Quantitative studies of the digestion of Rhodes grass hay by Zebu cross steers.

Abstract

142 , Proc. Aust. Soc. Anim. Prod. Vol. 17 QUANTITATIVE STUDIES OF THE DIGESTION OF RHODES GRASS HAY BY ZEBU-CROSS STEERS B. BAKRIE*, R.M. MURRAY*, J.P. HOGAN** and C.S. McSWEENEY** SUMMARY Six Zebu-cross steers were fed Rhodes grass (Chloris gayana) hay plus urea and min r s ad libitum each 12 hours. Organic matter intakes were equivalent to if % .d and digestibility was 557 g/kg, with 65% of the digestibility occurring in the rumen. Nitrogen leaving the stomach in forms other than ammonia was only about 63% of that predicted from other studies , possibly associated with rumen ammonia levels being less than 50 mg N/L for at least one third of each 12 hour period. A wet sieving technique applied to rumen and abomasal digesta indicated that the fraction forming approximately 89% of the particles in the abomasum also represented almost 60% of particles within the rumen. Keywords : intake, digestibility, Rhodes grass, steers. 5 7 g/w l INTRODUCTION The many studies linking digestive physiology and energy metabolism in temperate regions of the world have permitted the development of predictive models of the nutritive value of feeds of defined chemical composition. By contrast few similar data are available to describe the interaction of tropically adapted deriving nutrients cattle from forages also adapted to harsh environmental conditions. There is clearly need to remedy this situation if the nutritional problems of more' than one third of the national herd are to' be addressed. As part of this, a study has been made of various aspects of digestion by Zebu-cross steers offered Rhodes grass (Chloris gayana) hay. MATERIALS AND METHODS Six Zebu-cross steers two years old and of mean (ZSE) live.weight 219 (216) . kg, fitted with permanent rumen and abomasal fistulae, were kept in metabolism cages indoors. They were offered ad libitum in two equal lots, at 0800 h and 2000 h, Rhodes grass hay supplemented with minerals (Siebert and Kennedy 1972) plus 12 g urea/kg DM. Preliminary experiments -had shown a 6% increase (P>O.O5) in feed intake in response to this level of urea supplement. After three weeks' adaptation ,period, infusion of `two reference markers chromium-EDTA (Cr-EDTA) and ruthenium phenanthroline were commenced and continued .for eight days during which On the last three days samples of rumen and abomasal faeces were collected. digesta were collected .and after the infusions had ceased samples of rumen fluid were collected during 31 hours. On the next three days the rumen was emptied once daily to provide data'on the amount and composition of digesta 4, 8 and 12 hours after. feeding. . Organicomatter (OM) was estimated as loss in weight of dry matter (DM) after 2 hr at 600 C, total nitrogen (N) and ammonia by the auto-analyser method of Clare and Stevenson (1964), acid detergent fibre by the method of Goering and Van Soest (1970), chromium by atomic absorption and ruthenium by a modified Volatile fatty acids (VFA) were Megarrity' and Siebert (1977) technique. determined by gas liquid chromatography with iso butyrate as an internal standard. Particles from rumen and reconstituted abomasal -digesta were separated' by wet sieving (Poppi et al. 1980) with collection, of fines, but excluding * Grad. School of Trop. Vet. Science., James Cook University,TuwnwHle;~ld~4811 ** CSIRO Div. of Trop. Anim. Sci., Private tiil Bag, P.O. Aitkenvale, Qld.4814 Proc. Aust. Soc. Anim. Prod. Vol. 17 , 143 soluble DM, passing the smallest sieve. Data are presented as the means with the standard error of the mean given in brackets. RESULTS The hay contained 927 g/kg DM, which contained (g/kg) 79 ash, 6$T5N, and .d and The intake of OM was equivalent to 57 g/W 456 acid detergent fibre. digestibility was 557 g/kg (Table 1) of which approximately 65% of this occurred The N intake was 42.9 g/d to which the roughage component in the stomach. contributed approximately 54%. A net loss of nitrogen occurred in the stomach, the amount leaving the stomach in forms other than ammonia being equivalent to Approximately 55% of this