Effects of stage of maturity and method of drying on in situ nitrogen degradability of fresh herbage of Cassia rotundifolia, Lablab purpureus and Macroptilium atropurpureum

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

Department of Agriculture, Bindura University of Science Education, P. Bag 1020, Bindura, Zimbabwe
*Department of Animal Science, University of Zimbabwe, P.O. Box MP 167, Mt. Pleasant, Harare, Zimbabwe
tjmupangwa@yahoo.com 

Abstract 

 

The rumen degradability of nitrogen from Cassia rotundifolia (Cassia), Lablab purpureus (Lablab) and Macroptilium atropurpureum (Siratro) was investigated by the in sacco technique using three rumen fistulated Friesian steers. The legumes were harvested at 8, 14 and 20 weeks of growth and either sun or oven dried before being incubated for 6, 12, 24, 48, 72, 96 and 120 h.

 

The quickly degradable nitrogen content of the legumes was different at similar stages of growth. Lablab, at 8 weeks of growth, had a higher content of quickly degradable nitrogen, irrespective of drying methods, compared to cassia and siratro which also differed. However, at 14 weeks of growth, sun dried cassia had quickly degradable nitrogen content higher than lablab and siratro. Oven drying reduced the quickly degradable nitrogen content of cassia compared to that of lablab and siratro. The rate of degradation of the slowly degradable nitrogen fraction was greater for siratro, than either cassia, or lablab.. Oven drying reduced the rate of degradation at 8 weeks of growth but had no effect in forages harvested at 14 and 20 weeks of growth.

 

It was concluded that legumes provide variable amounts of degradable nitrogen that is dependent on species, stages of growth and drying treatments. 

Keywords: Degradability, legumes, maturity, nitrogen

Introduction 

The nutritive value of a feed is assessed by voluntary intake, the amount of nutrients it contains (chemical composition) and their flow to post-ruminal sites, and digestibility. Rumen degradability of dietary protein is an important factor influencing the amount of dietary N made available for rumen microbial growth and intestinal amino acid supply to the ruminant animal (Mupangwa et al 2003a).

 

Although the effects of stage of maturity and methods of drying fresh forage on chemical composition and apparent digestibility are well documented (Norton and Poppi 1995), relatively little information is available on the effect of stage of growth of forages on rumen N degradability of tropical herbaceous legumes. Because of the effects of stage of growth of forage on the supply of N in the rumen for microbial growth and total tract digestibility, it is reasonable to expect that the stage of growth and drying method have large effects on N degradability. Therefore, the objective of this study was to determine the effects of stage of growth and method of drying on the rumen N degradability of fresh forage herbaceous legumes. 
 

 

Materials and methods 

 

Forage production procedure

 

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

 

Legume samples were cut in six randomly selected rows to 10 cm stubble height at 8, 14 and 20 weeks of growth after germination. One portion of the samples was sun dried in the field, a practice used by farmers, and the other portion was dried in an oven at 60 ⁰C for 48 hours, as practiced in the laboratory. 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 diameter, were used to determine the degradability profiles of forage legumes using the nylon bag technique (Bhargava and Ærskov 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. The steers were fed ad libitum a basal diet (150 g CP/kg DM) made up of veld hay (dominated by Hyparrhenia species) and fine stem stylo hay (Stylosanthes guianensis) in a ratio of 60:40, respectively. The feed was given daily in two equal meals at 08:00 and 16:00 h. Fresh water was always available from automatic drinkers. A mineral-vitamin lick was freely available.

 

Incubation procedure

 

The dried forages were milled (2 mm screen) and approximately 5 g of sample were placed in nylon bags measuring 8 x 15 cm with pore size of 40 - 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 Ærskov 1987) and suspended in the rumen of each steer according to a randomised complete block design. The bags per sample were withdrawn at 6, 12, 24, 48, 72, 96 and 120 hours and were washed under running tap water and gently squeezed until clear water came out of the bags. The zero time loss of N was determined by soaking weighed nylon bags containing the samples of forages in cold water for 1 h, followed by washing of each bag under running tap water. The bags were dried in an oven for 48 h at 60 ^oC to constant weight.

