The effect of the source of inoculum on the in vitro digestibility of lucerne hay, oaten hay mixtures.

Abstract

THE EFFECT OF THE SOURCE OF INOCULUM ON THE IN VITRO DIGESTIBILITY OF LUCERNE HAY OATEN HAY MIXTURES K. SANTHIRASEGARAM* and JENNIFER CLARK* Summary The in vitro digestibility of mixtures of lucerne and oaten hay in varying proportions was determined using inocula from two cannulated sheep fed either lucerne hay or oaten hay. A significant interaction was observed between the sheep, their diets and the digestibilities of the test mixtures. The ability of a given rumen inoculum to digest the test mixtures varied with the amount of organic matter residue it contained. The in vitro digestibility values of the the various mixtures of lucerne and oaten hays showed a significant positie deviation from the expected values calculated from the in vitro digestibilities of the components. The maximum deviation occurred in the 50 per cent lucerne hay + 50 per cent oaten hay mixture for both inoculum sources. I. INTRODUCTION Various modifications of the technique of Tilley, Deriaz and Terry (1960) for the determination of the in vitro digestibility of herbage are widely used in animal nutrition studies and evaluation of herbage plants. There is, however, considerable difference of opinion as to the effect and contribution of the various factors involved. Kumeno, Dehority and Johnson ( 1967) found that the amount of dry matter disappearance of substrates in vitro was independent of the diet of the animal source of inoculum. Troelsen and Hanel (1966) and Quicke et al. (1959) made similar observations with cellulose digestibility. However, many workers, including Shelton and Reid (1960), Tilley, Deriaz and Terry (1960), Johnson (1966) and Bezeau ( 1965), have noted variations depending on the diet of the donor animals. Warner ( 1956) believes that the diet of the donor animal should approximate the substrate under test. In most of the studies reported, the plant materials compared were grass species or strains grown in monoculture but, in practice, the evaluated species and strains would usually be grown as mixed pastures with at least one companion * Department of Agriculture, State Research Farm, Werribee, Victoria. -. 522 legume. Harkess ( 1963) reported that the in w digestibility of a miE,e$ herbage could be accurately predicted from those of the components. The in &o'digestibility of a mixture of two feeds may deviate from the simple addition of the digestible portions of the components. This deviation, if any, may be positively or negatively biased due to interaction between the components. The present experiment was designed to study the effect of the sources of rumen liquor on the digestibility of various mixtures of the components, oaten and lucerne hay. Initially, the two donors of rumen liquor were fed lucerne hay and oaten hay separately and, after four determinations at weekly intervals of the irt vitro digestibility of the five test mixtures, the diets of the donor sheep were reversed and, after an adjustment period of three weeks, another four determinations were made. There were two such eight-week test periods. The sheep were two year old Corriedale wethers each fitted with a permanent rumen cannula. The rumen liquor was collected at 10 a.m. on each occasion, using a vacuum pump. The hays were passed through a hammer mill with a 2.5 cm screen, and were fed ad libitum. The material used for test mixtures (1) to (5) was drawn from these feeds, ground further in a hammer mill with a 0.5 mm sieve and mixed by weight of organic matter content. For all of the mixtures, the organic `matter `content as a percentage of the dry matter was 94.OeO.3. The proximate analysis of the test mixtures is given in Table 1. The method of Alexander and McGowan ( 1966) was used for the determination of in vitro digestibility except that no ammonium sulphate was added to the fermentation mixture so as to avoid masking differences due to the nitrogen content of the diet. TABLE 1 523 During the first test period of eight weeks, only the loss of organic matter due to fermentation with rumen liquor was determined. In the second test period, the total in vitro digestion including the pepsin stage was also determined for each of the five mixtures. A calculation of the expected in vitro digestibility of the mixtures was made, based on the in vitro digestibility of each hay component. III. RESULTS With both sheep on either diet, the total loss of organic matter, after digestion with rumen inoculum and pepsin, exhibited the same trends as that following the digestion by rumen inoculum alone. The corresponding curves were parallel in all instances, and only the results of the latter observations will be considered