three objectives: (1) to determine whether the techniques which use either fresh or ground and dried plant material are able to identify differences in physical strength (i.e. resistance to grinding or shear strength) between four different species of Brachiaria, (2) to determine if forages of different nutritional quality were ranked equally by both the grinding and shear strength techniques, and (3) to study relationships between measurements of physical strength and chemical composition. 2. Materials and methods 2.1. Samples collection Twelve accessions of Brachiaria, three for each of four species, were chosen from genetic selection plots at the EMBRAPA beef cattle (Campo Grande, Brazil). The species included Brachiaria brizantha (accession numbers B30, B140 and B144), B. decumbens (D40, D62 and D70), B. humidicola (H13, H17 and H112) and B. ruziziensis (R50, R128 and R134). Three accessions within each of the species were used as replicates, thereby,
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allowing interspecies comparison of data. Plant material was collected at two different stages of re-growth (i.e. 4 and 6 weeks) after a standardisation cut at 15 cm above ground. 2.2. Grinding resistance measurements Grinding resistance was measured as the proportion of dried and ground material that could be ground to a smaller size in a pre-determined period of time. Details of the technique used to measure grinding resistance are described by Mir et al. (1990). However, modi®cations to the technique were made in an attempt to increase reproducibility between replicates of the same sample and to adjust to the increased shear strength of the tropical forage species used in this study. The modi®cations made are described below. Material initially ground through a 5 mm mesh was sieved through a 1 mm hand sieve commonly used for sieving soil samples. This served to remove all material smaller than 1 mm. The proportion smaller than 1 mm was recorded, as this was assessed to be an important grinding characteristic of the forage. This ensured that material passing through the 1 mm sieve during the grinding test was a direct result of the grinding time to which it was subjected. There was a shortage of vegetative DM on the plots from which the sample plants were collected. These plants were part of a large germplasm collection and were kept in very small experimental plots (i.e. 5 m2 per accession) within an ongoing selection program for new promising varieties of Brachiaria. Therefore, it was necessary to reduce the amount of material to be used in the grinding test. All samples were in the weight range of 20±20.09 g and actual weights were corrected to a standard sample weight of 20 g. Preliminary grinding trials had been conducted to ensure that this reduction in test sample size was able to distinguish differences among accessions. Grinding time was increased to 25 s, allowing between 20 and 25% of the test material to be ground, as preliminary grinding trials had revealed that insuf®cient material was ground in the 10 s suggested by Mir et al. (1990). This was attributed to a higher shear strength of tropical versus temperate grasses, for which the technique had originally been developed. The arrangement of grinding tests were conducted to account for variability in: (a) mill behaviour between days by grinding a single replicate each day, (b) mill behaviour within any day by rotating sample test `blocks' of leaf and stem material, (c) mill behaviour according to the time that the mill had been running by alternating the order of sample grinding, (d) humidity to which the samples were exposed, and (e) the effect of power surges. 2.3. Shear strength measurements Shear strength was measured with the use of a Warner Bratzler meat shear. The purpose of the instrument, originally designed to determine the tenderness of meat, was modi®ed in order to measure the shear strength of grass leaf material as described by Hughes et al. (2000). All samples, consisting of ®ve bound leaves, were sheared at two horizons, being one-third and two-thirds of the leaf length and results averaged over these two horizons. There were ®ve replicates of groups of ®ve bound leaves. Replicates, and horizons within a replicate, were averaged for each accession. Additional measurements of leaf length, leaf
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width average of both horizons, leaf weight, and leaf area were taken from a sample group of 25 leaves. This information was used for the purpose of standardising shear strength parameters for leaf morphology, as there were large differences among species and accessions. 2.4. Chemical analysis Chemical components analysed in the samples were mainly those associated with structural carbohydrates although other components of nutritive value were examined. These were organic matter (OM; incineration at 4508C), nitrogen (N; Tedesco, 1982), neutral detergent ®bre (NDF; Van Soest et al., 1991), acid detergent ®bre (ADF), lignin (H2SO4 and KMnO4 methods) and silica (Robertson and van Soest, 1981) and in vitro digestibility (IVDMD; Tilley and Terry, 1963). Hemicellulose concentrations were calculated as NDF ADF, while cellulose concentrations were determined by subtracting the concentration of permanganate lignin from ADF. 2.5. Statistical analyses The GLM option of SAS (1996) was used to detect signi®cant differences between species, age of re-growth or their interactions on the variables of interest, and the relationships between chemical and physical constituents were analysed using linear correlations. Signi®cance was declared at the 0.05 level of probability. 3. Results and discussion 3.1. Morphological differences No signi®cant differences were found between the two ages of re-growth at 4 and 6 weeks, and there were no interactions between age and species for any of the morphological characters. Thus, data in Table 1 are presented by Brachiaria species only. Table 1 Morphological characteristics of leaves of four species of Brachiariaa Morphological attribute
Length (cm) Width (cm) Area (cm2) Weight (g) Area density (g/cm2) Linear density (g/cm) a b
S.E.D.b
Brachiaria species (n 6) Brizantha
Decumbens
Humidicola
Ruziziensis
39.0 a 1.6 a 43.7 a 1.35 a 0.026 a 0.029 a
16.4 b 1.1 b 13.7 b 0.27 b 0.020 b 0.016 b
16.9 b 0.6 c 7.5 b 0.22 b 0.028 a 0.011 b
16.7 b 1.4 ab 16.6 c 0.33 b 0.020 b 0.020 a
Means with different letters indicate significant differences (P < 0:05). S.E.D.: standard error of the difference between means.