 

Chemical analysis

 

The samples and rumen residues were analysed for 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 N degradability constants were determined using the iterative least squares procedure according to the exponential equation of Ærskov and McDonald (1979):

 

N degradability = a + b(1- e ^-ct)

 

Where: a = soluble fraction

            b = slowly degradable fraction

            c = rate of degradation of b

            t = incubation time

            e = exponential constant

 

The effective degradability (P) of N was calculated using assumed ruminal fractional outflow rates (k) of 0.02 and 0.05/h according to the equation of Ærskov and McDonald (1979):

 

P = a + [bc/(c + k)]

 

where a , b and c are as described above.

 

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:

 

Y_hijk = m + A_h + L_i + D_j + W_k + (LD)_ij + (LW)_ik + (DW)_jk + (LDW)_ijk + e_hijk

 

Where;

    Y_ijk is the dependent variable (rumen degradability or effective degradability of N)

    m is the overall mean,

    A is the effect of animal (h = 1, 2, 3)

    L is the effect of legume species (i =1,2,3)

    D is the effect of drying method (j = 1, 2)

    W 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 legume species, drying method and stage of growth and e_ijk is the error term

 

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

 

 
Results

 

Chemical composition

 

The CP content decreased while NDF and ADF increased with advanced maturity. The oven-dried forages had greater NDF and ADF than sun-dried materials. The ADL content of siratro and cassia increased, while that of lablab declined with advancing plant maturity. Siratro had higher ADL and ash content than that of either cassia or lablab. The ADIN content of the legumes declined with increasing maturity and was higher in oven-dried than in sun-dried forages (Table 1).  

 

 

 

Nitrogen degradability

 

The quickly degradable N (QDN) fraction (a), of the three legumes was influenced by the interaction of legume species, drying treatment and stage of growth (L x D x W) (Table 2). When harvested at 8 weeks of growth, sun- and oven-dried lablab had higher (P < 0.001) QDN content than either cassia or siratro which were dried similarly. The QDN content of sun-dried cassia was also greater (P < 0.01) compared to that of sun-dried siratro but the two legumes were not different (P > 0.05) in the QDN content of oven dried samples. At 14 weeks of growth, sun-drying resulted in cassia having a higher (P < 0.001) QDN content than either lablab or siratro which themselves were not different (P > 0.05). However, oven drying resulted in a reduction (P < 0.01) in the QDN content of cassia compared to that of lablab and siratro. Similarly, siratro also had a lower (P < 0.05) QDN content than lablab. In forages harvested at 20 weeks of growth, sun dried lablab had a greater (P < 0.001) QDN content than either cassia or siratro, while that of siratro was lower (P < 0.01) than cassia. When the forages were oven-dried, cassia and lablab maintained a higher (P < 0.001) QDN value compared to that of siratro while that of oven dried cassia was significantly greater (P < 0.01) than lablab. The observed differences in QDN content of the legumes due to species variation, drying treatment and stage of growth contributed to the three-way interaction. 

 

The slowly degradable N (SDN) content of the legumes was also dependent on the interaction of legume species, drying method and stage of growth. At 8 weeks of growth, sun-dried cassia and lablab had similar (P > 0.05) SDN values, which were lower (P < 0.05) than that of sun dried siratro. In contrast, oven-drying resulted in no significant difference (P > 0.05) in SDN content of lablab and siratro and both legumes had lower (P < 0.01) SDN values compared to cassia. When the legumes were harvested at 14 weeks of growth, sun-dried lablab and siratro had similar (P > 0.05) SDN contents which were higher (P < 0.001) than that of cassia. However, when the legumes were oven-dried siratro had lower (P < 0.01) SDN value compared to cassia and lablab but the latter two legumes did not differ. At 20 weeks of growth, sun-dried cassia and lablab had lower (P < 0.001) SDN values than sun-dried siratro, while that of cassia was higher (P < 0.01) compared to that of lablab. When oven-dried, lablab had SDN contents which were higher (P > 0.05) than that of cassia or siratro.

