1. How Much Weight Does a Pig Gain Each Day?
To understand the scale, imagine a 2 kg bag of rice. A newborn pig is slightly lighter than one of these bags. An adult pig weighing 110 kg is like carrying 55 of those bags. However, the pig does not just carry the weight; its body is built from the food it eats every day.
The Ehime Prefectural Livestock Experiment Station sets a target for low-cost production: reaching a shipping weight of 110 kg at 150 to 160 days of age [3]. If we subtract the birth weight of 1.5 kg from the final weight and divide by the number of days, the average daily weight gain is about 0.7 kg. This calculation is based on the figures provided in this article.
Different sources give slightly different numbers. An encyclopedia notes that pigs may reach 90 to 100 kg by 6 months [1]. This article uses the figure of about 110 kg and roughly 73 times, as reported by the Ministry of Agriculture, Forestry and Fisheries [2]. Pigs are omnivores, descended from wild boars. Their diet includes grains, bran, potatoes, leftovers from food production, and pasture [1]. Since they eat almost anything, gathering feed seems easy, but farmers face specific challenges.
2. Why Feed Makes Up About Two-Thirds of the Cost of Raising a Pig
According to the Ministry of Agriculture, Forestry and Fisheries, feed accounts for 67% of the cost of raising fattening pigs [6]. If it costs 100 yen to raise a pig, about 67 yen goes to food. Feed is a major expense for farmers.
Most of the feed used in pig farming operations relies on imports. A study from Tochigi Prefecture states that most feed for pig farming relies on imports, and prices remain high [5]. The Ministry reports that for the fiscal year 2023 (estimated), the overall feed self-sufficiency rate is 27% [6]. For roughage like pasture, it is 80%, but for concentrated feed made mostly of grains, it is only 13% [6]. Note that this 13% figure applies to concentrated feed for all livestock, including cattle and chickens, not just pigs.
What is the main imported grain? In a test in Hyogo Prefecture, the standard mixed feed contained 70% corn [4]. This specific ratio was used for that test and does not represent all mixed feed in Japan. Researchers are asking if domestic products can replace imported grains. They have turned their attention to rice.
3. Can Pigs Grow Well on Rice Instead of Imported Corn?
The Hyogo Prefectural Livestock Technology Center compared groups of pigs. One group ate standard feed with 70% corn. Other groups ate feed in which 30%, 50%, or 100% of the corn was replaced with ground rough rice (rice with the husk still on) [4]. The trial used groups of 7 pigs.
Even when all the corn was replaced with ground rice, there was no significant difference in carcass performance or in the physical and chemical properties of the meat, and the daily weight gain was similar to the standard group [4]. However, the feed conversion ratio (the amount of feed needed to gain 1 kg of weight) was higher [4]. This means more feed was needed to gain 1 kg of weight.
A study in Tochigi Prefecture focused on pigs in the later stage of fattening, from 70 kg to shipping weight. When 50% of the feed was ground rice, weight gain and feed efficiency were similar to commercial mixed feed [5]. The texture of the pork was rated better than the standard meat, though taste and smell showed no difference [5].
The shape of the rice matters. If whole brown rice is fed, the feed conversion ratio rises to 4.46, compared to 3.54 for the control group [5]. A higher number means more feed is needed. 4.46 divided by 3.54 is about 1.26, meaning pigs would need to eat about 26% more food (a calculation from this article). The reason for this difference is not clear from the data. These tests used small numbers of pigs, so results may vary on other farms.
4. Can Leftover Bread and Brewing Waste Become Pig Feed as Eco-Feed?
"Eco-feed" is animal feed made from by-products of food production (like soy sauce or shochu lees), unsold food (like bread or bento boxes), vegetable scraps from cooking, and non-standard agricultural products [6]. The term combines "eco" (representing environmental care or saving) and "feed".
In fiscal year 2022, food waste and by-products totaled 15.25 million tons. Of this, 11.21 million tons were recycled, and about 80% of that, or 8.63 million tons, became feed [6]. This means slightly more than half of all food waste and related by-products became feed (a calculation from this article). This 8.63 million tons includes feed for all livestock, not just pigs.
Using eco-feed has benefits. Farmers can reduce feed costs and sometimes improve livestock productivity and product quality. Food producers can save money on waste disposal [6]. Some farmers brand their meat as eco-feed products to sell them [6]. However, when food meant for humans is given to pigs, safety is a concern.
5. What Rules Apply to Feeding Pigs Food Leftovers?
Since April 2021, regulations require heating for food scraps that may have contacted meat [7]. The food must be stirred while heated to at least 90°C for 60 minutes, or treated with an equivalent process [7]. Previous guidance called for 70°C at the center for 30 minutes [7].
The rules were strengthened to prevent African swine fever. There have been many reports of this disease spreading through feed made from insufficiently heated food leftovers [7]. These rules apply only to leftovers that may have touched meat, not all feed [7]. These rules apply when the food scraps used in eco-feed may have come into contact with meat.
Imported grains, domestic rice, and food leftovers are all part of the mix. The best choice depends on balancing price, pig growth, and safety.
6. Try Calculating Pig Growth Yourself With a Calculator
You can check the math in this article with a calculator. Subtract 1.5 from 110, then divide by 150 and 160 to see the daily weight gain range. Dividing 110 by 1.5 confirms the 73-fold increase. Growth is not always steady, but this gives a rough estimate.
For information on eco-feed and the heating standards, see the Ministry of Agriculture, Forestry and Fisheries materials listed in the sources. Detailed test results are available from livestock research centers in Ehime, Hyogo, and Tochigi prefectures. You can also search for brand names of eco-feed livestock products to see real-world examples.