Water-extractable dry matter and neutral detergent fibre as indicators of whole tract digestibility in goats fed diets of different nutritive value

Pheng Buntha and Chhay Ty

Center for Livestock and Agriculture Development
(CelAgrid, UTA-Cambodia)
Phnom Penh, Cambodia
tha_vt@yahoo.com

Abstract

Fresh foliages of guinea grass (Panicum maximum), cassava (Manihot esculenta), stylo (Stylosanthesis guiensis, CIAT 183) and water spinach (Ipomoea aquatica) were fed to growing goats in a  Completely Randomized Block arrangement with 3 goats and 4 periods, each of 8 days. The aim was to compare the water-extractable DM of these feeds with NDF values as predictors of whole tract DM digestibility.

The proportion of leaf  in the foliages offered was highest in cassava with relatively lower values for stylo and water spinach which were similar to each other (only leaves were fed in the case of guinea grass) . Some degree of selection occurred during eating as there were indications that on the cassava the goats selected the same proportion of leaf as in the foliage offered, but ate more of the petioles than the stems. For stylo, leaves were preferred slightly more than stems while the opposite was the case with the water spinach. There was a close relationship between water-extractable DM and NDF in DM (R² = 0.77; n = 35) in the different botanical components of the four foliages. DM intakes were highest on guinea grass and lowest on cassava with intermediate values for stylo and water spinach. Highest DM digestibility was recorded on the water spinach diet with lowest values for cassava. N digestibility was also high for water spinach with the lowest value for guinea grass. Among the guinea grass, stylo and water spinach foliages, there was a close relationships between DM digestibility and the two chosen indicators:- NDF and water extractable DM , with the closest fit being observed for the  latter (R² = 0.85 and 0.98, respectively; n=3). The data for cassava did not fit in either of these relationships.

It is concluded that the water-extractable DM technique is a simple, low cost, and effective method that is equal to, or superior than,  NDF  as a predictor of the the DM digestibility of most forages. Forages for which the method is inappropriate (eg: cassava) are likely to be those which contain anti-nutritional substances known to affect negatively feed intake and digestibility. Urine excretion was 6 times higher, relative to live weight,  and 2 times higher, relative to water consumed from the forages, in goats fed water spinach compared with the other foliages.

Key words: Cassava, foliages, goats, guinea grass, NDF, nutritive value, stylo, water extraction,water spinach

Introduction

The use of foliage from trees and shrubs in animal nutrition has focused the attention of many researchers, due to the fact that these feed resources are locally available, perennial sources of feeds (Leng 1997), rich in protein and particularly appropriate for small ruminants (Seng Sokerya and Rodriguez 2001). Nowadays many researchers are interested in simple methods for measuring the digestibility of diets such as in sacco, and in vitro, and also the use of indicators of digestibility such as crude fibre and NDF. Another approach to the use of simple predictors of digestibility was developed by Ly and Preston (1997). They measured the DM extracted by water during a 90 minute cycle in a washing machine and suggested that this parameter could be used as an indicator of the potential degradability of the feed in the rumen ecosystem.  The washing loss was closely related with in sacco degradability  at 48 hr and with the in vitro gas production at 48 hr. It was recommended that the use of the washing loss method could be 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 (Ngugen Van Lai and Nguyen Thi Thu Huong 1999).

In an experiment with buffaloes to test the effect of supplementation of rice straw with Sesbania leaves, it was shown that the dry matter wash value (water-extractable DM), as determined by the method of Ly and Preston (1997), was more closely related to in vivo digestibility than were measurements of  in vitro and in sacco digestibility (Nguyen Van Thu 2005).

The aim of the present investigation was to compare the water-extractable dry matter with the NDF value as predictors of in vivo digestibility of Guinea grass, fresh cassava foliage, stylosanthes, and water spinach, each  fed to goats as the sole feed.
 

Materials and Methods

Location

The study was carried out at the Livestock Research Center, in Laos, from September to October, 2005.

Treatments and design

Fresh foliage of guinea grass (Panicum maximum) (GG), cassava (Manihot esculenta) (CF), stylo (Stylosanthesis guiensis) (ST) and water spinach (Ipomoea aquatica) (WS) were compared  in a  Completely Randomized Block arrangement with 3 goats and 4 periods (Table 1), each of 8 days. The first 3 days were for adaptation to the diet followed by  5 days for data collection.

