Post-ruminal protein supply and N retention of weaner sheep fed on a basaldiet of lucerne hay (Medicago sativa) with increasing levels of quebracho tannins

The ability of low to moderate levels (<5% DM) of dietary condensed tannins (CT) to promote useable rumen-escape protein was evaluated using a quebrac ho tannin extract (QTE) given with high quality (4.0% N) lucerne hay to wea ner sheep in a metabolism trial. The QTE (73.4% (w/w) CT in DM) was given a s a liquid drench at: levels of 0, 20, 40, and 60 g per day with 1 kg DM pe r day of hay. The trial comprised 12 animals in groups of three per dose ra te. The hay was offered continuously from automated individual feeders to a chieve 'steady-state' conditions for digesta flow measurement using Cr2O3 f rom intra-ruminal controlled release capsules. A diet acclimatisation perio d of 10 days was followed by 7 days for total collection of urine and faece s to determine digestibility and N retention, and microbial protein synthes is from total purine derivatives in urine. Animals were slaughtered at the end of the metabolism trial and digesta were sampled from the abomasum and the terminal ileum for determination of digesta flow rare, and rumen fluid for NH3-N concentration. Despite the low levels of CT given, we found no ev idence to support the generalised hypothesis that low to moderate amounts o f CT improve N utilisation by sheep. The dose response to QTE showed no eff ect for rumen digestion of N (mean NH3-N = 291 +/- 19.0 mg/l, r(2) = 0.43) and OM (mean DOMADR (%) = 81 +/- 23.5, r(2) = 0.03), and consequently the a bomasal NAN (mean 20.5 +/- 2.32 g per day, r(2) = 0.109) and microbial-N su pply (mean 11.5 +/- 1.38 g per day, r(2) = 0.57) were not affected, hence r umen-escape protein was also unaffected. Significantly though, the apparent N digestibility was reduced by 10% units (r(2) = 0.970) associated with a significant increase in faecal N (r(2) = 0.984) due to QTE, The excess faec al N was directly related to the decline in apparent absorption of NAN in t he small intestines from a baseline of 12.6 g per day to as low as 7.2 g pe r day (r(2) = 0.904). However, this decline was not sufficient to alter N r etention (mean 13.2 +/- 4.18 g per day, r(2) = 0.58). The failure of the QT E to improve the protein supply post-ruminally is at odds with the general hypothesis fur CT enhancing rumen-escape protein by reducing N degradation in the rumen. Nevertheless, the ability for CT to dramatically reduce prote in absorption in the small intestines was well highlighted. These results s uggest that QTE given as a dietary supplement is unlikely to improve the ab sorbable protein content of forages. (C) 2001 Elsevier Science B.V. All rig hts reserved.