The effect of supplementing a basal diet of veld grass hay with increasing levels of velvet bean hay (Mucuna pruriens) on nutrient parameters in sheep

J F Mupangwa, N T Ngongoni*, D E Daka^* and H Hamudikuwanda*

Department of Environmental Science, Bindura University of Science Education,
P. Bag 1020, Bindura, Zimbabwe
^*Department of Animal Science, University of Zimbabwe,
PO Box MP 167, Mt. Pleasant, Harare, Zimbabwe.

Abstract

Four wethers cannulated at the rumen and abomasum were used in an experiment with a 4 x 4 Latin Square arrangement to measure intake, digestibility, nitrogen balance, microbial protein production and nutrient flows to post-ruminal sites. The sheep were given ad libitum a basal diet of natural grass hay, dominated by Hyparrhenia species, supplemented with increasing levels (0 %, 10 %, 20 % and 30 %)of velvet bean hay (Mucuna pruriens) harvested at pod maturation stage.

The intakes of dry matter and organic matter were higher in animals given 20 and 30 % velvet bean hay compared to those on 10 % velvet bean and veld hay alone. The apparent digestibility of dry matter, organic matter and neutral detergent fibre increased with supplementation. The intake of nitrogen increased with the increase in legume inclusion from 3.42 to 9.51 g/d in the control animals and those receiving 30 % velvet bean hay, respectively. Incremental levels of legume inclusion resulted in increased total excretion of purine derivatives and microbial protein supply. The flow of DM, OM and NDF declined with increasing levels of velvet bean inclusion. However, the flow of total N to the abomasum increased with increasing level of legume inclusion. Flow of microbial N and "bypass" N to the abomasum increased with increasing levels of legume inclusion in the diet. Supplementation with velvet bean hay had a synergistic incremental effect on the digestibility of  the basal veldt hay diet.

The results show that the inclusion of velvet bean as a protein supplement to low quality diets improves digestion, nitrogen flow to post-ruminal sites and nitrogen retention in small ruminants.

Keywords: Velvet bean, Mucuna, legume, intake, digestion, nutrient flow, microbial protein synthesis.

Introduction

The most abundant feed resource for ruminants in the tropics is poor quality roughages, in particular natural pasture grasses (veld) and crop residues. The intake of these roughages by ruminants is usually too low to maintain body weight, especially during the dry season, due to their tough texture, poor digestibility and nutrient deficiency (Kaitho 1997). The poor nutritive quality of the roughages leads to slow rates of ruminal degradation, a high rumen load, low rumen fractional outflow rates, poor growth in young stock, loss of body weight and consequent sub-optimal productive and reproductive performance (Leng 1984; Ngongoni and Manyuchi 1993).

Forage legumes supply nutrients, especially nitrogen, that is highly degradable and digestible (Mupangwa 2000), thus providing sufficient quantities of rumen degradable nitrogen, peptides and amino acids to meet rumen microbial N requirements. This results in an increase in dry matter intake, fermentation of the roughage, fractional outflow rates and microbial protein supply from low quality roughages (Umunna et al 1995). Velvet bean (Mucuna pruriens) is one of the herbaceous legumes that can be incorporated within the smallholder farming sector to improve ruminant animal production during the dry season. However, there is a paucity of information on the extent to which velvet bean can be utilised by ruminants. Therefore, the objective of this study was to investigate the effects of supplementing poor quality natural grass hay with increasing levels of velvet bean (Mucuna pruriens) hay on feed intake, apparent digestibility, nitrogen metabolism, rumen microbial protein production and abomasal flow of nutrients in sheep.

Materials and Methods

Animals and management

Four mature wether sheep of the Sabi breed with mean live weight of 27.8 ± 5 kg, each surgically fitted with rubber cannulas (Piggot and Maskew Ltd, Bulawayo) at the rumen and abomasum were used in the compartmental digestion study. The four sheep were placed in metabolism cages measuring 0.6 x 0.75 x 1.0 m and raised 0.5 m above the floor in the Bioassay Laboratory, Department of Animal Science at the University of Zimbabwe. Each crate was equipped with a feed trough and nipple drinker to make water accessible at all times.

