The seeding rate is the weight of the seed planted per unit area of land for any crop. Typical seeding rates range from a few pounds per acre to over 100 lbs/acre.
The crop yield is the weight of the produce from a harvested crop. In many cases, the produce is itself a seed that can be replanted. Wheat, rice, corn, barley, potatoes, beans, and of course various edible seeds can each be planted to produce a second crop.
A crop cycle is one season for a particular crop, ending with harvesting. Some crops have a short cycle lasting only two or three months. Other crops require many months for each crop cycle. Tree crops often produce only one harvest per year, so they have a crop cycle of 12 months.
Suppose you have a small amount of seed for a particular crop, but you wish to plant a large area. One solution is to do a grow-out. You plant all the seed, harvest the crop when it is mature, and then use the seed from that harvest for a subsequent larger planting. In a grow-out, you plant a crop in order to produce seed for a second crop. The grow out ratio tells us the increase in seed due to one crop cycle.
For example, typical seeding rates for soybeans range from 50 to 80 lbs/acre. Recent commercial yields have ranged from 40 to 44 bushels per acre (at 60 lbs/bushel). To simply the numbers, let’s say that you plant soybeans at a seeding rate of 50 lbs/acre, and your crop yield is a modest 2000 lbs/acre. The grow-out ratio is 40:1. It is the yield divided by the seeding rate, for crops whose yield is a seed. If you plant only 1 pound of soybeans, you might expect about 40 pounds of soybeans in the harvest. However, seeding rates and crop yields vary, so the grow-out ratio is only an approximation.
Multiple grow-outs can be used to increase the amount of seed over the course of multiple crop cycles. Suppose you plant 1 lb of soybeans, and the first crop cycle produces a yield of 40 lbs. Replant those 40 lbs, and the harvest may be about 1600 lbs. (another 40x increase). A third crop cycle will then produce 64,000 lbs of soybeans at the same 40:1 grow-out ration.
But grow-out ratios vary a great deal. Grain amaranth has a seeding rate from 0.25 to 4.0 kilograms per hectare. Typical yields range from 1,000 kg/ha and up. Let’s suppose a moderate seeding rate of 1 kg/ha, and a modest yield of 1,000 kg/ha. In this case, the grow-out ratio is 1000:1. Your first harvest from planting one kg of amaranth seed on one hectare of land is 1,000 kg. If you replant all that seed, you will need 1,000 hectares of land, and the harvest will be one million kg of amaranth. A third crop cycle would require one million hectares of land, and would produce one billion kg of amaranth (one million metric tonnes).
Note the comparison between soybean and amaranth. Two grow-outs increases the amount of soybean seed by 1600 times, and the amount of amaranth by one million times. Three grow-outs increases the amount of soybeans by 64,000 times, and the amount of amaranth by one billion times.
Grow out ratios are important in any scenario where you need to produce a large amount of seed in a short amount of time. Plants with a higher grow-out ratio can ramp up to full production must faster.
The grow-out ratio also affects the cost of growing a crop, since seed is usually sold by weight. A high grow-out ratio is due, in part, to a low seeding rate, which usually implies a lower cost per acre for seed. Higher yields also increase the grow-out ratio, and of course a higher yield makes any crop more lucrative.
Crops with high grow-out ratios have several advantages in fighting world hunger.
1. More food is produced per kg of seed
If an aide organization provides 100 kg of seed for growing wheat to farmers in a particular region, they can plant about one hectare (ha) with that wheat. But if the organization instead provides 100 kg of amaranth, they can plant 100 hectares. The wheat harvest could be as high as 3000 kg/ha, times one hectare. But the amaranth harvest could be as high as 1000 kg/ha times 100 hectares, for a total of 100,000 kg of amaranth.
Other crops with a high grow-out ratio include: sesame seed, camelina (an oilseed), quinoa, and corn.
2. Farmers can more easily share seed from their harvest with neighboring farms and villages.
Quinoa has a grow out ratio of about 250:1. A farmer who grows quinoa can offer neighbors relatively small amounts of seed, and yet this will be sufficient to plant a substantial area. The seeding rate for quinoa is about 4 kg/ha, and typical yields are around 1000 kg/ha.
A similar analysis applies to other crops with high grow-out ratios. These crops can spread among local residents more quickly because a small amount of seed plants a large area.
3. Input costs may be lower.
The lower seeding rate can result in a lower cost for seed, making the crop less expensive to grow. For farmers who save seed from a previous harvest, the lower seeding rate lowers the labor and storage space needed to save the seed. These advantages help subsistence farmers and small commercial farmers in developing nations.
4. If agricultural disaster strikes a region or nation, the seed supply for crops with a higher grow-out ratio can be rebuilt more quickly.
This benefit applies to both developed and developing nations. A vast amount of seed is needed to feed the world population each year. Relatively few crops provide the vast majority of our macronutrients (protein, fat, and carbohydrates). A failure of one of these top staple crops could be disastrous. Rebuilding the agricultural system for that crop or a replacement staple crop is much faster if the crop has a high grow-out ratio.