The effect of supplementing Rhodes grass (Chloris gayana) hay with Acacia tortilis leaves and pods mixture on intake, digestibility and growth performance of goats

S A Abdulrazak*, E G Njuguna and P K Karau

Department of Animal Science, Egerton University, P.O Box 536, Njoro, Kenya
*Present address: Egerton University, Division of Research and Extension. P. O. Box 536 Njoro
sabdulrazak@yahoo.com

Abstract

An experiment was conducted for 63 days to examine the effects of supplementation of Rhodes grass hay (H) with mixtures of leaves and pods of Acacia tortilis on intake, digestion and growth performance of Small East African goats (SEAG). Twenty four SEAG of 18±4 kg BW, 9months old were assigned to six diets including ad libitum Rhodes grass hay alone (H) or H supplemented with various proportions of pods, leaves and mixture of the two as follows; 19 g DM/kg W^0.75 pods (P), 19 g DM/kgW^0.75 leaves (L), 9.5:9.5 g DM/kg W^0.75 pods +leaves (PL), 14:5 g DM/kg W^0.75 pods +leaves (PPL) or 5:14 g DM/kg W^0.75 pods +leaves (PLL). The diets were allocated to the goats in a completely randomised design, with 4 goats per treatment.

Dry matter intake, digestibility, nitrogen retention and live weight gains were all increased by supplementation. There was an indication of a better response in intake and weight gains when the hay was supplemented with equal proportion (9.5 g DM/kg W^0.75) of pods and the leaves.

It is concluded that mixtures of pods and leaves give more benefits than when either is offered singly to goats fed a basal diet of grass hay.

Key words: Acacia tortilis, goats, intake, leaves, live weight, pods, tree mixture

Introduction

In arid and semi arid (ASAL) regions of Kenya, goats are normally kept under extensive management system and mainly depend on rangeland pastures that are often deficient in nitrogen and digestible nutrients. These low quality feed will have an effect in optimum rumen digestion, thus low intake (Van Soest 1982). One practical solution to protein supply constraints in livestock is through supplement with leguminous tree fodder. Acacia trees dominate drier areas of Sub-Saharan Africa, rich in protein and digestible nutrients (Abdulrazak et al 2000), consequently, improves digestibility, intake and animal performance (Tanner et al 1990). When consumed in excess of certain concentrations, proanthocyanidins present in this species adversely affect palatability, feed intake, animal performance, digestion of proteins and carbohydrates (Reed et al 1990). Studies have indicated that dilutions involving feeding of forage mixtures at different levels have shown improvement in palatability and intake (Rosales and Gill 1997). Bosman et al (1995) noted increased digestibility and growth rate on supplementing with mixed tree fodders.

The conventional approach in the using of tree forage in various studies has been to exploit a single tree component, probably due to difficulties associated with mixing of different tree parts. However, animals in the field feed on the mixture of two or more tree parts. Also there are scanty information quantifying optimum inclusion levels of tree mixtures and their effects on utilisation of poor roughages by goats. The objectives of this study were to examine the effects of supplementation of leaves or / and pods on voluntary feed intake, diet digestibility and growth performance in Small East African goats (SEAG) fed ad libitum grass hay.

Material and Methods

Animals and feed preparation

Twenty-four SEAG (bucks) of 18±4kg body weight, 9 month old were divided into six similar groups based on their live weight, and later allocated to six diets with four goats each in a completely randomised design. The diets included ad libitum Rhodes grass hay alone (H) or H supplemented with Acacia tortilis pods, leaves and mixture of the two as follows; 19 g DM/kg W^0.75 pods (P), 19 g DM/kgW^0.75 leaves (L), 9.5:9.5 g DM/kg W^0.75 pods +leaves (PL), 14:5 g DM/kg W^0.75 pods + leaves (PPL) or 5:14 g DM/kg W^0.75 pods +leaves (PLL). The animals were penned individually in well-ventilated pens (1.5m x 2m x 1.5m) with timber slatted floors. Each goat was dewormed with Nilzan⁺® before the trial commenced and four weeks later to control internal parasites. They were also subjected to weekly spraying with Triatix®.to control external parasites.

