Chemical composition and enzymatic in vitro digestibility of cowpea cultivars and buffalo grass hay grown in Limpopo province of South Africa

K E Ravhuhali, J W Ng’ambi and D Norris

Department of Animal Production, University of Limpopo, Turfloop Campus, Private Bag, X1106, Sovenga 0727, South Africa
soulmankr@gmail.com

Abstract

The aim of this study was to determine chemical composition and in vitro enzymatic digestibility of four cowpea cultivars and buffalo grass hay. The four cowpea cultivars used were Pan 311, Red caloona, Black eye, Agripes and buffalo grass. Cowpea cultivars had higher (P<0.05) crude protein (CP) contents than buffalo grass hay. Buffalo grass hay had higher (P<0.05) NDF values than cowpea cultivars which had similar (P>0.05) NDF contents. Analysis of variance indicated that cowpea cultivars had higher in vitro DM, OM and protein digestibilities than buffalo grass hay. In vitro cowpea DM, OM and protein digestibilities ranged from 0.64 to 0.75. Chemical contents of the cowpea cultivars and buffalo grass had poor capacity to predict forage in vitro digestibility.

These results indicated that all cowpea cultivars have high in vitro digestibility and hence the legumes should be able to supply enough nutrients, particularly proteins, when given as supplements.

Keywords: Agripes, Black eye, buffalo grass, in-vitro digestibility, Pan 311, Protein, Red caloona

Introduction

A major problem facing ruminant livestock producers in tropical areas is poor nutrition for their animals during the dry season when pastures and crop residues are limiting in nutritional quality (Murphy and Colucci 1999). Normally, it is during this period when problems such as sickness and weight losses due to poor dietary profiles arise. One way of improving the utilisation of such low quality roughages is by proper supplementation with leguminous forages (Poppi and McLennan 1995). Leguminous hays are important sources of nutrients for herbivores in South Africa (Mokoboki et al 2000). These legumes are rich in protein. Though there are several forage plants that have the capacity to produce high yields of protein, they contribute little to the much needed improvement of livestock production because data on their nutritive values are scarce (Barro and Ribeiro 1983; Mokoboki 2007). Cowpea is such an important legume and a source of protein for livestock and humans (Giami 2005).

Factors that determine the feeding value of feedstuffs are very complex. All available information, both quantitative and qualitative must be used in making judgments on the feeding values of a particular plant species (Dzowela et al 1995). The highest level of efficiency of animal production can be achieved only by reaching the highest possible level of nutrient intake (Blaxter 1962). Forage intake by ruminants involves interactions between the forage itself, the microbes in the gastro-intestinal tract and the animal (Blaxter et al 1961; McDonald et al 2002). Thus, forage intake is a function of the forage rate of degradation by ruminal microbes, the rumen capacity, the forage digestibility and passage rate through the gut (McDonald et al 2002). These factors are important when considering the forage feeding value. Conventional in vivo digestibility and voluntary intake of a forage are consistently used to enable one to make an accurate and reliable assessment of forage feeding values in the majority of situations. However, in vivo digestibility and voluntary intake experiments with animals are laborious, time consuming, expensive and require large feed quantities (Karsli and Russell 2002).  A simple and inexpensive technique to estimate in vivo digestibility and voluntary intake quickly and accurately is a necessity. In commercial agriculture, chemical analysis is usually used to determine the forage quality, and their nutrient contents are then matched with requirements for different physiological stages of the livestock to develop feeding systems and rations (Chesworth 1992). The in vitro digestibility technique (Tilley and Terry 1963) is widely cited as the laboratory procedure of choice which ensures the most precise and accurate estimation of in vivo digestibility of forages (Abdouli and Attia 2006; Melaku et al 2003; Tessema and Baars 2004).

The objective of this experiment was to determine nutrient contents and in vitro digestibility of four cowpea cultivars and buffalo grass grown in Limpopo province and their correlations.

Materials and methods 

Feeds and experimental design

The study was conducted in the University of Limpopo Animal Production Department Laboratory University of Limpopo Turfloop campus. The feeds used in this experiment were four cowpea cultivars. The cowpea cultivars were Pan 311, Red caloona, Black eye and Agripes harvested at vegetative stage. Buffalo grass hay and cowpea hays were cut and dried in the shade. The forages were assigned as treatments, with three replications, in a completely randomized design.

Determination of in vitro enzymatic digestibility

In vitro enzymatic digestibility was determined using the method of Michalet-Doreau and Aufrère (1988) and Tilley and Terry (1963) for the four cowpea cultivars and buffalo grass hay. Three grammes of the sample were accurately weighed into a 50 ml plastic centrifuge tube with a screw cap in duplicate. Thirty milliliters of 0.1 NHCL containing 0.2 % (w/v) pepsin was added and incubated in a water bath of 39 °C for 48 hours. The tube was shaken at least three times a day. The sample was then filtered back into crucibles and then transferred back to the centrifuge tubes with quantitatively minimum amounts of sodium acetate buffer containing 2.5 % (w/v) cellulase. The volume in the tube was made up to 30 ml with acetic acid cellulase buffer. The tubes were then incubated in the water bath at 39 °C for 48 hours. The tubes were shaken twice a day. At the end of the incubation period, the contents of the tubes were filtered through dried and pre-weighed sintered glass. The residues were washed with distilled hot water. The residues were then placed in an oven to be dried overnight at 105 °C. The dried residues were analyzed for dry matter, organic matter, crude protein, neutral detergent fibre and acid detergent fibre.

