Areas of sulla (Hedysarum coronarium) and pasture (Lolium perenne/Trifolium repens/Holcus lanatus) were grazed by young sheep (29·5–34·8 kg initial liveweight) in four experiments, and effects upon body growth, wool growth and rumen metabolism were measured.
Sulla contained 40–50 g condensed tannins (CT)/kg DM, whilst the pasture contained small amounts of CT (2–6 g/kg DM). After chewing during eating, a lower proportion of total CT was readily extractable and greater proportions were protein-bound and fibre-bound. Nutritional effects attributable to CT were assessed by oral administration of polyethylene glycol (PEG), which binds and inactivates CT, to half the animals grazing each forage. Rates of body growth were consistently higher for lambs grazing sulla than those grazing pasture, with the CT concentration in sulla being neither stimulatory or inhibitory to body growth or voluntary feed intake (VFI). The action of CT reduced carcass fatness in sheep fed both diets, in the one experiment where this was measured. CT present in both sulla and pasture decreased rumen ammonia concentration and decreased molar proportions of iso-butyrate and iso and n-valerate. During spring and early summer, when wool growth rates were highest, CT present in both pasture and sulla increased wool growth rate; when wool growth rates were low during winter, CT had no effect upon the wool growth of sheep grazing either forage. Numbers of protozoa and molar proportions of n-butyrate in rumen fluid were increased by CT in sheep grazing sulla but not pasture.
It was concluded that the higher rates of body growth and VFI in lambs grazing sulla was most likely to be due to its very high ratio of readily fermentable: structural carbohydrate.