The rise and fall of Siratro (<i>Macroptilium atropurpureum</i>) − what went wrong and some implications for legume breeding, evaluation and management
Siratro (Macroptilium atropurpureum) cv. Siratro was one of the first tropical legumes released for commercial use in the 1960s. It initially showed great promise in experiments and commercial sowings. Early research showed it was unproductive under heavy grazing, but after some 15 years there was increasing concern about its persistence, even under light to moderate grazing pressure. Commercial usage subsequently declined markedly although siratro, usually as cv. Aztec, is still sown to a very limited extent. This paper examines some reasons for this decline and then discusses some implications for research into improving tropical pastures through the use of legumes. In general, early pasture research, such as that on Siratro, failed to recognize that original plants of herbaceous legumes had a limited life span and that, for long-term persistence, new plants had to develop through vegetative or sexual reproduction. However, many studies over a 20-year time span showed that, although Siratro could form new plants, in most cases these replacements were insufficient to maintain an adequate plant density in the long term. Data on stolon density, plant longevity, soil seed banks and seedling survival, under different rainfall regimes and stocking rates, are presented to illustrate this. The major limitation was that soil seed banks were generally inadequate to ensure persistence, especially through a period of drier years, when there would be little or no seed set and possibly the death of all seedlings, which emerged from isolated falls of rain. Autumn spelling of pastures to enhance seed set improved persistence, but not reliably enough to be of widespread practical use. The major implication is that evaluation studies failed to adequately recognize the need for introduced legumes to form new plants after the original ones died. This has implications for future experiments, in terms of: duration; the management regime(s) imposed; the measurements or observations taken; and the need for a more ecological approach in evaluation.