The use of nylon bags to characterise the potential degradability of feeds for ruminants.

Abstract

28 THE USE OF NYLON BAGS TO CHARACTERISE THE POTENTIAL DEGRADABILITY OF FEEDS FOR RUMINANTS T.J. KEMPTON* SUMMARY The nylon bag technique is a simple means of obtaining estimates of potential degradability of supplements and feedstuffs for ruminants. Rate of disappearance of test material from the bags is particularly sensitive to the basal diet of the cannulated animal. Inclusion of values for fractional clearance of undigested feed residues from the rumen into calculation of degradability provide estimates of rate of degradation of the various components of the test material which more closely approximate true degradability of the material in the rumen. INTRODUCTION . .The nylon bag technique has been used for many years to provide estimates of the rate and extent of disappearance of feed constituents from the rumen (Ouin et al. 1938; Rodriguez 1968; Mehrez and @rskov 1977). Under ce&ainzary and production conditions, ruminant diets must be supplemented with forms of rumen non-degraded dietary protein (bypass protein) to incre.ase the efficiency of nutrient utilisation and hence production (Kempton et al. 1977). Thusthere is need for a technique to quantitate the potential degradability in the rumen of commercially available supplements. Although accuracy of the technique is influenced by certain factors, it provides a relatively simple me'ans of grading supplements in terms of potential degradability. Factors influencing the accuracy of the nylon bag technique The technique involves suspending 4-6 nylon bags each containing a known weight (5g) of sample, on nylon string in the rumen of sheep or cattle fitted with appropriate rumen cannulae. Bags are removed from the rumen at known intervals over the following 24 to 72 h, depending on natureoof the sample, and then washed under tap water, Bags are oven dried (70 for. 24 h) and degradability is normally assessed from disappearance of dry matter (DM) and protein from the bag with time. Comparison of results from different experiments is complicated to . some extent by differences in bag size t porosity,of bag material, The preparation of test sample and time of incubationin the rumen. major source of variation however is associated with the composition of the basal diet and level at which it i&fed to the animals. * Department of Biochemistry and Nutrition, University of New England Armidale, NSW, 2351. 29 7 Poresize of the bag material and fineness of grinding Pore size of the material used for the manufacture of nylon bags apparently has no significant effect on DM disappearance from bags . during a 72 h incubation (Rodriguez 1968). ,Furthermore, there was no detectable .effect of particle size of the test material (dried lucerne) on DM disappearance (Rodriguez 1968). In studies in this laboratory, the rate of degradation of lucerne chaff when either chopped (0.5-l cm length) or ground (40 mm seive) was not significantly different (13 h half time for DM disappearance for both samples,. Kempton and Hiscox, unpublished). With grains however, cracking the glumes increases . degradability and with protein meals, degradability increases with reduction in particle size (Mohamed and Smith 1977). Effect of washing. . .The standard procedure is to wash the bag under tap water until the water is clear, which takes approximately 5 minutes per sample, although the method and time of washing apparently has no appreciable influence on coefficient of variation of DM disappearance/Van Keuren et al. (1962) and Mehrez and @rskov (1977). Diet and between animal variation The basal.diet of cannulated animals has a major effect on DM disappearance. For instance, half time for DM disappearance from rice hulls is considerably less.in sheep given a diet of chopped lucerne chaff in comparison with sheep given a diet of liquid molasses and 100 g .wheaten chaff (Table 1). There was no significant effect of between sheep ~variation on DM disappearance in this study, although a small ambunt of variation was associated with between sheep and between sampling days differences in the studies of Mehrez and grskov (1977). TABLE 1 Half time (hours) for disappearance of dry matter from alkali' treated rice-hulls when suspended in nylon bags in the rumens ' of sheep given diets of .either chopped lucer'ne or liquid molasses (Ker?pton and Gupta, unpublished observations) @rskov and De B Hovel1 (1978) demonstrated similar between diet differences for rate 'of degradation of chopped hay when placed in the. . rumens of Zebu cattle given diets of either chopped sugar cane or pangola .hay. DM disappearance was 18% lower in bags incubated in the rumen of animals given the drhoppedcanediet after 