Formulating a 30% protein carp feed on a small budget
Bikash Pradhan, a trainee at a vocational institute, manages a 0.8 ha carp pond near Puri and expects 1,500 kg of fish. He feeds a 1:1 mixture of de-oiled rice bran and groundnut oil cake at 3% of body weight, records an FCR of 3.4, and pays Rs 26 per kg of the mixture. Prices at the local mill: de-oiled rice bran Rs 18/kg (12% crude protein), groundnut oil cake Rs 34/kg (44% crude protein) and fish meal Rs 70/kg (60% crude protein). A mineral-vitamin premix costs Rs 150/kg and is used at 1% of the diet. He wants a 30% crude protein pellet, can buy fish meal only in 50 kg bags, and has Rs 9,000 for next month's feed. He has heard that oil cake alone can supply all the protein and asks whether adding fish meal is worth it at all.
1. Using Pearson's square, work out the rice bran to oil cake proportions for a 30% crude protein mix and its cost per kg. Comment on whether the mix is practically usable.
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Pearson's square compares each ingredient with the target:
- Oil cake above target: 44 − 30 = 14 parts
- Rice bran below target: 30 − 12 = 18 parts
- Total 32 parts, so oil cake = 18/32 = 56.25% and rice bran = 14/32 = 43.75%.
Check: (0.5625 x 44) + (0.4375 x 12) = 24.75 + 5.25 = 30.0% crude protein. Correct on paper.
Cost: (0.5625 x 34) + (0.4375 x 18) = 19.13 + 7.88 = Rs 27.00/kg, plus 1% premix at Rs 1.50 = Rs 28.50/kg.
The arithmetic is right but the mix is not practically usable. Groundnut oil cake at 56% of the diet brings fibre, residual oil that goes rancid in Odisha's humidity, and aflatoxin risk if the cake is mouldy. A practical ceiling for oil cake in carp feed is roughly 30–35%. So the 'cheapest' 30% protein mix is also the one most likely to give poor digestion and a worse FCR than the number suggests.
2. Recalculate with 25% fish meal included and decide whether fish meal is worth buying. Show the FCR at which the decision flips.
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Fish meal at 25% supplies 0.25 x 60 = 15% protein. The remaining 75% of the diet must therefore supply the other 15%, i.e. a 20% protein blend of bran and oil cake.
Pearson's square on that blend: oil cake = 20 − 12 = 8 parts; bran = 44 − 20 = 24 parts; total 32. So the blend is 25% oil cake and 75% bran, which over the whole diet is 18.75% oil cake and 56.25% bran.
Check: 15 + (0.1875 x 44) + (0.5625 x 12) = 15 + 8.25 + 6.75 = 30.0% protein.
Cost: (0.25 x 70) + (0.1875 x 34) + (0.5625 x 18) = 17.50 + 6.38 + 10.13 = Rs 34.00/kg, plus premix Rs 1.50 = Rs 35.50/kg.
Decision: his present feed costs 3.4 x 26 = Rs 88.40 per kg of fish. The bran-oil cake 30% mix at an expected FCR of 2.6 costs 2.6 x 28.50 = Rs 74.10. The fish meal version at FCR 2.2 costs 2.2 x 35.50 = Rs 78.10 - worse. Fish meal pays only if 35.50 x FCR falls below 74.10, i.e. if FCR is better than about 2.1. That is a real gain in protein quality, but not one he should assume. The 50 kg bag (Rs 3,500) fits his budget, so a trial is affordable; a blind switch is not.
3. He has no pelletizer and feeds dry mash at 3% of body weight. Explain how feed form and feeding rate affect his FCR, and give the low-cost changes you would make first.
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Mash is the hidden cost in his FCR of 3.4. Dry powder broadcast into water loses a large share to drifting fines and leaching - water-soluble vitamins and free amino acids dissolve before the fish find the particle, and uneaten mash sinks and decays, adding to oxygen demand. Fish must also spend energy picking at particles instead of taking a defined ration, so less of what he buys becomes flesh. A 3% body-weight rate on mash is not a true ration because he cannot see what was eaten.
Low-cost changes, in order of return:
- Moisten the mix and pass it through a hand mincer or screw press to make a soft, sinking pellet. Even crude pellets cut leaching and fines sharply.
- Feed at fixed times (about 9 am, 1 pm and 5 pm) in a few trays or at marked spots, and lift the trays an hour later to see what is left. Feed only what is cleared.
- Recalculate the ration monthly from actual biomass. Three percent of a guessed biomass is how ponds get overfed; if trays show leftovers, cut the rate before cutting the protein level.
- Store bran and oil cake on pallets, dry, and use within a month; rancid or mouldy cake destroys digestibility no matter how good the formulation looks.
Measuring FCR properly - total feed bought divided by total fish harvested, including the 1,500 kg at harvest - is the only way he will know whether the new formulation actually earned the extra money.