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XIII. Phosphorus and Nitrogen Fertilization of Grass

XIII. Phosphorus and Nitrogen Fertilization of Grass

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the requirement for this element. He found that if the phosphorus content

of kikuyu and Rhodesgrass exceeded 0.33% there was no response to

added phosphate, but if it was less that 0.23% a significant response was

obtained. In Trinidad Ahmad et al. (1969a) found that phosphorus content

of pangolagrass tended to increase with age and that 0.26% in the grass

did not limit growth.

Andrew and Robins (1970) investigated the effect of phosphorus on

growth and chemical composition of a number of tropical grasses; among

these, molassesgrass was the most responsive and kikuyu the least responsive. Critical percentages of phosphorus in the tops of molassesgrass,

Gayndah buffel, common paspalum, green panic, pioneer Rhodes, S .

almum, Nandi setaria, pangola, and kikuyu were 0.18, 0.26, 0.25, 0.19,

0.23, 0.20, 0.22, 0.16, and 0.22, respectively. Apart from increasing

phosphorus concentration in the tops, phosphate application decreased

nitrogen and potassium, did not affect calcium, increased magnesium in

most species, and increased sodium in four grasses. Pangola had the

lowest phosphorus and nitrogen concentrations, and Nandi setaria the

highest. Rhodesgrass, green panic, and pangola had high sodium and

Gayndah buffel was intermediate; these species had relatively low potassium and magnesium. Nandi setaria had high potassium and low sodium,

and molassesgrass, S . almum, and kikuyu had high magnesium, low

sodium, and intermediate potassium. S . almum and common paspalum

were relatively high in calcium, and pangola was low.


Most tropical grasses have a capacity for high photosynthetic rates

(Ludlow and Wilson, 1968), and high dry matter production of 10,000

Ib or more per acre in response to nitrogen fertilization is usual in the

humid subtropics and tropics (vide Prine and Burton, 1956; Romney,

1961 ; Vicente-Chandler et al., 196 1 ; Oakes and Skov, 1962; Ahmad et

al., 1969b). In southeastern Queensland Henzell(l963) found that tropical grasses (Rhodes, paspalums, S . almum) given adequate superphosphate and potash yielded only 1000-5000 dry matter per acre per year,

but up to 20,000 Ib per acre in wet years when as much as 400 lb of nitrogen was also applied. Nitrogen content of the grasses was relatively

low unless nitrogen was applied in excess of the requirements for maximum growth. The best nitrogen recoveries in plant tops were 40-50%

obtained at the higher nitrogen applications. It was considered that efficiency of dry matter production relative to applied nitrogen and recovery

of nitrogen in the tops could both be improved. Henzell and Stirk ( 1 963)



concluded that nitrogen rather than soil moisture is the primary limiting

factor to grass growth under natural rainfall in southeastern Queensland.

The relatively low nitrogen content of tropical grasses compared with

temperate is thought by Henzell and Oxenham (1964) to be due in part

to a greater degree of nitrogen deficiency in warm climates.

Pangolagrass, because of its adaptability and productivity, is being

used increasingly throughout the humid tropics and subtropics (Nestel

and Creek, 1962). In the Wallum of southeastern Queensland, Bryan and

Sharpe ( 1965) applied increasing rates of nitrogen as urea to pangola and

obtained maximum annual yields of dry matter of 2 1,000 Ib per acre and

of nitrogen of 200 Ib. The mean yield of nitrogen in plant tops was 45% of

that applied, and the nitrogen content was generally low. In Guadeloupe,

with dry season irrigation and 1735 lb of nitrogen per acre, Salette (1 966)

produced in a year 35,159 Ib of dry matter per acre from pangola, the

highest yield recorded for this grass.

The uptake of nitrogen by Rhodesgrass was followed with labeled

nitrogen fertilizers by Martin et al. ( I 963), Henzell et al. ( I 964, 1968),

and Vallis et al. (1967). A mean of 93.6% of added total nitrogen and

94.0% of added isotopic nitrogen was recovered from a soil-Rhodesgrass system. Nitrogen uptake by the grass was a linear function of the

quantity of labeled ammonium nitrate applied for rates up to the equivalent of 400 Ib of nitrogen per acre, but the proportion of fertilizer nitrogen

recovered in the plants fell significantly when 800 lb of nitrogen per acre

was used. Fertilizer nitrogen was distributed between tops and roots in

the ratio of 5.2 : 1 for total nitrogen and 4.5 : 1 for isotopic nitrogen. Rhodes

grass when grown in association with tropical legumes took up considerably more labeled nitrogen than the legumes.

I t is apparent that tropical grasses give a high dry matter response to

fertilizer nitrogen, but there are problems in uptake and recovery of applied nitrogen as well as in the maintenance of a sufficiently high nitrogen

content. Also, high yields of dry matter are of limited value if they cannot

be efficiently converted into high yields of animal products. Factors involved in feeding value of tropical grasses given adequate N , P, and K

were studied in central Brazil by Gomide et al. (1969b). They found that

mean dry matter percentage of molasses, pangola, napier, kikuyu, bermuda, and guinea grasses increased from 2 I .2 to 5 1.4%with time. Napier

had the lowest dry matter percentage, and bermuda the highest. Nitrogen

decreased dry matter percentage in the early stages, but not later. Cellulose percentage of molasses, napier, and guinea increased with age while

that of the other three grasses remained almost constant. Crude fiber



percentage increased with age, and in vitro cellulose digestibility decreased. The overall effect of nitrogen fertilization was to decrease in

vitro digestibility except in the young growth.