3.99 0.21 7.11 0.213 0.0026 0.0035
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B. brizantha had the longest, widest and heaviest leaves and consequently the largest leaf area of all species. Leaf length was similar between the other Brachiaria species. However, leaf width differed, with B. humidicola having a smaller leaf width and consequently the lowest leaf area. In addition, leaves of B. humidicola were observed to be more rolled than the other species. Measures of density also illustrate differences between Brachiaria species. Area density (g/cm2) was highest for B. humidicola and B. brizantha and lowest for both the B. decumbens and the B. ruziziensis accessions. The ranking for linear density (g/cm leaf length) differed substantially, with B. brizantha and B. ruziziensis being the most linearly dense. 3.2. Chemical analysis No differences between ages of re-growth occurred for chemical parameters, and there were no interactions between age and species. Thus, data in Table 2 are presented by Brachiaria species only. The CP concentrations varied between species with the highest concentration in B. ruziziensis followed by B. decumbens and B. brizantha, with B. humidicola containing the lowest CP level. The highest concentration of NDF was in B. brizantha and B. humidicola and the lowest level in B. ruziziensis. B. decumbens was in intermediate. Sulphuric acid lignin values did not suggest differences among species, whereas permanganate lignin outlined clear differences and two groups. The IVDMD were re¯ective of the NDF and permanganate lignin levels (r 0:77 and 0.78, respectively) with B. ruziziensis being the most digestible species, followed by B. decumbens, while B. brizantha and B. humidicola had lower digestibilities.
Table 2 Chemical composition of leaves of four species of Brachiariaa S.E.D.b
Chemical composition (g/kg DM)
Brachiaria species (n 6) Brizantha
Decumbens
Humidicola
Ruziziensis
Organic matter Crude protein NDF ADF Lignin (KMnO4) Lignin (H2SO4) Silica Cellulose IVDMDd
935 a 122 a 743 a 358 a 77 a 31 13 331 a 0.55 a
920 b 129 a 670 b 290 b 62 b 28 17 263 b 0.63 b
930 a 102 b 741 a 370 c 76 a 31 15 340 c 0.55 a
917 b 146 c 639 c 277 d 63 b 29 14 249 d 0.66 c
a
Means with different letters indicate significant differences (P < 0:05). S.E.D.: standard error of the difference between means. c NS: not significant. d IVDMD: proportion of DM digested. b
6.0 7.8 19.2 8.9 2.8 NSc NSc 7.5 0.019
154
Parameter
Shear strength (kg) Shear strength/weight (kg/g) Shear strength/area (kg/cm2) Shear strength/length (kg/cm) Shear strength/area density (kg/g cm2) Shear strength/width (kg/cm) Shear strength/linear density (kg/g cm) Grinding resistance test (g per 25 s) Less than 1 mm ground particles (%) a
S.E.D.b
Brachiaria species (n 6) Brizantha
Decumbens
Humidicola
Ruziziensis
2.75 a 3.13 a 0.08 a 0.081 a 106.0 a 1.85 a 93.7 a 4.2 a 38.3
1.40 c 5.41 b 0.10 a 0.087 a 71.0 b 1.25 b 85.7 b 3.9 a 32.2
1.93 b 9.83 c 0.26 b 0.116 b 70.3 b 3.06 c 160.2 c 3.8 a 38.9
1.23 c 3.80 a 0.07 a 0.074 a 62.4 c 0.89 b 63.2 d 5.1 b 41.2
Means with different letters indicate significant differences (P < 0:05). S.E.D.: standard error of the difference between means. c NS: not significant. b
0.171 1.05 0.015 0.0087 5.46 0.206 11.79 0.33 NSc
S.E.D.b
Age of re-growth 4 weeks (n 12)
6 weeks (n 12)
1.71 a 5.00 a 0.12 a 0.083 a 68.8 a 1.59 a 76.5 a 5.1 a 33.9 a
1.95 b 6.12 b 0.14 b 0.096 b 83.5 b 1.94 b 112.3 b 3.5 b 41.4 b
0.121 0.742 0.011 0.0062 3.86 0.146 8.33 0.23 3.3
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Table 3 Effects of species and age of re-growth on physical strength parameters of Brachiaria species leavesa
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3.3. Strength parameters Differences were detected among species and between ages of re-growth for strength parameters, although there were no interactions. Thus, the main effects means are presented in Table 3. The absolute measure of shear strength was re-calculated on the basis of morphological characteristics of the leaves being tested. It is important to express results relative to these morphological traits so that strength differences among samples can be assessed (Hughes et al., 2000). Shear strength alone (kg) was highest for B. brizantha, intermediate for B. decumbens and B. humidicola, and lowest for B. ruziziensis. However, when shear strength results were expressed per unit weight, area or leaf length basis, B. humidicola was tougher (Table 3). No differences were detected among