Animals and housing

Two female goats and one male goat averaging 13 kg live weight were used for the evaluation of each plant species. The goats were housed in metabolism cages (1.0x0.8 m), elevated 0.7 m and allowing the separate collection of faeces and urine. The cages were make of wood and were situated in a building with roof and open sides (Photo 1).

Photo 1: The metabolism cages made from local materials

Feeding management

The guinea grass, fresh cassava foliage, stylosanthes foliage and water spinach were harvested daily from plots in the Livestock Research Centre. They were offered fresh as the sole feeds, immediately after harvesting. The foliages were offered as whole branches and were suspended from the roof of the cage above the feed trough. Fresh quantities of foliage were given three times per day (7:00 AM, 12:00 AM and 4:00 PM) in amounts some 20% above observed intakes.

Measurements

Feeds offered and refused, and output of faeces and urine, were recorded daily during the last 5 days of each period. Samples of feed offered and refusals were separated daily into leaf, petiole and stem (cassava), and leaf and stem (stylo and water spinach). In the case of the guinea grass, only the leaves were offered. Samples of each component (offered and refused) were retained for analysis of DM, N, water extractable DM and N and NDF. A sub-sample (10%) of faeces was put each day in plastic bags in the freezer (-20 °C). Urine was collected in a bucket containing 10 ml of 25% sulphuric acid (H₂SO₄) to keep the pH below 4 so as to prevent escape of ammonia. A sub-sample (10%) was collected each day and kept in the freezer.

Chemical analysis

Samples of the foliages offered were dried by micro-wave radiation (Undersander et al 1993) and ground in a coffee mill. For the water extraction method (Ly and Preston 1997; Ly and Preston 2001), duplicate samples (about 1 g) were put in tea bags in a semi-automatic washing machine and thereafter washed for three consecutive cycles of 30 min each. The temperature and pH of the tap water utilized were on average 25^oC 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 and N in the residue in the bags were measured.

Feed and faecal samples were dried by microwave radiation to measure the DM content (Undersander et al 1993). Total N of samples of feed and faeces and urine, and of the residual material after washing, were determined by the Kjeldahl procedure as outlined by AOAC (1990). The ash content of feed and faeces was determined following the AOAC (1990) recommendations; organic matter was assumed to be the result of subtracting the percentage of ash from 100. The NDF was determined according to the method of Van Soest et al (1991).

Statistical analysis:

The data were analyzed using the general linear model (GLM) procedure in the ANOVA software of MINITAB (release 13.31). Sources of variation were treatments (foliages), blocks (goats) and error. Relationships between independent variables (wash value of DM  and NDF) and the dependent variable (DM digestibility)  were determined using the regression models in the Excel (2003) software.

Results and discussion

Feed selection and composition

The proportion of leaf  in the foliages offered was highest in cassava with similar, and relatively lower, values for stylo and water spinach (Table 2; Figures 1 to 3). Some degree of selection occurred during eating as there were indications that on the cassava the goats selected the same proportion of leaf as in the foliage offered, but ate more of the petioles than the stems. For stylo, leaves were preferred slightly more than stems while the opposite was the case with the water spinach. This pattern of eating contrasts markedly with the major degree of selection for leaves observed in sheep fed cereal crop residues (Aboud et al 1991), but is in agreement with the reports of Theng Kouch et al (2003), Toum Keopaseuht et al (2004) and Ammaly Phengvilaysouk and Lampheuy Kaensombath (2006) where goats preferred to have access to whole branches of foliage, which enabled them to eat some stems and petioles, as compared with having access restricted to leaves only.

Crude protein was highest in the leaves of cassava and water spinach, while guinea grass leaves were lowest with intermediate values for stylo leaves (Table 3).  Petioles and stems of cassava and stems of stylo and water had similar levels of crude protein in the range of 9.6 to 12.8% in DM.

