Intake, digestibility, urinary excretion of purine derivatives and growth by sheep given fresh, air-dried or polyethylene glycol-treated foliage of Acacia cyanophylla Lindl.

Effects of air-drying or spraying Acacia cyanophylla Lindl. leaves with polyethylene glycol (mol. wt 4000; PEG) upon aqueous methanol-extractable vanillin-HCl condensed tannins (CT) level were studied in Barbarine and Queue Fine de l'Ouest sheep. Drying was under sun or shade for 2, 7, 14 or 28 days and PEG treatment (PEG/CT ratios: 0.5, 1.0, 2.0 or 4.0). Sun-drying was slightly more efficient in reducing CT levels in acacia foliage than shade-drying (29.4 and 20.4% reduction in 48 h, 38.7 and 33.8% in 28 days). PEG treatment of freshly cut material at a rate of 0.5, 2 and 4 g g-1 CT reduced the CT content of acacia foliage by 29.0, 51.8 and 55.5%. Using fresh, field-dried and PEG-treated acacia foliage harvested at the same time, digestibility and growth trials were performed with five adult rams (total faecal collection, 28 days) and six growing 4-month old female lambs per treatment. Animals received daily acacia ad libitum and 400 g barley per head. Fresh leaves of acacia were field-dried for 14 days, or sprayed with PEG solution at a rate PEG/CT of 2.0 at the time of feeding. Acacia and total diet dry matter (DM) intakes by adult and growing sheep were similar across experimental diets. Fresh and dried acacia had the same low nutritive value. Sheep on PEG-containing diet gained more weight (p < 0.05) than those on fresh or dried acacia (95 g day-1 vs. 69 or 73 g day-1, respectively). Dietary crude protein and neutral detergent fibre (NDF) digestibilities were increased by PEG treatment when compared with fresh acacia (72.8 vs. 38.7% and 48.3 vs. 39.9%, respectively). In comparison with fresh or dried acacia, PEG treatment increased nitrogen retention (p < 0.05), the urinary excretion of purine derivatives (PD) by 17% (p < 0.05), suggesting an increase in outflow of microbial protein from the rumen. It is concluded that PEG may be used to improve the nutritive value of A. cyanophylla foliage. However, the reduced extractable CT induced by air-drying may simply reflect a reduced solubility of CT-protein complexes.