Tropical Forage Legumes

Harnessing the Potential of Desmanthus and Other Genera for Heavy Clay Soils

John R. Lazier, Nazeer Ahmad

• Publisher: CAB International
• Year: 2016
• ISBN:
  9781780648774
• Topic: Technology & Engineering
• Subtopic: Agronomy
• Index: Technology & Engineering

The development of legume use in agricultural production in the tropics lags far behind the temperate areas and extensive research over recent decades has aimed to rectify the lack of available leguminous fodder species available for heavy clay soils. This book draws together that research and explores the importance of heavy clay soils to agricultural productivity in the tropics and subtropics and the identification of adapted, productive forage legumes for these environments. Covering over four decades of international research, Tropical Forage Legumes: · Includes a detailed analysis of the forage germplasm available which is adapted to heavy clay soils· Covers the adaptation of a wide range of forages on Australian clay soils, and the evaluation of successful native and exotic forage legume species that have potential for those in Belize · Explores the genetics of the most promising genera, Desmanthus and Stylosanthes, and looks at the results from countries where new genotypes have been found to be productive and persistent · Provides details of a number of exciting new species, especially those in Desmanthus which have the potential to be, or have been commercialized · Makes recommendations for future research Providing an invaluable example of how a global search for adapted and productive forage germplasm has been - and can be - undertaken, and allowing access to a significant body of knowledge that was acquired before the digitalization of reports, this book will be a key resource for new scientists and experienced researchers in the areas of agriculture and forage agronomy.

1 Providing pasture and ley legumes for use on clay soils in tropical and subtropical environments

R.L Burt^†, J.R. Lazier^*1 and N. Ahmad^†

^*Formerly International Livestock Centre for Africa

¹E-mail: [email protected]

Almost 50 years ago, the FAO published a very informative review on the dark clay soils of the tropics and subtropics (Dudal and Bramao, 1965). It showed that the tropical and subtropical worlds contain very large areas of alkaline and clay soils; 83 million ha, one-third of the clay soils in sub-Saharan Africa, are vertisols; the Republic of the Sudan has 40 million ha of ‘dark clay soils’, India has 60 million ha and Australia 70 million ha. The soils are generally characterized by higher montmorillonitic clay content and are often situated under wetter conditions. They occupy very large areas and are of major importance for cropping and pastures because of generally higher fertility and their ability to ‘hold water’ and thus extend the growing season. Indeed, plants that form effective rhizobial relationships with bacteria from such soils are deemed to have ‘special rhizobial relationships’.

It has long been known that such soils are of huge agronomic significance for all types of farming (e.g. Dudal and Bramao, 1965). Eponymously termed black cotton soils, these soils are justly world famous, however in India smallholder cotton farmers have been faced with major challenges, including rising costs of inputs, erratic rainfall and an inefficient manufacturing industry. It may thus be necessary for such farmers to diversify their production in order to ensure income security (Ramesh et al., 2004). Yet sources of N for pasture growth and animal nutrition are currently largely limited to chemical fertilizers, which are uneconomic for many pasture-based livestock operations. Dudal and Bramao point to the need to utilize productive legumes to assist in improving productivity on these soils through enhanced soil fertility, organic matter content and greater water retention, and as a source of high-quality fodder. While there is already considerable interest in greater use of legumes in ‘no-till agriculture’ and ‘organic farming’, many livestock production systems are currently largely based on legumes. Their use can increase animal production levels and allow other land to be conserved. However, there is a general lack of pasture and ley legumes that are productive and persistent for such edaphic environments, and the need is worldwide not just restricted to India (Clem and Hall, 1994; Pengelly and Conway, 2000).

The need to collect, conserve and utilize the plant genetic resources of the world has long been recognized: one of the earliest recorded plant-collecting missions is that organized by Queen Hatshepsut of Egypt in 1481 BC (e.g. http://www.plantlives.com/docs/A/Ammi_majus.pdf). Today the importance of plant biodiversity is widely acknowledged and there is both national and international attention being paid to the collection and maintenance of genetic variability throughout the world.

