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XII. Feeding Value of Grasses versus Legumes

XII. Feeding Value of Grasses versus Legumes

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TROPICAL PASTURES



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maturity. Intake of 30-day-old growth of tropical grasses was relatively

high, whereas that of 150-day-old mature growth fell off markedly except

in a cultivar like Callide Rhodesgrass. By contrast, intake of Cooper

glycine and siratro remained high to maturity. Milford (1960, 1967)

found that intakes of digestible dry matter of 17 tropical grasses were

lower than those of seven tropical legumes and lucerne, the difference

being greatest in autumn and winter.

Dry matter digestibility of tropical grasses is generally lower than that

of temperate grasses. Minson and McLeod (1 970) found a high negative

correlation ( r = - 0.76) between dry matter digestibility and temperature

in both tropical and temperate grasses. They concluded that the difference in digestibility between tropical and temperate grasses is mainly

due to the conditions under which each is normally grown, the former

in warm to hot and the latter in cool. Legumes did not behave in the same

way since respective digestibilities of the temperate white clover and tropical siratro were similar in both summer and winter. Milford and Minson

( I 966b) showed that decline in digestibility with age was rapid in tropical

grasses compared with tropical legumes which retain relatively high digestibility at maturity and even after frosting (Milford, 1967). Cowpea

and Rongai lablab are examples of annual legumes which maintained a

high feeding value (digestibility and intake) with age but cowpea was

superior at the first harvest (Milford and Minson, 1968a) and both had

very high voluntary intakes compared with Rhodesgrass of the same

digestibility (Milford and Minson, 1968b). The fact that feeding value

of tropical legumes is superior to that of tropical grasses at all but the

earliest growth stages has important implications in the development and

management of legume-based tropical pastures.

Among a range of tropical grasses in Trinidad, Butterworth ( I 964)

found that pangola, jaragua, and Kazungula setaria had the highest digestible energy contents. In a study of pangolagrass, S.almum, and siratro, Minson and Milford ( I 966) found that age was the most important

factor determining the digestible energy content. Rate of fall in energy

digestibility percentage with maturity was least in siratro. Also siratro

gave a higher daily intake of digestible energy than the two grasses. These

results indicate that the relatively rapid decline in feeding value of tropical

grasses with age is due to increasing shortage of digestible energy from a

rapid buildup in crude fiber.

Milford and Minson (1966a) consider that crude protein content does

not limit intake of tropical grasses until it falls to 7%. Young growth of

tropical grasses usually provides enough protein for the animal, but Milford and Haydock ( I 965) showed that content and digestibility of crude



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protein declined more rapidly with age in tropical grasses than legumes

like siratro. Among the grasses the decline was least in kikuyu and greatest in buffel and pangola. Thus, although feeding value of pangolagrass is

high, animal intake near maturity can be limited by a deficiency of crude

protein. Minson (1967) showed that intake of pangola can be increased

54% when crude protein content is raised from 3.7 to 7.2% by urea application a month before harvesting. Voluntary intake of mature pangola

was also increased (Minson and Milford, 1967a) by including 10-20%

lucerne or white clover in the diet.

Playne (1969a,b) studied the effect of supplements on intake and digestibility of speargrass and Townsville stylo. Control sheep fed Townsville stylo gained weight, and those fed speargrass and urea lost weight.

Dicalcium phosphate supplement increased intake and liveweight of

sheep fed Townsville stylo, but there was no response in those fed speargrass and urea. Supplementing sheep with sodium sulfate, gluten, or

sodium sulfate plus gluten markedly increased intake and dry matter

digestibility of hay comprising four parts speargrass, one part Townsville

stylo, and resulted in substantial liveweight gains compared with losses

in the control sheep. Thus the phosphorus, sulfur, and sodium contents

of tropical forages appear to significantly affect their feeding value.

Several important points emerge from the research on feeding value

of tropical species. The higher intake of digestible energy and protein

of tropical legumes at all but the early growth stages makes it evident

that legume-based pastures should be managed to contain a high proportion of legume. Also to further increase the feeding value of tropical pastures, every effort should be made to select grasses with a relatively high

intake and crude protein content at maturity. This will not be easily

achieved as illustrated by the quite small differences in feeding value

obtained by Milford and Minson (1968b) between a number of Rhodesgrasses. It is now apparent that the physiological and chemical factors

associated with the relatively rapid increase in structural carbohydrates

in tropical grasses with time need intense study. Most of the work on

intake and feeding value has been done in indoor feeding experiments

with dried and chaffed material, and there could well be important differences in intake between housed and grazing animals, as shown by

T. B. Miller’s results (1969). Selective grazing occurs in the field and

species in the green state could have a different palatability and intake

fed dry although Minson (1 966) found no difference in intake between

fresh and dried material of three tropical grasses. Indoor trials have allowed rapid progress to be made in knowledge of the feeding value of

tropical pasture species, but the stage has been reached when intake of

the grazing animal animal needs investigation.



TROPICAL PASTURES



53



XIII. Phosphorus and Nitrogen Fertilization of Grass



Relatively cheap nitrogenous fertilizers are now becoming more generally available. In the subtropics and tropics it does not appear that they

will be used to any extent except in favorable situations with an extended

growing season and proximity to profitable markets. Regular and heavy

application of nitrogen to grass will not achieve high animal production

unless adequate amounts of other essential nutrients, such as phosphorus,

sulfur, potassium, and calcium, are also supplied.

Dry matter responses of tropical grasses to nitrogen have received

much attention. However, more studies of the interaction of nitrogen and

other elements on composition and yield are needed to define mineral

requirements of the various grasses. Burton et al. ( 1 969) found that omitting phosphorus and potassium from fertilizer which supplied 600 Ib of

nitrogen per acre to coastal bermudagrass growing on a sandy loam reduced its yield 45% without affecting its protein, carotene, and xanthophyll contents. It appeared that inadequate potassium was responsible

for this response and that applying large quantities of phosphorus and

potassium to coastal bermuda scarcely increases protein, carotene, and

xanthophyll contents. Mineral composition of molasses, pangola, napier,

kikuyu, bermuda, and guinea grasses growing on a sandy loam in central

Brazil was studied by Gomide et al. ( I 969a). Adequate amounts of ammonium sulfate, superphosphate, and potassium chloride were used at

planting, and different levels of nitrogen were applied at regular intervals

subsequently. Nitrogen fertilizer had no effect on any of the minerals

studied except manganese, which increased. With increasing age there

were significant decreases in potassium, phosphorus, magnesium, copper,

and iron. Average potassium content of all grasses at 4 weeks of age was

1.42%: and at 36 weeks, 0.30%. Kikuyu had the highest potassium content at 36 weeks. Average phosphorus content of all grasses was 0.26%

at four weeks, and 0.12% at 36 weeks. Pangolagrass was a poor source

of phosphorus at all ages.

In the rest of this section specific responses of tropical grasses to phosphorus and nitrogen will be discussed with emphasis on the animal production resulting from nitrogen fertilization.

A. PHOSPHORUS



E. W. Russell ( 1 966) investigated the effects of different amounts of

superphosphate on yield and composition of stargrass in Kenya; he found

that yield was not always increased and that phosphorus content increased most in the wet season. Other studies have been made of phosphorus levels in tropical grasses, but Birch (1 953) attempted to define



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E. M. HUTTON



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.



B. NITROGEN

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)



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