INSIGHTS
- Favorable planting conditions on April 5 resulted in significant yield benefits in this 2024 trial.
- Yield penalties increased significantly when planting after May 15.
- Seeding rates greater than 100,000 plants per acre maximized yield within each planting date.
Introduction
In recent years there has been more focus on early-planted soybeans. Even though colder soil temperatures in early spring limit growth rate, soybeans can benefit from early planting dates because it can help capitalize on photoperiod and maximize growth and development in vegetative and reproductive stages. This allows plants to have more time accumulating nodes and filling pods, which both contribute to higher yields.
2024 Agronomy in Action Trial
An Agronomy in Action research trial was established in Slater, IA, to examine the effect of planting date on soybean yield. Two varieties, GH2884XF and GH2925XF brands, were selected and established at seeding rates of 170,000, 140,000, 110,000, 80,000, and 50,000 seeds per acre. The two varieties and 5 seeding rates were planted on 4 dates: April 5, April 24, May 15, and June 10. Due to similar responses to planting date and seeding rates, results were averaged across varieties.
Planting Date Response
Weather conditions in early 2024 were generally wetter than in previous years. The April 5 planting date was the first date when soil moisture levels allowed tillage. The following weeks were cold and wet, and conditions were not conducive for planting again until April 24. Despite these conditions, on average the early April planting date yielded 3.9% and 6% better than the following two planting dates respectively (Graph 1). April 24 planting date yields were 3.2 Bu/A less than the April 5 planting date. Delaying planting until mid-May resulted in an additional 2.3 Bu/A decrease in yield compared to the late April planting date. The last planting date on June 10 resulted in the largest yield penalty of 6.3 Bu/A less than the May planting date and 11.8 Bu/A less than the April 5 planting date.
Graph 1. Soybean yields averaged across 5 planting dates in 2024.Seeding Rate Response
When averaging across planting dates, seeding rates of 110,000 or greater yielded similarly, however the 80,000 and 50,000 seed per acre rates lost 4% and 11.7% of their yield potential respectively (Graph 2). In general, delaying planting had a more negative effect than reducing plants per acre. The optimal seeding rate of ≥110,000 seeds per acre did not change with later planting dates (Graph 3). In previous trials however, late planting dates have been shown to be more responsive to increased seeding rates due to a reduced ability to branch and develop the same number of nodes compared to earlier planting dates.
Graph 2. Soybean yield and seeds per pound averaged across planting date.
Graph 3. Planting date effect on soybean yield by seeding rate.Seeding Rate Influence on Seed Size
To better understand how increasing or decreasing the number of plants per acre affects yield components such as seed mass, grain samples were collected to determine seeds per pound across seeding rates. Graph 2 shows the relationship of seeds per pound to yield at each seeding rate. In general, seed mass decreased (more seeds per pound) as seeding rates decreased. Increased seed mass combined with a lack of change in yield at higher seeding rates indicates that fewer seeds per acre are being produced when population increases. This is a great example of how dynamic soybean yield components can be. Changes in seed size is just one way that soybeans compensate for poor stands.
Summary
This trial demonstrates the importance of planting soybeans early to maximize yield potential. With early planting dates, soybeans can maximize time in vegetative and reproductive growth stages. Earlier planting dates can also aid in weed control and moisture retention through quicker canopy and row closure. When delaying planting date, there are fewer days available for plant growth and development, and soybeans can lose the ability to maximize yields. The small differences in yield between different seeding rates within each planting date demonstrate the plasticity of soybean yield components to compensate for stand reductions.
Risk of stand loss and emergence issues from crusting or planting into excessively wet soil early in the spring are also risk factors. Compared to soybeans, a delay in planting corn acres would likely result in higher economic losses. This risk should be balanced against the weather forecast and the total time required to plant all corn and soybean acres.
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