Advances in agronomy volume 22

ADVANCES IN

AGRONOMY
VOLUME 22


CONTRIBUTORS TO THIS VOLUME

K. P. BARLEY
J. R. BROWNELL
J. w.CARY
FRANCISE. CLARK
J. P. EVENSON
D. L. GRUNES
PAUL R. HENSON
E. M. HUTTON
D. S. MCINTYRE
H. F. MAYLAND
M. M. MORTLAND
ELDORA. PAUL
D. L. PLUCKNETT
W. G. SANFORD
ROBERTR. SEANEY
P. R. STOUT


ADVANCES IN

AGRONOMY
Prepared under the Auspices of the

AMERICAN
SOCIETY

OF

AGRONOMY

VOLUME 22
Edited by N. C. BRADY
Roberts Hall, Cornell University, lthaca, New York

ADVISORY BOARD
W. L. COLVILLE W. A. RANEY
I. J . JOHNSON
J . R. RUNKLES
R. B. MUSGRAVE G. W. THOMAS
1970

ACADEMIC PRESS

New York and London


COPYRIGHT
0 1970, BY ACADEMIC
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CONTENTS
CONTRIBUTORS TO

VOLUME 22 ....................................................................

....................................................................................................
PREFACE

ix
xi

TROPICAL PASTURES

E . M. HUTTON
1. Introduction

II.
111.
IV .
V.
VI .
VII .
VII1 .
IX .
X.
XI .
XI1 .
XI11.
XIV .

xv .

XVI .

....................................................................

Climate and Potential for Improved Pastures in the Tropics .................
Role of Plant Introduction ................................................................
Tropical Legumes ...
...............................
.................................
Temperate Legumes

d in the Subtropics ..........................................
Nodulation and Nitrogen Fixation of Tropical Legumes ..........................
Legume Nutrition Relative to Tropical Pasture Development ...................
Undesirable Compounds in Tropical Legumes .......................................
Physiology of Tropical Legumes ..........................................................
Breeding and Genetics of the Main Legumes .........................................
The Main Grasse
....................................................
umes ............................................
Phosphorus and Nitrogen Fertilization of Grass
Breeding and Genetics of Tropical Grasses
Beef Production from Legume-Based Tropi
...........................
Summary and Conclusions ...........................................
References ...........................................................................

CLAY-ORGANIC

2
3
5
8
21

23
27
33
36
39
44

50
53
57
60
65
66

COMPLEXES A N D INTERACTIONS
M . M . MORTLAND

I.

.............................................................................

I 1 . Bonding Mechanisms in Clay-Organic Complexes ..................................
I11 . Nature of Some Clay-Organic Complexes and Reactions ........................
IV . Nature and Importance of Some Clay-Organic Complexes in Soils and
Sediments ........................................................................................
V . Conclusions .....................................................................................

.................................................................................
V

75
77
95
108

113
I14

vi

CONTENTS

BIRDSFOOT TREFOIL
ROBERTR. SEANEYA N D PAULR. HENSON

....................................................

I. Introduction ..................

11. Morphology ................
................................
111. Physiology .....
......................................................

IV. Culture ..........
....................................................
V. Utilization .............................................................
V1. Genetics and Cytology............................................

.....................................................................................
......................................

References ....................................

120
122

125
127
139
143

147
153

THE CONFIGURATION OF THE ROOT SYSTEM IN RELATION
TO NUTRIENT UPTAKE

K. P. BARLEY
I. Introduction .....................................................................................

I59

11. Geometrical Description of the Root System .........................................
111. Nutrient Transference in the Soil .........................................................
IV. Physiological Conditions Governing Uptake ..........................................

161

V. The Influence of Configuration on Uptake.. ...........................................
VI. Conclusions ..........................................
....................
References .................
................
.....................

167
17 1
177
197
198

FROST AND CHILLING INJURY TO GROWING PLANTS

H. F. MAYLAND
AND J. W. CARY
I. Introduction
Ill.
IV.
V.
VI.

................................................................
f Protein Structure .....................................

