6.3 Feeding and fertilization

This section of the manual concentrates on practical feeding in the grow-out stage, and some farm-made feeds for freshwater prawns are described in its tables. The feeds and feeding strategies given apply equally to prawns reared in nursery facilities. Detailed information on the nutritional requirements of this species can be found in D’Abramo and New (2000), and on the digestive system in Ismael and New (2000). Feeding strategies for broodstock and feeds and feeding strategies for the larval stages of freshwater prawns have been discussed earlier in this manual.

Figure 74
Macrobrachium rosenbergii farming can be integrated with crop and other livestock production; in this case prawn culture is associated with rice culture and vegetable production (Viet Nam)

 

SOURCE: MARCY WILDER, REPRODUCED FROM NEW AND VALENTI (2000) WITH PERMISSION FROM BLACKWELL SCIENCE

 

 

It is necessary to maintain an adequate phytoplankton density, to provide cover and control the growth of weeds in freshwater prawn ponds. This is done by encouraging the growth of phytoplankton.
However, it is often unnecessary to fertilize, because this is rapidly achieved by the feeding regime. However, ponds built in a sandy-clay soil may require fertilization for this purpose. Where necessary, 25 kg/ha/month of triple superphosphate (Na3PO4) will keep the water green.
Benthic fauna are very important features in the ecosystem of freshwater prawn ponds, forming part of the food chain for prawns. Fertilisation to encourage the development of benthic fauna is therefore recommended. Animal manures have been used for this purpose (e.g. 1 000-3 000 kg/ha of cattle manure) but the use of animal manure is not encouraged in this manual, for the reasons explained in Box 18. Animal manures can be substituted by other organic materials, such as distillery by-products or other plant resides. The rest of this section of the manual is devoted to the use of feeds.

FEED TYPE

You can get a small production level of freshwater prawns (perhaps 200-300 kg/ha/year, as shown in Box 14, Level 1) by relying on the natural productivity of the ponds. However, successful semi-intensive farming must involve supplementary feeding. Some farms claim to rely on fertilisation, rather than feeding, at the beginning of the rearing period. Some stimulate an initial algal bloom through the addition of an inorganic fertilizer (such as a liquid 0-36-0 formulation, applied to provide about 9 kg/ha of phosphorus). Others find that providing feed from the beginning of the rearing period improves performance and is costeffective.
However, the dividing line between the effectiveness of feed as a direct nutritional input to the prawns and what is acting as a fertilizer is blurred. Whether the feeds are pelleted mixtures or individual ingredients (such as distillery or brewery by-products), they actually act as both feeds and fertilizers. At the beginning their primary use may be as an organic fertilizer that enhances the availability of natural feeds in the rearing ponds.
Later, as the prawns grow, the feeds become more and more directly consumed by the prawns. The application of feeds/fertilizers from the beginning of the rearing period not only increases the availability of natural food but also decreases the transparency of the water, therefore reducing the growth of weeds.
The types of feed used in freshwater prawn farming vary widely and include individual animal or vegetable raw materials and feed mixtures prepared at the pond bank; both of these are generally referred to as ‘farm-made feeds’. In addition, commercial feeds designed for freshwater prawns are available in some countries, sometimes from several aquafeed manufacturers. Freshwater prawns are omnivores and, so far as is known at present, their nutritional requirements are not very demanding. Some farmers utilize commercial feeds designed for marine shrimp in freshwater prawn nurseries or during the first

