Seeding and Reseeding Approaches have Changed
Published: Friday, April 9, 2021
The following is from Jeff Burbrink, Elkhart County Extension educator.
For at least 10 years now, agronomists have planted test plots and discussed what the optimum amount of seed per acre to plant corn and soybeans is. Emerson Nafziger, an agronomist from University of Illinois, recently shared some thoughts about planting issues.
Some of the early research seemed to suggest that corn may benefit from higher plant populations. But as further research is done, the benefits of higher corn plant populations have not continued to increase yield as much as some people once thought it would. In large part, that is because hybrids have added more yield per plant, without needing more plants per acre.
In most corn varieties, 34,000 to 38,000 plants per acre often maximizes yield. Planting 42,000 to 45,000 plants per acre usually does not lower yields compared to planting 36,000 to 40,000, but the cost of the additional seed does not cover the average increase in yield.
Variable-rate seeding, a technology that can drop more seed in more productive areas of a field, does makes sense for corn, but it needs to be done without lowering the population too much on the least-productive soils. In fields that are very uniform in soil type and fertility, the economic returns from variable rate seeding will be modest.
In soybeans, the opposite trend has occurred, with seeding rates trending towards lower rather than higher rates. Part of that trend is due to better quality, cold-tolerant seed, as well as major improvements in planting technology. In the past, farmers aimed for 175,000 to 200,000 seeds per acre, which helped assure enough plants when seed germinated. Today, a final stand of 110,000 to 120,000 plants per acre, using seeding rates adjusted for conditions and equipment, is the recommended practice.
Nafziger pointed out that the biggest yield-killer to stand establishment is heavy rain shortly after planting. Saturated soils lack oxygen, and seeds need oxygen to germinate. Deciding when to replant is as simple as counting plants and replanting if numbers are below a certain level.
After heavy rain, if soybean stands end up as uniformly-spaced, intact plants with more than two plants per square foot, replanting may not increase yield very much. In the more likely case that stands are poor in some places and not others, supplementing the existing stand with a partial seed rate might be worthwhile.
Corn stand problems from wet soils typically require replanting low spots only, but if there are gaps all over the field, a full replant following destruction of the existing stand may be necessary. Frost after emergence might kill enough plants to require replanting, but plants with some leaf damage tend to recover to produce full yields.
Bob Nielsen, Purdue's corn agronomist, says that from his experience in evaluating "patching in" new seed versus "destroy and replant" suggests that "patching in" without killing the original stand should not be attempted unless surviving stand is roughly 25 percent or less of the original population.
The risk with "patching in" surviving stands with populations higher than 25 percent is the original survivors will provide too much competition for the newly emerging replant population. There is a tendency to "patch in" at the original seeding rate, assuming that the planter will destroy quite a bit of the original stand, and then ending up with a final stand that is 1.5 times or more what you intended because a lot of the original stand survived.
More information on corn replanting can be found at https://bit.ly/31GnWi7 and soybean replanting at https://bit.ly/31HO5NG.
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