The degradation of the cell walls of lucerne, Italian ryegrass and wheat straw when fed to cattle, sheep and rabbits

Abstract

Dried lucerne (Medicago sativa), dried Italian ryegrass (Lolium multiflorum) and wheat (Triticum aestivum) straw, in the latter case supplemented with soyabean meal, were each fed to cattle, sheep and rabbits in each of 2 years. In both years, plant parts of the three diets were tested for in vitro digestibility, in both milled and chopped (1 cm lengths) form, and for cell wall content (as NDF). In the first year, the plant parts were analysed for lignin and both the plant parts and the faeces were examined microscopically for the proportions of thick-walled, thin-walled and epidermal cells in cross-sectional area and for the thickness of the cell walls.

The plant parts with the lowest proportion of thick-walled cells in cross-sectional area (0·05) were the lucerne leaflets and those with the highest proportion of thick-walled cells (0·68) were the stems of wheat straw. The cell walls of the thick-walled tissues were thinnest (0·7–0·8 μm) in Italian ryegrass leaf blades and sheaths. Within each cell type for the whole crop, the order of cell wall thickness was wheat straw > lucerne > Italian ryegrass. In vitro digestibility of DM was lower (by 0·031–0·085 g digestible DM/g total DM) in chopped than in milled stems of lucerne, ryegrass and wheat and in leaf sheaths of wheat. This suggests incomplete and/or delayed access of rumen microorganisms to some of the cell wall in chopped material in vitro and probably, therefore, also in chewed material in vivo. The concentrations of NDF and lignin in both ryegrass and wheat were in the order leaf blades < leaf sheaths < stems.

The lucerne crops were more mature than the ryegrass crops and there was no consistent difference between lucerne and ryegrass in intake of DM or intake of NDF. The intake of wheat straw DM was 0·52 that of lucerne and ryegrass, whereas the intake of straw NDF was 0·89 that of lucerne or ryegrass NDF. Intake of both DM and NDF in relation to metabolic body weight was highest (87–93 g DM and 45–48 g NDF/kg W^0·75) with cattle on lucerne and ryegrass and rabbits on ryegrass and lowest (33–34 g DM and 29–30 g NDF/kg W^0·75) with sheep and rabbits on straw. The output of faeces/kg W^0·75 was particularly high (38–41 g DM and 30–32 g NDF) from rabbits fed lucerne or ryegrass. Digestibility of DM was highest (0·726–0·732 g/g) with cattle and sheep fed ryegrass, followed by cattle and sheep fed lucerne and sheep fed straw. Digestibility of NDF was highest (0·708–0·752 g digestible NDF/g total NDF) with cattle and sheep fed ryegrass and sheep fed straw. Digestibility of NDF with rabbits was lower than with cattle or sheep, but was higher than might have been expected, in a small, hind-gut fermenter, with ryegrass (0·339 g/g) and straw (0·492 g/g).

The proportion of thin-walled cells was much lower in the faeces than in the diets, but there was an appreciable proportion (0·10–0·27) of these cells in the cross-sectional area of faecal particles. The cell walls of all cell types were thinner in the faeces than in the diets, e.g. those of the thick-walled cells were thinner by 0·35 μm in lucerne, by 0·11 μm in Italian ryegrass and by 0·41 μm in wheat straw. The faeces from rabbits had higher proportions of thick-walled and epidermal plant cells in cross-sectional area, and a lower proportion of thin-walled cells, than the faeces from cattle and sheep.