Rhodes grass hay (Chloris gayana) was purchased from a local farm and chopped with a chaff cutter to a length of about 4 - 6 cm to minimise wastage and selection. Acacia tortilis leaves and pods were harvested for 5 consecutive days from mature plants. Leaves were hand harvested by pruning the lower branches, air-dried under shade for maximum of three days. Dry pods were harvested by use of hooked stick and later milled to pass through a 6-mm screen. The dried leaves and pods meal were stored in separate gunny bags for later use in the experiment. The supplement diets were offered twice per day (08.00 and 14.00 h) in separate trough to that of basal diet. Rhodes grass hay was offered ad libitum allowing a proportional refusal of 0.20 of total amount offered. The amounts of supplements (on DM base) were adjusted weekly based on animal's body weight. The refusals (hay and supplements) from individual pens were separately collected and weighed the following morning before fresh feeds were offered. Daily intake was calculated as the difference between the amount offered and refusals. Feeds offered were sampled fortnightly and bulked for chemical analysis. Clean drinking water and multi-mineral mix (Maclik plus) was provided free choice to each goat during the entire feeding period. Weekly live weight of individual goat was recorded using a weighing scale for the whole period but intakes were recorded from day 8 to day 63 of the trial.

Digestibility and nitrogen balance trial

At the end of feeding trial, three goats (bucks) were randomly selected from each treatment group and transferred to individual metabolic cages, which allowed for separate faecal and urine collection. The trial comprised of a five-day period for animals to adapt to the cage followed by a 7-day collection period. During the trial, animals were maintained on similar treatment diets as indicated in the feeding trial. Faeces and urine aliquot were collected in separate plastic buckets, with urine buckets containing 10% (vv) H₂SO_4. Total daily (24h) faecal and urine output were measured and a 10% aliquot of each was sampled daily. Collected faecal and feed samples were oven dried at 60^oC for 48 h, milled to pass through 1 mm screen and stored in polythene bags. The urine samples were deep frozen pending analysis.

During the last two days of the digestibility trial, rumen liquor samples were taken from each animal using a stomach tube and vacuum pump at 0, 3, 6, 9, and 12h after offering the supplements. The rumen pH of the samples was immediately determined using an ionizable pH metre. Samples were strained through a clean double layer of cheese cloth and sub-samples of 20 ml of the liquor fraction taken, acidified with 2 ml of 10% H₂SO₄ (vv)and stored at -20^oC for later NH₃-N analysis.

Chemical analyses

Rhodes grass hay, supplements and faecal samples were milled to pass through a 1mm screen. The dry matter (DM), ash, Kjeldahl nitrogen analyses (AOAC 1990) were performed in duplicate on dried samples and CP calculated as (N x 6.25). Neutral detergent fibre (NDF) was determined by the method of Van Soest et al (1991) and rumen liquor analysed for NH₃-N as described by Abdulrazak and Fujihara. (1999).

Statistical analyses

Results of mean intake, digestibility, rumen pH, NH₃-N, N-balance and live weight gains were analysed using the General Linear Model (GLM) procedure (SAS 2000). Initial live weight was used as a covariate in determining the intake. Treatments means were separated using Duncan New multiple range test (Steel and Torrie 1980).

Results and discussion

The data presented in Table 1 shows the chemical composition of feeds offered during the trial. Rhodes grass hay had the highest NDF content, whereas lowest value was recorded in pods. However, the CP of hay was slightly above 70 g kg^-1 DM, level indicated as minimal for microbial growth and roughage intake (NRC 1981). The CP of the pods was within the range reported in other work (Shayo et al 1997) whereas the leaves CP was slightly lower than values reported by Abdulrazak et al (2000). This variation was partly attributed to site, variety, and stage of maturity as the forages were from mature plants. Work conducted on Acacia tortilis forages have indicated that seeds contained the more CP concentration than the empty pods (Shayo et al 1997), emphasizing the importance of grinding the pods before feeding to take advantage of the nutrients that would otherwise escape digestion as the seeds were tiny.

Woodward and Reed (1997) established a negative relationship between palatability, voluntary intake and the level of NDF in leguminous tree fodder. Mangan (1988) demonstrated a positive relationship between the levels of NDF and proanthocyanidin, which were inversely related to palatability of browse forage. However, in our study, the proportion of individual components in the mixture strongly influenced their acceptability, as reflected by the decline in intake of high leaf supplements (Table 2). Moreover, palatability of the supplements improved on mixing the components (pods and leaves), subsequently increasing their intake. This could be explained by dilution of fibre level in the supplements as the proportion of pods in the mixtures increased. Hay dry matter intake  decreased (P<0.05)  with increasing consumption of pods, similar to the findings of Tanner et al (1990). This may have resulted from high intake and low digestibility of pod supplements, leading to displacement of more rumen volume, consequently reducing the rumen space for the basal diet.