 

The rate of degradation (c) of the SDN fraction of the legumes was dependent on the interaction between drying method and stage of growth, and also due to the main effect of legume species. Oven-drying forages harvested at 8 weeks of growth resulted in a reduction (P < 0.01) in the rate of degradation of the SDN compared to sun-drying, 0.07 vs 0.09/h. However, drying method had no effect (P > 0.05) on the SDN content of the legumes harvested at 14 and 20 weeks of growth. Among the legumes, siratro had a higher (P < 0.001) rate of degradation with a mean value of 0.09/h compared to mean values of 0.07 and 0.05/h for cassia and lablab, respectively, which were also different (P < 0.01).

 

The interactions of legume species, drying method and stage of growth influenced the potentially degradable N contents of the legumes. At 8 weeks of growth, there were no significant (P > 0.05) differences among the sun-dried legumes in their potentially degradable N contents except that of cassia which was lower (P < 0.001) than that of lablab. In contrast, oven-dried cassia and lablab had similar (P > 0.05) potentially degradable N contents which were higher (P < 0.001) than that of siratro (Fig 5.7). Sun-drying the forages harvested at 14 weeks of growth resulted in lablab having a greater (P < 0.01) potentially degradable N content compared to either cassia or siratro, which did not differ (P > 0.05) between themselves. A similar result was observed in oven-dried forages. At 20 weeks of growth, there were no significant (P > 0.05) differences among sun-dried legumes in their potentially degradable N contents. However, oven dried-lablab had a greater (P < 0.01) potentially degradable N content compared to either cassia or siratro which themselves were not different (P > 0.05).

 

The effective rumen degradable N (ERDN) content of the forage legumes at an estimated rumen outflow rate of 0.02/h was dependent on the interaction of legume and stage of growth. When the forage legumes were harvested at 8 weeks of growth, cassia and siratro had similar (P > 0.05) ERDN values of 878 and 880 g/kg N, respectively, which were higher (P < 0.05) than the mean value of 862 g/kg N for lablab. At 14 weeks of growth there were no significant (P > 0.05) differences in the ERDN content of the legumes except that of cassia which was lower (P < 0.05) than that of siratro. The mean ERDN values for the legumes were 845, 852 and 861 g/kg N for cassia, lablab and siratro, respectively. Similarly at 20 weeks of growth, the three legumes did not differ (P > 0.05) in their ERDN contents and had mean values of 790, 789 and 780 g/kg N for cassia, lablab and siratro, respectively.

 

The ERDN content of the legumes at an estimated rumen outflow rate of 0.05/h was influenced by the interaction between legume species and stage of growth; and between method of drying and stage of growth. When harvested at 8 weeks of growth, the three legumes had similar (P > 0.05) ERDN values of 821, 826 and 828 g/kg N for cassia, lablab and siratro, respectively. In contrast, at 14 weeks of growth, cassia and lablab had lower (P < 0.01) ERDN contents with mean values of 783 and 789 g/kg N, respectively, compared to a mean value of 809 g/kg N for siratro. At 20 weeks of growth, cassia had a greater (P < 0.01) ERDN content of 709 g/kg N compared to that of either lablab or siratro which had mean values of 694 and 682 g/kg N, respectively, and were not different (P > 0.05). The drying treatment did not have a significant (P > 0.05) effect on the ERDN values of the legumes at 8 and 14 weeks of growth. However, at 20 weeks of growth, oven-dried forages had higher (P < 0.05) ERDN content compared to that of sun-dried forages thus causing the observed interaction between drying method and stage of growth.

