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

Abstract

The ability of low to moderate levels (<5% DM) of dietary condensed tannins (CT) to promote useable rumen-escape protein was evaluated using a quebracho tannin extract (QTE) given with high quality (4.0% N) lucerne hay to weaner sheep in a metabolism trial. The QTE (73.4% (w/w) CT in DM) was given as a liquid drench at levels of 0, 20, 40, and 60 g per day with 1 kg DM per day of hay. The trial comprised 12 animals in groups of three per dose rate. The hay was offered continuously from automated individual feeders to achieve ‘steady-state’ conditions for digesta flow measurement using Cr 2O 3 from intra-ruminal controlled release capsules. A diet acclimatisation period of 10 days was followed by 7 days for total collection of urine and faeces to determine digestibility and N retention, and microbial protein synthesis 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 rate, and rumen fluid for NH 3-N concentration. Despite the low levels of CT given, we found no evidence to support the generalised hypothesis that low to moderate amounts of CT improve N utilisation by sheep. The dose response to QTE showed no effect for rumen digestion of N (mean NH 3-N=291±19.0 mg/l, r 2=0.43) and OM (mean DOMADR (%)=81±23.5, r 2=0.03), and consequently the abomasal NAN (mean 20.5±2.32 g per day, r 2=0.109) and microbial-N supply (mean 11.5±1.38 g per day, r 2=0.57) were not affected, hence rumen-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 faecal N was directly related to the decline in apparent absorption of NAN in the small intestines from a baseline of 12.6 g per day to as low as 7.2 g per day ( r 2=0.904). However, this decline was not sufficient to alter N retention (mean 13.2±4.18 g per day, r 2=0.58). The failure of the QTE to improve the protein supply post-ruminally is at odds with the general hypothesis for CT enhancing rumen-escape protein by reducing N degradation in the rumen. Nevertheless, the ability for CT to dramatically reduce protein absorption in the small intestines was well highlighted. These results suggest that QTE given as a dietary supplement is unlikely to improve the absorbable protein content of forages.