Total DMI ranged from 3.1 to 3.9% of body weight and was within the range 2.5 to 3.9% reported for tropical goats (Devendra and Burns 1983). Total dry matter intake increased (P<0.05) with supplementation, but was not significantly different among supplemented groups. Apart from showing high preference for mixed supplements, animals fed on mixed supplements had relatively higher DM intake compared to those on single components. The low protein status of poor quality roughage limits voluntary dry matter intake in ruminants. Browse forage supplement supplied more nutrients (nitrogen) to the rumen microbes. This may have reduced the rumen retention time by increasing the outflow rate stimulating the intake. Improvement in intake especially in groups supplemented with pods and leaf mixtures may also have resulted from dilution of anti-nutritive factors that includes the fibre levels. Similarly, Bosman et al (1995) reported improvement in intake when goats were offered browse mixtures.

Supplementation with leaves alone depressed the organic matter (OM) digestibility (Table 2). The tannins present in the OM of leguminous tree forages form complexes with NDF and may interfere with true availability of OM (Dutta et al 1999). Furthermore, an alternative source of carbohydrate depresses digestion of cellulose as the cellulolytic microbes in the rumen have more preference for it (Sutton et al 1993). The digestible carbohydrate in conjunction with the high tannin levels in browse forages may be responsible for partial depression in OM digestibility noted in supplemented groups.

Values of ruminal pH (Table 3) were within the range indicated as optimal for microbial growth and fibre digestion (Ørskov 1982). The NH₃-N concentration tended to increase with supplementation.

However, comparisons within supplemented groups showed lower NH₃-N concentration in rumen fluids from animals offered supplements high in proportion of leaves. This was attributed to inhibition of protein deamination in the rumen by high level of NDF noted in leaves. The results of our experiment indicate that the NH₃-N concentration in all the groups was above the range of 50 - 80mg L^-1 (Satter and Slyter 1974), reflecting abundant rumen degradable nitrogen (RDN) supply for rumen fermentation. The high NH₃-N in the control group could be explained by goats' ability to produce and concentrate urea in their saliva when fed on fibrous roughage.

Nitrogen intake and faecal nitrogen loss increased (P< 0.05) with supplementation but urinary nitrogen loss remained low and similar among the diets (Table 3). However, faecal nitrogen is an important aspect in ruminant nutrition as it indicates degree of protein degradability in the rumen. On comparing the diets, L and PLL supplemented groups had relatively higher faecal nitrogen loss as percentage of total nitrogen intake. The increase in faecal nitrogen output observed with L and PLL supplemented groups is typical of ruminants fed fibrous browse forage. The explanation for this can be found in dietary nitrogen complexing with NDF excreted in the form of NDF-N (Woodward and Reed 1997). Groups fed on mixed supplements retained relatively more nitrogen compared to those on single components possibly due to positive associative effects between nitrogen and organic matter present in pods and leaves (Rosale and Gill 1997). Generally, the positive nitrogen balance in the present study is in agreement with data from Reed et al (1990) using tropical browse supplements in ruminant diets.

The daily weight gains were noted to improve with supplementation (Table 2). However, the gains were higher (P< 0.05)  in P, PL and PPL supplemented groups. The groups offered mixtures had relatively higher weight gains than those offered single component (pods or leaves). The gain in weight on supplementation was associated with sufficient supply of fermentable substrate to ruminal microbes enhancing their growth and protein synthesis, subsequently improving availability of microbial protein in the small intestine. Mixing (pods and leaves) presumably synchronized fermentability of individual chemical constituents (nitrogen and carbohydrate) leading to associative effects in DM intake and digestibility (Rosales and Gill 1997), hence the difference in weight gains. However, the observed growth rates were lower than expected from the retained nitrogen. It is likely that it was due to underestimation of faecal nitrogen loss, possibly caused by binding of NDF and dietary nitrogen resulting in overestimation of nitrogen balance. In this trial, all the faecal samples were dried at 60^oC for 48h and this may have contributed to underestimation of faecal nitrogen as a result of volatile N loss. Other studies have similarly reported lower growth rates than those expected from retained nitrogen (Reed et al 1990; Kibon and Maina. 1993; Nherera et al 1998).
 

Conclusions

• The mixtures if pods and leaves of  Acacia tortilis  supported similar or greater growth rates than single components.

• Pods were superior to leaves, although the maximum weight gain was obtained with 1: 1 mixture (pods: leaves).

• Mixing of pods and leaves could therefore be an alternative approach of supplementing low quality roughage for goats at a lower cost.

Acknowledgements

We wish to express our gratitude to the anonymous reviewers for their useful comments; and the Ministry of Livestock Fisheries Development, Kenya, for provision of experimental animals and the facilities used during the experiment.

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