further. Figure 1 shows the observed and expected values of the in vitro digestibility of organic matter (D.O.M. ) of the five test mixtures for each inoculum source. A summary of the analyses of variance of the in vitro D.O.M. results is shown in Table 2. With sheep A, the digestibilities of the five test mixtures decreased with! increase in the percentage of lucerne hay in the mixture. Items 2 and 5 in Table 2 show that a single regression equation adequately describes the data for sheep A fed either diet. Similar values and trends were observed with sheep B when fed lucerne hay. However, when sheep B was fed oaten hay, all the values were lower 524 TABLE 2 than those of sheep A. The in vitro digestibilities of the mixtures increased with increasing content of lucerne h.ay up to 50 per cent and, thereafter tended to level off. The in vitro digestibility of the test mixtures showed a significant positive deviation from the expected values. The maximum deviation occurred for mixture No. 3 (50 per cent lucerne hay + 50 per cent oaten hay) for both sheep on either diet (Figure 1). Item 8 in Table 2 shows that quadratic equations are required to describe the data for each inoculum source. The mean organic matter residue in the control tubes using rumen liquor from sheep A on either diet, and fro,m sheep B when fed lucerne hay, was 0.7-t-0.2 g/100 ml of strained rumen liquor. However, when sheep B was fed oaten hay, the value was only 0.3tO.l g/100 ml. IV. DISCUSSION The trends in the proximate analysis and the in vitro digestibilities of the mixtures of oaten and lucerne hay were also noted by Knippel and Troelsen ( 1966) in mixtures of lucerne hlay, wheat straw and barley except that, in the mixtures they studied, there was little change in the crude protein content. These results tend to support the conclusions of Kumeno, Dehority and Johnson (1967) that the in vitro digestibility of a feed depends more upon the crude fibre and nitrogen free extract content than on the crude protein content. The data suggests thlat there are two factors involved in the different effects of diet on the-fermentation activity of rumen liquor - the type of feed offered to sheep, and the individuality of the sheep. The interaction between these two factors may account for the contradictory observations made by various workers conceming the effect of the feed offered to the donor on ~PZ vitro digestibility values. 525 The relatively low mean organic matter residue in the strained rumen liquor from sheep B when fed oaten hay indicates that the bacterial concentration in the rumen liquor was low, suggesting that some factor peculiar to sheep B affected the bacterial concentration and thus the fermentation activity of the rumen liquor. This apparent relationship between the organic matter residue of the rumen liquor and the ability to digest material in vitro may give some guide to the effectiveness of the liquor from a donor on a particular diet. Further work under standard conditions may reveal a critical organic matter content which could be adopted as a standard by workers conducting routine in vitro digestibility studies. The observations that the in vitro digestibility values of the test mixtures increased with increasing percentages of lucerne hay, when rumen liquor of sheep B fed oaten hay was used, supports the suggestion of Knippel and Troelsen ( 1966) that lucerne hay may contain a microbial growth promoting factor which, in the present experiment, may be nitrogen. For sheep A, when fed either diet, or for sheep B fed lucerne hay, this factor had little effect on the in vitro digestibility values, presumably because the bacterial concentration in the inoculum used was adequate. The existence of the. positive deviation of the in vitro digestibility values from those expected was also observed by Knippel and Troelsen ( 1966) when rumen liquor from sheep fed wheat straw and barley was used. Further work with various mixtures of other herbages should reveal the circumstances and conditions under which this deviation occurs. V. ACKNOWLEDGMENTS The authors would like to thank Mr. C. Kat for his assistance during the collection of rumen liquor and Miss S. Maynard and Mrs. E. Bolsdon for laboratory assistance. VI. REFERENCES A LEXANDER , R. H., and M C G OWAN, M. ( 1966). J. Br. Grassld. Soc. 21: 140. B EZEAU, L. M. (1965). J. Anim. Sci. 24: 823. H ARKESS, R. .D ( 1963). J. Br. Grassld. Soc. 18: 62. J OHNSON, R. R. (1966). J. Anim. Sci. 25: 855. K NIPPEL , J. E., and TROELSEN, J. E. ( 1966). Can J. Anim. Sci. 46: 9 1. KUMENO, F., D EHORITY , B. A., and J OHNSON, R. R. (1967). J. Anim. Sci. 26: 867. Q UICKE , G. V., B ENTLEY , 0. G., Scm~, H. W., and MOXQN, A. J. 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