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XIV. Breeding and Genetics of Tropical Grasses

XIV. Breeding and Genetics of Tropical Grasses

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58



E. M. HUTTON



cies of Digitaria. Setaria is almost entirely cross-pollinated (Gildenhuys,

1960), which facilitates hybridization, and although S. almum is selfcompatible, hot water can be used to emasculate whole inflorescences

(Pritchard, 1965a). A number of the important tropical grasses are apomictic. These include buffel (Snyder et al., 1955), guinea (Warmke, 1954),

green panic, molasses, paspalums (Bashaw and Holt, 1958), and species

in the genera Brachiaria (Pritchard, 1967), and Urochloa. In obligate

apomicts, the only variation available for selection is that which exists

between the accessions collected from different ecological niches. Most

produce some functional pollen, so crosses are possible if sexual forms of

the apomicts can be found. Burton and Forbes (1960) overcame the

apomictic barrier in P . notatum by crossing the common apomictic tetraploid and fertile induced tetraploids from the sexual diploid Pensacola

bahiagrass. The search for a sexual type in other important apomicts has

been successful only in buffelgrass (Bashaw, 1962), and the resultant

crosses have released considerable variation and have shown apomixis

to be recessive to sexuality.

A. SETARIA

The main objectives of the breeding work with setaria at the Cunningham Laboratory are to produce cultivars with frost resistance, high feeding value, low oxalic acid content, and an extended growing season. The

diploid Nandi (2n = 18) and tetraploid Kazungula ( 2 n = 36) belong to the

S . sphacelata complex, which also contains pentaploid, hexaploid, octoploid, and decaploid races (Hacker, 1966). Crosses have been obtained by

Hacker ( 1967) between all proximate ploidy levels except diploid and

tetraploid, and also between high and low levels. Thus for seed production, lines of setaria should be isolated from each other. Hacker (1968a)

has cast doubt on the validity of the separation of species in the S.sphacelata complex as he has been able to hybridize diploid forms of S . anceps and S . trinervia, and S.anceps and S . splendida, and hexaploid lines

of all three species. From Hacker's work (1 968b) it appears that the S .

sphacelata complex forms an autopolyploid series.

B. Sorghum almum

The aim is to breed lines of S . almum with higher yield and persistence

than the Australian cultivar Crooble and possessing juicy stems, distinctive brown glumes, late flowering, and tolerance to leaf diseases. Pritchard ( 1965a) crossed S . almum and perennial sweet sudangrass (Hoveland, 1960) and found that juicy stem and brown glume and plant color of

the latter were linked and mainly tetrasomically inherited. Selection was



TROPICAL PASTURES



59



facilitated by an association between translucent midrib, juicy stem, and

high soluble carbohydrate, and some of the advanced lines have a 20%

higher soluble carbohydrate in the stem than Crooble.

Using a tetrasomically inherited albino seedling character in S . almum,

Pritchard ( 1965b) studied natural crossing in S . almum and between S .

almum and the weed S . halepense and concluded that there was a degree

of genetic isolation between these species. From cytological examination

of aneuploid plants of S . almum and S . halepense I’ritchard (196%) suggested that these species were autotetraploids. Pritchard ( 1965d) crossed

diploid Sorghum with S . almum and obtained tetraploids and triploids.

Segregation in tetraploid progeny resulted in tramfer of certain characters from the diploid to tetraploid level. Slight fertility of the triploids also

enabled transfer of characters from the diploid to tetraploid (or near tetraploid) by backcrossing and selfing. The triploids could also be used to

transfer such characters as perenniality from tetraploid to annual diploid

sorghums.

C. COASTAL

BERMUDAGRASS

Burton’s notable development ( 1 947, 1954) of (coastal bermudagrass

has been followed by further work aimed at improving its agronomic

characters and feeding value. Dry matter digestibility of the many genotypes was determined by Burton er al. ( 1 967), who found that quality of

a number of clones decreased as age of forage increased and that genotype

X age interactions were not significant. A coastal >: Kenya 56 F1 hybrid

averaged 12.3% more digestible dry matter than coastal over a four-year

period. Several Midland X Kenya 6 1 hybrids had higher yields and better

digestibility than either parent. This work has indicated that the quality

of C . ductylon may be improved by breeding.



D. BUFFELGRASS

At the Cunningham Laboratory, Pritchard (196’7) crossed the sexual

buffelgrass from Texas (Bashaw, 1962) with the main apomictic cultivars

and obtained more variation than has been assembled in over thirty years

of introduction. A number of promising leafy lines which flower later than

the parents and have greater cold tolerance have been selected. The in

vitro digestibility at maturity of some of these is siiperior to that of the

parents, so it may be possible to improve the feeding value of buffelgrass.

