Livestock Research for Rural Development 15 (2) 2003

Citation of this paper

The effect of stage of growth and method of drying fresh herbage on in sacco dry matter degradability of three tropical forage legumes 

J F Mupangwa, N T Ngongoni* and H Hamudikuwanda* 

Department of Agricultural Science, Bindura University of Science Education,
P Bag 1020, Bindura, Zimbabwe
jmupangwa@excite.com
*Department of Animal Science, University of Zimbabwe,
PO Box MP 167, Mt. Pleasant, Harare, Zimbabwe


Abstract 

Cassia rotundifolia (Cassia), Lablab purpureus (Lablab) and Macroptilium atropurpureum (Siratro) were harvested at 8, 14 and 20 weeks of growth and either sun- or oven-dried prior to evaluation by the in sacco technique using three rumen fistulated Friesian steers. The samples were incubated for 6, 12, 24, 48, 72, 96 and 120 h.

After oven-drying the three legumes had similar quickly degradable dry matter contents at each of the growth stages. However, after sun-drying, cassia and lablab had a higher quickly degradable dry matter content than siratro. The quickly degradable dry matter content of the forages declined with increasing maturity, irrespective of drying method.  For the slowly degradable fraction of the dry matter, the rate of degradation was greater for siratro, 0.08/h, than either cassia or lablab that were similar (0.05/h). Sun-dried forages harvested at 14 and 20 weeks of growth had greater potentially degradable dry matter content than oven-dried forages. At all stages of growth and irrespective of drying method, the proportions of potentially degradable dry matter were highest for lablab followed by cassia then siratro.

It is concluded that oven-drying reduced the in sacco dry matter degradability of siratro, cassia and lablab forages compared with sun-drying.   

Keywords: Herbaceous legumes, in sacco degradability,  stage of growth, sun-drying


Introduction 

The rumen degradation characteristics of a feed can be a guide to it's nutritive value for ruminants. The potential degradability is indicative of overall digestibility while the rate of degradation can be a guide to the extent that digestible dietary nutrients escape the rumen fermentation. The extent to which a forage is degraded in the rumen depends on many factors, among which are stage of maturity, presence of  tannins, microbial proteolytic activity and rumen retention time (Stern et al 1994; Hadjipanayiotou et al 1996).   In the tropics, sun-drying is often used by farmers as a method of conserving protein-rich forages. However, there is little information on the effect of drying method and it's possible interaction with stage of maturity, on the nutritive value of forages. It was therefore considered appropriate to determine the effects on chemical composition and rumen degradability of three herbaceous legumes of sun-drying compared with oven-drying, as practiced in the laboratory.


Materials and Methods

Forage production procedure 

The chosen legumes were: Cassia rotundifolia (Cassia), Lablab purpureus (Lablab) and Macroptilium atropurpureum (Siratro). They were established in rows 0.45 m apart in plots measuring 15 x 50 m on sandy soils (pH 5.5 on CaCl2 scale). Each of the plots was fertilized with single superphosphate at 200 kg/ha as recommended from soil analysis results.  

Samples (cutting height 10 cm above soil level) were taken from six randomly selected rows at 8, 14 and 20 weeks of growth after germination. One portion was sun-dried in the field; the other portion was dried in an oven 60 C for 48 hours. During sun drying in the field the forages were turned twice a day for four days to ensure even drying. 

Animals 

Three mature Holstein-Friesian steers weighing 440 20 kg, each surgically fitted with a rumen cannula of 8.5 cm internal diameter, were used to determine the degradability profiles of the dried foliages using the nylon bag technique (Bhargava and rskov 1987).

Housing and diets 

The steers were housed in individual pens measuring 3 x 2 m in the bio-assay laboratory of the Department of Animal Science, University of Zimbabwe. They were fed ad libitum a basal diet (150 g CP/kg DM) made up of veld hay (dominated by Hyparrhenia species) and the forage legume fine stem stylo (Stylosanthes guianensis) hay in the ratio of 60:40, respectively. The feed was given daily in two equal meals at 08:00 and 16:00 hours. Fresh water was always available from automatic drinkers. A mineral-vitamin lick (Hamish-Cameron, Harare) was freely available.