fraction was digested in the about 85% of intake. intestines. Rumen ammonia levels pre-feeding were approximately 40 mg N/L and rose to about 100 mg N/L within four hours of feeding but then declined rapidly so that for five hours of the twelve they were below 50 mg N/L (Fig. 1). VFA levels were relatively steady at 108 (2) mmole/L with the molar proportions of acetic 74.3 (cv 1.9) propionic 19.2 (cv 8.4) and butyric acids 6.5 (cv 6.9) percent also showing little variation. The DM (kg) in the rumen increased from the pre-feeding level (= 12`h post-feeding) of 4.59 (0.24) to 5.72 (0.36) at 4 hours after feeding and then declined to 5.49 (0.34) by 8 hours. The mean volume (L) of liquid in the rumen, estimated by dilution of Cr-EDTA at 41.6 (2.5), agreed well with that measured by rumen emptying 41.4 (1.7), but at 4, 8 and 12 hours after feeding, the volumes were 46.1 (2.1), 43.1 (1.9) and 36.1 (1.8). l Table 1. Variables relating to the intake and digestion of organic matter and nitrogen (g/d) in cattle fed on Rhodes grass plus urea and miner'als 144 Proc. Aust. lSoc. Anim. Prod. Vol. 17 The distribution of size of particles in rumen digesta and in abomasal digesta is given in Fig. 2. Approximately 89% of abomasal digesta DM was able to pass through a 0.60 mm sieve and this fraction represented on average 57% of the rumen digesta DM, the values at 4, 8 and 12 hr after feeding being 56, 53 and 63% respectively. The apparent fractional passage rates (Table 2) suggest that the larger fractions pass from the rumen at considerably slower rates than the smaller fractions. Table 2. Weights of partixulate ftactions in the rumen (kg), leaving.the abomasum (kg/d) and apparent fractional passage rates (/d) of particulate fractions * Proc. Aust. Soc. Anim. Prod. Vol. 17 DISCUSSION 145 Digestion of organic matter and the concentrations and proportions of rurnen However, crude protein flow was only 63% of that predicted from relationships that appear with Similar observations have sheep fed tropical forages (Hogan and Weston 1981). been reported by Kennedy (1982) and Hunter and Sibbert (1986), although those authors presented other results more in accord with the predictions. All these results have involved digesta flow rates measured with non-radioactive ruthenium the analysis of which presents difficulties; adverse comments on the use of ruthenium phenanthroline to measure digesta flow with mature forages have also However in the present experiment, OM been made (Egan and Doyle 1984). transactions showed no indication that digesta flow rate are too low. Possibly microbial protein synthesis was lower than anticipated. Certainly rumen ammonia levels for at least one third of the day were below those needed for maximum fibre fermentation (Boniface et al 1986) and probably for maximum protein The true synthesis (Elliott and Armstrong 1982; McAllan and Smith 1984). situation regarding the amounts of protein derived by cattle from mature tropical forages need to be established urgently as at present it is not possible to predict the protein status of such cattle with any confidence. The distribution of feed particles in abomasal and ruminal digesta suggests that at any time approximately 60% of particles are small enough to pass from the rumen, but the fractional passage rate of these particles was low in comparison with values for sheep given oaten hay (Egan and Doyle 1984). The low fractional passage rates of large particles indicate that particle size reduction and digestion are the major avenues of removal of large particles from the rumen. VFA conformed to the pattern expected from temperate forages. ACKNOWLEDGEMENTS Grateful thanks are dedicated to Mr. L.L. Conlan, Mr. A.L. Neish, Mr. A.N. Boniface and Mrs. C. McGrath for their invaluable assistance during both sampling and laboratory analyses. REFERENCE S BONIFACE, A.N., MURRAY, R.M. and HOGAN, J.P. (1986). Proc. Aust. Soc. Anim. Prod. 16: 151. CLAREx.?nd STEVENSON, A.E. (1964). N. 2'. J.` Aqric. Res. 7: 39% EGAN; J.K. and DOYLE, P.T. (1984). Aust. J. Agric. Res. 35: 2%. ELLIOT, R. and ARMSTRONG, D.G. 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