So you go out and do your hula-hoop counts in soybean fields this week. Some of the counts, especially in parts of the field, aren't what you like. If everything is up that's coming up, is there enough stand there that you should leave it, or should you tear it up and start over?
Recognize from the very start that this question is somewhat different today, when seed is $40 to $50 per unit, or $50 to $70 per acre, vs. the days when seed was perhaps $20 per acre. Replanting, notwithstanding any help from your seed supplier, represents a substantial cost just in seed alone.
Fortunately, data collected at Purdue and other places over the years gives a reasonable indication of what you can expect form thin stands. The data illustrates the soybean's ability to branch, bush and compensate to make up for missing plants. You'll find the information you need to make intelligent replant decisions about soybeans on pages 104 and 105 of the 2010 Purdue University Corn & Soybean field pocket guide.
For example, suppose you seeded 160,000 seeds per acre. If you have 120,000 plants per acre and they are relatively evenly distributed, then you can still expect 100% of yield. You've lost nothing. It's an illustration of the large safety gap built into soybean seeding rates.
Suppose you come out with 80,000 plants per acre. That's a stand that's going to look mighty thin when you first walk the field. But data says that in solid-seeded situations, you can still expect 96% of original yield. In 30-inch rows, expect 100% yield! For 15-inch rows, the number would lie somewhere in between.
If your yield goal was a lofty 60 bushels per acre, you can still expect $57.6 bushels per acre. You've forfeited, on average, 2.4 bushels. T $9 per bushel, that's $18.36. That won't pay half the cost of the seed for replanting alone if you have to cover the entire cost of replant seed.
Now suppose there are only 60,000 plants per acre. This is a stand 9 farmers out of 10 would walk into, walk out of, and go get the disk. However, data says you can still achieve 92% of original yield in solid seeded beans, and 94% if you're in 30-inch rows. An 8% loss off a 60 bushel potential is 4.8 bushels. You would just about pay the cost of the replant seed in what you could expect to gain in yield by replanting, assuming it isn't so late now in the season that you're losing potential due to late planting.
A whole bunch of assumptions apply. First, these examples assume you get a good stand the second time. Also, they assume plants are fairly evenly distributed. Other data shows that if there are 2 foot skips in 50% of the row area, you can drop to 945 of original yield based on skips alone.
Weed control also becomes a factor. Thin stand may require an extra application since you don't get much help on the soybean canopy.
Do the math, put emotion aside, and make the decision that looks most profitable in the long run.