Cold Lability of Enzymes ...................................................................
Membrane Composition and Permeability ...................
Protection from Freezing.. ........................................
Conclusions ...........................................................

...................................................

203
206
2 15

vii

CONTENTS

THE PLATINUM MICROELECTRODE METHOD FOR SOIL
AERATION MEASUREMENT

D . S. MC ~ NT YRE

I . The Method .....................................................................................
I1. Electrochemistry ...............................................................................
111 . Physical Effects of Electrode Insertion .................................................
IV . Models and Microelectrode Response ..................................................
V . Conclusions .....................................................................................
V1 . O2 Flux and Plant Response ...............................................................
VI1 . Summary .........................................................................................
References .......................................................................................

235
241
266
268
276
278
281
281

RATOON CROPPING
D . L . PLUCKNETT.

J . P . EVENSON.
AND W . G . SANFORD
1. Introduction .....................................................................................
I1 . Genetic Aspects ...............................................................................
111. Botanical and Physiological Considerations ..............................
IV Ratooning and Environmental Factors .....................................
V . Soil Relationships .............................................................................
VI . Pests and Disease .............................................................................
VII . Management ....................................................................................
VIII . Future Outlook .................................................................................
References ................
....................................................

.

286
293
294
300
305
312
317
323
326

GRASS TETANY OF RUMINANTS

.

D . L. GRUNES.P . R . STOUT.AND J . R BROWNELL

1. Introduction .....................................................................................
11. Incidence. Climate, and Season ...........................................................

I11 . Symptoms of Animals ........................................................................
Iv . soils ...............................................................................................
V . Forage ............................................................................................

332
332
336
340
342


...

CONTENTS

Vlll

.

v1 Animal Factors in Hypomagnesemia ........................................
VII . Treatment of Affected Animals .................................
VIII . Prevention of Grass Tetany .............................................
IX Magnesium Deficiency in Humans .......................................................
..... ....................................................
X . Summary
References .......................................................................................

.

354
363
363
368
369
369

THE MICROFLORA OF GRASSLAND

FRANCIS
E. CLARKA N D ELDORA . PAUL
1. Introduction .......................................
.....
I1 . The Microflora of the Living Plant .......................................................
I11 . The Microflora of Grassland Litter ......................................................
IV . The Microflora of Grassland Soils .......................................................
V . Biomass and Bioactivity Measurements ................................................
VI . The Humic Component of Grassland Soil .............................................
VI1 . Nitrogen Transformations in Grassland Soils .........................................
References .......................................................................................

375
376
385
395
403
409
416

426

Author Index ..............................................................................................
Subject Index .............................................................................................

431
459


CONTRIBUTORS TO VOLUME 22
Numbers in parentheses indicate the pages on which the authors’ contributions begin.

K. P. BARLEY(159), Waite Agricultural Research Institute, Glen
Osmond, South Australia
J . R. BROWNELL*
(331), Kearney Foundation of Soil Science, Davis,
Calif0rn ia
J. W. CARY(203), Snake River Conservation Research Center, Soil and
Water Conservation Research Division, Agricultural Research Service, U.S. Department ofAgriculture, Kimberly, Idaho
FRANCISE. CLARK
(373, U. S. Department OfAgriculture, Fort Collins,
Colorado
J . P. EVENSON
(28% University of Queensland, Brisbane, Australia
D. L. GRUNES(33 11, U. S. Plant, Soil and Nutrition Laboratory, U.S.
Department of Agriculture, Ithaca, New, York
PAUL R. HENSON( 1 19), U. S. Department of Agriculture, Beltsville,
Maryland
E. M. HUTTON( l ) , Commonwealth ScientiJic and Industrial Research
Organization, Division of Tropical Pastures, Cunningham Laboratory, Brisbane, Australia

