Comparison of the in sacco rumen and washing loss methods to estimate the potential energetic value for livestock of leaves from tropical trees, shrubs and crop residues

Abstract

Samples of leaves from ten tropical species: six trees (Acacia mangium, Acacia auriculiformis,Gliricidia sepium, Leucaena leucocephala, Indigofera teysamii, Jackfruit - Artocarpus heterophyllus), three crop plants (Cassava - Manihot esculenta, sugar cane - Saccharum officinarum, banana - Musacea)  and a crop residue (Rice straw - Oryza sativa) were put in nylon bags and the losses of dry matter were estimated for washing times of 30, 60, 90 and 120 minutes in a commercial semi-automatic washing machine, and incubation times of 6, 12, 24 and 48 hours in the rumen of a cow fed rice straw and fresh grass (Panicum maximum).

The relationships between losses of dry matter in the washing machine and time of washing were curvilinear (range of R² from 0.86 to 0.99) for all leaves except those from banana and Acacia auriculiformis where the relationship was linear (R²=0.99).  There were close relationships between in sacco dry matter loss after 48 hours incubation and dry matter losses by washing (R²=0.73, 0.80, 0.86 and 0.80 for washing times of 30, 60, 90 and 120 minutes [average values for all samples at each time were used in this analysis]). Selecting the 90 minute washing time and correlating the data obtained with this method (range 15-47% for the ten species) with those from the 48 hour in sacco dry matter loss (range 25-61%) gave an R² of 0.86. The two methods ranked the species similarly and were considered to be equally effective in rating the chosen materials as potential sources of digestible dry matter for both ruminant and monogastric livestock. The use of the washing loss method is recommended as a first approximation to estimation of nutritive value as it is simple, rapid, and low cost, and dispenses with the need for surgically modified animals. 

Key words: Washing losses, nutritive value, leaves, trees, crop residues, in sacco, analytical methods

Foreword

Much effort has been expended over the last three decades to develop laboratory methods to predict the digestibility and the nutritive value of feeds for both ruminant and monogastric animals. A major aim of this research was to generate values for feeds from which to calculate feeding standards and least-cost formulations. The most widely used methods (eg: the in vitro technique of Tilley and Terry 1963; the in sacco method popularised by Ørskov et al 1980; and the in vitro gas production approach of Menke et al 1979) rely on fermentation by rumen micro-organisms as the means of deriving an index of digestibility.   However, in the ruminant animal, digestion is a two stage process involving both fermentation (in the rumen, caecum and colon) and gastric processes   (in the abomasum and intestine).  For monogastric animals, gastric digestion is the predominant mechanism preceding limited fermentation in the hind gut. It is clearly impossible to simulate these processes in the laboratory by in vitro or in sacco processes.  The best that can be achieved is a relative ranking of feeds according to their potential energetic value.  The final assessment of nutritive value can only be ascertained in animal feeding trials which permit the expression of performance according to the specific nutrients provided by feeds and the balance of these nutrients in relation to the needs of the different productive traits (Preston and Leng 1987).

From the time of Kellner there has been almost an obsession with refining methods for estimating the composition and nutritive value of the fibrous cell wall complex of feeds yet this is the fraction which contributes least to nutritive value. By contrast, methods for measuring the soluble components of feeds have been neglected, relegated to being determined "by difference". 

The following paper by Nguyen Van Lai and Nguyen Thi Thu Huong provides supporting evidence for the original proposal of Dr Julio Ly that a simple washing loss value, requiring neither chemicals nor surgically modified animals, can be as useful as the  in vitro or in sacco methods when the aim is to have an indication of  the potential energetic value of a feed for either ruminant or monogastric livestock.

The editors

Introduction

The idea of using a commercial, semi-automatic washing machine for deriving a nutritive value index, based on the water soluble components of feeds, was  first proposed by Ly and Preston (1997).  They found that a total wash time of 90 minutes, comprising three consecutive wash cycles of 30 minutes each, gave the most consistent results using as test materials a range of sun-dried leaves from trees and shrubs. When the values obtained with the wash value method were compared with the in vitro gas production test (Menke et al 1979), for the same range of feeds, the coefficients of determination (R²) were 0.49, 0.77, 0.70, 0.63, 0.50 and 0.56 for the variables 90 minute wash value and the gas production at times 6, 12, 24, 48 and 96 hours (Ly et al 1997).

