By Darren Hefty
When I was younger, my dad would often switch planting populations from field to field. On the sandier fields, he’d cut back. On the good ground, he’d bump the population up. However, what didn’t make sense to me was that even in the sandy fields there were some areas of really good ground that could’ve used a higher planting population. It was “too much work and too much hassle” to stop and change things as we went through the field.
Now, it’s easy to set up planters to automatically switch planting populations as you drive across the field. The tough part, though, is to know what population to plant in each area of your farm.
With corn, the right planting population really varies based on soil type, geography, weather, etc. Seed companies have been pushing for higher populations over the last decade. My belief is we’ve gone too far in some instances, though.
The question I ask farmers when we have this discussion is this: “What is your yield divided by your plant population in thousands?” If you haven’t looked at planting populations this way, it gives a frame of reference when talking to others.
Example: Let’s say your corn yielded 210 bushels per acre and your planting population in that area was 30,000. 210 divided by 30 equals 7. The factor of 7 is a very average number in our area. A few years back, Brian and I had corn plots at the Ag PhD Field Day that yielded just over 300 bushels. The planting population was 34,000. 306 divided by 34 equals a factor of 9. That’s really good, and I wish ours was all that way! Look at David Hula’s planting population when he broke the world record with 532 bushel corn. You’ll find he did even better.
Make adjustments and build your variable rate planting maps for each field. Start by figuring out what yields are better than average (i.e. the factor of 7) for the population you’re seeding. You may benefit from a higher population in those areas. By the same token, if you’re only getting 5 bushels of yield per 1000 plants, your population is likely too high for that area.
Side note: if you’re getting a factor of 5, there may be an undiagnosed fertility or drainage issue, as well.
Companies like Farmer’s Edge are working on tools to automate this process and to make it easier to implement.
We’ll talk more next month about the right planting population for soybeans, but having variable population in beans can help in some situations, too. For example, if you have IDC (high pH) in spots, bump your population to minimize yield loss. If you have had white mold pockets, cut your population in those areas to slightly reduce your risk for sclerotinia. Varying population in beans isn’t usually as critical as corn, but it can still be helpful.