Grasses which give high yields of digestible nutrients with nitrogen

fertilization are required. Two of the most promising are pangola and

kikuyu, shown by Milford and Minson ( 1 966b) to retain a relatively high

digestibility with age. Kikuyu has the added advantage of maintaining

high crude protein with age (Milford and Haydock, 1965). Nitrogenfertilized signalgrass ( B . decumbens) is giving high animal production

in the Queensland wet tropics (Grof, 1969), so it is another promising

species for a nitrogen system.

A number of experiments on animal production from nitrogen-fertilized grass have been reported, and only a few of these will be quoted.

Suman et al. (1962) in the southeastern United States measured beef

liveweight gains on coastal bermuda, common bermuda, and Pensacola

bahia grasses fertilized annually with 100, 200, and 400 Ib of nitrogen

per acre; they obtained the highest gains from coastal bermuda. During

three years, with 400 Ib of nitrogen coastal bermuda produced a mean of

91 3 to 938 Ib liveweight gain per acre per annum. Over four years in the

humid tropics of Puerto Rico, Caro-Costas et al. (1965) grazed young

beef steers on separate pastures of five grasses which annually received

280 Ib of nitrogen per acre and also phosphate and potash. Pangola, guinea, and napier grasses produced similar yields averaging 1058 Ib liveweight gain and 7559 Ib total digestible nutrients per acre per annum at

2.5 steers per acre. Para and molasses grasses were inferior and produced

an average of only 636 Ib liveweight gain and 5030 Ib total digestible nutrients per acre per annum at 1.6 steers per acre. On the Wallum of southeastern Queensland, T. R. Evans ( 1 969a) obtained, over two years, mean

liveweight gains of 1 139 Ib and 12 I5 Ib per acre per annum from pangolagrass fertilized annually with 400 Ib and 800 Ib of nitrogen per acre, respectively, and grazed at a mean of three beasts per acre. Using 400 Ib

of nitrogen per acre per annum, T. R. Evans ( 1969b) has improved these

results markedly by attention to timing of the split applications of nitrogen. In north Queensland at the Parada Research Station of the Queensland Department of Primary Industries, annual beef liveweight gain

averaged 1800 Ib per acre in the last three years on irrigated and nitrogen

fertilized (300 Ib N per acre per annum) pangolagrass stocked at a mean

of three beasts per acre (J. Evans, 1967). Both the Wallum and Parada

experiments also received adequate annual amounts of superphosphate

and potash.

At Wollongbar Research Station in the subtropics of northern New



South Wales, Holder ( I 967) reported that fertilizing kikuyugrass with

300 lb of nitrogen per acre per annum increased both stocking rate and

butterfat production per acre per annum two to three times. In the humid

tropics at Turrialba, Costa Rica, Blydenstein et al. ( 1969) obtained over

a year a mean milk yield of 5367 Ib per acre from pangolagrass fertilized

with 205 lb of nitrogen together with phosphate and potash. Efficiency

of dry matter conversion in this experiment was about 12%.

In Jamaica, Nestel and Creek (1966) have shown that intensive beef

production or dairying on nitrogen-fertilized pangolagrass gives a good

return on money invested provided attention is given to stocking rate and

effective managerial practices. Henzell ( 1 968) has outlined the problems

associated with use of nitrogen fertilizers on grass. These include requirements for other nutrients such as superphosphate and potash, loss of

nitrogen to the air, acidifying effects, management needed, and the relatively low feeding value of the bulk of grass produced. This last problem is

the most important in animal production. Although nitrogen fertilization

markedly increases dry matter yield of tropical grasses, all the evidence

indicates that except in the young growth stage it does not increase concentration of digestible nutrients and efficiency of conversion into animal


XIV. Breeding and Genetics of Tropical Grasses

A major aim in breeding tropical grasses is to produce varieties that

do not decline rapidly in feeding value as the plants mature. In vitro digestibility (Minson and Milford, 1967b) and crude protein content could

be used to select from large numbers of samples the few with potentially

higher feeding value. These selections could then be grown in larger

plots where their response to nitrogen fertilization could be measured and

their feeding value determined from a bulk of dried material fed to sheep

(Minson and Milford, 1968). The final criterion for selection is the animal

production obtained from the one or two superior selections grown in a

pasture either associated with a standard legume or fertilized with nitrogen.

Compatibility with a legume is essential in most grasses, as for many

years legume-based pastures will be the main source of cattle feed in a

majority of the areas now being developed in the Tropics. It may be necessary to breed slower growing grasses which do not compete strongly

with legumes at the peak of the season to achieve a balanced pasture.

Standard breeding programs can be adopted with cross-pollinating

grasses, which include setaria, S. almum, Rhodes, P. coloratum, and spe-



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.


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

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XIII. Phosphorus and Nitrogen Fertilization of Grass

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