BOX 22

Examples of integrated freshwater prawn culture in Viet Nam

Example 1:
In Viet Nam, most PL and juveniles for stocking grow-out areas still come from the capture fisheries, where brushwood (Figure 75), stow and straw nets, and shelter traps are commonly used, although hatcheries are beginning to be established.
Ponds and garden (crop production) canals are typically stocked at 4-6 juveniles/m2 and paddy fields at 0.5- 2/m2. The ponds are usually rectangular and small (0.1-0.2 ha), while the rice fields are 0.5-2.0 ha, of which 15 to 20% of the space consists of internal canals. In the paddies, the water level is 1.0-1.2 m in the canals and 0.2-0.6 m over the rice growing area (when flooded).
Both farm-made and commercial feeds are used in the ponds and, intermittently, in the rice fields.
Ponds have about 2-4 months downtime between crops, during which liming and predator eradication is practised. Urea (CH4N20)and diammonium phosphate [(NH4)2HPO4] are used in the rice fields.
Monocultured pond productivity ranges from 0.60-0.75 mt/ha. In polyculture, total aquaculture production is 2.1-3.0 mt/ha, of which about 10% is freshwater prawns, the rest being finfish: silver carp, silver barb, tilapia, river catfish, and common carp. The prawn productivity in rice fields is about 0.1-0.3 mt/ha/yr.
It is normal for there to be two rice crops per year but the prawn crop spans both of them; the prawns remain in the irrigation ditches (or are temporarily transferred to an adjacent pond) during the first rice harvest and are allowed back into the paddies for a total of 8-12 months rearing period. Selective harvesting occurs several times before the final drain harvest.
Example 2:
In areas of Viet Nam where only one rice crop is possible because the salinity of rivers and canals is too high for growing rice during the dry season, the addition of M. rosenbergii to an integrated system may be beneficial. Experiments in crustacean- crop integration were conducted in three fields (0.4, 0.5, and 0.6 ha). Rice was planted in mid- June, freshwater prawns were stocked (1.5/m2) at the end of June, the rice was harvested in mid- November, and freshwater prawns were harvested in early February. In one field, marine shrimp (Penaeus monodon) were also stocked (1.5/m2) in mid-February and harvested in early June (just before the rice planting of the second year).
The farmed crustaceans used natural food until the final month of culture, when supplemental food was supplied. Yields ranged from 2.9 to 3.4 mt of rice and 434 to 596 kg/ha of freshwater prawns (average individual weights ranged from 62 to 76 g). In the one field where tiger shrimp were stocked, an additional 390 kg/ha of shrimp was harvested (mean weight 42g). The researchers concerned stated that this form of farming was a low-investment, highrevenue, no-pollution opportunity.
SOURCE: NEW (2000b) AND ZIMMERMANN AND NEW (2000)

Figure 75
If hatchery-reared Macrobrachium rosenbergii are not available, brushwood can be used to capture wild postlarvae (Viet Nam)

SOURCE: MICHAEL NEW, REPRODUCED FROM NEW AND VALENTI (2000) WITH PERMISSION FROM BLACKWELL SCIENCE

few weeks of the grow-out phase when prawns are stocked as PL. Marine shrimp feeds have a much higher protein content than is needed for freshwater prawns, so cheaper commercial feeds that have either been specifically designed for freshwater prawns or for a species of fish (e.g. catfish) must be used in grow-out ponds stocked with nursery-reared juveniles, or substituted as soon as possible in those stocked with PL. You can assess the relative suitability of commercially available feeds by asking your local extension agent from your government fisheries department and checking with other freshwater prawn farmers.
Commercial feeds may be the most productive and reliable to use but they are expensive, are not always available to the small farmer, and do not take advantage of locally available ingredients. You may also have problems in storing compound feedstuffs in humid conditions where deliveries cannot be made regularly in small quantities. If you make your own feeds, some of your ingredients can be locally available. You can also produce them with your own farm labour and simple equipment, such as small bakery mixers and meat mincers. Usually, no extra labour is required. Feed making just forms another job which can be fitted in between the other duties of your farm labourers.
Many different ingredients could be used in your farm-made feeds, either individually or combined into ‘compound feeds’. Commercial feeds for freshwater prawns tend to use ingredients which are available in large quantities; many of them are global commodities, such as fish meal or soybean meal. You can also include some of these ingredients in the feeds you make on-farm, as shown in the formulae given in this manual. In addition, you could include so-called ‘unconventional feeds’ (feeds not normally used in commercial feeds because they are only available in small quantities, often only locally and seasonally); some of these are listed in Table 18. In addition to ‘trash’ fish, molluscs and prawn wastes form valuable animal protein sources. Meal made from the leaves of the Ipil ipil bush (Leucaena sp.) has formed a constituent of shrimp and prawn diets but its use is cautioned by the toxicity of mimosine, which is a problem in its use for terrestrial animals. Some farmers add other materials to their ponds, including pig manure (added as a feed, not a fertilizer, where ethnically acceptable) and the mortalities from chicken farms, staked out around the periphery of the pond. Other locally available materials may also be satisfactory.
If you use individual raw materials (not made into a mixed and bound compound feed), especially wet materials (such as trash fish and beef liver), you stand more risk of causing your pond water to become polluted. Compounded feeds, especially when they are water-stable, cause less problems of this type. Compounded chicken and pig feeds, either unmodified, or re-extruded through a mincer with trash fish or prawn meal, have been used in freshwater prawn farming. Some are included in the formulae given in this manual.
However, be careful about using chicken and pig feeds because they often contain growth promoters, antibiotics, and other substances which may have unpredictable effects on prawns. Their presence in prawn tissues may also make the product unacceptable.
Using water-stable feeds provides your prawns with a balanced ration. It also stops the prawns selecting individual ingredients. Using well-bound compounded feeds also results in less water pollution and makes your task of judging how much feed to give each day easier. Feeds can be made water-stable by including a wide range of naturally occurring and modified gums and binders, by adding pre-gelatinized starch, and by certain processing techniques used by feed manufacturers. Some typical formulae for freshwater prawn diets are given in Annex 9.
The methods for making farm-made feeds are not described in this manual because there are other publications available. Details are provided in another FAO manual (New 1987) and the use of farm-made aquafeeds generally is discussed in New, Tacon and Csavas (1995). If you are formulating your own diet it is necessary to determine [by analysis and/or