 

Discussion

 

The new protein systems (AFRC 1993) base the evaluation of dietary protein on the concept of quickly (QDP) and slowly (SDP) degradable protein to meet the needs of rumen microflora, and undegradable dietary protein to meet host animal requirements. The QDP comprises non-protein-nitrogen, free amino acids and small protein molecules (AFRC 1993). The QDN fraction of the legumes in this experiment varied between 290 and 709 g/kg N and was higher than the range of 214 to 496 g/kgN reported by Mgheni et al (1993) for Desmodium uncinatum, Neotonia wightii and Pueraria phaseoloides. This difference may be attributed to species variation and different stages of maturity of the materials used in the two experiments. Therefore, the high QDN content of the legumes even, at advanced stages of maturity, shows that they can supply sufficient quantities of N to meet rumen microbial requirements. The reduction in QDN content of the legumes with increasing maturity is due to a decline in the soluble cell contents while cell wall contents increased (Balde et al 1993).

 

The oven-dried forages tended to have a higher QDN content compared to sun-dried material at all stages of growth. This was possibly due to a more rapid drying, resulting in reduced autolysis of plant proteins or due to an increase in leaf loss in the field dried forages. The leaf fraction of legumes has been reported to have higher protein content than the stem (Hendricksen et al 1981). However, other studies have reported a decrease in QDN of forages dried at much higher temperatures (100 ^oC) than those used in this study (Yang et al 1993).

 

The slowly degradable N content of the forages ranged from 262 to 485 g/kg N for cassia, 251 to 486 g/kg N for lablab and 246 to 622 g/kg N for siratro. These values are comparable to those of 216 to 423 g/kg N reported by Mgheni et al (1993), except that of siratro, which seems to have a higher range. There was an increase in the SDN content of the legumes with maturity and siratro had a higher SDN content than the other legumes at 20 weeks of growth although at 8 weeks the legumes did not differ. These differences could be due to species differences in their ability to retain the leaf fraction which has a higher protein content than the stems with advancing maturity (Hendricksen et al 1981; Sanderson and Wedin 1989). In an earlier study, siratro was observed to maintain a higher CP content than the other two legumes with increasing stage of growth (Mupangwa et al 2003b) and this possibly explains its higher SDN content. Similar increases in SDN content in forage legumes have been reported in other studies (Llamas-Llamas and Combs 1990).

 

The rate of N degradation was similar in materials harvested at either 8, 14 or 20 weeks of growth. Balde et al (1993) reported similar findings, while Vik-Mo (1989) reported decreases in degradation rate of N with increasing maturity in clover. The reduced rate of degradation in oven-dried materials as compared to sun-dried forages at 8 weeks of growth may be due to their higher proportion of cell wall covalently linked to the soluble nitrogen to form ADIN that is unavailable for microbial degradation (Yang et al 1993). An increase in ADIN content with heat treatment indicates an increase in heat damaged indigestible protein. Heating facilitates the Maillard reaction between sugars and free amino acids forming an amino-sugar complex which is more resistant than normal peptides to enzymatic hydrolysis (Stern et al 1994).

 

The potentially degradable N values obtained in this study are higher than the range of 430 to 880 g/kg N reported by Mgheni et al (1993) for some tropical legumes. Norton and Poppi (1995) reported potential N degradability values of 850 and 750 g/kg N for siratro and cassia, respectively, while Umunna et al (1995) reported potential N degradability of 866 g/kg N in lablab. The data from this study are in agreement with those reported by Balde et al (1993) for alfalfa that had potentially degradable CP fraction of 900 g/kg CP or more. The high potentially degradable N content of the legumes shows that they are suitable protein supplements to low-quality roughages, with lablab providing greater quantities of degradable N followed by cassia and least siratro. However, excessive protein degradation in the rumen may lead to a reduction in the amount of UDP flowing to post-ruminal sites for digestion and absorption.

 

Conclusions

 

• The findings of the present study indicates that legume species, drying method and stage of maturity have a marked influence on in situ N degradability of forage legumes. The legumes have a highly degradable N content, even at advanced stages of maturity, that is sufficient to meet microbial N requirements. However, excessive legume protein degradation in the rumen may occur, leading to a reduction in dietary protein made available for intestinal absorption.

• Oven-drying reduced protein degradation compared with sun-drying, and this may lead to increased dietary protein flow to the intestines.

• The differences in degradability of the legumes according to stage of maturity and drying method could explain variations in reported values in the literature, which suggests that fixed degradability values for tropical legume forages are inappropriate.

 

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