Incubation procedure 

The dried foliages were milled (2 mm screen) and approximately 5 g were placed in nylon bags of 8 x 15 cm and pore size of 40 to 45 mm (Polymon, Switzerland). The bags were tied using rubber bands to three slits on a flexible vinyl tube, 40 cm long, of 6 mm outer diameter (Bhargava and rskov 1987) before being suspended in the rumen of each steer according to a randomised block design. The bags were withdrawn from the rumen at 6, 12, 24, 48, 72, 96 and 120 h. They were washed under running tap water and gently squeezed until clear water came out of the bags. The zero time loss of DM was determined by soaking four weighed nylon bags containing the samples of forages in cold water for 1 hour, followed by washing of each bag under running tap water. The bags were dried in an oven for 48 h at 60 C to a constant weight.  

Chemical analysis 

The legume samples were analysed for nitrogen (N) using the Kjeldahl procedure (AOAC 1984). Neutral detergent fibre (NDF), acid detergent fibre (ADF), acid detergent lignin (ADL) and acid detergent insoluble nitrogen (ADIN) were determined according to the procedure of Goering and Van Soest (1970).  

Calculations and statistical analysis 

The DM degradability constants were determined using the iterative least squares procedure according to the exponential equation of rskov and McDonald (1979): 

DM degradability = a + b (1 – e-ct) 

Where a = soluble fraction; b = slowly degradable fraction; c = rate of degradation of b;   t = time of incubation; e = exponential constant. 

Analysis of variance was carried out on the degradability and effective degradability data using the General Linear Model Procedure (SAS 1990). The analytical model for each variable was as follows: 

Yhijk = m + Ah + Li + Dj + Wk + (LD)ij + (LW)ik + (DW)jk + (LDW)ijk + ehijk 

Where: Yhijk is the dependent variable (rumen degradability of DM or effective DM degradability),
           
m is the overall mean,
           
A is the effect of animal (h = 1, 2, 3),
           
Li is the effect of legume species (i =1,2,3),
           
Dj is the effect of drying method (j = 1, 2),
           
Wk is the effect of stage of growth (k = 1, 2, 3),
           
(LD)ij is the interaction between legume species and drying method,
           
(LW)ik is the interaction between legume species and stage of growth,
           
(DW)jk being the interaction between drying method and stage of growth,
           (LDW)ijk being the interaction of the legume species, drying method and the stage of growth and        
            eijk is the error term.

The means were compared using the Tukey Studentised Range Test of SAS (SAS 1990).


Results
 

Chemical composition

As expected, crude protein decreased and NDF, ADF and ADL increased with increasing age at harvesting.Siratro had a higher crude protein content at all stages of growth than the other legumes (Table 1).  Oven drying increased the amount of N linked with the fibre (ADIN).

Table 1: Chemical composition (g/kg DM) of cassia, lablab and siratro harvested at three stages of growth and dried in the sun or in an oven

Legume (L)

Week (W)

Drying(D)

CP

NDF

ADF

ADL

ADIN

Cassia

8

Sun

225

343

282

84.4

53.3

Oven

242

426

288

76.0

70.2

14

Sun

221

507

368

73.7

16.1

Oven

246

506

408

74.2

21.5

20

Sun

184

545

323

70.3

6.5

Oven

173

547

357

106

10.0

Lablab

8

Sun

252

375

294

89.3

35.3

Oven

254

328

282

95.8

60.0

14

Sun

221

455

331

76.2

35.5

Oven

216

473

405

62.5

20.5

20

Sun

162

522

386

78.3

9.0

Oven

183

566

353

58.5

11.0

Siratro

8

Sun

282

351

323

101

59.9

Oven

279

413

322

91.7

72.9

14

Sun

238

546

433

56.1

15.2

Oven

252

534

472

72.7

20.6

20

Sun

229

465

334

143

12.0

Oven

191

509

418

114

8.00

SED

LxDxW

5.94

5.05

4.68

0.90

0.39

Significance

LxDxW

***

***

***

***

***

Means within the same column are different at * = P < 0.05; ** = P < 0.01; *** = P < 0.001; NS = not significant.
SED  Standard error of difference

 

Dry matter degradability 

 The content of quickly degradable  DM (QDDM) did not differ between legumes at similar stages of growth, but decreased with increasing maturity (Table 2).  