D. S . MCINTYRE(235), Commonwealth ScientiJic and Industrial Resea reh organization, Canberra,Australia
H. F. MAYLAND
(203), Snake River Conservation Research Center, Soil
and Water Conservation Research Division, Agricultural Research
Service, U.S. Department of Agriculture, Kimberly, Idaho
M. M. MORTLAND( 7 3 , Michigan State University, East Lansing,
Michigan
ELDORA. PAUL (373, The University of Saskatchewan, Saskatoon,
Canada
D. L. PLUCKNETT
(285), College of Tropical Agriculture, University of
Hawaii, Honolulu, Hawaii
W . G. SANFORD
(285), College of Tropical Agriculture, University of
Hawaii, Honolulu, Hawaii
ROBERTR. SEANEY
( 1 19), Cornell University, Ithaca, New York
P. R. STOUT(331), Kearney Foundation of Soil Science, Davis, California
*Present address: Fresno State College, Fresno, California

ix


This Page Intentionally Left Blank


PREFACE

In keeping with one of the basic objectives ofAdvances in Agronomy,
this volume covers a variety of subjects of concern to crop and soil

scientists. Likewise, the sixteen North American and Australian authors
who contributed these papers have a breadth of backgrounds and interests.
They have covered topics vital to both public and scientific concerns.
The continuing world-wide attention to tropical agriculture is recognized in this volume. An analysis is made in one chapter of the potential,
and the problems, of ratoon cropping -a practice of considerable importance with tropical crops such as bananas, sugar cane, and pineapples. An extensive review of research to improve and utilize tropical
pastures relates to the potential for forage production in the tropics. The
problems involved in the production and utilization of a number of important tropical forage species are also emphasized. While one is impressed
with the research contributions, the opportunities and .problems ahead
present challenges which dwarf these accomplishments of the past.
Two other chapters relate to the animal industry as well as crop production. One is a review of the research on birdsfoot trefoil (Lorus cornicularus L.), an important pasture and forage legume of especial value
in the North Central and Northeastern states. The second deals with
grass tetany, a ruminant animal malady associated with forages low in
magnesium and often relatively high in nitrogen and potassium. Factors
affecting the animal disorders, probable reasons for them, and therapeutic techniques are reviewed.
A review of recent research findings on frost and chilling damage to
plants includes evidence as to the mechanism of damage, and information on methods of preventing or reducing this damage. In another
chapter the relationship of geometric configuration of roots to nutrient
uptake is examined. Research is reviewed which identifies the conditions
under which nutrient transfer in the soil and root system configuration
1im it nutrient uptake.
Growing public concern for environmental quality has forced a realistic consideration of the part tne soil might play as a sink for various
kinds of wastes. Included are pesticides and other exotic chemicals,
sewage, and similar wastes. One chapter is addressed to a review of the
reaction of soils (clays) with organic compounds. The increasing specificity of our knowledge is impressive but the need for greater understanding of these reactions in soil is even more obvious.
The soil environment for plant roots and other living organisms is
considered in two chapters. A critical review of the platinum electrode
xi


PREFACE

xii

method for measuring soil aeration casts some doubt on the interpretation
of earlier findings, especially those wherein so-called critical values of
oxygen flux for root and plant growth were established. The microflora
of grasslands and grassland soils is discussed in one chapter. These contributions will be especially helpful as background for those concerned
with ecosystems and how man is modifying them.


ADVANCES IN

AGRONOMY
VOLUME 22


This Page Intentionally Left Blank


TROPICAL PASTURES

.

E M. Hutton
Commonwealth Scientific and Industrial Research Organization.
Division of Tropical Pastures. Cunningham laboratory.
Erisbane. Australia

Page

I . Introduction ....................................................................................
I1 . Climate and Potential for Improved Pastures in the Tropics .....................
111. Role of Plant Introduction ..................................................................

IV . Tropical Legumes .............................................................................

..............................................................................
ecies .......................................................................
The Desmodiums .......................................................................
Glycine ( G . wightti) ....................................................................
Leucaena ( L. leucocephala) ......
......
Miles Lotononis ( L. bainesii) ........................................................
Dolichos and Vigna Species .........................................................

C.
D.
E.
F.
G.
H . Centro (Cenfrosemapubescens) ...................................................
1. Calopo (Calopogonium mucunoides) and Puero (Pueraria

..................................................