the other species for these parameters, with the exception of shear strength/weight where B. decumbens was greater than B. brizantha and B. ruziziensis. McClelland and Spielrein (1957) and Prince (1961) suggested that intrinsic differences in leaf length and weight between species could be accounted for by expressing shear strength measurements on the basis of linear density (g/cm). Shear strength per unit of linear density demonstrated that the toughest species was B. humidicola, with B. ruziziensis having the softest leaf tissue. Similar results were obtained when using area densities (g/cm2) as covariates of shearing strength. These results were expected based on qualitative assessments of the experimenters who appreciated clear differences in plucking resistance and texture when collecting samples in the ®eld. Resistance to plucking at leaf sheath level, with care so as not to damage the leaf itself, could be described as tensile strength, a measure describing breakage resulting when two forces are applied along the longitudinal axis of the leaf. However, shear strength and tensile strength are not necessarily positively correlated (Kennedy and Doyle, 1993). The implications of plant material having lower resistance to physical breakdown have already been outlined. Mackinnon et al. (1988) studied effects of reduced intake of pasture and found that the ®rst animal response to pastures with reduced shear strength was a faster rate of DM consumption. These authors also outlined the potential use of the shear strength technique for selecting grasses of higher nutritive value. Results in the current study support the ®ndings of Mackinnon et al. (1988), in that reduced shear strength was correlated with reduced levels of structural carbohydrates in plant tissues. It is common to misinterpret grinding results as a high value indicates reduced resistance to grinding (i.e. reduced strength). Overall grinding results differed from shearing results. There were effects of age of re-growth and species in grinding resistance. However, these differences appeared to be small for most, but not all, parameters (Table 3). Both grinding and shearing measures identi®ed older material to have increased strength with material collected at 4 weeks of re-growth having lower grinding resistance than material collected at 6 weeks of re-growth (Table 3). There were no correlations between any measures of shear strength and grinding resistance. This suggests that the two measures are not associated and raises the question as to which technique is preferable to obtain a measure of physical strength. Unlike measurements of shear strength, grinding is not amenable to standardisation to account for morphological characteristics because the method uses dried and pre-ground samples. Shear strength certainly has the advantage that it relies on fresh green material, thus,
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maintaining the anatomical structure and the bonds between cell wall constituents in the samples, factors recognised as important in determining the physical strength of forages (Wilson, 1997; Wilson and Hat®eld, 1997). Also, the Warner Blatzer apparatus used for measuring shear strength is very sensitive to changes in the force (kg) required to split groups of leaves. Therefore, it is able to detect the small physical strength changes associated with changes in the chemical composition of the leaf samples. Even when substantial modi®cations were made to the grinding resistance test for increasing its sensitivity to the tougher tissue from tropical grasses, it is possible that the magnitude of the differences in the physical strength and chemical composition between the species used was small for the test to detect them. These different sensitivities may explain the lack of relationships found between the two methods. 3.4. Hand sieving and separation of 1 mm grindings There was an increase in the proportion of small particles separated through hand sieving with increased age of re-growth (Table 3). It is likely that this effect is due to the fracture properties of the dried and ground samples used. This effect may be due to increased brittleness, as there were no changes in the levels of structural components between 4 and 6 weeks of re-growth. The effect of increased brittleness is likely to be occurring through changes in the patterns of anatomical association between structural components of the cell walls as forages mature (Wilson, 1997). 