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VIII. Undesirable Compounds in Tropical Legumes

VIII. Undesirable Compounds in Tropical Legumes

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34



E. M . HUTTON



and concentrates for several years at the Hawaiian Agricultural Experiment Station (Anonymous, 1948). Mimosine is an undesirable depressant

of cell division (Hegarty et al., 1964a) and accounts for about 0.5% of

the N of leucaena herbage. It has been known for some time that feeding

leucaena affects reproduction in monogastric animals like rabbits (Willet et d., 1947)and sows (Wayman and Iwanaga, 1957).

Hegarty et al. ( 1 964b) developed methods for the extraction and determination of mimosine present in leucaena leaves and urine. This enabled

them to study the reaction of sheep to consumption of leucaena and mimosine. Sheep shed their fleeces on a sole diet of leucaena because the mimosine in it suppressed mitotic activity in the follicle bulb of the growing

wool fiber and caused follicle degeneration. Follicle regeneration occurred

when sheep were taken off leucaena but occurred also in animals on a

continuing leucaena diet. During leucaena feeding, only small quantities

of mimosine were excreted, most being degraded by rumen flora to 3,4dihydroxypyridine, the main urine component. Sheep cannot detoxicate

mimosine after absorption beyond the rumen. It was found that sheep

could be conditioned to a sole diet of leucaena without ill effect due to

increased detoxication from adaptation of rumen microorganisms.

In Queensland continuous grazing of leucaena pastures for extended

periods has adversely affected steers. Symptoms include shedding of

hair on the rump and tail and loss of weight indicating incomplete breakdown of mimosine in the rumen. Hamilton er al. (1 970) made a close

study of reproduction in dairy heifers fed a complete diet of this legume.

Leucaena did not affect estrus cycle length, conception rate, gestation

length, calving rate, milk production or composition. However, mild

incoordination occurred briefly during gestation in some cows and birth

weight of calves from cows fed leucaena was lower than of control calves.

There was no residual effect of leucaena on calf growth rate, as the resulting calves grew at the same rate as the controls.

C. INDOSPICINE

Indigofera spicata (syn. I . endecaphylla) is found in a number of areas

including India, Ceylon, Indonesia, Philippines, Hawaii, Central America,

Brazil, and west Africa, and is regarded as a promising pasture legume because of its vigorous, prostrate, stoloniferous habit and high level of N

fixation (Henzell, 1962). Trials in the early 1950’s with several introductions at coastal sites in southeastern Queensland confirmed the potential of this legume. However, its widespread use was prevented by the

work of Emmel and Ritchey (1941) and Nordfeldt et al. (1952), who fed



TROPICAL PASTURES



35



it to rabbits, cows, and sheep, and found that it caused liver degeneration

and that pregnant animals aborted. In studies of Nordfeldt et al. (1 952),

the guinea pig was less susceptible, and Freyre and Warmke (1952)

showed that guinea pigs survived indefinitely on I. spicata but pregnant

females aborted. All these findings indicated the presence of an unidentified hepatotoxin in 1. spicata.

The chick test of Rosenberg and Zoebisch (1952) for investigating

toxicity of forage legumes was used by Morris et al. (1 954) to study I.

spicatu, in which they identified hiptagenic acid (3-nitropropionic acid)

which was considered to be the toxin. Cooke (1 955) also supported thi;

view. Britten et al. (1959a) using the chick test concluded that 3-nitropropionic acid was probably the sole toxic agent in I . spicatu. A high

correlation between the chick test and a chemical test for 3-nitropropionic

acid was found by Britten el al. (1959b), who recorded differences in

toxicity between I. spicata plants. Later Britten et al. (1963) showed a

positive correlation between the amount of 3-nitropropionic acid in the

ration and toxicity to chicks.

Hutton et al. (1958a) obtained a similar type of liver damage in rabbits whether green leaf, dried leaf, or seed of I . spicata was fed. 3-Nitropropionic acid did not appear to be the hepatotoxin involved, as it was

not present in the seed and force-feeding the pure compound did not produce liver damage in rabbits. Further studies with mice (Hutton et d.,

1958b) indicated that 3-nitropropionic acid was not the hepatotoxin in

1. spicatu, so preliminary work was commenced to isolate the compound

implicated (Coleman et ul., 1960). Hegarty and Pound ( 1 968) finally

reported the isolation from I. spicata of the first naturally occuring hepatotoxic amino acid, which they named indospicine. It is ~-2-amino-6amidinohexanoic acid, and when injected subcutaneously into mice it

produces fat accumulation and cytological changes in the liver (Hegarty

and Pound, 1970). Fat accumulation was inhibited by simultaneous injection of arginine but not by canavanine, so indospicine may produce

its hepatotoxic effects by interference with arginine metabolism. This

could explain the results obtained by previous workers with chicks which

are uricotelic and may have a different metabolic pathway from mammals. Hegarty and Pound (1970) found that a substantial part of the hepatotoxicity of extracts of I. spicatu seed was accounted for in terms of

indospicine.