V . Temperate Legumes Used in the Subtropics .......................................
A . Hunter River Lucerne ..............
......................................
.........................
B. Barrel Medic .................................

C . White Clover ..........................
VI . Nodulation and Nitrogen Fixation of Tropical Legumes ..........................
VII . Legume Nutrition Relative to Tropical Pasture Development ..................
VIII . Undesirable Compounds in Tropical Legumes ......................................
A . Estrogens and Substances Causing Bloat and Milk Taints .....
B . Mimosine .................................................................................
C . Indospicine ................................................................................

IX .

..............................................................................
pica1 Legumes .................................

A . Temperature and Growth in Several Tropical Legum

B. Townsville Stylo ........................................................................
C . Glycine wightii ...
..............................................................
D Siratro ......................................................................................
E. African Trifoliums ............................
.................................
Breeding and Genetics of the Main Legumes
.................................
A Breeding Systems ..............................
.................................
B. Townsville Stylo ........................................................................
C . Siratro ......................................................................................
D . Desmodiums ..............................................................................

.

X.

.

1

2
3

5
8
9
12
14
16
17
19
20
21
21

21
21
22
22
23
27
33
33

33
34
36
36
36
37
37
38
39
39
39
40
41
41


2

E. M. HUTTON

............ ,f............. .........
Leucaena .................
Indigofera .................
Lucerne .........................................
.................
Main Grasses .................
..................
A. Brachiaria .................
.................
B. Cenchrus ......................

........................................
C. Chloris ....................
........................................
D. Cynodon ...................
........................................
E. Digitaria ....................................................................................
F. Melinis .........................
........................................
G. Panicum .....................................
........................................
H. Paspalums .........................
........................................
I. Pennisetum ..............................................................................
J. Setaria ......................................................................................
K. Sorghum ...................................................................................
L. Urochloa ................................................................
.........
XII. Feeding Value of Grasses versus Legumes ..........................................
~

F.
G.
H.
XI. The

Phosphorus ..................
B. Nitrogen ........................
................
XIV. Breeding and Genetics of Tropical Grasses
A. Setaria .............................................

A.

C. Coastal Bermudagrass .

................
A. Wet Tropics

C. Humid Subtropics .......................................................................
D. Subhumid Subtropics ........................
XVI. Summary and Conclusions .........................................,.......................

.................

1.

42
42
43
43
44
44
45
45
46
46
47
47
48
48
49

49
50
50
53
53
54
51
58
58
59
59
60
61
61
63
64
65
66

Introduction

Improvement of tropical grasslands was neglected for many years because most of the areas involved are in developing countries with pressing
sociological and economic problems. Also, it had been concluded (Whyte
et al., 1953) that it would be very difficult to introduce a legume into
tropical grasslands and establish legume-based pastures as productive as
those in temperate areas. It was left to grassland scientists in several
countries, including Hawaii (Takahashi, 1956), Jamaica (Motta, 19561,
the Congo (Germain and Scaut, 1960),and Australia (J. G. Davies, 1960),
to pioneer research on legume-based pastures for the tropics. This work
was intensified and expanded mainly at experiment stations in north-

TROPICAL PASTURES

3

eastern Australia and Hawaii. At some of the centers significant progress
has been made in solving the problems of tropical legume-grass pastures.
The staff of the C.S.I.R.O. Cunningham Laboratory in Brisbane summarized their findings (C.A.B. Bull. 47, 1964) for the benefit of pasture
workers in other tropical areas. More recently, in 1966, W. Davies and
Skidmore edited a book, “Tropical Pastures,” which presents the modern
approach to tropical pastures.
Now that reliable tropical legumes and grasses are available as well
as knowledge of their fertilization, management, and productivity, considerable interest has been stimulated in the tropics in the use of improved
tropical pastures for animal production. A rapid expansion in the areas
planted with tropical pasture species can be envisaged with a concomitant
increase in production of beef and milk so that the “protein drought” in
many countries will gradually disappear. The development of pasture ecosystems for the wide range of environments throughout the tropics will
present many new problems. This will require increasing research activity
in many countries on pasture and animal production and a swing away
from the preoccupation with veterinary and animal health problems.
II. Climate and Potential for Improved Pastures in the Tropics

