The availability in the reticulo-rumen of organic matter and crude protein form senescing subterranean clover.

Abstract

Proc. Aust. Soc. Anim. Prod. Vol. 16 THE AVAILABILITY IN THE RETICULO-RUMEN OF ORGANIC MATTER AND CRUDE PROTEIN FROM SENESCING SUBTERRANEAN CLOVER C.E. McLAREN * and P.T. DOYLE* SUMMARY The rate and extent of digestion in the reticula-rumen of organic matter (OM) and crude protein (CP) was measured for three cultivars of subterranean clover harvested on three occasions between flowering and senescence. Both the rate and extent of ruminal degradation decreased as the clover matured. There was a tendency for this decline to occur earlier in the early maturing cultivars. CP was more degradable than OM at early flowering, but had a lower rate of degradation and potential degradability than OM when the clover was dead. (Keywords: clover, ruminal availability, organic matter, crude protein). INTRODUCTION Fels et al. (1959) and Hume and Purser (1975) have reported that sheep eating dry mature subterranean clover (Trifolium subterraneum) excrete more faecal nitrogen (N) per 100 g digestible organic matter (OM) intake than when this feed is immature. This has been attributed partly to an increase in the excretion of undigested dietary protein (Hume and Purser 1975). This paper reports on changes in the rate and extent of digestion in the reticula-rumen of OM and crude protein (CP) in three subterranean clover cultivars as they underwent flowering, senescence and death. The study also examined whether there were differences in the ruminal digestion of OM and CP. MATERIALS AND METHODS Plant material The three subterranean clover cultivars used were Nungarin, Trikkala and Tallarook. They were grown in plots (3 x 2 m, three replicates) at Mount Derrimut Field Station, 17 km west of Melbourne, during 1982/83. Sub-plots (0.75 x 0.75 m> were harvested on September 28, November 19 and January 20, by cutting to ground level with hand shears to give three sub-plot samples for each cultivar at each harvest. The harvested material was dried at 50�C for 48 h, and hand sorted to remove burr, weeds and stones. The remaining clover, comprising leaf, petiole and stem material was ground through a 1 mm screen in a laboratory mill. Animals and management Two 2-year-old Hereford steers (about 475 kg liveweight), fitted with 8 cm cannulae in the dorsal sac of the rumen, were kept in stalls and fed a ration comprising chopped ryegrass/subterranean clover (58%), green subterranean clover (28%) and lucerne (14%) hays. The OM digestibility of the diet was about 60%. The feed intake of each steer was restricted to 6.7 kg dry matter (DM) per day, which was given in two equal portions at 0930 and 1630 h. * i- School of Agriculture and Forestry, University of Melbourne, Parkville, 3052. Present address: Department of Agriculture and Rural Affairs, P-0. Box 500, East Melbourne, 3002. 275 Proc. Aust. Soc. Anim. Prod. Vol. 16 Nylon bag method Nylon bags measuring 150 x 70 mm (internal dimensions 120 x 60 mm) were sewn with nylon thread and the seams glued. The cloth ('Nytal' ASTM 324-44, Swiss Screens Pty. Ltd., Melbourne) had a mean pore size of 44 7-m. Samples were prepared in batches of three sub-plot harvests for analyses as follows. Approximately 5 g DM of ground clover was weighed into a bag which was sealed, tied to a 1 m nylon cord and weighted with a 50 g weight. Four bags were prepared for each sample and each time of incubation, two of these being suspended in the rumen of each animal. Bags were inserted into the rumen at 0900 h and withdrawn after 3, 6, 9, 12, i6, 24, and 48 h incubation. The bags and residual samples were then frozen until the completion of collections, when they were thawed, washed, and dried in a forced-draught oven at 50�C to constant weight. Bags containing samples of the clovers which had not been suspended in the rumen were also washed to estimate losses of soluble material. For each sub-plot sample, residues from each incubation time were bulked prior to chemical analyses. Analytical methods DM was determined by drying to constant weight at 50�C, OM after ignition at for 3 h, and N content was measured by a micro-Kjeldahl technique (AOAC 1970). In vitro OM digestibility (IVOMD) of the clover samples was measured by the pepsin-cellulase method (McLeod and Minson 1978). The relationships between OM and CP disappearance (g/100 g) and time (h) were described by fitting a 'Mitschlerich' equation in the form: 550�C where p = OM or CP disappearance (g/100 g) A = Potential degradability (asymptote) (g/100 g) b= Slowly degradable fraction which disappears at rate c (g/100 g) C = Degradation rate (/h) t = Time (h) The equation was fitted by an iterative least squares procedure to derive values of A, b and c for each sub-plot sample. Differences