Now that the hepatotoxin in I. spicata has been identified, breeding

a nontoxic line of this valuable legume is possible. Perhaps mutagenic

techniques may be the most appropriate to achieve this result.



36



E. M. HUTTON



D. TANNIN

The desmodiums have a high tannin content (Rotar, 1965) and are the

only important tropical legumes containing this chemical complex (Hutton and Coote, 1966). Whether tannin is an undesirable component in

them has yet to be determined. No doubt their tannin content would

preclude them causing bloat. However, the tannin may reduce their digestibility, as the mean in v i m digestibility of 30 bred lines of D . intorturn

was 53.1% whereas that of the same number of P . atropurpureus lines

was 64.9%. R. J. Jones (1969) found a similar difference in digestibility

between these species.

IX. Physiology of Tropical legumes,



All the legumes need to be characterized physiologically to quantify

their growth potential for any particular environment in the subtropics

and tropics. This involves studies on the effects of energy input, photoperiod, temperature, and moisture and the interrelationships between

these on growth, maturation, and seed production of the different legumes.

Interactions between these environmental factors and nodulation and application of essential mineral elements should also be studied because of

their fundamental importance in the pasture environment. Proper physiological characterization of the legumes will not only explain their adaptation to the various conditions, but indicate how they can be improved and

how they can be used more profitably in the pasture system. Also it should

be possible to correlate data from controlled environment facilities with

climatic data and so predict the field behavior of a legume in any particular region.

Research in controlled environments needs to be linked with studies

on the physiological reactions of both tropical legumes and grasses in

pastures under grazing. In this way the factors involved in legume-grass

competition, dry matter and protein production, and persistence could be

elucidated so that ways of improving output from the pasture ecosystem

could be devised. Research on physiological parameters of tropical pasture plants in laboratory and field has barely commenced and needs to be

intensified.

A. TEMPERATURE

A N D GROWTH

I N SEVERAL

TROPICAL

LEGUMES

Whiteman (1968) studied the effects of temperature on growth in a long

day (16 hours) of the six tropical legumes Murray lathyroides, siratro,

silverleaf and greenleaf desmodiums, D . sandwicense, and Tinaroo glycine. At the first harvest (14 days) seedling dry weight (stems, leaves)



TROPICAL PASTURES



37



was highly correlated with mean seed weight of each species, siratro

having the greatest seed and seedling weights and greenleaf desmodium

the smallest. At the second harvest, the real differences between the legumes in growth rate were expressed. Growth was abnormal and reduced

markedly at the lowest temperatures 15110°C and 18/ 13°C (daylnight).

Optimum temperature for growth of all the legumes was 30/25 2 3°C

which is lower than for tropical grasses and higher than for temperate

legumes and grasses. Above 33/28"C growth rate declined, particularly

in the Desmodium species, but not so markedly in siratro.



B. TOWNSVILLE

STYLO

In experiments with Townsville stylo, 't Mannetje (1965) extended the

photoperiod of 8 hours sun with incandescent light. He found that it was

a short-day plant in temperatures of 30°C (day) and 25°C (night) and that

dry matter yields in 12- and 14-hour photoperiods were greater than those

in 8- and 10-hour ones. Sweeney's results, quoted by Humphreys (1967),

showed optimum dry matter production at 33/28"C. In a study of the flowering behavior of seven selections (early to late) of Townsville stylo, D.

F. Cameron ( 1967a) showed that day length was the main factor controlling flowering and that they all had a strong short-day response. At normal temperatures, maximum day lengths (critical day lengths) at which

all plants flowered were 13 hours for the early selections, 12 hours for

the midseason and late midseason, and 1 1.5 hours for the late. Both high

night temperature and low day temperature delayed or inhibited flower

initiation in the early and midseason selections, and these effects were

greater at the critical day length.

D. F. Cameron's field and shadehouse experiments (1967b) with different sowing dates and locations gave similar results to his controlled

environment studies. In the early December sowing, the range in flowering time between maturity groups was 56 days because the longer day

lengths promoted flowering in early types and prevented floral initiation in

late types. With the late March sowing, day length was short enough to

promote flowering in all maturity types so the range in flowering time was

only 8 days. At the southerly locations, most selections flowered later

because the longer day lengths delayed flowering time.

C. Glycine wightii

Edye and Kiers observed ( 1966) variation in maturity, stolon development and frost resistance in 50 accessions ofglycine at Lawes, southeastern

Queensland. The discontinuous variation in flowering enabled definition



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VIII. Undesirable Compounds in Tropical Legumes

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