The tropics and its agricultural development are discussed by Phillips
(196 1) and Webster and Wilson ( 1 966). Wet equatorial climates with constant heat, rainfall, and humidity occur mainly within 5” to 1O”N and S of
the equator over a large area of South America and in West Africa, Malaysia, Indonesia, the Philippines, New Guinea, and various Pacific Islands.
Annual rainfall is usually 80- 120 inches, but a higher maximum is obtained in a number of places. The crops grown include rubber, oil palm,
banana, coffee, coconut, cocoa, and rice, and experiments and some
commercial plantings have shown that productive pastures can be established in the wet tropics.
Between the low latitude zones of wet equatorial climate and the Tropics of Cancer and Capricorn (23 M”N and S) there are extensive areas with

annual rainfalls of 20-80 inches and with a tropical monsoon climate in
which there is an alternation of wet and dry seasons. These areas occur
in South America, West and Central Africa, India, and in countries of
Southeast Asia including Burma, Thailand, Laos, Cambodia, and Vietnam, and also in northern Australia. Four climatic zones can be distinguished in these monsoonal areas as follows: annual rainfall of 40-80
inches in two rainy seasons with short dry seasons; annual rainfall of
25-50 inches in two short rainy seasons with pronounced dry seasons;


4

E. M. HUTTON

annual rainfall of 30-50 inches in one fairly long rainy season with a long
dry season; and annual rainfall of 20-40 inches with one short rainy
season and a long dry season. Rainfall reliability decreases at the lower
rainfalls. In the wetter parts, perennial tropical crops are grown as in the
wet tropics. Where dry seasons are well defined, annual crops like rice,
cotton, and maize are important, and in the driest areas sorghum, bulrush
millet, and peanuts are grown. The tropical monsoonal areas have considerable potential for improved pastures and cattle production, as shown
by some of the results obtained in different regions (vide Norman and
Arndt, 1959; Shaw, 196 1 ; Stobbs, 1969a).
The moist tropical climate extends from the Tropics of Cancer and
Capricorn to about latitudes 35"N and S, respectively, to give zones with
a humid subtropical climate (McIntyre, 1966; C. L. White et al., 1968).
These encompass southeastern United States, the middle Orient, a zone
including southern Brazil, southern Paraguay, northern Argentina, and
Uruguay, and areas along the eastern coasts of South Africa and Australia.
These humid subtropical transition zones have a rather variable rainfall
(Griffiths, 1959), which is predominantly in summer but with a winter
increment. Usually referred to simply as the subtropics, they are important agriculturally and grow the hardier tropical crops and, as shown in

Australia (J. G. Davies and Eyles, 1965), have considerable potential for
the development of cattle pastures based on tropical legumes and grasses.
The subtropics of eastern Australia is described by Coaldrake (1 964) as
the region where plants may be subjected to water stress or surplus any
month of the year and where water, rather than energy, is more likely to
limit plant production. Coaldrake also commented on the drastic reduction of herbage quality caused by a relatively small number of rather mild
winter frosts in these areas. Their sudden onset allows no hardening of
plants, and in any case most tropical pasture plants originated in frostfree areas and their above-ground portions, though usually not the roots,
are killed by frost. Most of the recent cultivars that are having an impact
on tropical pasture development in Australia and elsewhere were selected
and developed at research centers in Australia's subtropics (J. G. Davies
and Eyles, 1968). Thus, in Australia, the subtropics and Tropics form a
continuum in which the factors governing pasture production vary in
degree rather than in principle.
As outlined, the tropical zones where there is distinct potential for
increased cattle production on improved pastures include the wet equatorial, the tropical monsoonal, and the humid subtropical covering approximately 27% of the world's area. Of the moist tropics, 33% is wet
tropics, 49% monsoonal, and 18% subtropics. The arid tropical zone,