3.5. Relationships between physical strength parameters and chemical composition Shear strength (kg) had the strongest correlations with structural constituents such as NDF, ADF, cellulose and KMnO4 lignin (r 0:65, 0.74, 0.76 and 0.68, respectively, Table 4). When measures of shear strength were expressed relative to the morphological characteristics of the leaves being tested, it was the measure of shear strength per width that had the strongest correlations with chemical parameters (r 0:73, 0.85, 0.85 and Table 4 Linear correlations between physical strength and chemical composition parameters of leaves of four species of Brachiaria NDF Shear strength (kg) Shear strength/area (kg/cm2) Shear strength/length (kg/cm) Shear strength/area density (kg/g cm2) Shear strength/width (kg/cm) Shear strength/linear density (kg/g cm) Grinding resistance test (g) a
Lignin: KMnO4 method. NS: not significant, P > 0:05. * P < 0:05. ** P < 0:01. b
**
0.65 0.45* 0.44* 0.50* 0.73** 0.61* NSb
ADF **
0.75 0.55** 0.51* 0.52* 0.85** 0.75** NSb
Cellulose IVDMD Lignina **
0.75 0.53** 0.50* 0.47* 0.86** 0.74** NSb
**
0.61 NSb NSb 0.43* 0.65** 0.54* NSb
**
0.68 NSb NSb 0.45* 0.70** 0.58* NSb
Lignin/cellulose NSb NSb 0.47* NSb 0.50* 0.45* 0.57**
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0.70 for chemical constituents of NDF, ADF, cellulose and lignin (KMnO4), respectively). This is not surprising, since shear strength is estimated by applying a perpendicular force to split groups of bound leaves, hence, it is partly a function of leaf width. This shows the importance of expressing shear strength results on the basis of leaf morphological characteristics for enabling comparisons between grasses with different leaf morphology. Several shear strength expressions were positively correlated with leaf morphological measurements (correlations of 0.67, 0.70 and 0.82 between shear strength (kg) and area, weight and length were found, respectively), thus, demonstrating the importance of standardising strength parameters for morphological traits. Linear densities (g/cm) were also positively correlated to shear strength (r 0:68). A similar relationship was observed by McClelland and Spielrein (1957) and Prince (1961). A measure of `linear density' can be used to relate measurements of strength with other quality parameters. Even though the IVDMD and grinding resistance methodologies both use dried and ground material, there was no correlation (r 0:18) between these two parameters nor any of the cell wall constituents. This lack of relationship may have been caused by the relatively small difference in most of the chemical composition parameters and the lack of sensitivity of the grinding test. However, there was a negative correlation (r 0:61) between shear strength per se and IVDMD. This relationship re¯ects a process common to the disruption of tissues that occurs during the application of a shearing force, and accessibility of tissues by microbes during digestion. Use of techniques to evaluate this relationship in green/fresh material more characteristic of consumed herbage, might reveal a closer relationship, specially if kinetic aspects of digestion are described by use of nylon bag or gas production techniques (Nagadi et al., 1998). 4. Conclusions 1. Both the grinding resistance and the shear strength technique were able to detect differences between Brachiaria species and ages of re-growth. However, the shear strength technique was more sensitive for identifying physical strength differences at the species level. 2. Both techniques identified the same species (B. ruziziensis) as the softest, but subsequent ranking of species by the shear strength technique depended on the leaf morphological characteristic used to express the results. Similar rankings were obtained when shear strength measurements were expressed on the basis of leaf width (kg/cm) or on the basis of linear density (kg/g cm). 3. Shear strength measurements were correlated to the cell wall components and IVDMD of the samples. The highest correlations were obtained for ADF, cellulose and lignin with shear strength measurements expressed per unit of leaf width (kg/cm), per unit of linear density (kg/g cm) or for the raw shear strength data (kg). Grinding resistance was not correlated to the chemical composition and IVDMD of the samples. 4. The shear strength technique provides a sensitive measure of the physical strength of forage leaf tissue and is a suitable indicator for identifying nutritive quality differences between Brachiaria species.