40 h incubation* Similarly in sheep, rate of disappearance of protein from various plant protein meals was faster when the meals were incubated in the rumens of. 30 sheep given a grass based diet as compared with a whole barley diet. The faster rates of degradation on the grass diet would mostly result from faster rumen turnover on the grass diet compared with the straw diet. The rate of disappearance of an animal protein meal (fishmeal) however was not different in sheep given the whole barley or dried grass . diet (Ganev et al. 1979). It is important therefore that the cannulated animals are given standard rations when used to assess the rates of degradation of most feed materials. Between animal variation can be overcome by replicating the measurements in not less than three animals. Practical aspects Rumen cannulae in sheep should be approximately 40 mm in diameter . to facilitate insertion and removal of bags. Ideally, a maximum of six bags can be placed in the rumens of sheep at any time, although considerably more can be,used in cattle. Bags are suspended in the rumen on nylon string with a free length inside the rumen of between 20-25 cm. The strings often entwine and make removal of the initial bags difficult, although`this can be overcome if the strings are covered with thin plastic tubing. Calculation and interpretation of results Disappearance of material from the rumen is the sum of material degraded by microbial fermentation and material of suitable particle size washed from the rtien. Disappearance of material from nylon bags'over time is therefore an.estimate of degradation by microbial activity. The relation between DM or protein disappearance (D) from bags over time (t) as shown in Figure 1 can be described by the equation: where A is the proportion of material present in the bag at zero-time, B is the proportion of material immediately soluble in rumen fluid and c is the rate of degradation. FIGURE 1 'Disappearance of dry matter from rolled barley s,uspended in nylon bags in the rumens of sheep given grass based diets, over time (from Mehrez and @rskov 1977) 31 Disappearance of DM or protein calculated from this equation is overestimated howeverbecause the nylon bag prevents movement of unfermented sample particles from the rumen. @rskov and McDonald (1979) therefore treated soya-bean meal with sodium dichromate (after Uden et al. 1979) to render the sample completely insoluble in the rumen. Rate of passage bf treated meal from the rumen was the.n calculated by . first order dilution principles from rate of disappearance of chromium in samples of rumen contents after a single injection of treated meal. Effective degradation of material at an3 time (t), which includes rate of passage of undigested residues' from the rumen is given,by the equation: where k is the fractional rate `of outflow of meal from the rum-en (h.ours) (after erskov and McDonald 1979). Measurements of proportion of soyabean meal disappearing from nylon bags as calculated from the equations given above are given in Table 2. TABLE 2 Percentage disappearance of soya-bean meal protein from nylon bags in the rumens of sheep (from erskov and McDonald, 1979) The potential degradability of soya-bean meal protein (given bv the asvmptote at time infinity) was'amarently 100% when predicted from . equation 1, or 66071% from equation 2. Therefore, unless a value for fractional clearance of undigested, meal,from the rumen is included in calculations. of degradability, potential degradability after 24 h incubation may be overestimated by at least 20%. Values for fractional clearance of undegraded residues will vary both with level of feeding (grskov and McDonald 1979) and nature of the basal diet given to animals. To obtain realistic estimates of potential degradability therefore, a measurement of-particle turnover in the rumen at the time of nylon bag measurements is necessary. For most purposes however, measurements of liquid turnover in .the rumen (using Cr-EDTA as a nonabsorbable liquid flow marker) may provide values for rumen turnover suitable tc completethe model for rumen degradability of feed materials. where DR is the-proportion of material remaining in the bag at any time 32 The DR-time relaticnship, plotted.on semi-logarithmic co-ordinates (Figure 2) is described by the equation: FIGURE 2 Proportion of soya-bean meal protein remaining in nylon bags, suspended in the &men overtime (from @rskov and McDonald 1979). In this relationship, A is the proportiori (80%) of material in the bag at zero-time which is degraded at a rate given by the slope (c) of the exponential component, The proportion of material which is readily soluble in rumen fluid (B) is given by 100-A (ie 20%). The soluble component can also be physically