Effect of supplementing tropical tannin-free and tanniniferous legumes to grass-fed sheep on the utility of their manure as nitrogen fertiliser

A pot experiment was conducted to assess the fertiliser value of faeces from sheep fed with a diet supplemented with legumes free of or containing condensed tannins. Ten animals received the same five diets in different experimental runs of a double Latin Square design, all consisting of a low quality tropical grass ration, supplemented with 450g/kg legumes. Legume treatments were either solely Vigna unguiculata (a tannin free herbaceous legume) or mixtures of V. unguiculata with either Calliandra calothyrsus or Flemingia macrophylla (both shrub legumes rich in condensed tannins) in ratios of 2:1 and 1:2. Faeces from animals receiving the same diets were pooled and applied at two levels (20 and 80 mg N/kg soil) to pots with seedlings from the cultivar Mulato II, a Brachiaria grass hybrid. An acidic infertile tropical soil was used. Aerial biomass yield and plant N content were evaluated. The results seem to indicate that the N fertiliser value of faeces from sheep receiving a diet supplemented with tanniniferous legumes is not substantially reduced compared to diets free of condensed tannins. However, the data also indicate that nitrogen was not the most limiting element in the soil used since mineral fertilisation without extra nitrogen was efficient, too

Nutrient composition and in vitro ruminal fermentation of tropical legume mixtures with contrasting tannin contents

Various combinations of a low-tannin herbaceous legume (Vigna unguiculata) and foliage of tanniniferous shrub legumes (Calliandra calothyrsus, Flemingia macrophylla and Leucaena leucocephala) or a low-tannin shrub legume (Cratylia argentea), all mixed together with a low-quality tropical grass (Brachiaria humidicola), were tested in vitro for differences in the effects on ruminal fermentation. Two experiments with the gas transducer technique were carried out, where each forage mixture was tested either with or without polyethylene glycol in order to be able to identify tannin-related effects (n = 3). In Experiment 1, a stepwise replacement of V. unguiculata by C. calothyrsus (5:0, 4:1, 3:2, 2:3, 1:4, 0:5) at a legume proportion of 1/3 or 2/3 in the mixture was evaluated. Together with two grass-alone and four pure legume treatments this added up to 30 treatments. In Experiment 2, V. unguiculata was gradually replaced by each of the four shrub legumes (3:0, 2:1, 1:2, 0:3) in grass legume ratios of 2:1, adding up, together with two grass-alone treatments, to 28 treatments. When added alone, V. unguiculata resulted in high fermentative activity as measured by gas production and kinetics as well as low proportion of undegraded crude protein. When V. unguiculata was replaced by the low-tannin C. argentea in Experiment 2, there was no noticeable difference (P>0.05) in fermentative activity. In both experiments, the effect of the substitution of V. unguiculata by tanniniferous shrub legumes resulted in a declining gas production and an increasing proportion of undegraded crude protein (P<0.001). However, the extent of these changes depended on the level of replacement and the shrub legume species (P<0.001). The results of Experiment 2 illustrate that this was the consequence not only of different tannin contents (less adverse effects with L. leucocephala than with C. calothyrsus) but also differences in the chemical properties of the tannins present in these shrub legume species (much less adverse effects with L. leucocephala than with F. macrophylla despite similar tannin contents). Furthermore these results indicate that, once the extent of the effects of a tanniniferous legume is known, one may calculate the maximal level of replacement of a low-tannin legume in a grass diet possible without negative effects on ruminal fermentation. This allows to improve dry season grass-based diets with as few as possible of the expensive and less well growing low-tannin legume

Effect of freeze drying on in vitro ruminal fermentation dynamics of three tropical shrub legumes with and without condensed tannins

Extensive comparisons of the effects of tropical shrub legumes rich in condensed tannins (CT) require well-conserved material. It is, however, unclear if the application of even gentle methods like freeze drying (lyophilization) affects the results in comparison to fresh material. Therefore, an experiment with the gas-pressure transducer technique, simulating ruminal fermentation dynamics in vitro, was conducted to investigate the effect of freeze drying on the ruminal nutrient degradability of three tropical multipurpose shrub legumes. Leaves of the CT shrubs Calliandra calothyrsus and Flemingia macrophylla and of the CT-free shrub Cratylia argentea were tested either in fresh form or lyophilized. In order to simulate practical feeding conditions, the legume leaves were incubated together with Brachiaria humidicola (1:2) for 144 h. Additionally, incubations were carried out either with or without polyethylene glycol (PEG) to be able to separate effects either dependent or independent of the CT. Only few differences were found between fresh and lyophilized leaves. These included that the proportion of apparently undegraded nitrogenous compounds was higher in fresh than in lyophilized leaves of Flemingia macrophylla, and freeze drying had a limited influence on volatile fatty acid production in Calliandra calothyrsus. The variables related to degradation dynamics (i.e., total gas production, the time until the point of inflection, apparent dry matter degradability), however, were not influenced. There was also no difference between CT and non-CT plants in that respect, as is also obvious from the lack of interactions of state of the plant material and PEG addition. This indicates that effects of freeze drying of shrub leaves on overall ruminal nutrient degradation in mixed grass-legume diets were minor