Editor’s note: The following was written by Mark Licht, Iowa State University assistant professor and Extension cropping systems specialist, for the university’s Integrated Crop Management News website.
This year, let’s all toss out planting windows for corn. Farmers should be focused on when soil conditions are prime to plant.
Soil temperatures need to be on the rise — 50 degrees and rising. Planting 24-36 hours before a cold spell — regardless of how long it will last — is putting your seed at risk of cold injury and reduced germination.
The critical corn planting date for Iowa is before May 18 and soybean is May 20, after which you can expect yield losses. Switching to a well-adapted maturity doesn’t pay off until after June 1 for corn and about June 15 for soybean, but at that point a seed switch would be more about reducing fall frost risk.
There might be some hope when it comes to variable rate seeding, not necessarily from increasing yields, but from seed cost savings. Typical seeding rate response range for common hybrids is going to be 32,000-35,000, or 2,000-4,000 less for economic seeding rate, depending on seed costs.
The weather and its interaction with topography and soil is a huge influencer on how variable-rate seeding works. Dry weather typically means depressions and field areas that collect water can support higher seeding rates. However, if conditions are wet then upland areas and coarse soils can support higher seeding rates.
For soybean, using the same 125,000-140,000 seeding rate is still a solid recommendation. Farmers could possibly drop 25,000 on high-productivity field areas. Stress areas from soybean cyst nematode or iron deficiency chlorosis or similar may benefit from higher seeding rates, while areas with high white mold potential may benefit from lower seeding rates.
Row spacing on corn may have higher yields for 20-inch rows compared to 30-inch rows when yield potential is greater than 240 bu./acre. However, below 240 bu./acre, 20-inch and 30-inch rows are yield neutral.
I’m quite confident of this for north of Hwy. 30 and west of I-35, less certain for southern Iowa.
Yield benefits to changing row widths could be realized in soybean (unless white mold potential is high), which could help pay for equipment changes. The problems are that a planter, combine head and tires are a huge capital expense, and even a soybean yield response may not be able to cover those costs.