Diets and experimental design

The four treatment diets comprised 100 % veld hay (VH) that predominantly consisted of Hyparrhenia species as a negative control, 90 % veld hay : 10 % velvet bean hay (VBH), 80 % veld hay : 20 % velvet bean hay and 70 % veld hay : 30 % velvet bean hay. The four dietary treatments were randomly allocated to four cannulated wethers in a 4 x 4 Latin Square design, with each period lasting for fourteen days.

Feeding and management

The feeds offered and refused were weighed and recorded daily in the morning over a five-day total collection period. The refusals were bulked for each sheep for the total collection period. Faeces for each sheep were collected in metal trays, bulked and kept frozen at – 4 ^oC. A 100g sample of faecal pellets was taken daily from each animal for DM determination. The total urine produced daily was collected into 10 ml of concentrated sulphuric acid to maintain a pH of 2 to 3. About 10 % of the daily urine excreted from each animal was bulked and kept frozen at –20 ^oC pending analysis for N. A second sample of urine, 10 % of daily output, was collected daily and diluted five times with distilled water and stored frozen until analysis for purine derivatives (PD).

Nutrient flows

Chromic oxide was used as a particulate phase marker according to the procedures described by Robinson et al (1979), in the estimation of dietary CP, dry matter (DM), organic matter (OM) and neutral detergent fibre (NDF) flowing to the abomasum. It was made up in four times its weight of corn starch, mixed with water and baked at 100 ^oC for 24 h. The mixture was then milled and added in powder form to dietary treatments at the rate of 1.25 kg/100 kg feed during the last day of total collection.

The wethers were fed ad libitum with the daily ration given five times a day over a period of 24 h at 0800, 1200, 1600, 2000 and 2400 h. Abomasal digesta was taken through the abomasal cannula at 2 h intervals over a 24 h period. The digesta was kept frozen at –20 ^oC until preparation for freeze-drying and analyses.

Chemical analyses

The samples of whole digesta were freeze dried. The DM and NDF of feed, faeces, refusals and freeze-dried abomasal digesta were determined according to the procedures of AOAC (1984). The ash content of feed, refusals, faeces and abomasal digesta was determined by ignition of a dry sample in a muffle furnace at 550 ^oC for 24 h. The N in feed, urine, faeces, refusals and freeze dried abomasal digesta was determined using the methods described by AOAC (1984).

The chromium concentration in feed, refusals, faeces and freeze-dried whole abomasal digesta was determined by atomic absorption spectrophotometry. Total purine derivatives in urine were determined according to the procedure described by Chen et al (1990). The amount of absorbed microbial purine and microbial protein supply were estimated from the daily excretion of PD based on the model described by Chen and Gomez (1992).

Calculations

The flow of nutrients to the abomasum was calculated with reference to the marker on the assumption that it is indigestible and non-absorbable such that:

DMI (g/d) x Chrome concentration (Cr %) in diet = Abomasal DM flow x Cr. (%) in abomasal digesta.           (1)

Abomasal DM flow (g/d) = DM intake (g/d) x Cr. conc. (%) in diet                                                                            (2)

                                                               Cr. conc. in abomasal digesta

Abomasal OM flow (g/d) = Abomasal DM flow x OM conc. in abomasal digesta                                                   (3)

Abomasal N flow (g/d) = Abomasal DM flow x N conc. in abomasal digesta                                                           (4)

Abomasal NDF flow (g/d) = Abomasal DM flow x NDF conc. in abomasal digesta                                                (5)

Feed N escaping rumen fermentation plus endogenous N was considered to be equal to total N flowing to the abomasum minus rumen microbial N. Supplemental N flowing to the abomasum was calculated as the difference between the control diet and supplemented diets assuming no change in endogenous nitrogen.

