| Livestock Research for Rural Development 9 (3) 1997 | Citation of this paper |
Finca Ecológica, University of TropicalAgriculture,
Thu Duc, Ho Chi Minh City, Vietnam
Washing losses of dry matter (WL) were determined in samples of leaves from 9 different tropical trees and shrubs locally available either in fresh or sun-dried basis, during 30, 60, 90 and 120 min in four successive cycles of 30 min each in a semi-automatic washing machine. A significant effect (P<0.001) of time on the WL of the leaves was observed. The lowest values were recorded for 30 and 60 min. There was no difference in WL of leaves washed during 90 and 120 min. The regression analysis did not reveal differences in predicted WL at 0 min when linear and quadratic equations were fitted to the data (overall means, 21.0 and 19.5% for linear and quadratic equations, respectively). The WL of fresh leaves was significantly lower (P<0.001) than the corresponding value for sun-dried samples (22.2 and 30.6%, respectively). Ninety min of washing is suggested for leaves from the trees evaluated in this study.
Washing losses (WL) of dry matter have usually been identified as the water-soluble, readily fermentable fraction of roughages in ruminant degradation studies (rskov and Shand 1997). On the other hand, it has been suggested that WL is related to the cell content of plant materials (Chermiti et al 1996). The quantification of WL in feeds for livestock could be of immediate importance.
Several approaches have been reported in the estimation of washing losses in a variety of feed samples for ruminants. Tuah et al (1996) determined the washing losses of a range of feeds in Ghana by soaking them in warm water (39oC), but the washing time was not recorded. Others have washed the sample without soaking (Nsahlai et al 1995).
In practice, sample washing has usually been carried out in a washing machine, for 10 min (Khazaal et al 1995), 15 min (Chermiti et al 1996), 20 min (Kibon and rskov 1993), 30 min (Nsahlai et al 1995; Siaw et al 1993) or 60 min (Khazaal et al 1993). In other studies, WL has been determined by hand-washing the bags under running water after soaking the bags in warm (37oC) water during 15 min (Abdulrazak et al 1996). It is obvious that there has been no agreement as to the time required to wash a feed to determine the WL.
The aim of the present communication is to describe a preliminary approach to the estimation of WL in some leaves from tropical trees and shrubs.
Samples of 9 different species of tropical trees and shrubs growing in the Thu Duc district were employed in triplicate for this study. The leaves were from Acacia auriculiformis, Acacia mangium, Anacardium occidentalis, Artocarpus hetorophyllus, Gliricidia sepium, Hibiscus rosasinensis, Leucaena leucephala, Trichantera gigantea and Nypa fruticans. The samples were obtained at 08.00 am from terminal branches (less than 0.5 m length) from trees and shrubs periodically cut according to a tree cropping system. The leaves were separated from stems, weighed and cut in very small pieces with a knife and thereafter used in the fresh state or after sun-drying during 3 days. Dry matter content and ratios (fresh basis) of leaf:stem are shown in Table 1.
Nylon bags (50 x 150 mm) were used and they were made from nylon filter cloth with a pore size of 45 to 55 m. The bags were used in duplicate to estimate the WL in a semiautomatic washing machine. About 5g (fresh) or 1g (dry) of sample in either fresh or sun-dried state were introduced in the bags and thereafter washed at random in one, two, three or four consecutive cycles of 30 min each. The temperature and pH of the tap water utilized were on average 25oC and 6.6 respectively. The volume of water used in every cycle was in the ratio of 3 litres per bag. After washing, the DM content in the residue contained in the bags was estimated by microwave radiation to constant weight as described by Undersander et al (1993).
A linear model taking into account the type of sample used (fresh or sun-dried) and the
time spent for washing (30, 60, 90 or 120 min) was employed to study the characteristics
of the WL of leaves (SAS 1993). The analysis of variance was carried out using Minitab 9
software (Minitab Inc, State College, PA 16801, USA). All statements of significance are
based on a probability of less than 0.05. When appropiate, the means were separated by the
Duncan's multiple range test.
The analysis did not reveal any significant interaction of time of washing x
processsing method (P>0.10) but there was a highly significant effect of time of
washing (P<0.001), the lowest values being obtained for 30 and 60 min. There was no
difference in WL of leaves washed during 90 and 120 min (Table 2).
An analysis of regression made on the pooled data (mean values for each leaf at every washing time, on fresh or sun-dried basis; n = 72), did not reveal differences in WL at time "t"=0 min, when linear and quadratic equations were compared (predicted means for WL at zero time were 21.0 and 19.5%, for linear and quadratic equations, respectively). Moreover, the R2 values, although significant (P<0.05), were very low and almost equal for the equations of first or second degree, respectively: 0.053 and 0.054. This could be due to the fact that WL from some leaves was practically independent of the time of washing, such as the case of Hibiscus rosasinensis and Artocarpus heterophyllus (R2<0.1 in each case). In contrast, WL from Gliricidia sepium were highly dependent on time of washing. It was found that R2 values for leaves of Gliricidia sepium were about 0.85 in fresh basis and 0.7 in sun-dried basis, respectively. These results could suggest that the time needed for washing certain tropical leaves could be highly dependent on the type of tree or shrub studied. This assumption might lead to the necessity of a very careful estimation of WL which, in turn, could be very important when the water solubility of samples is very high, thus leading to a decisive influence on the nutritive index of feeds.
Individual values for WL of every sample of leaf according to the washing time and the
method of processing is shown in Table 3. In addition, calculated WL for t = 0 according
to a linear or quadratic effect are also shown.
The WL values of fresh leaves were significantly lower (P<0.001) than the corresponding values for sun-dried samples (Table 2). The reasons for these results are not apparent. In this connection, it has been observed that there are differences between fresh and dried leaves from the point of view of ruminal degradation parameters, but species differences were apparent (Siaw et al 1993). If the WL index is a reflection of leaf topoarchitecture, it is logical to assume that a different response in dry matter degradability could be possible. Thus WL could be, as a first approach, a simple parameter to assess the overall process of leaf degradation.
It is difficult to make a firm conclusion from this experiment, since there appear to
be no previous comparative studies concerning the estimation of washing losses in leaves
of tropical trees. Nevertheless, according to the results reported here, a washing time of
90 min could be suggested as a tentative recommendation for leaves of the species used in
this study and when a semi-automatic washing machine with cycles of 30 minutes is used.
Taking maximum advantage of the opportunity offered by the electronic medium we attach some comments by Dr Orskov, the aim being to stimulate debate on the topic and to assist other researchers working on simple methods of feed evaluation. Some of the points raised by Dr Orskov will be the subject of forthcoming papers to be published in future numbers of LRRD. Dr Orskov’s comments were: "I think there is a need to compare the method with a measure of dry matter loss from small particles by letting water drip through the substrate on a nylon bag filter or compare it with the rapid gas production if possible, ie gas production during first 2 to 3 hours and if possible both. The differences in some samples due to length of washing period may be associated with particle loss". Dr Ly replies that "with increasing time of washing there could be an increase in the amount of material solubilised but that particle size is unlikely to be the explanation as the samples (fresh and dried) were not ground but were cut with a knife into pieces of about 10mm diameter"
The Editors: LRRD
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Received 20 May 1997