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Acknowledgements This study was conducted as part of the joint research project no. 545MHY `The Relationships between the Physical Structure of Tropical Pastures and their Nutritive Value for Grazing Ruminants' carried out between IERM, University of Edinburgh, Scotland and EMBRAPA Gado de Corte, Brazil. The ®nancial assistance from both institutions as well as a scholarship from the Tropical Agricultural Award Fund are appreciated. References Hughes, N.R.G., do Valle, C.B., Sabatel, V. de O., Boock, J., Jessop, N.S., Herrero, M., 2000. Shearing strength as an additional selection criterion for quality in Brachiaria ecotypes. J. Agric. Sci. (Camb.) 135, 123±130. Kennedy, P.M., Doyle, P.T., 1993. Particle-size reduction by ruminants: effects of cell wall composition and structure. In: Jung, H.G., Buxton, D.R., Hatfield, R.D., Ralph, J. (Eds.), Forage Cell Wall Structure and Digestibility. ASA/CSSA/SSSA, Madison, pp. 499±534. Mackinnon, B.W., Easton, H.S., Barry, T.N., Sedcole, J.R., 1988. The effect of reduced leaf shear strength on the nutritive value of perennial ryegrass. J. Agric. Sci. (Camb.) 111, 469±474. McClelland, J.H., Spielrein, R.E., 1957. An investigation of the ultimate bending strength of some common pasture plants. J. Agric. Eng. Res. 2, 288±292. Mir, P.S., Mir, Z., Hall, J.W., 1990. Physical characteristics of feeds and their relation to nutrient components and dry matter disappearance in sacco. Anim. Feed Sci. Technol. 31, 17±27. Nagadi, S., Herrero, M., Jessop, N.S., 1998. A comparison of the gas production profiles of fresh and dry forage, Proc. Br. Soc. Anim. Sci., 62. Prince, R.A., 1961. Measurement of the ultimate strength of forage stalks. Trans. ASAE 4, 208±209. Reid, C.S.W., Ulyatt, M.J., Monro, A., 1977. The physical breakdown of feed during digestion in the rumen. Proc. New Zealand Soc. Anim. Prod. 37, 173. Robertson, J.B., van Soest, P.J., 1981. The detergent system of analysis. In: James, W.P.T., Theander, O. (Eds.), The Analysis of Dietary Fibre in Food. Marcel Dekker, NY, Chapter 9, pp. 123±158. SAS Institute 1996. SAS: Statistical Analysis System, Version 6.12. SAS Institute Inc., Cary, NC. Tedesco, M.J. 1982. ExtracËaÄo simultaÃnea de N, P, K, Ca e Mg em tecido de plantas por digestaÄo com H2O2± H2SO4. S.l., IPAGRO, 1982, 23 pp. (IPAGRO. Informativo Interno, 1±82). Tilley, J.M.A., Terry, R.A., 1963. A two-stage method technique for the in vitro digestion of forage crops. J. Br. Grassland Soc. 18, 104±111. Van Soest, P.J., Robertson, J.B., Lewis, B.A., 1991. Methods for dietary fiber, neutral detergent fiber, and nonstarch polysaccharides in relation to animal nutrition. J. Dairy Sci. 74, 3583±3597. Wales, W.J., Doyle, P.T., Pearce, G.R., 1990. The feeding value of straws. I. Wheat straws. Anim. Feed Sci. Technol. 29, 1±14. Weston, R.H., 1985. The regulation of feed intake of herbage fed ruminants. Proc. Nutr. Soc. Aust. 10, 55±62. Wilson, D., 1965. Nutritive value and the genetic relationships of cellulose content and leaf tensile strength in Lolium. J. Agric. Sci. (Camb.) 65, 285±292. Wilson, J.R., 1997. Structural and anatomical traits of forages influencing their nutritive value for ruminants. In: Gomide, J.A. (Ed.), SimpoÂsio Internacional sobre ProducËaÄo Animal em Pastejo. Universidade Federal de VicËosa, VicËosa, Brazil, pp. 173±208. Wilson, J.R., Hatfield, R.D., 1997. Structural and chemical changes of cell wall types during stem development: consequences for fibre degradation by rumen microflora. Aust. J. Agric. Res. 48, 165±180.