between cultivars and harvest dates analysis of variance for a split plot design, in which cultivar and plot, with harvests being examined at the for OM and CP were compared at each harvest date using RESULTS In vitro organic matter digestibility and CP content (Table I) declined more rapidly in Nungarin than in the later maturing cultivars. Tallarook was always highest (PcO.05) in IVOMD and CP and Nungarin generally lowest. Table 1 were examined using an the main effects were sub-plot level. A, b and c a paired t-test. In vitro organic matter digestibility (IVOMD) and crude protein (CP) content (% DM) of three cultivars of subterranean clover harvested on three occasions Values within columns with differing superscripts are different (PcO.05) 276 Proc. Aust. Soc. Anim. Prod. Vol. I6 The Mitschlerich equation accounted for 98 loss and 95 to 100% for CP loss. The potential degradable fraction (b) and degradation rate (c) cultivars at each harvest are given in Tables 2 Table 2 to 100% of the variation in OM degradability (A), slowly of OM and CP of the three and 3 respectively. , The potential degradability (g/100 g), slowly degradable fraction (g/100 g) and degradation rate (/h) of organic matter from three cultivars of subterranean clover harvested on three occasions * At this harvest b lower (P(O.05) for Nungarin than for the other cultivars The slowly degradable fraction (b) of OM at the first harvest was lower (P<O.O5) for Nungarin than for the other cultivars. Apart from this no significant differences (P<O.O5) were found between cultivars in any of the curve parameters at any harvest for either OM or CP. This was despite a consistent trend for the potential degradabilities (A) and rates of degradation (c) to be higher in the later maturing cultivars. Potential degradability and degradation rate decreased (P<O.Ol) as the season progressed and the clover matured. In contrast, the slowly degradable fraction increased (P<O.Ol) indicating that the amount of rapidly degradable or soluble OM and CP, (A - b) also declined as the clover matured and died. Table 3 The potential degradability (g/l00 g), slowly degradable fraction (g/100 g) and degradation rate (/h) of crude protein from three cultivars of subterranean clover harvested on three occasions 277 Proc. Aust. Soc. Anim. Prod. Vol. I6 Comparisons of curve parameters for OM and CP at each harvest date indicate that A and b were higher (P<O.O5) for CP than for OM on 28 September; there were no differences on 19 November; whilst on 20 January, A (P<O.O5), b (P<O.Ol) and c (P03.05) were all lower for CP than OM. DISCUSSION The marked decline in the rate and extent of ruminal digestion of OM and CP as the clover matures, wilts and dies follows the expected pattern of changes associated with increasing cell wall content, lignification of plant cell walls, and decreasing digestibility and protein content. There is some evidence from this work to suggest that the protein in mature subterranean clover is more resistant to ruminal degradation than the other components of the plant OM. There were no significant differences between cultivars in the rate or extent of OM and CP digestion in the rumen, although a trend existed for higher values of A and c for Tallarook. Following wilting, Nungarin had a lower IVOMD and CP content than did Tallarook. These findings differ from those of Rossiter and Klein (1985), who have suggested that there is little variation in the digestibility and chemical composition of subterranean clover cultivars as they mature and senesce. The results presented here indicate that for subterranean clover grown at this location there was a marked decline in the ruminal digestibility of OM and CP as the plants matured and senesced. There was also a trend for this decline to occur earlier for the early maturing cultivars. This suggests that where the growing season will support later maturing cultivars, there may be some advantage in including such cultivars in pasture mixtures, as they appear to have the potential to provide more energy and essential nutrients to grazing animals for a short period at the end of the growing season. ACKNOWLEDGEMENTS The study was supported by the Australian Wool Research Trust Fund, who also provided a post-graduate studentship for one of us (C.E.McL.). REFERENCES 'Official Methods of Analysis' 11th ed. (AOAC, Washington, DC.). AOAC (1970). FELS, H-E., MOIR, R.J. and ROSSITER, RX. (1959). Aust. J. Agric. Res. 10 : 237. HUME, I.D. and PURSER, D.B. (1975). Aust. J. Agric. Res. 26 : 199. McLEOD, M.N. and MINSON, D.J. (1978). Anim. Feed Sci. Technol..3 : 277. ROSSITER, RX. and KLEIN, L. (1985). In 'Ruminant Physiology : Concepts and Consequences', p-187, editors SK. Baker, J.M. Gawthorne, J.B. Mackintosh and D.B. Purser. (University of Western Australia). 278