Statistical analysis

The measured variables were subjected to analysis of variance for a 4 x 4 Latin Square design using the GLM procedure of SAS (1990). The model of analysis was:

Y_ijkl = µ  + T_i + A_j + P_k + e_ijkl

Where Y_ijkl  is the dependent variable (eg: DM intake), µ  is the overall mean, T_i is the fixed effect of treatment (i = 1, 2, 3, 4), A_j is the fixed effect of animal (j = 1, 2, 3, 4), P_k is the fixed effect of period (k = 1, 2, 3, 4) and e_ijkl is the random error. The differences between treatments were assessed using the Tukey Studentised test.

Results

Intake and apparent digestibility

The veld hay used was a characteristic low quality forage as shown by the low CP content of 36 g/kg DM and the high NDF content of 839 g/kg DM (Table 1).  The velvet bean hay that was used had higher CP and low NDF contents of 168 and 517 g/kg DM, respectively.

The animals given incremental levels of velvet bean hay had greater (P < 0.05) DM and OM intake compared to those on the control treatment (Table 2). However, NDF intake was higher (P < 0.05) in animals supplemented with 20 and 30 % velvet bean than those receiving the control and 10 % supplemented diets. The apparent DM, OM and NDF digestibility were greater (P < 0.05) in sheep given the legume hay than in animals on the control diet. Among the supplemented animals, apparent digestibility of DM, OM and NDF increased significantly (P < 0.05) with increasing level of legume inclusion.

Nitrogen balance and microbial protein production

Total N intake was higher (P < 0.01) in animals given the legume hay than in those on the control diet (Table 3). Among the supplement treatments, N intake increased with increasing level of velvet bean hay inclusion. The urinary N output by sheep given veld hay alone and that of animals supplemented with 10 and 20 % velvet bean hay were not different (P > 0.05) but were lower (P < 0.05) than that of animals receiving 30 % velvet bean hay supplement. The total faecal N output did not differ (P > 0.05) among the treatments except for animals given 10 % velvet bean hay which was significantly lower (P < 0.05). Apparent N digestibility was higher (P < 0.01) in sheep given 20 and 30 % velvet bean hay, while those on 10 % velvet bean inclusion level were intermediate and the control diet had the least. Nitrogen retention increased (P < 0.01) with increasing levels of velvet bean inclusion (Table 3).

There was a significant (P > 0.01) treatment effect on total daily purine derivative excretion by the sheep. Legume inclusion resulted in increased total PD excretion compared to the control. The total PD excretion was highest in animals on 70 % veld hay and 30 % velvet hay followed by those on diets 80 % veld hay and 20 % velvet hay and on 90 % veld hay and 10 % velvet hay and least in animals on veld hay alone. When calculated on the basis of apparent digestible organic matter in the rumen (ADOMR), the efficiency of microbial protein production did not differ (P > 0.05) among the treatments.

Nutrient flows

DM, OM and NDF flows to the abomasum decreased linearly (P < 0.01) with increasing level of velvet bean hay in the diet  (Table 4). In contrast, the flow of total N to the abomasum increased linearly (P < 0.01) with increasing level of legume inclusion. Microbial N as a proportion of total N flowing to the abomasums also increased with increasing levels of legume inclusion with values of 57.9, 57.2, 60.7 and 66.5 % for the 0, 10, 20 and 30 % legume inclusion level, respectively. The difference between total N flow to the abomasum and the microbial protein flow from the rumen was assumed to be dietary and endogenous N. This fraction increased linearly  with increasing level of velvet bean hay in the diet (Table 4).

Discussion

The natural grass hay used in this study had a chemical composition typical of low quality roughage characterised by a low CP and high fibre content (Osuji et al 1995). The protein content of the veld hay was below the minimum level of 60 to 80 g/kg DM which depresses appetite (Minson 1981). The CP and NDF content of velvet bean hay were within the range reported for tropical herbaceous legumes (Norton and Poppi 1995). 