In the 1960s the lack of adapted legumes for tropical and subtropical pasture conditions generally (e.g. Burt and Williams, 1975) resulted in the development of worldwide forage germplasm collection and evaluation programmes. In Australia, programmes initiated in the late 1960s had, by the early to mid-1970s, resulted in the commercial release of previously undomesticated species, mainly of the genus Stylosanthes. The International Centre for Tropical Agriculture in Colombia (CIAT), which commenced research in 1969, also identified and released domesticated lines of legumes suitable for livestock fodder, mainly additional Stylosanthes species. The bulk of the research was focused on adaptation to acid, sandy-textured soils, which are of major importance in the higher rainfall areas of Australia and South America. Today, Stylosanthes is undoubtedly the genus of major utility for tropical pastures throughout the tropics and the genus most widely tested on clay soils (e.g. Ramesh et al., 2004 for India). Because little emphasis has been placed of the provision of legumes adapted to dark clay soils, the genotypes of productive legumes identified and disseminated are in general not very productive or persistent on them (Cook et al., 2005).

As there are only a limited number of pasture cultivars currently available and these do not provide varieties that are consistently productive over the range of farming systems and environments requiring them, there is a need for new cultivars to expand the range of environments in which legumes can increase the efficiency of cropping and livestock production. These cultivars may be found in species, or within species’ forms already in use, and could provide such characteristics as improved disease resistance or slightly different agronomic characteristics (e.g. Jones and Clem, 1997). However, other situations, such as the hugely important tropical clay soil areas of Sudan, India and Australia (Dudal and Bramao, 1965), require the development of ‘new genera’ and ‘new species’ (Burt, 1993a; Clem and Hall, 1994; Chapter 6, this volume). There is a similar need for new legumes for use in ley cropping systems (Pengelly and Conway, 2000).

Enthusiasm for the development of leguminous species for pastures grew from the 1960s to the 1980s, after which interest faded for a number of reasons: new areas of research became popular and attracted funds, it was thought that the genetic variation had been well sampled, a lack of interest in agricultural development in general and livestock and complex pasture and ley agricultural systems in particular. National and international funding has dried up, and scientific research establishments drastically reduced or eliminated.

This need for adapted forage legumes is exacerbated by the rapidly rising price of chemical fertilizers because of the high cost of energy, the increasing awareness of the pollutant effects of these fertilizers, the pressure for greater productivity on the limited global arable land resources and the deteriorating condition of much of the world’s cultivated soils. The identification of productive and persistent species and genotypes for heavy clay soils is increasingly threatened by expanding land use and global climate change, which result in the loss of potentially valuable native plant germplasm. These facts emphasize the urgent need for the collection and conservation of genetic resources with agricultural potential.

The requirement for new adapted and productive legume germplasm is likely to increase with global warming and climatic change. Even slight rises in temperature or changes in patterns of rainfall are already causing reductions in the livestock carrying capacity of many traditional grazing areas, resulting in overgrazing, plant loss and soil erosion (Anon, 2006; DeMeo, 2006). Re-establishing native pasture species, many of which were poorly adapted in the first instance (Moore, 1967) – and as shown herein – may not always be possible. Diversification of the native sclerophyll vegetation to meet such climatic changes, such as that which occurred during earlier long-term climate changes in Western Australia (White, 1986, p. 191), may well be too slow to cope with the relatively rapid changes of climate likely to occur (e.g. Bättig, 2007). Such changes are likely to be most rapid and most severe in tropical South America, the region most likely to be the source of new pasture legumes of value (Burt and Reid, 1976).