TABLE 18 Examples of major ingredients either used individually or in mixed freshwater prawn grow-out feeds

10 Shrimp and prawn processing wastes (heads, shells, etc), sometimes used in farm-made feeds, may be virus disease carriers, if used raw (not processed). It is better to use commercially available shrimp meals. The diseases they carry may not produce obvious symptoms in freshwater prawns but could induce them to transfer the disease to other crustaceans. Meat and bone meal is a banned feedstuff ingredient in some countries (because of BSE); prawns reared on aquafeeds containing meat and bone may face consumer resistance. Compound feeds made for other species may contain antibiotics and/or levels of other substances harmful to prawns. Ipil ipil contains the toxin mimosine. Some other high-protein plant ingredients also contain toxins but these are removed by adequate procesing.

TABLE 19 Tentative specifications for semi-intensive freshwater prawn grow-out feeds

consulting published information (Fonnesbeck, Harris and Kearl 1977; Gohl 1981; New 1987; Tacon 1987, 1993a, 1993b)] the composition of your locally available ingredients.
Some specifications for freshwater prawn grow-out feeds are given in Table 19. Recently (Anonymous 2001a), it was reported that a farm-made aquafeed and feed mill unit has been launched in Cochin, India. Here, farmers can manufacture their own feed using communal equipment. This kind of development was recommended in a meeting in Thailand in 1992 (New, Tacon and Csavas 1995) and it is hoped that more units like this will emerge to assist small farmers to make their own, cheaper feeds. However, if you choose not to make your own feeds, consult your local aquafeed manufacturers and ask if they make feeds for freshwater prawns. You will find that they are keen to help you.

MEASURING FEED EFFICIENCY

You should not judge the value of a feed only by its unit cost (price per mt of feed). What you must consider is:

what weight of prawns you will get by using this feed (mt/ha/crop) ?

what proportion of the prawns produced will be marketable (the correct size for your market; good appearance, etc.) ?

what will your total feeding costs be (this includes not only the cost of the feed itself, but how much does it cost you to store it, transport it to the ponds, feed it, and solve any problems it may cause in pond management) ?
The unit of measurement most commonly used on the farm is the feed conversion ratio (FCR). This is the actual weight of feed presented divided by the actual weight of animals produced (no adjustments are made for the differing moisture contents of the feed and the prawns). An FCR of 2:1 to 3:1 would be typical for a dry (~10-12% moisture) compounded diet. The FCR of wet feeds, such as trash fish, is much higher, perhaps 7:1 to 9:1.
A semi-moist feed (typically with a moisture content of 35-40%), consisting of a mixture of dry and wet ingredients, might have an FCR of 4:1 to 5:1.
However, FCR is a rather crude measurement, because it only refers to total productivity.
This is not the whole story. Time from stocking to harvest (the growth rate achieved), the prawn size and quality obtained, and the cost of storage and feeding are just three of the other factors that are important. For example, suppose that two feeds have an equal unit cost and the same FCR, the use of one may result in prawns reaching the average marketable size in 5 months, the other may take 6 months. The first is obviously the more efficient. FCR alone does not tell you this. This illustration is provided simply to make you think about your choice of feed more carefully.

FEEDING RATE

There can be no exact general recommendation for daily feeding rates, because these depend on the size and number of prawns (and, in a polyculture system, fish) in the pond, the water quality, and the nature of the feed. Some farmers start feeding rates very high at first (perhaps as much as 100% of body weight at the PL stage). If juveniles are stocked, the rate might be 20-10% of body weight (depending on juvenile size) and it would decline gradually to about 2% by harvest time. This works quite well if the ponds are batch-harvested.
However, if you are culling out the larger animals, this may result in some underfeeding for the others. It is also very difficult to calculate even a reasonably accurate estimate of the total body weight in your pond.
This manual recommends that you should start by feeding a fixed amount, which depends on the pond size, to encourage the growth of natural food (as measured by transparency, see below). Then, you should continue by feeding ‘to demand’ (in other words, giving as much feed as the prawns will eat but no more). Spread the feed around the periphery of the pond in the shallows, which are good feeding zones. Putting the feed in defined ‘feeding areas’ a few metres apart makes it easier to observe how much is consumed. This practice also leaves the areas in between the feeding zones clean, thus lessening pollution and promoting more healthy rearing conditions. Some farmers operating large ponds use boats to distribute feeds more evenly (Figure 76). Others use rafts, which are towed around fixed routes by means of a series of ropes guided by fixed wood or bamboo stakes within the pond or on its banks, for this purpose. Whether you confine feed to the periphery of your pond or distribute it more widely throughout your pond, the use of defined feeding areas, rather than general broadcasting, is recommended.