The addition  of velvet bean hay to the low quality veld hay resulted in increased total DM and OM intake relative to the control treatment. This finding is in agreement with results reported by Umunna et al (1995), who observed an increase in DM intake by sheep given a basal diet of oat straw supplemented with “lablab” hay. Other studies have reported similar findings (Matizha et al1997). The observed increase in intake was due to the additive effect of the velvet bean hay, as  the intake of the basal veldt hay was decreased only marginally (approximately 554, 598 and 568 g DM/d for 10, 20 and 30 % velvet bean inclusion levels compared with 600 g DM/d on the control).  Similar findings have been reported in earlier studies (Abdulrazak et al 1996; Mupangwa et al 2000).

The increase in the apparent digestibility of DM, OM and NDF as a result of legume hay supplementation is in agreement with results reported by Umunna et al (1995) and Matizha et al (1997). Rumen cellulolytic and amylolytic bacteria are reported to require ammonia, amino acids, peptides and branched-chain fatty acids as growth factors (Ndlovu and Buchanan-Smith 1985). The inclusion of the legume hay, which has higher levels of available nutrients, could have provided additional nutrients for microbial growth, especially fermentable N. This could have been accompanied by an increase in the attachment of rumen microbes to dietary fibre, thus increasing fibre digestion (Akin et al 1974) through the action of surface-bound cellulase and hemicellulase enzymes secreted by rumen bacteria and fungi. The overall effect would be an increase in apparent digestibilities of DM, OM and NDF as was observed in this study.

The synergistic effect of supplementation with the velvet bean hay on digestibility of the basal diet is apparent if a calculation is made of the apparent digestibility of the velvet bean hay assuming a constant digestibility of the veldt hay DM of 51% (as on the control diet). Values for DM digestibility of the velvet bean hay calculated by difference are then: 111, 96 and 101%. Clearly these values are not feasible, which confirms that one effect of the legume hay is to increase the digestibility of the veldt hay.

Nitrogen intake increased with increasing level of legume inclusion, leading to higher nitrogen retention in animals offered the velvet bean hay as a protein supplement.  The increasing levels of legume inclusion resulted in an increase in microbial N production from 1.94 to 6.52 g/d; however, the efficiency of microbial growth (microbial protein produced per unit rumen digestible organic matter) did not appear to have been affected. The increase in the abomasal flow of endogenous N and N derived from the supplemental velvet bean hay implies that part of the protein in the velvet bean hay was escaping the rumen fermentation and that could have been  the stimulus for the greater DM intake which in turn would lead to increased rumen microbial protein production. This is in line with the general theory proposed by Preston and Leng (1987) to explain one of the beneficial effects of  ”escape” or “bypass” protein in basal diets of low digestibility and low protein content.  The fact that the efficiency of microbial synthesis from rumen fermentable organic matter was not affected by the legume inclusion does not support the theory that the effect of supplementation with protein (as opposed to fermentable N) is to increase the efficiency of rumen microbial synthesis through provision of peptides and amino acids (Ndlovu and Buchanan-Smith 1985). The effect of the increased OM intake in increasing microbial protein production through provision of higher amounts of rumen fermentable energy is the more likely explanation and is in line with results reported by Clark et al (1992).

The decline in the quantities of DM, OM and NDF flowing to the abomasum with increasing level of legume inclusion is a logical consequence of the increased rates of rumen degradation and hence of total tract digestibility of these elements. These findings are similar to those reported by McAllan (1991).

Conclusion

The results of this study indicate that inclusion of velvet bean hay in a low quality grass hay diet increases:

·       Protein availability to the host animal by increasing the supply of both rumen degradable and “bypass” protein.

·       Rumen degradability (digestibility) of the basal diet

·       Intake of organic matter and nitrogen retention

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Received 29 April 2002

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