Pigs, Feed Spillage and a Salty Babai Pit
At a school farm on South Tarawa, 10 weaner pigs are kept in a concrete pen. Each pig starts at 18 kg and the agriculture teacher aims to sell at 70 kg live weight after 120 days. The only feed available is a commercial grower ration costing $1.10 per kg, and records show each pig eats 2.4 kg per day; about one third of this is routinely spilled into the drain and hosed away. The local buyer pays $3.20 per kg live weight. Separately, the teacher wants to use the pig manure to rescue a 60 m² babai (giant swamp taro) pit beside the pen. The pit was flooded by a king tide three months ago; the plants are yellow and stunted, and a simple conductivity test shows the soil water is very salty. The teacher plans to dig in 300 kg of fresh pig manure with coconut husks and replant immediately, saying 'the manure will feed the plants and the rain will wash the salt away'. The school has no irrigation water except rain tanks.
1. Calculate the feed conversion ratio and the margin per pig at the planned sale weight. State whether the project is viable and identify the single number that most needs fixing.
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Gain per pig. 70 − 18 = 52 kg over 120 days = 0.43 kg/day.
Feed per pig. 2.4 kg/day × 120 days = 288 kg.
FCR. 288 ÷ 52 = 5.54 : 1. A well-managed pig on a grower ration should be nearer 3.0–3.5, so 5.54 is the first warning.
Margin per pig. Feed cost = 288 × $1.10 = $316.80. Sale value = 70 × $3.20 = $224.00. Result = −$92.80 per pig, i.e. −$928 on ten pigs before labour or housing.
Break-even check. Break-even FCR = $3.20 ÷ $1.10 = 2.91, which this system cannot reach while a third of the feed is wasted.
The number to fix - spillage, not price. Feed actually eaten = 288 × 2/3 = 192 kg, so FCR on feed eaten = 192 ÷ 52 = 3.69, and cost = $211.20, giving $224.00 − $211.20 = +$12.80 per pig. Simply stopping the waste turns a $92.80 loss into a small profit, without buying better feed or changing the sale weight. Recommend troughs with lips, feeding twice daily, and weighing feed in and out for one week to measure the real wastage.
2. Evaluate the teacher's plan to 'feed the plants and wash the salt away' with 300 kg of fresh manure. Distinguish what organic matter can and cannot fix, and state the risks of using it fresh.
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What manure can fix. It adds nitrogen, phosphorus, potassium and organic matter, improves water-holding and structure, and supports the microbes that make nutrients available. It does not remove chloride. Salinity is an osmotic problem: high salt in the soil water makes it hard for roots to take up water, which matches the yellowing and stunting seen.
Why fresh manure is a risk. Fresh pig manure is high in ammonia and is still heating; dug straight into a planting hole it can burn roots and kill new plants. It also carries pathogens and weed seeds, and it contains salts of its own, so it will raise the conductivity reading further before it helps.
Why 'the rain will wash it away' is unreliable. Leaching needs a lot of freshwater plus somewhere for the salt to drain to. A babai pit is a closed depression with a high water table; if drainage is poor, salts stay. If the next king tide overtops the pit again, it re-salts regardless.
Correct sequence: (1) repair the pit wall/bund to keep out spring tides; (2) compost the manure with the coconut husks for 4–6 weeks, turning it, until it is dark and earthy; (3) flush with stored rainwater and check conductivity again before planting; (4) replant on raised mounds, apply the compost as a surface mulch, and keep a salt-tolerant crop on the bank in the meantime. Compost feeds the plant; only freshwater and drainage move the salt.
3. The teacher has 300 kg of fresh manure and 100 kg of coconut husk. Composting loses about 40% of the fresh mass. At an application rate of 2 kg of compost per m², will the material cover the 60 m² pit? Explain the role of the husk.
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Compost mass. 300 kg fresh manure loses about 40% of its mass (water and carbon lost as CO₂): 300 × 0.60 = 180 kg of compost, plus whatever remains of the husk.
Requirement. 60 m² × 2 kg/m² = 120 kg. So 180 kg covers the pit with 60 kg to spare - but only if the husks are mixed in and the heap is turned and kept damp. If the heap is left in a dry, unturned pile, losses can exceed 60% and the quantity will fall short.
Role of the husk. Fresh pig manure has a low carbon-to-nitrogen ratio (roughly 10–15:1), which is too rich: it loses nitrogen as ammonia and smells. Coconut husk is very fibrous and carbon-rich (roughly 80–100:1). Mixing about 3 parts manure to 1 part husk here lifts the C:N ratio towards the 25–30:1 range that composts hot and fast, absorbs moisture, and gives the heap the air spaces that keep it aerobic.
Judgement on quantities: the tonnage is sufficient, so the limiting factors are heap management and salt, not volume. Do not spread all 180 kg on newly flushed soil; keep part for a second application after the next conductivity check, because a single heavy dose on stressed ground can do more harm than good.