Table 2: The DM degradability (g/kg) of cassia, lablab and siratro legumes harvested at 8, 14 and 20 weeks of growth and dried in the sun or in the oven

Legume

(L)

Week           (W)

Drying

(D)

a

b

c

a + b

p (k=0.02)

P
(k=0.05)

Cassia

8

sun

388

394

0.07

782

691

615

oven

397

379

0.05

776

664

581

14

sun

395

338

0.04

733

612

539

oven

335

368

0.04

703

570

490

20

sun

257

468

0.05

725

560

464

oven

215

442

0.06

657

545

455

Lablab

8

sun

407

440

0.05

847

726

633

oven

415

421

0.05

836

706

615

14

sun

368

479

0.04

847

659

556

oven

365

416

0.05

781

663

574

20

sun

211

526

0.05

737

575

471

oven

227

521

0.04

748

576

461

Siratro

8

sun

384

376

0.09

760

685

619

oven

389

349

0.06

738

652

582

14

sun

386

322

0.07

708

635

572

oven

369

306

0.08

675

611

554

20

sun

171

550

0.08

721

610

508

oven

218

419

0.08

636

545

468

SED

L

8.73

11.2

0.01

9.82

6.04

7.58

D

7.13

9.13

0.004

8.02

4.94

6.19

W

8.73

11.2

0.01

9.82

6.05

7.58

L x D

12.3

15.8

0.01

13.9

8.55

10.7

L x W

15.1

19.4

0.01

17.0

10.5

13.1

D x W

12.3

15.8

0.01

13.9

8.55

10.7

L x D x W

21.4

27.4

0.01

24.1

14.8

18.6

Significance

L

ns

***

**

***

***

**

D

*

**

NS

***

***

**

W

***

***

NS

**

***

***

L x D

***

**

NS

NS

*

NS

L x W

NS

*

NS

NS

**

*

D x W

NS

NS

NS

*

NS

NS

L x D x W

NS

*

NS

NS

NS

NS

Means in the same column are significantly different at     * = P < 0.05, ** = P < 0.01,
*** = P < 0.001    NS = non-significant
a = quickly degradable  b = slowly degradable  c = rate constant 
p = effective DM degradability at outflow rates (k) of 0.02 and 0.05/h.

Forages harvested at 8 weeks of growth had greater (P < 0.01) amounts of QDDM content than of forages harvested at 14 and 20 weeks of growth. Similarly, legumes harvested at 14 weeks of growth had higher (P < 0.001) QDDM compared to forage harvested at 20 weeks of growth.  

The slowly degradable DM (SDDM) content of the legumes was dependent on the interaction of legume species, drying method and stage of harvesting (L x D x W). At 8 weeks of growth there were no significant (P > 0.05) differences in the SDDM content of cassia and lablab, and cassia and siratro on both drying methods. However, the SDDM content of sun and oven dried lablab was greater (P < 0.05) than that of siratro which received similar drying treatments. Sun drying of the forages harvested at 14 weeks of growth resulted in higher (P < 0.05) SDDM content in lablab compared to that of cassia while no differences (P > 0.05) were observed between the two legumes in oven dried forages and with siratro forage. Lablab had a higher (P < 0.05) SDDM content in both sun and oven dried materials compared to siratro forage which was either sun or oven dried. Oven dried cassia also had a greater (P < 0.05) amount of SDDM content compared to oven dried siratro while no differences (P > 0.05) were observed between the sun dried materials of the two legumes. At 20 weeks of growth, lablab that was either sun or oven dried had a higher (P < 0.05) SDDM content than of cassia which received similar drying treatments. The SDDM content of oven dried lablab was also greater (P < 0.05) than that of oven dried siratro while no differences (P > 0.05) were observed between the two legumes in their sun dried material. Sun dried cassia had a lower (P < 0.05) SDDM content compared to that of sun dried siratro while the two legumes did not differ (P > 0.05) in their SDDM contents in oven dried forages. These variations among the legume species, drying method and stage of harvesting in SDDM contents contributed to the observed three-way interaction.  

