Effect of season, soil type and fertilizer on the biomass production and chemical composition of five tropical shrub legumes with forage potential T. T. Tiemann*,†, L. H. Franco†, M. Peters†, E. Frossard‡, M. Kreuzer*, C. E. Lascano†,§ and H. D. Hess– *Department of Agricultural and Food Science, ETH Zurich, Universitaetstrasse 2, CH-8092 Zurich, Switzerland, †Tropical Grass and Legume Project, CIAT, Cali, Colombia, ‡Institute of Plant Science, ETH Zurich, Eschikon 33, CH-8315 Lindau, Zurich, Switzerland, §CORPOICA, Corporacio ´ n Colombiana de Investigacio ´ n Agropecuaria, Bogota ´, Colombia, and –Agroscope Liebefeld-Posieux, Research Station ALP, P.O. Box 64, CH-1725 Posieux, Switzerland Abstract The biomass production and the nutritive value of three tropical shrub legumes with condensed tannins (CT) (Calliandra calothyrsus, Flemingia macrophylla, Leucaena leucocephala) and two without CT (Cratylia argentea, Desmodium velutinum) as nitrogen-rich feed supplements for ruminants were assessed in two field experiments in Colombia. In one experiment, conducted on two different typical tropical soils (mollisol vs. oxisol; pH, 7Æ8 vs. 5Æ0; P content, 43 vs. 2 mg kg DM )1 ), the effects of low and high levels of fertilizer application with P, K, Ca, Mg and S were tested in both the dry and rainy seasons. In a second experiment on the oxisol, the effect of a lower level of application of either P or S fertilizer was assessed. On the oxisol, C. calothyrsus and F. macrophylla had the highest biomass production (93 and 100 g DM plant )1 in 9 weeks respectively) but the dry season caused extremely low DM yields in all species tested. Leucaena leucocephala did not grow on the oxisol but had the highest biomass production on the mollisol (454 g DM plant )1 in 9 weeks). On the oxisol, the mineral concentrations of the forage were below the requirements of ruminant livestock. Fertilizer treatment had no clear effects on the nutrient compo- sition of the forage. The non-CT shrub C. argentea had the highest concentrations of most minerals in its forage. Keywords: Calliandra, Cratylia, Desmodium, dry season, Flemingia, Leucaena Introduction Cultivation of feed crops for ruminant livestock in the tropics is often limited by low plant performance due to acidic, infertile soils and extended periods of drought. Some shrub legumes, such as Leucaena leucocephala and Calliandra calothyrsus, are adapted to low fertility soils and drought (Chen et al., 1992; Norton, 2000). Their suitability as feeds for livestock has been confirmed in various studies, and they are typically given as supplements to grass in proportions of up to 0Æ25 of the diet (Hess et al., 2004; Stu ¨ rm et al., 2006). Compared to grasses, leguminous forages are less well researched. They are generally characterized by higher crude protein and lower fibre concentrations than grasses. In addition, many of these forage species are significant sources of minerals for ruminants (Minson, 1988). However, tropical shrub legumes adapted to acid soils often contain substantial amounts of condensed tannins (CT) (Peters et al., 2003). At higher dietary concentrations, CT frequently have anti-nutritive effects (Waghorn, 2008) because they impair the digestibility of nutrients, espe- cially crude protein and fibre (Tiemann et al., 2008a,b). There is, however, limited information about the impact of environmental factors, such as soil type, fertilizer and season (i.e., rainy vs. dry season), on the nutritive value of tropical shrub legumes as a feed for ruminant livestock. The aim of the present study was to assess the biomass production and chemical composition of the forage of five different shrub legume species, with a focus on their nutritive value for ruminant livestock. Correspondence to: Dr. H. D. Hess, Agroscope Liebefeld- Posieux, Research Station ALP, P.O. Box 64, CH-1725 Posieux, Switzerland. E-mail: dieter.hess@alp.admin.ch Received 27 October 2008; revised 8 May 2009 doi: 10.1111/j.1365-2494.2009.00691.x Ó 2009 Blackwell Publishing Ltd. Grass and Forage Science, 64, 255–265 255