TROPICAL PASTURES

5

with its deserts, the dry subtropical with xerophytic scrub, herbs, and
grasses, and the semiarid tropical of grassland steppes are zones where
the vegetation can be improved with range management techniques and
not usually with the use of sown species. As pointed out by Hutton
(1968a), 60% of the world’s cattle are in the moist tropics, where 10%
is cropped, 20% is pasture, 35% is forest, and a third is wasteland. Forest
areas and wasteland are often in hilly areas and attempts to crop them

result in soil erosion. With the use of the new tropical pasture species and
fertilizer, idle uplands and unimproved native pastures can be improved
markedly with significant effects on beef and milk production.
Ill. Role o f Plant Introduction

The vital role of plant introduction in the development of pastures
has been recognized for many years in the United States and Australia,
both of which are deficient in promising native pasture species in their
temperate and also tropical areas. Although the tropics of Latin America
and Africa are rich in indigenous legumes and grasses with potential value
for improved pastures, Kenya is the only country in these continents
where a study has been made of the important species of native grasslands (Edwards and Bogdan, 195 I).
The U.S.D.A. plant introduction services, which commenced in 1898
(vide Yearbook of Agriculture, 1962) and which are now vested in the
New Crops Research Branch, have not been particularly concerned with
introducing tropical pasture plants. However, pangolagrass (Digitaria
decumbens), which was introduced as vegetative material from South
Africa in 1935 by the then U.S.D.A. Division of Plant Exploration and
Introduction, has made a major impact on tropical pasture improvement
(Oakes, 1960). Interest in Digitaria was stimulated and led to the collection by Oakes (1965) of an extensive range of species and ecotypes
within the genus. The breeding of coastal bermudagrass (Burton, 1954),
which has significantly increased pasture production in the southeastern
United States, was achieved through the use of two tall-growing South
African introductions of Cynodon dactylon. U.S.D.A. plant introduction
work has also assembled species in the genus Paspalum, such as P. notaturn, and P . dilatatum and obtained species for the Hawaiian Experiment
Station in the legume genus Desmodium.
As a result of the work on indigenous grasses in Kenya (Edwards and
Bogdan, 1951) the tropical world has obtained valuable ecotypes of a
number of grasses including Rhodes (Chloris gayana), buffel (Cenchrus
ciliaris), star (Cynodon dactylon), molasses (Melinis minutiflora),guinea

6

E. M. HUTTON

(Panicum maximum), kikuyu (Pennisetum clandestinum), and Setaria
(Setaria sphacelata).The indigenous legumes were also studied in Kenya,
and valuable ecotypes of Glycine wightii (formerly G.juvanicu)(Bogdan,
1966a) and species of Dolichos and Vigna have been made available to
pasture scientists in other countries. Kenya workers have studied both
native and introduced pasture plants at the Grassland Research Station,
Kitale, since I95 1. The cultivated varieties of herbage plants resulting
from this work were described by Bogdan ( 1965).
The Kenyan example could well be emulated in the countries of Central
and South America, where there is a wealth of indigenous legumes waiting to be collected, classified, and assessed. Latin America is the source of
a few important grasses, notably in the genus Paspalum, but does not
possess as valuable a grass flora as Africa. Although Africa has an extensive range of native legumes, it has not as yet contributed as many
promising pasture legumes as Latin America. There is an awakening
interest in many tropical countries in introduction of tropical legumes and
grasses that have shown promise elsewhere. Most of them still show a
reluctance to investigate their own rich native flora for promising pasture
plants.
As pointed out by Hutton (1970), Australia is singularly deficient in
indigenous legumes and grasses that can be used as the basis for improved
pastures and increased animal production. As a result, there has been a
continuing interest in Australia in pasture plant introduction, which commenced on a random basis about the 1880’s and became organized in
1930 with the establishment of the Plant Introduction Section of the
C.S.I.R. Division of Plant Industry (McTaggart, 1942). Up to the present,
50,000 introductlons have been brought into Australia and 6 I % of these

are pasture and forage species. Much of the current pasture development
in the Australian Tropics is based on the chance annual introduction
Townsville stylo (Stylosanthes humilis) which was recognized at Townsville around 1900 and known formerly as Townsville lucerne. Introduction of tropical pasture species has been a major aim of C.S.I.R.O.
plant introduction work since its inception, and a large number of legume
and grass accessions from tropical countries have been evaluated over
the years. The selection of grasses adapted to northern Australia has been
relatively easy, whereas obtaining adapted legumes has proved difficult,
particularly for the subtropics, where rainfall is variable and frosts can
occur.
Since the turn of the century, the Australian wet tropics of about 4
million acres of northeastern coastal country between Mossman and Mackay has had adapted introductions of tropical grasses such as guinea,