The rate of degradation (c) of the SDDM fraction was influenced only by the main effect of legume species. Siratro had a significantly higher (P < 0.01) rate of degradation at all stages of growth and drying methods than either cassia or lablab that did not differ (P > 0.05). The mean rates of degradation were 0.05, 0.05 and 0.08/h, respectively, for cassia, lablab and siratro.  

The potentially degradable DM fraction of the legumes was influenced by an interaction of drying method and stage of growth (D x W), and the main effect of legume species. Forages harvested at 8 weeks of growth and either sun or oven dried did not differ (P > 0.05) in their potentially degradable DM contents. However, at 14 and 20 weeks of growth, sun dried forages had greater (P < 0.05) mean content of potentially degradable DM compared to oven dried forages. 

The variations in the potentially degradable DM content of the forages due to drying method and stage of growth resulted in the observed two-way interaction. Lablab had a greater (P < 0.001) mean potentially degradable DM content compared to cassia and siratro, respectively. Similarly, the potentially degradable DM content of cassia was also significantly higher (P < 0.001) than that of siratro. .

The estimated effective DM degradabilities (EDMD) at a rumen fractional outflow rate of 0.02/h were influenced by the interaction between legume species and drying method (L x D). Oven drying reduced (P < 0.05) the EDMD compared to sun drying in cassia and siratro forages (Figures 1 and 2), while in lablab forages there was no significant (P > 0.05) effect of the drying method.  Among the legumes, sun dried cassia had a lower (P < 0.05) mean EDMD content compared to that of sun dried lablab and siratro. In contrast, oven dried cassia and siratro had similar (P > 0.05) EDMD contents which were lower (P < 0.05) than that of oven dried lablab  These differences among the legume species due to drying treatment resulted in the L x D interaction.

Figure 1. Effective DM degradability (g/kg) of either sun- or oven-dried cassia forage at different stages of growth. Figure 2. Effective DM degradability (g/kg) of either sun- or oven-dried lablab forage at different stages of growth.

The EDMD contents of the three legumes at an outflow rate of 0.02/h were also influenced by the interaction between legume species and stage of growth (L x W). Lablab harvested at 8 weeks of growth had a higher (P < 0.01) EDMD compared to those of cassia and siratro which were not different (P > 0.05). At 14 weeks of growth, lablab maintained a higher (P < 0.01) EDMD content compared to cassia and siratro. Cassia harvested at 20 weeks of growth had a lower (P < 0.01) EDMD content compared to that of lablab and siratro. The differences in EDMD content between cassia and siratro at 8 weeks of growth and lablab and siratro at 20 weeks of growth resulted in the interaction observed. 

At an estimated rumen fractional outflow rate of 0.05/h the EDMD contents of the forages were influenced by an interaction between legume species and stage of growth, and the main effect of drying method.  The three legumes had similar (P > 0.05) EDMD values at 8 weeks of growth. In contrast, at 14 weeks of growth, lablab and siratro had similar EDMD contents that were greater (P < 0.05) than that of cassia. When harvested at 20 weeks of growth, the three legumes were not different in their mean EDMD content except cassia which maintained a lower (P < 0.05) EDMD content compared to that of siratro. Oven drying of fresh forages reduced (P < 0.01) the EDMD content of the legumes compared to sun drying.


