Abstract:
The objective of the study was to determine the effects of fertilisation with bio-digester slurry and
the inclusion of carbohydrate additives at ensiling on the fermentation characteristics, chemical
composition, ruminal degradability, and in vitro digestibility of Napier grass silage. Napier grass
planted at the School of Agriculture Experimental Farm, University of Venda in 5 m x 4 m plots
replicated three times in a completely randomised design and was irrigated with either biodigester
slurry or no bio-digester slurry (tap water) for a period of 12 weeks. After 12 weeks, the
Napier was freshly cut and ensiled for 90 days in 1 litre glass jars in a 2 (Control - tap water and
slurry irrigation) x 4 (No additive, molasses, maize meal and brown sugar) factorial arrangement.
Fermentation quality and nutritive composition were determined using standard protocols. The
dry matter (DM) and crude protein (CP) ruminal degradability was determined in sacco by
incubating feed samples in nylon bags (external dimension: 6 × 12 cm, pore size of 46 μm) in the
rumen in three Bonsmara steers fitted with rumen cannulae for 0, 6, 12, 24, 48, 72, 96 and 120
hours (h). Parameters to describe the dynamics of ruminal degradability of DM and CP were
obtained by fitting the data on the exponential equation P = a + b (1 - e-ct) using the NEWAY
computer program, where “a” is the rapidly degradable fraction, “b” is the slowly degradable
fraction and “c” is the outflow rate. The in vitro DM and CP degradability of rumen undegradable
residue collected after 12, 24 and 48 h incubation was determined by sequential digestion in
pepsin (abomasal) and pancreatin (small intestine) solutions. Fertilisation with bio-digester slurry
increased (P <0.05) CP content of fresh cut Napier grass pre-ensilage. Bio-digester slurry
fertilisation with molasses inclusion improved (P <0.05) the silage DM content which improved (P
>0.05) fermentation characteristics with pH of 4.2 and lowest NH3-N of 13.3 g/kg. Other chemical
compositions and fermentation characteristics were not affected (P >0.05) due to fertilisation x
additives treatment combinations. No bio-digester slurry fertilisation with maize meal inclusion
increased (P <0.01) DM degradability at 0 h incubation. As time progressed to 24 h, no biodigester
slurry fertilisation with no additive included reduced (P <0.01) DM degradability with no
difference (P >0.05) on other treatments. Potential DM degradability (a + b) of no bio-digester
slurry fertilisation with no additive inclusion silage was reduced (P <0.01). The reduction was
associated with low levels (P <0.01) of slowly degradable fraction “b”. In vitro DM and CP
digestibility were not affected (P >0.05) due to fertilisation x additives treatment combinations. In
conclusion, bio-digester slurry application improved the quality of fresh cut Napier grass, with the
combination of bio-digester slurry fertilisation and molasses addition yielding the best silage
quality.