The aim of the study reported in this paper was to compare the wash value method with the in sacco technique for assessing the relative energy value of feeds..

Materials and methods

The feed samples that were used, arrayed according to their crude protein content (determined according to AOAC 1984), are shown in Annex Table 1.  They comprised the leaves taken from branches (less than 0.5 m length) of trees and crop plants grown in the ecological farm of UTA where they are periodically harvested at 2-3 month intervals according to a tree cropping system. Leaves taken from recently harvested rice straw and immature sugar cane and banana trees were also included in the study.  The leaves were separated from stems, cut in very small pieces (<5mm) with a knife and thereafter sun-dried during 3 days.

Wash value measurement

The washing losses (WL) were determined in duplicate for three representative samples of each of the leaves following the procedure described by Ly and Preston (1997). The samples (1 g) were put in bags (50 x 150 mm) made from nylon filter cloth with a pore size of 45 to 55 microns and thereafter washed at random in one, two, three or four consecutive cycles of 30 min each. The volume of water used in every cycle was in the ratio of 3 litres per bag. After washing, the dry matter in the residue was estimated by microwave radiation to constant weight as described by Undersander et al (1993). 

In sacco measurement

The procedure used to determine the dry matter degradability of the samples was that described by Ørskov et al (1980). The sun-dried samples were milled in a kitchen coffee grinder and weighed amounts (5 g) put into nylon bags with the same characteristics as those used for the wash value measurement. The bags (in triplicate for each feed sample) were attached to plastic tubes (for description see Preston 1995) and incubated in the rumen of one rumen-fistulated Bos indicus heifer (diet was rice straw and fresh grass [Panicum maximum], 50:50 fresh basis)   for 3, 6, 12, 24 and 48 hours. After incubation, the bags were washed by hand under running tap water, until the water ran clear, and dried in a microwave oven to a constant weight (Undersander et al 1993).

Statistical analysis

Regression analysis (Microsoft Excel software, MS Office 7) was used to relate (i) dry matter washing loss (y) with washing time (x) for individual samples; and (ii) 48 hour in sacco dry matter loss (y) with 90 minute wash value (x) for each individual sample. For (i) either polynomial or linear regressions were selected according to which gave the highest coefficient of determination; for (ii) a linear regression gave the best fit

Results and Discussion

Mean values for dry matter losses determined by the washing and in sacco procedures are in Annex Table 1. The curves fitted to the data relating dry matter loss to washing time are shown in Annex Figure 1 for each individual feed.  Losses of dry matter in the washing machine were related curvilinearly (range of R² from 0.86 to 0.99) to time of washing for all leaves except those from banana and Acacia auriculiformis where the relationship was linear (R²=0.99). 

Table 1: Coefficient of determination (R²) between in sacco dry matter loss at 48 hours and washing loss of dry matter at different wash times.
Wash time, minutes	R² value
30	0.73
60	0.80
90	0.86
120	0.80

There were close relationships (Table 1) between in sacco dry matter loss after 48 hours incubation and dry matter losses by washing (R²=0.73, 0.80, 0.86 and 0.80 for washing times of 30, 60, 90 and 120 minutes [average values for all samples at each time were used in this analysis]).

Selecting the 90 minute washing time and correlating the washing losses (range 15-47% for the ten species) with the 48 hour in sacco dry matter losses (range 25-61%) gave an R² of 0.86 (Figure 1). The relationship of 90 minute washing loss with 48 hour in sacco rumen dry matter loss observed in this study was stronger (R²=0.86) than was reported by Ly and Preston (1997) for 90 minute washing loss and in vitro gas production (R²=0.76) for a similar range of tree leaves. The feed samples were ranked similarly by the two methods as can be seen in Table 2. 