Pritchard (1 967) found that the original sexual plant has a chromosome

number of 2n =36 and that selfed progeny numbers range from 2n = 35 to

2n = 38. Chromosome numbers of the apomictic cultivars are 2n = 36 for

Molopo and Lawes and 2n = 43 for Tarewinnabar, Nunbank, and Biloela.



60



E. M. HUTTON



Progeny of crosses between these apomicts and the sexual type have

chromosome numbers from 2n = 34 to 2n = 45.

The examples given indicate that agronomic characters and feeding

value of tropical grasses can be improved by breeding. Of the grasses not

mentioned in this context, pangola and kikuyu merit attention although

they pose special difficulties for the breeder. There is a need to increase

their adaptability and it is possible that even their relatively high feeding

value and response to nitrogen could be improved. In future it is hoped,

as more information becomes available on the physiological and biochemical characters which control growth and adaptation of grasses, that

more precise selection in breeding populations will be possible.

Xv. Beef Production from Legume-Based Tropical Pastures



The degree to which tropical pasture research and development is

successful can be measured only in terms of animal production and its

profitability. Productivity of nitrogen-fertilized grass systems has already

been dealt with, and in this section animal production from legume-based

pastures is discussed. Because of their flexibility and cheap production of

N for pasture and grazing animal, legume-based pastures will continue to

predominate in development of tropical areas. Tropical legumes are not

able to produce enough N for the associate grasses to attain their potential dry matter production, but does this matter? Feeding value of the

pasture is the main determinant of animal production, so a higher proportion of legume and smaller bulk of less digestible grass is an advantage.

In Norman and Stewart’s experiments (1 964) (see Table 2) at Katherine

in the Northern Territory, liveweight gain of cattle was directly related

to the proportion of Townsville stylo in the pasture.

TABLE I1

Dry Season Performance of Cattle on Sown Pastures with

Varying Proportions of Grass and Legume

Composition of pasture

(%)

Birdwood

grass



Annual

grasses



Townsville

stylo



Nitrogen content

of pasture at

start of grazing

(%)



51.5

9.9

-



25.1



45.4

31.4



22.8

44.7

62.6



0.75

1.12

1.34



Liveweight

gain

(Ib/head)

20

99

196



Period

of

gain

(weeks)



8

20



22



TROPICAL PASTURES



61



In general, progress in tropical beef production depends on persistent

and adapted legumes, regular application of superph,osphate (giving both

P and S), and use of adapted tropical cattle with tick. resistance and heat

tolerance (Schleger and Turner, 1965). Significant advances have been

made despite Whyte’s pessimistic conclusions ( 1962) concerning improvement of tropical grasslands. The following examples from areas in

the main tropical climates will make this clear.

A. WET TROPICS

Younge et al. (1964) estimated that about one-quarter of the Hawaiian

rangelands are unproductive low wetlands but capable of trebling the

current annual beef production. On the island of h4aui, pangolagrassD . intortum pastures fertilized once per acre with lime at about 3000 Ib

and a starting fertilizer comprising 44 Ib N, 84 P, 104 K, 3.5 B, and 2.5

Mo gave a mean over two years of 764 Ib liveweight gain per acre per

annum at a stocking rate of about two beasts an acre, which was highly

profitable. Also on Maui, pangola, dallis, kikuyu, and native grasses each

mixed with kaimi clover and given one dressing per acre of 6 tons of

calcium carbonate and starting fertilizer as in the previous experiment

produced over four years annual liveweight gains per acre of 720, 630,

575, and 524 Ib, respectively. At the Kauai Branch !station on an aluminous-ferruginous latosol, Younge and Plucknett (1966) with a pangolagrass-D. intortum pasture given an application of basic fertilizers and

four rates of P produced as high as I164 Ib liveweight gain per acre per

annum with yearling steers. Mean stocking rate varied from 1.18 beasts

an acre at the lowest P level to 2.38 an acre at the highest. There was a

curvilinear response to P, but the highest liveweighi: gain was obtained

from the heaviest P application. Unimproved pastures produce about 30

Ib liveweight gain per acre per annum.



B. MONSOONAL

TROPICS

Over two dry seasons at Katherine in the Northern Territory, liveweight of steers at a beast to 2 acres of Cenchrus-Townsville stylo pasture increased and that of steers at a beast to 17 acres on native pasture

declined substantially (Norman and Arndt, 1959). Shiiw (196 I ) at Rodd’s

Bay, Queensland, showed that year-round productivity of native speargrass pasture could be increased markedly by oversowing with Townsville stylo and topdressing annually with 1 cwt molybdenized superphosphate an acre. Carrying capacity of native pasture was trebled from a

steer to 9 acres to a steer to 3 acres, annual liveweight gain per acre was



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