Farming freshwater prawns

MEASURING FEED EFFICIENCYYou should not judge the value of a feed only by its unit cost (price per mt of feed). Whatyou 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 yourmarket; 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 solveany problems it may cause in pond management) ?The unit of measurement most commonly used on the farm is the feed conversionratio (FCR). This is the actual weight of feed presented divided by the actual weight of animalsproduced (no adjustments are made for the differing moisture contents of the feed andthe prawns). An FCR of 2:1 to 3:1 would be typical for a dry (~10-12% moisture) compoundeddiet. 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 ofdry 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 rateachieved), the prawn size and quality obtained, and the cost of storage and feeding are justthree of the other factors that are important. For example, suppose that two feeds have anequal unit cost and the same FCR, the use of one may result in prawns reaching the averagemarketable size in 5 months, the other may take 6 months. The first is obviously themore efficient. FCR alone does not tell you this. This illustration is provided simply tomake you think about your choice of feed more carefully.FEEDING RATEThere can be no exact general recommendation for daily feeding rates, because thesedepend 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 highat 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 declinegradually 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 underfeedingfor the others. It is also very difficult to calculate even a reasonably accurate estimateof the total body weight in your pond.This manual recommends that you should start by feeding a fixed amount, whichdepends 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, givingas much feed as the prawns will eat but no more). Spread the feed around the peripheryof 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. Thispractice also leaves the areas in between the feeding zones clean, thus lessening pollutionand promoting more healthy rearing conditions. Some farmers operating large ponds useboats to distribute feeds more evenly (Figure 76). Others use rafts, which are towed aroundfixed routes by means of a series of ropes guided by fixed wood or bamboo stakes within thepond or on its banks, for this purpose. Whether you confine feed to the periphery of yourpond or distribute it more widely throughout your pond, the use of defined feeding areas,rather than general broadcasting, is recommended.CHAPTER 6119