Effect of condensed tannins in birdsfoot trefoil (Lotus corniculatus) and sulla (Hedysarum coronarium) on body weight, carcass fat depth, and wool growth of lambs in New Zealand

The effects of dietary condensed tannins (CT) on growth and performance of young male weaned lambs were measured during a 4-month grazing trial when either birdsfoot trefoil (Lotus corniculatus) cv. Grasslands Goldie or sulla (Hedysarum coronarium) cv. Grasslands Aokau were fed. Each legume was offered at restricted and an libitum allowances and the effects of CT were determined by giving half the lambs in each treatment a twice-daily drench of polyethylene glycol (PEG; mol wt 3350) to deactivate the CT. Both agronomic and animal performance characteristics were recorded. Sulla had higher (P < 0.01) pre-grazing herbage masses than birdsfoot trefoil (11.6 cf. 7.1 t DM ha(-1)) and sulla stem (<6 mm diam.) was eaten, in contrast to birdsfoot trefoil where stem was rejected by the lambs. The sulla and birdsfoot trefoil diets had similar protein concentrations in the DM, but sulla had a higher concentration of total CT (88 cf. 50 g CT kg(-1) DM) and less structural carbohydrate (189 cf 328 g kg(-1) DM) relative to birdsfoot trefoil. Lambs fed sulla had a similar performance to those grazing birdsfoot trefoil at restricted allowances, with average liveweight gains of 168 g d(-1). At ad libitum allowances liveweight gains averaged 273 g d(-1), but the CT in sulla reduced carcass weight from 21.2 kg (with PEG drenching) to 18.8 kg (P < 0.05). The CT in birdsfoot trefoil did not affect wool growth, liveweight gain, or carcass characteristics. The trial has shown that both forages are able to provide a high level of productivity, but the CT in sulla can be detrimental to carcass yield from young lambs given this forage as a sole diet for a prolonged period. However, the sulla produces substantially more DM and is better utilised by lambs than birdsfoot trefoil, and is able to sustain high levels of animal production per hectare.