How Do We Define Early Planted Soybean?

By Jarrod Mille

Traditionally, soybean planting has occurred after corn planting is completed. However, as Delmarva farmers explore methods to increase yield, earlier planting dates are being considered. With earlier planting comes cooler soil temperatures and a higher risk of seedling losses, raising the question: how early is too early?

Understanding Soybean Growth and Development

Corn plants are determinate, meaning they have a fixed vegetative growth stage before tasseling and producing ears (reproductive stages). Some soybean varieties are also determinate and are primarily grown in regions with longer growing seasons, such as the southern U.S. These varieties typically belong to maturity groups V and later.

In contrast, indeterminate soybeans, which are better suited to shorter growing seasons, continue producing leaves even after entering reproductive stages (flowering and pod formation). Soybean reproductive development is influenced by day length, with flowering typically beginning after the summer solstice as nights gradually shorten.

Indeterminate Soybean and Season Length

With our moderate season lengths, we typically grow indeterminate group III or IV soybean on the Peninsula. To maximize leaf production before the summer solstice, planting as early as possible is beneficial. However, early planting also introduces risks, such as freezes and wet soils, which can hinder germination and early growth. On the other end of the season, delayed planting can limit leaf production and increase the risk of early freezes reducing yield (Figure 1).

In the 1980s, UD recommended planting soybeans from mid-May to early June for maximum yield, as yields typically declined with later planting. However, more recent research suggests that soybeans planted from mid-April to mid-May can produce comparable yields. This trend has been observed across locations from Keedysville, MD, to Georgetown, DE. Given this expanded planting window, the key question becomes: how do we define early-planted soybeans?

Definitions of Soybean Planting Dates

Soybean planting dates began to be re-evaluated in the 1980s, with the standard practice at the time referred to as conventional planting. Since then, research has identified an optimum planting window a period of several weeks during which soybean yield remains relatively stable (Figure 2). Once this window closes, planting is considered late, and yields may decline rapidly due to a shorter growing period for leaf area development and sunlight capture.

The definitions of early and ultra-early soybean planting are less clearly defined. Generally, early planting occurs at the very beginning of the optimum window, while ultra-early planting is riskier and often falls outside crop insurance coverage periods. The exact timing of early planting varies by region. For instance, in Delmarva, the second week of April may be considered early, whereas planting in late March would be considered ultra-early.

It's important to note that extension recommendations and research findings from different regions may define "early planting" differently based on local climate and growing conditions. Understanding these regional differences is key when interpreting planting date guidelines.

Should You Plant Early?

While recent findings indicate that soybean yields remain stable from mid-April to late May, in unusually warm springs, we have observed a yield boost with April planting. Additionally, even when early planted soybeans experienced freeze damage and stand losses, their yields in Delaware were comparable to those of soybeans planted in May.

Given this flexibility, planting decisions can be guided by equipment availability and field conditions. Farmers have a window of opportunity to adjust their planting strategy, allowing them to pivot between corn and soybean as conditions dictate.