Figure 76
Feed can be distributed within the pond by simple boats, which can be lifted from one pond to another; manual feeding along at least one side of the pond would be quite difficult in this case because of the method of construction, which has set a water channel in a very narrow pond bank (Thailand)

SOURCE: HASSANAI KONGKEO

The best way of measuring food consumption is to use feeding trays (Figure 77), which can be lifted out of the water for inspection. You can construct lift nets from any netting with a mesh small enough to retain the feed particles. If you use this system, lift the tray out of the water for inspection to see how much feed has been consumed before you distribute the next feed. If there is no feed left on the following day, the feeding rate should be increased. If there is excessive food left, the feeding rate should be decreased. In cases of severe over-feeding, which may cause water quality problems, feed may even be omitted for a day. The need for the operator to be able to see the unused feed after 24 hours highlights one of the advantages of a water stable diet.
Where you have enough water available to allow it to flow through the pond all the time, you could adjust the phytoplankton density by altering the water flow rate. Even if your pond is normally static you could flush the pond if the phytoplankton density becomes too high (or there are other reasons to suspect poor water quality) by partially draining and refilling the pond. However, the best means of controlling phytoplankton density without wasting water (and money!) is to carefully monitor the effect of feeding rate and aeration (which is often used in nursery ponds and always in intensive grow-out) on water transparency, and make alterations as necessary. By this means panic situations caused by gross over-feeding can be avoided.
Exact daily feeding rates are site and management specific. However, an example is given in Box 23, which also describes how to adjust feeding according to water transparency. This example

Figure 77
Using a lift net for observing feed consumption (Puerto Rico)

OURCE: HAROLD PHILLIPS

BOX 23

Example of feeding rate for freshwater prawns
ASSUMPTIONS:

Monoculture

Location is a tropical zone with an optimum water
temperature

Stocking rate 5 PL/m2

Expected yield 1 250 kg/ha in 6-8 months

A dry diet is fed
FEEDING REGIME:

BEGIN by feeding about 6 kg/ha/day. This is far more than the prawns will consume when they are young PL but the diet also acts as a fertilizer for enhancing the natural food available. This will increase the availability of benthic fauna and will build up the plankton density to a level which will provide cover for the prawns and prevent the growth of rooted aquatic plants.

CHECK the transparency (governed by the amount of phytoplankton present) of the pond water regularly with a Secchi disc (Figure 78).

CONTINUE feeding about 6 kg/ha/day until the Secchi disk reading shows that a visibility of between 25 and 40 cm has been reached. A cruder method of making this measurement is to immerse your arm in the water up to your elbow. If you can easily see the tips your fingers the water is too clear. If you cannot see your wrist then the phytoplankton density is too high.

WHEN YOUR MEASUREMENTS SHOW that the phytoplankton density has reached the desired level, start to adjust the amount of feed you give by examining the daily consumption of the prawns, preferably by inspecting your lift nets. This is called demand feeding.

YOU ARE RECOMMENDED to put all the daily feed ration into the pond once per day in the late afternoon [however, many farmers prefer to split the daily ration into two feedings. If you do this, give 30% in the early morning and 70% in the late afternoon].

YOU WILL FIND that the daily amount of feed, judged by consumption, will begin to rise gradually from the initial 6 kg/ha/day. By harvest time it will be much higher. The exact peak in feeding level will depend on the growth and survival rate of the prawns. You can expect the feeding rate to build up to nearly 40 kg/ha/day at the time just before a pond is harvested.

IF YOUR FEEDING adjustments have been accurate, and if the prawns have grown and survived normally, you should find that your total use of feed should not exceed about 3 000 kg (assumes an FCR of about 2.4) for each rearing cycle.

assumes that PL are stocked directly into the final grow-out pond. The use of feedstuffs to induce phytoplankton growth may seem rather expensive (compared to using fertilizers) but it is simple and effective. However, rearing PL to juvenile size in a nursery system, as described earlier in this manual, is a more efficient way of using feeds for the first 2 months or so after metamorphosis.