A review of available leguminous fodder species (Cook et al., 2005) has shown that, as had been suggested earlier by Dudal and Bramao (1965), there are few leguminous species available for heavy clay soils: ‘Legumes have not yet come to occupy the place they deserve for soil improvement, pasturage and fodder production. Research and extension in this field are still needed for tropical and subtropical countries’ and, as will be shown subsequently in this book (Chapter 6, this volume), development of legume use in agricultural production in the tropics lags far behind the temperate areas. As an example, the genus Medicago, which has provided many temperate legume cultivars for use on clay soils, has a USDA genetic resource collection of over 7000 accessions, while Desmanthus, its tropical equivalent (see later chapters in this volume), has 300 accessions in the largest collections.

Desmanthus is a leguminous genus whose species have particular potential for clay soils and has had little research attention. What information has been available has indicated that they have important potential as long-term pasture legumes (e.g. Clem and Hall, 1994), as ley legumes in crop/pasture systems (Cook et al., 2005), as cut and carry fodder in fodder banks, and for revegetation programmes (e.g. Muir et al., 2005 for use of both native and introduced Desmanthus species to regenerate and renew blackland prairie in Texas). A further approach to utilizing legumes and their genetic variation is the development of pulse/forage crops (e.g. Glover et al., 2007); species such as Desmanthus illinoensis can be used as a perennial pulse crop and soil cultivation avoided.

The research reported herein is international in scope; it involves two Australian programmes, one designed to provide pasture legumes for use on tropical clay soils and one undertaken to provide tropical legumes in general (Chapter 6, this volume). It also involves an IDRC-sponsored programme at the University of the West Indies (UWI) based in Belize, Antigua and Trinidad (Spurgeon, 1977), which was developed based on the need for such plants in the West Indies and Central America. The Caribbean has areas of economically important heavy clays, although these are too small to appear on the world soil map used by Dudal and Bramao (Fig. 1.1). These are of particular potential for global pasture development as their unique and diverse leguminous flora have provided adapted material of potential for similar soils in tropical and subtropical environments worldwide. The UWI research was loosely coordinated with that undertaken in Australia. Many of the results obtained have remained unpublished or, if published, not in a format that allows their ready appreciation. This volume seeks to address this situation.

Fig. 1.1. Dark clay soils of tropical and subtropical regions: Areas of major distribution. (Source: Food and Agriculture Organization of the United Nations (Dudal and Bramao, 1965). Reproduced with permission.)

The research also updates information provided by Dudal and Bramao, particularly that which relates to the distribution and major characteristics of the soils involved. It is now possible to give some specific examples of areas with heavy clay soils, problems involved in utilization, and relevant programmes undertaken to overcome these. Mention is made, for instance, of mineral nutrition and rhizobial studies, as the result of which legume growth can be made possible or stimulated (e.g. P and trace elements; Chapter 17, this volume), thereby promoting N fixation and alleviating the need for frequent application of costly chemical N fertilizer or scarce animal manure. Such strategic use of fertilizers to improve ‘soil health’ can be a major factor in promoting a ‘Green Revolution’ for smallholder farmers and commercial farms in Africa, for example. Appendices to chapters provide more detailed information about the clay soils that have yielded the majority of the important legume accessions currently in genebanks.

A further aim of this volume is to highlight the fact that considerable knowledge exists on numerous leguminous genera with forage potential for tropical and subtropical conditions, but is not readily accessible to today’s researcher having been acquired before digitalization of reports. As a result, research may be repeated, and some views may be currently held that are not in accordance with previously established results. The approaches utilized in these studies to identify, collect and screen genera, species and genotypes for particular environments are presented as illustrative of efficient and effective approaches to identification of adapted forage germplasm.

A global perspective was utilized in undertaking the research. It involved the worldwide identification of areas of legume genetic diversity, heavy clay soils and climatic conditions similar to those of target environments. Collection has been undertaken on a broad scale internationally with the collection of legume germplasm from selected previously unexplored environments in South and Central America, and the Caribbean representative of important livestock areas elsewhere in the tropics and subtropics. These were then tested for adaptation and persistence in a range of target environments along with promising materials from earlier global collections. The sites in which the research...