IMPROVING THE NUTRITIVE VALUE OF BARLEY STRAW FOR RUMINANTS : EFFECTS OF TREATMENT WITH LIGNINASE ENZYME OR WHITE-ROT FUNGI ON COMPOSITION AND DIGESTIBILITY IN VITRO

Kamal  A.R.  KHAZAAL

The white‑rot fungus Phanerochaete chrysosporium is a known delignifier from which ligninase enzyme has recently been isolated. The literature is confined to describing the ability of P.chrysosporium ligninase to degrade lignin model compounds. The objective of the present study was to define the potential of P.chrysosporium ligninase to delignify straw and thereby improve its digestibility for ruminants.

Experiments were on a laboratory scale involving milled straw and measuring effects of treatments on chemical composition and digestibility in vitro. In Experiments 1 and 2, involving a wide range of treatment conditions (enzyme concentration, pH, time, presence or absence of H₂0₂ and veratryl alcohol), there was no detectable effect of ligninase.

In Experiment 3, involving presence of P.chrysosporium in conditions similar to production of ligninase used in Experiments 1 and 2 and addition of extra ligninase, there was no ligninase activity as measured by veratryl alcohol oxidation to veratraldehyde at days 3 and 4.

Experiment 4 showed that an extract of barley straw (water extract after 2.0 h) was a powerful inhibitor of ligninase activity; 10μ1: of extract inactivated 5.0 units of ligninase in 1.0 ml solution. SDS‑gel electrophoresis in Experiment 4 showed that the enzyme as protein had not been degraded. The results of Experiments 1‑4 (ligninase in liquid medium) pose the question "how does P.chrysosporium degrade lignin under solid state fermentation (SSF) without having its ligninase inactivated?".

SSF of 75 g quantities of straw in experiment 5 under varying conditions (time, sterilization) using P.chrysosporium, Abortiporus biennis or Dichomitus squalens showed differing effects of fungus type and condition upon degradation of chemical emponents and digestibility; use of sterilized straw was necessary. A.biennis and D.squalens  degraded more hemicellulose and lignin compared to cellulose, and improved digestibility; P.chrysosporium did not improve digestibility but degraded lignin, hemicellulose and cellulose to similar extents, and more rapidly than the other fungi. Experiment 6 showed no ligninase activity or presence of the enzyme as protein in extracts of straw after SSF with P.chrysospori or A.biennis for 30 or 45 days.

Much further research, particularly regarding the mechanisms involved in degradation of lignin in straw by ligninase and other enzymes, using active microbial cultures, needs to be undertaken before practical methods for upgrading straw can be developed.