Chemical analysis

Dry matter (DM) and ash of feeds, feed refusals and faeces were determined according to AOAC (2000). Acid and neutral detergent fibres (ADF and NDF) were determined by the method described by van Soest et al (1991). Nitrogen content was determined for feeds and faeces using the Kjedahl method (AOAC 2000). Extracted condensed tannins were done using the method described by Porter et al (1986).

Statistical analysis

Analysis of variance was used to test the effect of cowpea and buffalo grass hays on their in vitro enzymatic digestibility using General Linear Model (GLM) procedures of SAS (2004) as in a completely randomized design. Duncan multiple range test was used to separate the means (SAS 2004). Linear regression was used to determine the correlation between chemical composition parameters and in vitro digestibility.

Results and discussion   

The results of the nutrient composition of cowpea cultivars and buffalo grass hay are presented in Table 1. Buffalo grass contained low crude protein content of 3.3 g/kg DM. This value is similar to the ones reported by Aganga et al (1999), Giacomini et al (2006) and Motubatse et al (2008).  All cowpea cultivars had over 15 % crude protein content. However, Black eye hay had higher protein content than the other cowpea cultivars.  These values are similar to the ones reported by Savadogo et al (2000), Baloyi et al (2001), Chakeredza et al (2002) and Rivas-Vegas et al (2006). All the cowpea cultivars contained traces of condensed tannins but the total amounts varied between the cultivars. Pan 311 contained highest amounts of condensed tannins while Red caloona contained the lowest amounts of condensed tannins (Table 1). Baloyi et al (2001) also indicated that some cowpea cultivars contained high proportions of protein–binding tannins. No traces of tannins were found in buffalo grass hay. This is similar to the findings of Motubatse et al (2008). It can be concluded that cowpea cultivars used in the present experiment have high crude protein contents and thus offer great potential as protein supplements for goats and sheep fed low quality roughages.

The results of the enzymatic in vitro digestibility of cowpea and buffalo grass hays are presented in Table 2. Buffalo grass had lower (P<0.05) in vitro dry matter, organic matter, crude protein, neutral detergent fibre and acid detergent fibre digestibility values than those of cowpea hays. However, cowpea cultivars had similar (P>0.05) in vitro dry matter and organic matter digestibility values. Pan 311, Red caloona and Agripes had similar (P>0.05) in vitro neutral and acid detergent fibre digestibility values. In vitro dry matter, crude protein and organic matter digestibility values of cowpea cultivars are within the ranges reported by Kiflewahid and Mosimanyana (1987). NDF and ADF digestibility values reported in the present study are similar to those reported by Hoffman et al (2003) who found that in vitro NDF and ADF digestibility values of leguminous hays ranged from 35 to 40 %. Black eye and buffalo grass hays did not differ (P>0.05) from each other on ADF digestibility. In vitro dry matter, organic matter and crude protein digestibility values of buffalo grass hay are similar to those reported by Aregheore et al (2006). The authors found that buffalo grass hay had 49.6 % in vitro dry matter, 47.4 % organic matter and 52.9 % crude protein digestibility values. In vitro NDF digestibility of buffalo grass hay in the present study is also within the range reported by Hoffman et al (2003) in grasses. The authors found that in vitro NDF digestibility of grasses ranged between 21.7 and 31.1 %.

The correlation co-efficients between chemical composition parameters and in vitro digestibility were generally low (Tables 3). Indeed, forages of different chemical composition gave similar in vitro enzymatic digestibility. These results show that chemical contents of the cowpea cultivars and buffalo grass have poor capacity to predict forage in vitro digestibilities. For example, the digestibility of cellulose carbohydrates was so variable that the acid and neutral detergent fibres were not well related to the in vitro enzymatic digestibilities of the forages. This may be due to the different environmental factors promoting lignification as opposed to cell content (van Soest 1996). The studies reported by Barber et al (1984), Givens et al (1988) and Ng’ambi (1995), also, indicated that chemical measurements of forages provided no indication of forage digestibility.  However, van Soest (1980) showed that NDF and ADF fibres were negatively and significantly correlated to in vitro digestibility of forages. The explanation was that NDF and ADF are analytical products having nutritional characteristics that describe those forage components that have low solubility in specific systems and are relatively less digestible than starch. Similarly, Linn and Kuehn (1993) found that high NDF and ADF fibre contents reduced digestibility of plants and could be used to predict net energy contents of feeds in ruminant animals.

Condensed tannin contents provided no reliable indication of in vitro enzymatic digestibility of the forages (Table 3), thus suggesting that condensed tannin contents have limited potential for predicting the feeding value of forages. This is similar to the observations of Mashamaite (2004) who found that diet digestibility in rabbits was poorly predicted by condensed tannin contents of leaves of acacia species.  

Conclusion 

• Chemical composition and in vitro digestibility values of the cowpea cultivars found in the present study are quite high and hence the legumes should be able to supply enough nutrients, particularly proteins, to ruminant animals when given as supplements. Chemical contents of the cowpea cultivars and buffalo grass hay have poor capacity to predict forage in vitro digestibilities.

Acknowledgement  

The senior author acknowledges financial support for scholarships from the National Research Fund and National Department of Agriculture in South Africa. 

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Received 21 June 2010; Accepted 8 July 2010; Published 1 September 2010

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