The role of condensed tannins (CT) in the nutritive value of fresh Leucaena forage, especially in relation to their varying levels in promising new Leucaena forage accessions, was evaluated using sheep metabolism pen studies.
In the first study, two metabolism experiments were conducted to measure the in vivo digestibility and N retention of four Leucaena species and a hybrid with total CT contents ranging from 0.4 - 12.2% in DM (L. pallida CT equivalent). A 6% CT appeared as a threshold level beyond which there were significant reductions in N retention. This apparent threshold level of CT was consistent with the level found in L. leucocephala and associated with the maximum level of OM digestibility (c. 60% OMD) obtainable for the fresh Leucaena diets evaluated. The apparent N digestibility was reduced (-15% units) relative to the 'tannin-free' Leucaena (L. collinsii) diet but was nevertheless reasonably high (>65%) at this threshold CT level. The ranking from high to low nutritive value of the Leucaena diets was inversely related to their CT contents. It was concluded that L. collinsii, L. leucocephala and Leucaena KX2 were of a high nutritive value as sole diets, whereay L. pallida and L. trichandra can not be fed as sole diets for sheep and other ruminant livestock.
A subsequent metabolism and slaughter trial evaluated the L. leucocephala CT activity on in vivo digestion and digestible protein supply to the small intestines and N retention using polyethylene glycol (PEG, MW. 4000) and relative to lucerne (Medicago sativa). The use of PEG to neutralise the tannins clearly demonstrated the ability of L. leucocephala CT to reduce N degradation in the rumen and increase rumen-escape protein and total protein flow at the abomasum and to the intestines. However, the additional protein supply of the CT-active diet, although just as digestible as the abomasal NAN of the PEG/Leucaena and control lucerne diets, was not as efficiently utilised as indicated by the lack of an advantage in N retention. The OMD of the three diets were similar and it was concluded that the relative energy supply may be a limiting factor in the effective utilisation of the extra digestible protein of fresh L. leucocephala forage. Moreover, the quality of L. leucocephala is comparable to lucerne hay but this was not a direct consequence of the Leucaena CT action on nitrogen digestion.
The dose response relationship of dietary CT and the availability of protein in terms of delivery to and amount digested in the small intestine and effect on N retention were evaluated in another terminal sheep metabolism trial. A concentrate quebracho tannin extract (QTE) was given as a oral drench at 6 hr intervals at the rate of 0-60 g/kg of DM intake with a basal lucerne hay diet. The QTE (from the Schinopsis spp.) was used to evaluate the exclusive effects of varying CT contents because fresh forage of different Leucaena accessions might vary in nutritional aspects other than their CT levels and activity. However the QTE did not improve the flow of protein post-ruminally which was at odds with the general hypothesis for CT enhancing rumen-escape protein. This was possibly a consequence of feed protein being degraded before the quebracho tarmins had the opportunity to bind to it and for this reason the protocol did not appropriately test the “low CT = improved protein supply” hypothesis. Nevertheless, the ability for CTs to dramatically reduce protein absorption in the small intestines was well highlighted. It was concluded that QTE given as a dietary supplement is unlikely to improve the absorbable protein content of forages.
The effect of CTs on metabolism of the visceral tissues was also investigated in a study run in conjunction with QTE dose response experiment. There were no changes in estimates of tissue hyperplasia (increased DNA concentration), hypertrophy (increased protein: DNA ratio) and protein synthetic capacity (RNA:DNA and RNA:protein ratios) of visceral organs, including the rumen, abomasum, the small intestines and the pancreas and liver. The estimates for protein fractional synthesis rate in the jejunum similarly showed no changes in response to the QTE, however the variability within treatments was high. Allowing for factors such as intake level and the relative sensitivity of visceral organs and the type of tissues analyzed, the methods adopted in this study showed potential for assessing the effect of CT on endogenous N losses. This is an important line of investigation as literature shows that the visceral tissue turnover and energy utilisation is substantial relative to whole body synthesis and metabolic requirements.
Future metabolism trials could further evaluate the provision of additional digestible energy with Leucaena forage diets as this may improve the efficiency of utilisation of the extra digestible protein supply, especially with feeding of 100% fresh forage diets of L. leucocephala.