Leaf tannins and maize yields
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CTA. 1995. Leaf tannins and maize yields. Spore 59. CTA, Wageningen, The Netherlands.
Permanent link to this item: http://hdl.handle.net/10568/47163
Farmers may choose a particular multipurpose tree because it produces copious amounts of leafy material suitable for fodder and soil mulch. But, whereas quantity is easily judged, quality is less easily measured. In Zimbabwe scientists at the...
Farmers may choose a particular multipurpose tree because it produces copious amounts of leafy material suitable for fodder and soil mulch. But, whereas quantity is easily judged, quality is less easily measured. In Zimbabwe scientists at the Agroforestry Project of the International Centre for Research in Agroforestry (ICRAF) and the Southern African Development Community (SADC) are comparing a number of multipurpose trees by examining the nutritive value of the leaves and assessing the amount of secondary compounds, such as tannins, that they may contain. Nitrogen is bound to these compounds and this affects the way livestock absorb or excrete the nutrients and once excreted, how much the soil benefits. Likewise, when leaves are used as a mulch for soil, tannin levels can affect the availability of soil nitrogen. The compounds govern how well the material fed to livestock is digested or degraded in the rumen, and can modify the way nitrogen is excreted, in urine and faeces. If excreted in urine, the nitrogen is quickly vapourized and lost to the soil, whereas in faeces it is insoluble and breaks down very slowly. Low levels of tannin lead to up to 60% of nitrogen being excreted in the urine, whereas if tannin levels are high, over 60% of the nitrogen is excreted in the faeces. The amount of secondary compounds also controls how well the leafy material is degraded in the rumen. The more nitrogen is bound to the tannins, the less degradable it is, and the greater the loss of nitrogen in the faeces. The researchers are now looking at ways of protecting nitrogen in the rumen so that there is more chance of it being absorbed in the lower gut. The same problem applies to the leafy material if it is added directly to the soil. Material with low levels of tannin compounds degrades and releases nitrogen more quickly than material with high levels. If tannin levels are high it means that the current crop is unlikely to get much nitrogen from the mulch but subsequent crops will benefit. Studies show that the leaves of Leucaena leucocephala have low tannin levels and degrade very rapidly, whereas the leaves of Acacia angustissima, which has recently been brought in from Central America have high levels of tannins, leading to slow breakdown in the soil. This has been illustrated by Dr. Ben H. Dzowela, who leads the Project. In 1994 he obtained 3.5t/ha of maize from a crop which received leaves of A. angustissima. This compared with 5.8t/ha from a crop mulched with L. Ieucocepitala leaves. In 1995' on the same land, the A. angustissima treated crop yielded 2.1t/ha, whereas the L. leucocephala crop yield fell to 1.4t/ha. Dr. Dzowela attributes the differences in maize yields in 1994 and 1995, between the Acacia and Leucaena treated crops, to the high and low levels respectively of tannins in the respective mulches. SADC/lCRAF Agroforestry Project c/o Department of Research and Specialist Services P O Box CY 594 Causeway, Harare, ZIMBABWE