Table 2: Relative ranking of a range of leaves from trees and crop residues according to the wash value and in sacco degradability
	90 minute wash value		48 hr in sacco degradability
Sugar cane leaf	15	Rice straw	25
Rice straw	20	Acacia auriculiformis	30
Acacia auriculiformis	25	Sugar cane leaf	30
Acacia mangium	29	Acacia mangium	32
Jack fruit	33	Indigofera	45
Indigofera	38	Banana	47
Banana	38	Jack fruit	50
Gliricidia	43	Leucaena	58
Leucaena	46	Gliricidia	61
Cassava leaf	47	Cassava leaf	61

Conclusions

It is concluded that:

• The dry matter loss after a 90 minute washing cycle in a semi-automatic washing machine is closely related to the in sacco dry matter loss after 48 hours of ruminal incubation and can be used equally effectively as a means of ranking feeds for their potential digestibility in ruminant and monogastric livestock.
• The wash value method is simple and rapid and, when used together with micro-wave radiation for drying the samples, will deliver results within a total time period of less than 3 hours after receipt of the feed sample.

References

AOAC 1984 Official methods of analysis . Association of Official Agricultural Chemists, Washington, DC.

Ly J and Preston T R 1997 An approach to the estimation of washing losses in leaves of tropical trees. Livestock Research for Rural Development (9) 3:http://www.cipav.org.co/lrrd/lrrd9/3/ly931.htm

Ly J, Nguyen Van Lai and Preston T R 1997 A study of washing losses and in vitro gas production characteristics of nine leaves from tropical trees and shrubs for ruminants. Livestock Research for Rural Development (9) 3:http://www.cipav.org.co/lrrd/lrrd9/3/ly932.htm

Menke K H, Raab L, Salewski A, Steingass H, Fritz D and Schneider W 1979 The estimation of the digestibility and metabolizable energy content of ruminant feedingstuffs from the gas production when they are incubated with rumen liquor. Journal of Agricultural Science, Cambridge 93:217-222

Ørskov E R, Hovell D and Mould F 1980 The use of the nylon bag technique for the evaluation of feedstuffs. Tropical Animal Production (5) 3: 195-213.

Preston T R and Leng R A 1987 Matching Ruminant Production Systems with Available Resources in the Tropics and Subtropics. PENAMBUL Books Ltd: Armidale NSW, Australia pp: 245

Preston T R 1995 Tropical animal feeding. A manual for research workers. APHP Series 126. FAO, Rome
(http://www.fao.org/WAICENT/FAOINFO/AGRICULT/AGA/AGAP/FRG

Tilley J M and Terry R A 1963 A two-stage tgechnique for the in vitro digestion of forage crops. Journal of the British Grassland Society (18): 104-111

Undersander D, Mertens D R and Thiex N 1993 Forage analysis procedures. National Forage Testing Association. Omaha pp :154

Annex Table 1: Mean values for losses of dry matter at different wash times in a washing machine and different incubation times in the rumen
		Wash loss of dry matter, %					In sacco dry matter degradability, %
	N*6.25, % in DM	30	60	90	120		6	12	24	48
Rice straw	5.3	9	14	20	24		13	17	21	25
Sugar cane leaf	9.6	8.2	10	15	22		17	22	28	30
Acacia auriculiformis	14.0	16	19	25	28		15	21	24	30
Acacia mangium	15.6	16	22	29	30		10	12	22	32
Banana	15.6	33	35	38	40		38	28	30	47
Jack fruit	19.3	20	27	33	33		34	25	29	50
Gliricidia sepium	20.0	30	34	43	40		30	40.6	48	61
Leucaena	20.1	31	36	46	48		25	25.6	41	58
Indigofera	23.6	32	35	38	42		24	33.9	31	45
Cassava leaf	24.5	32	41	47	49		38	40	46	61
Average	16.8	22.8	27	33.4	35.6		24.7	26.5	32.1	43.9

Annex figure 1: Trend lines for dry matter washing loss of leaves from trees and crop plants according to time of washing

Received 1 January 1999

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