determined from the DM loss after'washing the material in a nylon bag. Time taken for half the material in the pool described by the exponential component to be degraded (T$, hours) is given by the equation: Thus for the soya-bean meal in Figure 2, 20% of the protein was soluble in rumen fluid and the T% for disappearance of the remaining 80% of the protein <was 8 h. This suggested that the material was completely Some materials however are not completely degraded in the rumen. degraded in the bag such that A+B < 100 and the In (DR) - time. relationship is not linear. This indicates that the material contains . where n is the number of components and i is the component identification. For example, disappearance of DM in rolled barley (from !-Tehrez and drskov 197.7) when plotted on semi-logarithmic co-ordinates suggests' there are two identifiable pools of DM with different rates of degradation (Figure 3). 3 3 The more slowly degraded pool (1) is described by the final rectilinear component of the curve from 15 h through 24 h. T h e proportion of DM in the original material degraded at that rate described by the terminal exponent is given by the zero-time intercept The more rapidly degraded pool (2) is then .described by the (A1) exponent from 3 h to 13 hl Since both pools of DM are degraded simultaneously, it is necessary to subtract the terminal exponent from the earlier exponent (a process known as *'peeling''). Pool size (A2), and T for DM.disappearance from the more rapidly.degraded pool (2) is % .glven by the exponent through the peeled values* (see Figure 3), Values .for pool size and T for DM .and protein disappearance from the various ey pools in rolled-bar 2. are given in Table 3,. TABIS 3 Pool size (%) and half time (hours) for dry matter and protein disappearance from roIled-barley suspended in nylon bags in the rumens of sheep given a grass based diet, The values were calculated by multiLexponential analysis. (equation 6 in the text) .from the data presented by Mehrez.and @rskov (1977). These results suggest there are two identifiable pools of DM and. . protein in rolled,barley with measurable rates of degradation and that ,there. was no pool of 'DM or`protein immediately soluble in rumen fluid. That the total pool of protein exceeds 100% suggests considerable contamination of the undigested residues with microbial protein. O n l y 'trace amounts of DAPA' were detected in the sample (Mehrez ,and @rskov 1977), although the reliability of DAPA as a marker of microbial protein 34 i's questionable (Siddons et al. 1979). In most other studies however, values for size of the total protein pool in various meals suggest that microbial contamination of undegraded residues is of minor importance. Application of exponential analysis to estimates of protein disappearance from soya-bean meal calculated to include fractional clearance of undegraded residues from the rumen (from Table 2) identifies three pools of protein. Apparently 40% of the soya-bean meal protein is degraded relatively slowly (T+ 55 h), 35% is degraded more rapidly (T 5.5 h) and 25% is immediately soluble. By comparison, analysis 0 2 estimates of soya-bean protein disappearance uncorrected for undigested particle outflow -indicates 80% of the protein was degraded at Exponential analysis of the relationship between proportion of material remaining in the'bag with time therefore identifies the major pools of material with different rates of degradation which are characterisitic of that particular supplement under the specified feeding conditions, Inclusion of values for fractional clearance of undigested residues into the calculations would provide estimates of degradability which more closely approximate true degradability of the material in the rumen. Practical application of the ny$on bag technique l . The nylon bag technique has been used to provide comparative estimates of degradability of feedstuffs'used as supplements or components of the basal diet for ruminants (Table 4). TABLE 4 Pool size (%> and half time (hours) for dry matter disappearance of material fromthat pool for various feedstuffs suspended in nylon bags in the rumen. 35 TABLE 4 Cant- Supplements in which the major proportion of the DM or.protein is 'undegraded in the rumen would therefore typically have a small soluble pool and a large relatively slowly degraded pool (pool 1). Since . pool size and rate of degradation, are very sensitive to the dietary condition of the animals, it is necessary to assess the degradability of the supplements in animals given the diet to which this supplement is to be added. The potential digestibility of feedstuffs by ruminants dan be predicted from the half time for DM disappearance of the material in nylon bags. .For instance, low quality feeds including rice hulls, bagasse and barley straw were treated'with various alkalis and then either placed in nylon bags in the rumens of sheep given lucerne chaff, or incubated in vitro (Tilley and Terry 1963)'. From the relationship between in vitro digestibility. (Sk) and T4 DM disappearance (hours) (Figure 4), materials .with a potential digestibility greater than 40% have a T4 DM disappearance of less than 50 hours. 