There were wide variations in NDF with guinea grass leaves having levels close to 80% while water spinach leaves and stems had only half that level (Table 4). Stems of cassava and stylo had higher NDF than leaves of the same species and the levels were considerably higher than was observed in stems of water spinach. Water extractable DM was high in all components of cassava and water spinach foliage. Lowest values were in leaves of guinea grass. Water extractable N values were high in all components of cassava foliage with stems of stylo and water spinach having higher levels than the leaves.

There was a close relationship between water-extractable DM and NDF in DM (Figure 4). This was to be expected since, as pointed out by Chermiti et al (1996), the water-extractable DM is mostly derived from the cell contents and in the NDF method is represented by the detergent soluble fraction (ie: 100-NDF).

Figure 4. Relationship between water-extractable DM and NDF (data derived from plant
components in offered and refused foliages of cassava, guinea, stylo and water spinach)

Voluntary feed intake

DM intakes were highest on guinea grass and lowest on cassava with intermediate values for stylo and water spinach (Table 5; Figure 5). The intakes of cassava foliage as a function of live weight were much lower (17 g DM/kg LW) than those recorded by Theng Kouch et al (2003) for cassava foliage as the sole feed for goats (42 g DM/kg LW). The reason for the low intakes on cassava foliage is not immediately apparent except that the origins of the cassava plants were quite different., and there could have been differences in the concentrations of non-nutritional factors such as cyanogenic glucosides and tannins, which may have affected intake. Total intakes of  N were similar for stylo, cassava and water spinach but the contribution from the different plant parts were quite different with the leaves supplying almost all the N in cassava while it was divided equally  between leaves and stems for stylo and water spinach (Figure 6).

An interesting observation concerned the excretion of urine which was some 6 times higher for the water spinach diet compared with the other foliages (Table 5; Figure 7). In contrast, water consumed as part of the foliage was only between 2 and 3 times higher on water spinach compared with the other treatments. Thus the ratio of urine excreted to water consumed in the foliage was twice as high for the water spinach diet compared with the rest (Figure 8). There is no obvious explanation for these differences, although presumably it reflects differences at the level of nutrient metabolism.

Digestibility and N retention

Highest DM digestibility was recorded on the water spinach diet with lowest values for cassava (Table 5). N digestibility was also high for water spinach with lowest values for guinea grass.  The values for daily N retention reflected those for N intakes.

NDF and water-extractable DM

It is apparent from the comparison of the data for digestibility with those for NDF and water extractable DM that the results for cassava were atypical (Figure 9), in that NDF and water extractable DM values were comparable with those for water spinach yet there were 15 units of difference in DM digestibility. Leaving out the cassava data (Figure 10), there were close relationships between digestibility and the two chosen indicators:- NDF and water extractable DM - with the closest fit being observed for the latter (Figures 11 and 12)  

Support for the superiority of water extractable DM, compared with NDF, as an indicator of digestibility can be seen in data reported for tropical grasses in Cameroon (Figure 13). The coefficient of determination (R²) between potential DM digestibility (in sacco 72 hour degradability method) was 0.74 for water soluble DM compared with 0.31 for NDF (Enoh et al 2005). A close relationship between water-extractable DM and in vivo DM digestibility in buffaloes (Figure 14) was reported by Nguyen Van Thu (2005). Russell and Karsli (2002) observed that the water soluble "a" fraction  determined by the in sacco method was comparable with NDF as predictors of whole tract digestibility in cattle (R² of 0.83 and  0.85, respectively) and better than NDF for the prediction of voluntary feed intake (R² of 0.80 and 0.69, respectively).

Conclusions

• The water-extractable DM technique is a simple, low cost, and effective method that is equal to, or superior than,  NDF  as a predictor of the the DM digestibility of forages.
  
  
• Forages for which the method is inappropriate (eg: cassava) are likely to be those which contain anti-nutritional substances known to affect feed intake and digestibility.
  
  
• Urine excretion was 6 times higher, relative to live weight,  and 2 times higher, relative to water consumed from the forages, in goats fed water spinach compared with the other foliages.

Acknowledgment

The author wishes to thank the MEKARN programme for providing the opportunity to conduct this mini-project in order to improve and develop his knowledge and analytical skills.Mr Chhay Ty is acknowledged for timely advice and assistance in the laboratory analyses.

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