Different means of administering polyethylene glycol to sheep: Effect on the nutritive value of Acacia cyanophylla Lindl. foliage

Polyethylene glycol-4000 (PEG) was used to inactivate tannins in Acacia cyanophylla Lindl. foliage. In the first of two experiments, four groups of five Barbarine sheep were held in metabolism crates so that intakes, apparent digestibilities, nitrogen balances and urinary excretion of allantoin could be measured. The second experiment involved four groups of three male Queue Fine de l'Ouest sheep fitted with rumen cannulae and housed in individual pens to measure rumen fermentation parameters and dry matter in situ degradation of A. cyanophylla foliage. All animals received fresh A. cyanophylla foliage ad libitum and 330 g concentrate on a daily basis. In each experiment, three groups of sheep received 20 g PEG daily, either mixed with concentrate (PEG-concentrate), dissolved in drinking water (PEG-water) or sprayed as a solution on A. cyanophylla foliage at the point of feeding (PEG-treatment). The fourth group was not supplied with PEG (control). Dry-matter intake of A. cyanophylla was low (28-3 g/kg metabolic live weight (M0.75) per day) and increased in sheep given the PEG-concentrate diet (38.2 g/kg M0.75 per day). PEG-concentrate and PEG-water diets resulted in an improvement in protein utilization as indicated by an increase of crude protein apparent digestibility (2.1 and 1.9 fold, respectively), nitrogen retention (3.2 fold with both dietary treatments) and urinary excretion of allantoin (1.9 and 1.5 fold, respectively). Improvements obtained with PEG-treatment diet were low and in general not significant (P > 0.05). Low neutral-detergent fibre and acid-detergent fibre apparent digestibility coefficients of diets led to the conclusion that conventional detergent extraction techniques are questionable in determining the in vivo digestibility of cell wall constituents for tannin-rich forages. Results from rumen fluid analyses indicated that sheep given PEG-containing diets had higher ammonia-nitrogen and volatile fatty acid (VFA) concentrations (P < 0.05). These results, coupled with the increase of allantoin excretion gave clear evidence that the efficiency of microbial synthesis was improved with PEG addition. The absence of change in ruminal pH and molar proportions of individual VFA suggested similar fermentation patterns among all dietary treatments. PEG supply increased the slowly degradable fraction of A. cyanophylla foliage incubated in the rumen (P < 0.05), thus dry matter potential degradability (a + b) was highest in sheep given PEG-containing diets. It is concluded that the affinity of acacia tannins to PEG, increased the availability of degradable proteins, which resulted in an improvement of the nutritive value of acacia foliage. However, for practical situations, adding PEG to concentrate or to drinking water is recommended for sheep browsing A. cyanophylla trees in the field or fed indoors.