Supplementation of grass (Eragrostis spp.) hay with Vachellia karroo leaves: effect on chemical composition and in vitro ruminal fermentation

This study was designed to investigate the effect of supplementing a grass (Eragrostis spp.) hay basal diet with increasing levels of Vachellia karroo leaves (VKL) on chemical composition and in vitro ruminal fermentation. Fresh VKL were harvested, dried (60 degrees C), milled and used to supplement grass hay at a rate of 0 (VKL0), 5 (VKL5), 10 (VKL10), 15 (VKL15), 20 (VKL20) and 25% (VKL25). The effect of VKL condensed tannins on in vitro ruminal fermentation was assessed with the aid of a tannin-binding compound, polyethylene glycol (PEG Mr 4400), using the Reading Pressure Technique. VKL25 had the highest CP, soluble condensed tannin (SCT) and total soluble phenolics ( TSPh) contents (P<0.05) than VKL0. Supplementing grass hay with VKL reduced (P<0.05) NDF and ADF contents but had no influence on ADL (P > 0.05). With or without PEG inoculation, VKL0 had the highest (P<0.05) rate of gas production. Cumulative gas production was higher (P<0.05) for the inoculation with PEG, but no differences (P > 0.05) were observed among the treatments. PEG inoculation had (P<0.05) a shorter lag phase and lower Pgas but had higher in vitro ruminal organic matter degradability (ivOMD), effective gas production (Egas) and partitioning factors (PF). VKL0 had the highest gas production rate constant for the slowly fermentable fraction c (0.007 %/h) compared to the other treatments, which did not differ (P>0.05). VKL25 had the lowest Egas (43.7 ml/g OM) than VKL0, VKL5 and VKL10. It was concluded that Vachellia karroo leaves could be used as a potential protein source and, with the aid of PEG, the leaves could improve ruminal fermentation efficiency.