The role of condensed tannins in the nutritional value ofLotus pedunculatusfor sheep

Abstract

1. Three experiments were conducted using Lotus pedunculatus containing high concentrations of condensed tannins (CT), and utilizing the principle that polyethylene glycol (PEG) application (molecular weight 3350) will irreversibly bind a portion of the CT and thus reduce the dietary reactive (i.e. non-PEG bound) CT concentration. Lotus diets containing 95, 45 and 14 g total reactive CT/kg dry matter (DM), induced by spraying with three PEG rates, were given to sheep at hourly intervals (600 g DM/d) for 21 d (Expt 1). In Expts 2 and 3, lambs grazed areas oversown with either lotus (89 g CT/kg DM) or clovers (Trifolium repens and Trifolium pratense; < 1 g CT/kg DM) for 42 and 92 d respectively. In Expt 2 half the animals grazing each forage received oral PEG (75 g/d), whilst in Expt 3 half the lambs were sired by rams selected respectively for low or high levels of sub- cutaneous fat deposition.2. Hormone concentrations in plasma (Expt 1 only) were determined by radioimmunoassay. Rates of [U-14C]-acetate and D-[U-14C]glucose incorporation and oxidation by subcutaneous and abdominal adipose tissue removed at slaughter, together with rate of glycerol release, were determined during in vitro incubation in all three experiments.3. Plasma concentration of growth hormone was positively and linearly related to dietary reactive CT concentration, whilst 3, 5, 3'-triiodothyronine (T3) concentration tended to be negatively and linearly related to dietary reactive CT concentration. Diet CT concentration had no effect on plasma concentrations of the other hormones measured.4. Feeding of lotus high in CT was associated with a consistent but non-significant increase in the rate of glycerol release from adipose tissue, which was reduced as dietary reactive CT concentration was lowered through PEG application, and a reduction in the lipogenesis: lipolysis value. Selection for leanness decreased acetate incorporation and increased glycerol release from adipose tissue, with the effect not interacting with the diet.