Comparison of the digestive ability of crop fluid from the folivorous Hoatzin ( Opisthocomus hoazin) and cow rumen fluid with seven tropical forages

Abstract

We explored the hypothesis that the Hoatzin, a tropical bird which feeds on tree leaves and has a crop resembling a rumen, may have a better ability to digest tanniniferous feeds than do cattle. Comparisons were made between Hoatzin crop fluid (HCF) and cow rumen fluid (CRF) in in-vitro digestion studies using freeze dried leaves of six tropical tanniniferous shrub legumes and the tropical grass Panicum maximum. In-vitro dry matter digestibility (IVDMD) and in-vitro nitrogen digestibility (IVND) were measured in the presence and absence of polyethylene glycol (PEG) to minimise the effect of condensed tannin. For all plant species, and in the presence or absence of PEG, the CRF gave higher IVDMD and IVND than did HCF ( P<0.01). The interaction of PEG×animal ( P<0.01) for IVDMD was due to an absence of response with CRF (62.1 v 63.0%) and a large response with HCF (43.5 v 37.4%). Similarly for IVND, the response to PEG with CRF was much smaller (83.4 v 75.6%) than for HCF (51.9 v 26.5%) ( P<0.01). PEG had no significant effect on IVDMD or IVND of the CT-free Panicum maximum by HCF or CRF ( P>0.05). However, values were much higher with CRF: 74.2 v 30.6% and 81.3 v 29.0%, respectively. The only legume to have reasonable levels (>55%) of IVDMD and IVND with HCF in the absence of PEG was Gliricidia sepium, which has little or no free condensed tannins. Acacia boliviana, Calliandra calothyrsus and Leucaena trichandra gave low values and those for L. leucocephala and L. pallida were intermediate. The low IVDMD and IVND with HCF, together with the large response to PEG, clearly show that high tannin content in the diet cannot be handled by the Hoatzin. Low tannin content in the crop contents indicates that it avoids highly tanniniferous leaves. This supports other findings that the Hoatzin is extremely selective of a high quality leafy diet. It is concluded that the Hoatzin is an unlikely source of bacteria to improve the feeding value of tropical tanniniferous shrub legumes for cattle.