36 FIGURE 4. Relationship between in vitro digestibility of fibrous material and the half time for dry matter disappearance of that material suspended in nylon bags in the rumen of sheep given lucerne chaff (Kempton and Gupta, unpublished obse.rvations). on the basis of the relation in Figure 4, the nylon bagtechnique has been used to quantitate rate of degradationof clover and grass ' pastures conkned by grazing lambs. Samples of oesophageal extrusa were collected .from lambs grazing either lucerne or 'phalaris/fescue swards and the material placed in nylon bags in the rumens of animals from which the samp.le was collected. Since the first bags were not removed until 'after 19 h incubation,, values for analysis of the terminal exponent only are given in Table 4. These preliminary observarions suggest there was a considerably larger pool of slowly degraded DWinthe grass diet (40%) cowared with the.legume diet (28%). CONCLUSIONS The nylon bag technique provides a useful means of evaluating the rate of degradation and potential degradability of feedstuffs and 37 supplements. The technique can be used in field situations to assess the digestibility of forage consumed by grazing animals. The major sources of variation about measurement of degradability are associated with between diet and betwe,en animal variation; the betweenanimal variation can be sufficient reduced by replicating the measur.ements in. not less than three animals. Provided samples are homogeneous, preparation of fibrous dietary material has little effect on degradability. Degradability of grains and protein meals however increases progressively with decrease in particle size.. Bag size, porosity of the bag material and method of washing are easily standardised and of comparatively minor importance. Analysis of the relationship between the,proportion of material remaining in the bag with time by compartmental analysis provides information about the size of the identifiable degradable and non-degradable components of the material and,rates of degradation of the deg,radable components. The soluble component is also that proportion of material that can be physically removed from the bag by washing. Inclusion of e&&m&ties ' . for the fractional clearance of unfermented residues of the ,sample from the rumen provide realistic estimates of. degradability of the material in the rumen under the specified feeding, condition. REFERENCES . GANEV, G-r ORSKOV, E.R. and.SMART, R. (1979).. J. agric., Sci. C&b. 93, 651. mMPTOc T-J., NOLAN, J. V. and LENG, R.A. (1977). Wld Anim. Rev. .22, 2,. MEHREZ, A-2. and jZ&SKOV, E.R. (1977). J. agric. Sci., Camb. 88, 645. MO-D, 0-E. ,and SMITH, R.H. (1977). Proc. Nutr. Soc. 36, 152A. j&SKOV, E.R. and DE B HOVELL (1978). Trop. Anim. Prod..z9-11. P)RSKOV, E.R. and MCDONALD, I. (1979). J. Agric.Sci., Can&. 92, 499. Q U I N , VAN DER WATH, J.G. and MYBURGH, S. (1938). 0nder.J. Vet.' J.I., . Sci. Anim. Ind. 11; 341. RODRIGUEZ, H: (1968). Revista Cubana de Cienca Agricola 2,. 77. SIDDONS, R.C., BEEVER, D.E., NOLAN, J.V., McALLAN, A.B. and MacRAE, J.C. Ann. Rech. Vet. 10, 286-287. (1979). TILLEY; J.M.A. and TERRY, 'R.A. (1963). J. Br. Grassl. Soc. 18, 104. UDEN, P., CALLUCCI, P.E. and VAN SOEST, P.J. (1979). J. SciTFd. Agric. VAN KEUREN, R.W. and HEINEMANN, W.W. (1962). J, Anim.' Sci. 21, 3405. ' APPENDIX Recommended nylon bag technique for comparative estimates of degradability of supplements 5. Animals and diet - 6 bags per animal with not less than 3 animals if using sheep - standard diet tvpical of that which is to be -supplemented with the test material . - 5 minutes under running tap water' - dry at'70�C . 6. Washing the bag 7. 8. Soluble component - wash a sample of material in a nylon bag under tap water Calculation Plot the disappearance of material -- ._ time relationship on semilogarthmic coordinates and fit the terminal exponent to the curve. Use the value measured for the pronortion of soluble material in the sample to set the zero-. time .intercept of the curve. If more than one component, 'peel' the exponbnts and fit the second exponential. Calculate proportional size and T$ for disappearance for each pool of material 9. Rumen turnover If necessary, inject 200 mg CrEDTA/kgDM into the rumen and measure water turnover from the . disappearance of Cr from`rumen fluid with time.