Discussion

The high oven drying temperatures of 60 oC reduced the QDDM content of the three legumes at all stages of growth compared to sun drying. This finding is consistent with results reported in earlier studies (Moshtaghi Nia and Ingalls 1995).  Siratro, with a higher NDF content in sun dried material, had lower QDDM content than cassia and lablab at all the three stages of maturity. However, in oven dried forages the NDF contents of the three legumes were similar and this was reflected in similar QDDM contents. The lower QDDM content of oven dried forages may be associated with an increase in NDF content which is less readily degradable and could also be also due to physio-chemical changes arising from non-enzymatic reactions of soluble proteins and carbohydrates (Van Soest 1994). The QDDM values obtained in this experiment were higher than values of 177 to 299 g/kg reported by Mgheni et al (1993) for tropical herbaceous legumes Desmodium uncinatum, Neonotonia wightii and Pueraria phaseoloides. These differences in QDDM contents of the legumes at different stages of growth are due to a decline in cell contents and an increase in cell wall with advancing maturity. These findings are in agreement with those reported for alfalfa (Llamas-Lamas and Combs 1990; Balde et al 1993).  

The SDDM contents of the legumes were within the range of 355 to 457 g/kg reported for other tropical herbaceous legumes (Mgheni et al 1993). Lablab maintained a higher SDDM content than either cassia or siratro at all stages of growth and this was due to its lower NDF and lignin content compared to cassia that was intermediate and siratro which had the highest (Table 1). The low degradability of forages with high NDF and lignin is due to reduced penetrative ability of rumen microbes through lignified plant cell walls (Akin 1989). 

The potential DM degradability values of these legumes are within the range of 485 to 870 g/kg (Kimambo et al 1994) but higher than the range of 532 to 740 g/kg reported for other herbaceous tropical legumes (Mgheni et al 1993). The decline in potentially degradable DM contents of the legumes with increasing drying temperature and forage maturity may be explained by increases in less degradable fractions accumulating in the forages such as lignin. Lignified cells reduce the penetration of rumen fungal rhizoids (Akin et al 1974; Latham et al 1978) which results in reduced degradability. In addition, lignin is reported to physically encrust structural carbohydrates, preventing enzyme attack and inhibiting attachment of rumen microorganisms (Coombe 1981). Because cell contents are virtually all degradable, differences in DM degradability between the forages may be associated with the fibrous components such as structural polysaccharides whose degradation varies among forages (Singh and Makkar 1992).  

The rate of degradation (c) is important in determining effective degradation as well as rumen fill. The rates of DM degradation were not different between the stages of growth but varied among the legume species. This is in agreement with results reported by Balde et al (1993) in which alfalfa cut from early bud to full bloom had similar rates of DM degradation. In contrast, Hadjipanayiotou et al (1996) reported a decline in degradation rate of DM in vetch with increasing maturity. The rates of DM degradation for cassia, lablab and siratro are comparable to the values reported for other herbaceous tropical legumes (Mgheni et al 1993; Umunna et al 1995). The high CP content and the fragility of legume cell walls and high proportions of readily digestible thin-walled, non-lignified mesophyll tissues of the tropical legumes (Wilson and Kennedy 1996) could have resulted in the maintenance of high degradation rates even at advanced maturity. 

The effective DM degradability decreased with advancing maturity and high drying temperature in the three legume forages. The changes in effective DM degradability with increasing maturity and high drying temperature correspond closely with the changes in the proportions of potentially degradable DM and increases in NDF content of the legumes. These results are consistent with those reported in earlier studies (Balde et al 1993; Hadjipanayiotou et al 1996). Lablab maintained the highest effective degradability at all stages of maturity, with cassia intermediate and siratro was the least. These differences could have been caused by the species variation in fibre content. Forages with low fibre content have been reported to have higher effective DM degradabilities compared to those with high fibre content (Llamas-Lamas and Combs 1990). 


Conclusion
 

Based on the findings of this study it can be concluded that legume species, drying method and stage of maturity have a marked influence on chemical composition and in sacco DM degradability of forage legumes. Among the three legumes, siratro showed a higher sustained CP content followed by cassia, and lablab was the least. The differences in chemical composition and dry matter degradability of the legumes with stage of maturity and drying method could explain variations in reported values in the literature and suggest that fixed degradability values for tropical legume forages are inappropriate. 


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Received 10 November 2002; Accepted 4 December 2002

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