Characterization and evaluation of Indigofera species as potential forage and cover crops for semi-arid and arid ecosystems

Abstract

The potential of Indigofera species as forage and/or cover crops for semi-arid and arid environments was investigated in several experiments conducted on the Hatfield Experimental Farm in Pretoria, South Africa. Dormancy associated with hard seededness is the main constraint for uniform germination and large-scale propagation of these species. In this study, pretreatment increased germination in most accessions with scarification being more effective than boiling water treatment in six accessions, but not in the case of I. vohemarensis 8730. In five accessions (I. cryptantha 7067, I. brevicalyx 7517, I. arrecta 7524, I. spicata 8254 and I. vohemarensis 8730), scarification improved the total germination percentage, though it simultaneously resulted in higher seed mortality of I. brevicalyx 7517, I. arrecta 7524 and I. vohemarensis 8730 than in the control. In four accessions (I. brevicalyx 7517, I. arrecta 7524, I. vohemarensis 8730 and I. trita 10297), boiling water treatment improved germination percentage without causing any significant risk of seed mortality in the latter three species. In a field study, 41 Indigofera accessions were characterized in terms of morphological and agronomic parameters, using multivariate techniques to describe their phenotypic variability. Eight morpho-agronomic groups with various potentials were identified along with eight determinant characteristics that can be regarded as the core attributes for future Indigofera germplasm characterisation. Further evaluation of promising accessions revealed remarkable differences, both between and within species, in terms of plant height, canopy spread diameter, forage biomass, crude protein content, in vitro organic matter digestibility and indospicine level of the forage. These suggest the possibility of directly selecting accessions with forage potential for subsequent evaluation with target animals. The response of four selected Indigofera accessions under simulated moisture deficit stress and non-stress conditions exhibited significant variation. I. amorphoides was relatively sensitive while I. vicioides was able to maintain growth under water stress conditions, while the response of the two I. arrecta accessions were intermediate. The influence of season and species on forage quality was also studied. Spring growth had a significantly higher (P< 0.05) CP content than autumn growth in all species. In vitro digestibility of dry material also tended to decrease from the spring of 2004 to the autumn of 2004. Higher levels of Ca, P, Mg, Zn and Cu concentration were revealed in the leaf meal of the first harvest than in the re-growth harvest. All of the species had Ca, Mg, Zn and Mn concentration levels that could support the requirements of ruminants. P and Cu were slightly deficient for some of the species in the autumn suggesting the need to supplement P and Cu from other sources. Compared to Leucaena forage, Indigofera forage had higher apparent organic matter and dry matter digestibility coefficients and higher crude protein and neutral detergent fibre digestibility coefficients. The difference between Indigofera and Leucaena forage in terms of DM intake per unit of metabolic body weight (DMI g BW-0.75 day-1) was not significant (P> 0.05). The digestible organic matter intake (DOMI) and digestible crude protein intake (DCPI) of the sheep on Indigofera forage was similar to that of sheep fed Leucaena. In this study, lack of differences between Indigofera and Leucaena forage in terms of DOMI, DCPI and DNDFI means that Indigofera forage would likely support similar weight gains as that of Leucaena, but lower than that of M. sativa forage.