Herbage digestibility in sheep and corresponding estimates of digestibility in vitro.

Abstract

HERBAGE DIGESTIBILITY IN SHEEP AND CORRESPONDING ESTIMATES OF DIGESTIBILITY IN VITRO R. J. WILKINS* and R. C. GRIMES* Summary The in vivo dry matter digestibility of 27 herbages, which ranged from 39% to 83%, was significantly related to in vitro estimates of organic matter digestibility in one equation and cellulose digestibility in a second equation. The inclusion of the independent variable cellulose percentage in the second equation significantly reduced the residual standard deviation. For herbages of below 65% dry matter digestibility, the relationship between in vivo and in vitro estimates was slightly improved by the addition of urea to the basal medium used in the in vitro tech nique. I. INTRODUCTION The in vitro digestion technique is a promising laboratory method for estimating the digestibility of herbage (Tilley and Terry 1963). Ground herbage samples are incubated firstly with a rumen inoculum and secondly with acid pepsin. In many studies the pepsin stage has been omitted and cellulose digestibility determined. Reid, Jung and Murray, (1964) found dry matter digestibility in vivo was closely related to in vitro cellulose digestibility, but anomalies in the relationship have been reported by Quicke et crl. (1959) and Naga and el Shazly (1963). The technique provides a means for estimating the digestibility of grazed herbage collected through an oesophageal fistula (Van Dyne and Weir 1964), but these samples will be contaminated with ash from saliva and there is a need to develop a relation between in vivo digestibility and in vitro organic matter digestibility. In the experiments reported here the relationships between cellulose digestibility and organic matter digestibility determined in vitro and dry matter digestibility in vivo were examined. II. MATERIALS AND METHODS Twenty-seven herbages, comprising cocksfoot (3), phalaris ( 3)) ryegrass (5), cereal chaff (4), other grasses (6), clover (2), and lucerne (4), were examined. The dry matter digestibility of these herbages when fed to sheep ranged from 39 % to 83 % . Feed refusals were less than 15 % of the feed offered and all herbages contained more than 85% organic matter. The in vitro digestion technique used was modified by Wilkins (1966). Rumen liquor was sheep maintained on lucerne. Determinations of cate. When in v,ivo digestibility was less than 65 that of Tilley and withdrawn from a digestibility were % , determinations Terry (1963) as rumen-fistulated made in dupliwere also made zzAgronomy Department, University of New England, Armidale, N.S.W. ORGANIC MATTER DIGESTIBILITY u VITRO c%, O(I) Fig. l.-Relationship between organic matter digestibility in vitro and dry matter digestibility in vivo. with urea added to the medium at the rate of 9 mg per 0.4 g herbage. This increased the nitrogen content of the herbage by approximately 1%. Preliminar y experiments indicated that the addition of urea did not influence the in vitro digestibility of samples of more than 65% digestibility. Additions of urea above 9 mg/0.4 g produced, in some cases, a depression in digestibility. III. RESULT S In vivo dry matter digestibility (Y) was significantly correlated with in vitro organic matter digestibility (Xi) and with in vitro cellulose digestibility (X2) (Figures 1 and 2). Y = 8.888 + 0.871 X, t_ 4.19 (r = 0.939***) . . . (1) Y = 21.576 + 0.704 X2 t 3.83 (r = 0.950***) . . . (2) The error in predicting digestibility in the sheep was significantly reduced by including cellulose % (X,) in Equation 2 but, not in Equation 1. 335 Fig. 2. -Relationship between cellulose digestibility in vitro and dry matter digestibility in vivo. Y = 53.342 + 0.492 X2 - 0.664 X3 AZ 2.99 (r = 0.971***) . . . (3) The individual digestibility figures are shown in Table 1. Urea was added to the medium for the nine herbages of less than 65% dry matter digestibility in vivo. The addition resulted in small increases in digestibility (Table 2) but these were significant (P < 0.05) in only two cases for organic matter digestibility and five cases for cellulose digestibility. Urea addition produced some improvement in the correlation between digestibility in vivo and in vitro, but the increase in the correlation coefficient was not significant (P > 0.05). III. DISCUSSION The residual standard deviations of the regressions were higher than the value of about t- 2 reported by Tilley, Deriaz and Terry (1960) for the prediction of dry matter digestibility in vivo from dry matter digestibility in vitro. The accuracy of the regressions reported here could have been reduced by errors from two 336 The digestibility of herbages in vivo and in vitro TABLE 1 sources. Firstly, the level of feeding in the in vivo trials varied from about half maintenance to maintenance. Waite, Johnston and Armstrong (1964) showed that differences in intake of this order could change the in vivo organic matter digestibility of ryegrass by up to six digestibility units. Secondly, differences in drying TABLE 2 The influence of <urea on digestibility in vitro for herbages of below 6.5% dry matter digestibility in vivo procedures (Reid, Jung and Murray 1964) and milling (Baumgardt and Oh 1964) have been reported to influence in vitro digestibility and with the use of samples prepared in several laboratories some differences in preparation procedure were inevitable. It may be suggested that standardisation in the determination of in vivo digestibility and in the preparation of samples prior to in vitro digestion ma y have effected considerable reduction to the error in predicting in vivo digestibility. The precision of estimates of in vivo digestibility based on the two measures of in vitro digestion were similar. The significant reduction in residual variation produced by the inclusion of cellulose content in Equation 3 suggests that variation in cellulose content could have been responsible, in part, for the poor relationships between in vitro cellulose digestibility and in vivo dry matter digestibility reported by Quicke et al. (1959) and Naga and el Shazly (1963). In addition to avoiding the pepsin stage of digestion, determination of cellulose digestibility has the advantage that this measure will not be directly influenced by losses of labile plant components during drying. In circumstances where losses may be particularly large, such as during the processing of extrusa collected through oesophageal fistulae (Grimes, Watkin and Gallagher 1966), cellulose digestibility may be the more suitable measure of in vitro digestion. The increases in digestibility in vitro produced by urea supplementation were small, but as only nine samples were considered, the influence of urea on the overall relationship between digestibility in vivo and in vitro was not resolved. IV. ACKNQWLEDGMENTS We wish to thank University of Adelaide, Canberra, and Mr. T. Armidale, for making digestibility trials. Mr. W. G. Allden, Waite Agricultural Research Institute, Dr. G. W. Arnold, Division of Plant Industry, C.S.I.R.O., F. Reardon, C.S.I.R.O., Pastoral Research Laboratory, available samples of herbage they had fed to sheep l1 in V. REFERENCES B AUMGARDT , B. R., and O H , H. K. (1964). Evaluation of forages in the laboratory. IV. Within and among trial variability of the Wisconsin artificial rumen procedure. Journhl bj Dairy Science 47: 263. G , R. C., W ATKIN , B. R., and G ALLAGHER , J. R. (1966). An evaluation of pasture quality with young grazing sheep. 2. 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