TROPICAL PASTURES

7

molasses, and para (Brachiaria mutica). Schofield ( 1941) eventually
obtained successful legumes for this area including stylo (Stylosanthes
guyanensis), centro (Centrosema pubescens), puero (Pueraria phaseoloides), and calopo (Calopogonium mucunoides).
Much of the plant introduction work for northern Australia over the
last thirty years has aimed at obtaining legumes and grasses for pasture
development in the extensive tropical monsoonal and humid subtropical
areas between latitudes 30"sand 11"sand comprising about 260 million
acres (J. G. Davies and Eyles, 1965). Miles ( 1 949) made distinct progress
with this problem by evaluating an extensive range of introduced legumes
and grasses in central coastal Queensland from 1936 to 1946. He showed
that the low mineral and protein status of the native pastures could be
raised by perennial legumes in a number of genera including Arachis,
Centrosema, Desmodium, Glycine, Indigofera, and Stylosanthes. The

most promising grass introductions included ecotypes of Chloris gayana,
Cenchrus ciliaris, Digitaria sp., Panicum maximum, Paspalum notatum,
Setaria sphacelata, and Urochloa sp. Miles' results (1 949) stimulated
the first work in overseas plant exploration by Australia. Hartley (1949)
joined a U.S.D.A. expedition to subtropical South America and collected
mainly ecotypes of species in the genera Arachis, Desmodium, Stylosanthes, and Paspalum. From these introductions the cultivars Oxley
Fine-stem stylo and Hartley plicatulum (Paspalum plicatulum) (Bryan
and Shaw, 1964) have been selected.
Another ten important overseas collections of pasture plants have
been made by Australians in tropical monsoonal and humid subtropical
areas during the period 1952-1968 (Hutton, 1970). A range of material
was collected, particularly in the legume genera Centrosema, Desmodium,
Glycine, Phaseolus, and Stylosanthes, and the grass genera Cenchrus,
Panicum, Paspalum, Setaria, and Urochloa. Only the introductions from
J. F. Miles' visits to South Africa and east and west Africa in 1952 have
been fully evaluated. These have yielded Miles Lotononis (L. bainesii)
(Bryan, 1961), Rongai Dolichos lablab (W,ilsonand Murtagh, 1962), and
Samford Rhodes grass. R. J. Jones' collections ( 1 964) of Setaria sphacelata from East Africa have already produced the frost-tolerant cultivar Narok setaria, and it is anticipated that further promising lines will
come from these. The systematic exploration in 1965 of legumes and a
few of the grasses by Williams ( 1966) in the main states of Brazil, and in
Bolivia, Paraguay, and northern Argentina, has substantiated that these
areas are rich in indigenous species potentially valuable as tropical and
subtropical pasture plants. Williams found annual types of stylo similar to
Townsville stylo in a number of regions.


E. M. HUTTON

8

Due to the progress made on plant exploration and introduction for
the tropics, it is now more difficult to find native legumes and grasses
which are superior to existing pasture cultivars. This is no reason to curtail this activity, as only a fraction of the almost unlimited variation in the
indigenous flora of countries like South and Central America and Africa
has been investigated. With the advances in knowledge of the feeding
value of pastures relative to species and management, variants of present
cultivars or even new species could be required and may well be found
among the native plants of these and other countries. In any case, the
pasture plant breeder needs a continual flow of new genetic material
which can be obtained only through plant exploration and introduction.
IV.

