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.