Abstract:
Proc. Aust. Soc. Anim. Prod. 1994 Vol. 20 DRY MATTER DIGESTIBILITY OF SUBTERRANEAN CLOVER DURING SENESCENCE AND AFTER DEATH C.E. McLARENAB and P.T. DOYLE AC *School of Agriculture and Forestry, University of Melbourne, Parkville, Vic. 3052 BPrescnt address: Victorian Institute of Animal Science, 475 Mickleham Rd., Attwood, Vic. 3049 CPresent address: Sheep Industries Branch, Department of Agriculture, Albany, W.A. 6330 SUMMARY The rate and extent of disappearance of dry matter (DM) in the reticula-rumen was measured, using a nylon bag technique, for 5 cultivars of subterranean clover harvested on 3 occasions between flowering and death. Both the rate and extent of DM disappearance decreased as the clover senesced. Differences were found between cultivars, and between leaf and petiole-stem fractions in aspects of their ruminal DM availability. Keywords: subterranean clover, nylon bag digestibility. INTRODUCTION Differences in the digestibility of cultivars of subterranean clover (Trifolium subterraneum) at flowering and maturity have been reported by Hume et al. (1968) and McIvor and Smith (1973). Such differences could be exploited through grazing management or in plant breeding programs to improve the nutritive value of pastures late in the pasture growing season. This paper reports on changes in the rate and extent -of digestion of the vegetative parts of 5 cultivars of subterranean clover, harvested on 3 occasions between flowering and plant death. MATERIALS AND METHODS Five cultivars of subterranean clover (Nungarin, Trikkala, Woogenellup, Mt Barker and Tallarook) were grown in ungrazed plots (3 m x 2 m, 3 replicates) at Mount Derrimut Field Station, 18 km west of Melbourne, during 1982/83. The plots were sown on 18 March. Sub-plots (0.75 m x 0.75 m) were harvested on 28 September, 19 November and 20 January, by cutting to ground level with hand shears. On 28 September, all cultivars were flowering, and Nungarin and Trikkala had burrs. All cultivars had burrs on 19 November, and wilting and senescence were evident in Nungarin. By 20 January there was no green clover, and all cultivars had completely senesced and were dead. The harvested material was dried at 50�C to constant weight, and sorted by hand to remove burr and weeds. Burr was not analysed, because it was contaminated by adhering clay soil, and no satisfactory method was found to remove this soil without also affecting the chemical composition and digestibility of the burr. The clover remaining after sorting was ground through a 1 mm screen in a laboratory mill. At each harvest date, clover from 1 plot of each cultivar was separated by hand into 2 fractions, leaf and petiole-stem, prior to grinding. Two Hereford steers (about 475 kg liveweight), fitted with an 8 cm cannula in the dorsal sac of the rumen, were used to evaluate the disappearance of clover samples from nylon bags. The management of these animals, and the nylon bag procedure used in this experiment were previously described by McLaren and Doyle (1986). The relationship between the disappearance of DM from the bags (g/l00 g) and incubation time for the samples was described by fitting a Mitschlerich equation: p = A - be-et where p = disappearance of DM (g/100 g), A = potential disappearance (asymptote) (g/l00 g), b = slowly disappearing fraction (SDF, g/l00 g), c = rate of disappearance of slowly disappearing fraction (g/100 g-hour), and t = time (hours). The equation was fitted by an iterative least squares procedure to derive values of A, b and c for each cultivar and harvest, and accounted for 99% (range 97.4-99.9%) of the variance in DM disappearance. Two other fractions (g/100 g) were estimated from the fitted curve parameters: (1) a readily soluble fraction (RSF) calculated as A - b (the disappearance at t = 0), and (2) an apparently indigestible fraction (IF), calculated as 100 - A. The disappearance data were examined using an analysis of variance for a split plot design. For the 221 Proc. Aust. Sot. Anirn. Prod. 1994 Vol. 20 whole plant data, the main effects were cultivar and plot, and harvest date and the cultivar x harvest date interaction were examined at the subplot level. In the case of the leaf and petiole-stem data, the main effects were morphological fraction and plot, with harvest date and the interaction at the subplot level. Comparisons between means for cultivars, harvest dates or morphological fractions were made using the least significant difference (LSD) method when the relevant F statistic from the analysis of variance was significant. Table 1. Proportions of dry matter (DM, g/100 g) in readily soluble (RSF), slowly disappearing (SDF) and insoluble (IF) fractions of subterranean clover plants, and rate of disappearance (c, g/100 g.hour) of DM from the slowly disappearing fraction RESULTS The proportion of DM in the RSF (Table 1) declined (P c 0.01) at successive harvests, while that in the SDF and the IF increased (P < 0.01) with harvest date. The rate of disappearance of DM from SDF decreased (P c 0.01) at each successive harvest. There were no significant differences between cultivar means for the RSF, SDF or the rate of disappearance of the SDF. Nungarin had the highest (P < 0.05) mean proportion of DM in the IF over the 3 harvests. Woogenellup and Mt Barker had more (P < 0.05) DM in the IF than Tallarook. The cultivar x harvest date interactions for the RSF and SDF were significant (P c 0.01). Nungarin and Trikkala had more (P c 0.05) DM in the RSF and less in the SDF (PC 0.05) than Tallarook in September, whilst in November, Nungarin had less (P < 0.05) in the RSF than Trikkala, Mt Barker and Tallarook, more (P < 0.05) in the SDF than Mt Barker, and a lower (P < 0.05) rate of disappearance of DM from the SDF than Tallarook at this time. 222 Proc. Aust. Sot. Anim. Prod. 1994 Vol. 20 Table 2. Proportions of dry matter (DM, g/100 g) in readily soluble (RSF), slowly disappearing (SDF) and insoluble (IF) fractions of leaf and petiole-stem of subterranean clover plants, and rate of disappearance (c, g/100 g.hour) of DM from the slowly disappearing fraction For both leaf and petiole-stem (Table 2), the RSF decreased (P < O.Ol), the IF increased (P < O.Ol), and the rate of disappearance of DM from the SDF decreased (P < 0.01) as the plants matured. Leaf had more (P c 0.05) DM in the SDF and less (P c 0.05) in the RSF than petiole-stem in September. The mean SDF of leaf was greater (P < 0.01) than in petiole-stem, while the mean RSF was less (P < 0.01). There were no significant differences between leaf and petiole-stem in either the IF or the rate of disappearance of DM from SDF, though there was a significant (P < 0.05) morphological fraction x harvest date interaction for both the RSF and the SDF. DISCUSSION The decline in the rate and extent of intra-ruminal DM digestion in flowering and senescent clover followed the expected pattern of changes associated with increasing cell wall content and lignification of plant cell walls. The rate of DM disappearance and the proportion of DM in the IF observed in the clover harvested in September are similar to those reported by Stockdale (1992) for subterranean clover cut in mid-September, though Stockdale reported a larger SDF and a smaller RSF than was found in this study. The values for the dead clover harvested in January are also similar to those reported for mature Junee subterranean clover (Li et al. 1992). The differences between cultivars were small, and tended to indicate a lower availability of DM in the early maturing Nungarin. Cultivars varied in RSF and SDF at the first 2 harvest dates, the difference reflecting the physiological maturity of the cultivars at these harvests. 223 Proc. Aust. Sot. Anim. Prod. 1994 Vol. 20 The lower proportions of DM in the RSF and higher SDF of leaf compared to petiole-stem are consistent with the observations of higher content of cell wall fibre in leaves and the characteristics of ruminal digestion of subterranean clover reported by Stockdale (1992) and lower in vitro DM digestibility reported by Hume et al. (1968). The major changes with increasing time from sowing to harvesting were an increase in the IF and a decrease in the RSF, and these changes were much greater than any differences between cultivars. Branching in subterranean clover stops when floral initiation commences, and severe defoliation during vegetative growth can delay flowering by 30 to 42 days (Collins and Aitken 1970; Davidson et al. 1970). Hence it may be possible to delay the decline in digestibility of subterranean clover by grazing management early in the growing season, though care would need to be exercised to ensure that seed setting and persistence of the clover was not compromised. ACKNOWLEDGMENTS The authors thank Mr Andre Thalen, who assisted with the field work and harvesting of the plots, and acknowledge the financial support of the Wool Research and Development Corporation. REFERENCES COLLINS, W.J. and AITKEN, Y. (1970). Aust. J. Agric. Rex 21: 893-903. DAVIDSON, J.L., GIBSON, A.H. and BIRCH, J.W. (1970). Proceedings of the XIth International Grasslands Congress, Surfers Paradise, Australia, pp. 542-5. HUME, I.D., SOMERS, M. and McKEOWN, N.R. (1968). Aust. J. Exp. Agric. Anim. Husb. 8: 295-300. 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