Tropical Legumes

Australian research centered in Queensland is now in the forefront
on the introduction, selection, and development of legumes for tropical
pastures. This has resulted from the realization that legume-based pasture
is the most economical method for the development of the cattle industry
in the vast unused coastal and subcoastal areas of northern Australia
(J. G. Davies and Eyles, 1965). Maintenance of around 40% of a phosphate-responsive legume in a tropical pasture is the cheapest way to
provide nitrogen for the pasture and grazing animal (Hutton, 1968b).
In this section the origin and agronomic features of the principal tropical legumes commercialized in Australia will be discussed. For their
detailed descriptions, see Barnard (1967). Some, like the drought-resistant Townsville stylo and siratro (Phaseolus utropurpureus), are
adapted to a wide range of conditions in northern Australia, whereas
those including centro, glycine, the desmodiums, and Miles lotononis, are
less drought tolerant and more restricted in their adaptation. With the
exception of Miles lotononis, aboveground growth of all the tropicals is
killed by frost, which is a constant feature of the subtropics in winter.
However, the perennial crown and root systems survive and regenerate
unless subjected to intense and repeated frosting. As larger areas of the

different tropical legumes are established, they will become hosts to
various diseases and pests which could affect persistence of some cultivars. Fortunately the main legumes do not appear to be affected by rootknot nematodes, and siratro is highly field resistant (Hutton and Beall,
1957). The root-damaging Amnemus weevil (Amnemus quadrituberculutus) is a serious pest of glycine and the desmodiums on the north
coast of New South Wales, whereas Miles lotononis and lucerne are re-


TROPICAL PASTURES

9

sistant and siratro is seldom attacked severely (Braithwaite, 1967; Mears,
1967). Other native weevils have damaged a number of the legumes in
north Queensland. The bean fly (Melanagromyzaphaseoli) can seriously
damage Murray lathyroides (Phaseolus lathyroides) throughout the
season but siratro, affected only in the seedling stage, can be protected
by seed treatment with dieldrin (R. J. Jones, 1965). The viruslike disease
“legume little leaf” due to a mycoplasma (Bowyer et al., 1969) affects a
number of the legumes and under relatively dry conditions markedly reduces stands of the desmodiums and Miles lotononis, and causes some
loss in siratro. In high rainfall areas, varying amounts of defoliation is
caused by Rhizoctonia solani in several legumes, particularly siratro.
Commercial seed production of the different legume cultivars is increasing in Australia and several other countries. In Kenya Desmodium
intortum, silverleaf desmodium ( D . uncinatum), Glycine wightii, Stylosanthes guyanensis, Dolichos lablab, and Trifolium semipilosum are
sold whereas in South Africa and Brazil G . wightii is the one usually
harvested for sale.
A. STYLOS
1. Townsville Stylo (S. humilis)

The history and potential of the annual Townsville stylo is summarized by Humphreys ( 1967), who noted that its natural spread is confined to the north of Western Australia, the Northern Territory, and
Queensland. It has thin, fibrous stems and narrow elongated and pointed
leaves, and forms a dense stand under favorable conditions. Flowers are

yellow and inconspicuous and arranged in a short compressed spike. The
brown pods are hooked, have two segments, but usually contain only
one true seed. The hooked pods cluster into small balls and comprise the
commercial seed, the yields of which range from 400 to 700 Ib per acre.
As a result of D. F. Cameron’s work (1965) with naturalized ecotypes,
three vigorous upright cultivars, Gordon (late), Lawson (midseason), and
Paterson (early black seeded) are being commercialized.
Townsville stylo (Fig. 1) is grown widely in Australia, particularly from
latitudes 1 1“ S to 24”sand where the annual rainfall is between 25 and 7 0
inches. It flourishes on poor sandy soils but does not establish readily on
deep cracking clays and in waterlogged areas. Seed is sown at 3-4 lb per
acre in conjunction with 1 cwt superphosphate per acre by aerial or
ground methods into grazed open woodlands or cleared and cultivated
areas. It is susceptible to shading from vigorous associate grasses, so it


10

E. M. HUTTON

FIG. I . Townsville stylo, Narayen Research Station, near Mundubbera, Queensland.