Why Soybean Seeding Date is a Critical Decision Each Yea

With the spring we had in 2015, it left a lot of farmers wondering whether they should take the chance on seeding soybeans. The variability of spring weather can create a challenging decision making process at the time of year when most farmers think they have their plan fully laid out and ready to go. With increased acres of a farm dedicated to soybeans, the risk factors need to be weighed and measured.

One of the biggest challenges with growing soybeans is choosing the right timing for planting, taking into consideration all the factors necessary to give the plants their best start possible. The most important factors are soil temperature and the weather forecast during the planting window. Even though soybeans generally produce higher yields when planted early, the seed and seedling can be damaged by both low soil temperature and frost.

According to data collected by Manitoba Agricultural Services Corporation (MASC), the general trend for seeding many crops appears to be the earlier the better, but not too early, especially with soybeans. When soybeans are planted too early they can be exposed to an increased risk of spring frost.

Manitoba Agriculture, Food and Rural Development (MAFRD) recommends the optimum time to plant soybeans is between May 15 and May 25, or when the average soil temperature has warmed up to at least 10° C with 18-22° C being ideal. Soybean seedlings can tolerate a light spring frost for a short period but will not recover if damaged.

“That’s traditionally where it’s been set for the last 10 years,” said Terry Buss, Farm Production Extension Specialist – Pulse Crops, MAFRD regarding the recommended seeding window. “I think we’re coming to a time when the way we decide when to seed will become more sophisticated. But generally, by the May 15, soil temperatures are warm enough and the risk of frost is lower.”

“The 24 hours after seeding are the most critical. The soil temperature and the soil water temperature should be warm enough by May 15 for the seed to do well but chilling injury can still be a risk. If seeds imbibe soil water colder than 4.5° C within the first 24 hours, significant seed damage will occur if not germination failure. A cold rain in the first 24 hours can also cause similar damage to the seed.”

By mid May a killing frost is still possible, as well. Research has documented that beans at the early seedling stage have survived exposure to temperatures ranging from -1.1° C to -1.8° C and very brief periods up to -2.2° C. It should be noted that soybeans bring their growing point above the soil surface during emergence which decreases their ability to harden off to the degree that canola can. Frost or freeze damage extending below the cotyledons translates to complete death of the seedling.

When the only option is to seed a little early with less than optimum temperatures, seeding depth is critical. Seeding as shallow and uniformly as possible, while still placing seeds into moisture, will help maximize soil temperatures, as well as seeding into the field with the blackest soil first. Controlling depth at seeding from a maximum of 1.5 inches to as little as 0.75 inches is recommended.

“There isn’t much difference in harvest dates even if the beans are in the ground a couple of weeks early if those couple of weeks were cold ones,” suggests Buss. “Early seeded beans don’t usually emerge as quickly as later seeded plants due to colder soil temperatures and they can struggle more. Plants that take longer to emerge can be weaker and may have decreased tolerance to diseases such as Phytophthora wilt, which shows up later in the season.”

Beans are photoperiod sensitive, which means they react to changes in night length, beginning to produce flowers during the growing season after nights start getting longer and days start getting shorter. As the days get shorter the plant starts to move into the reproductive growth stages, flowering then setting pods and seeds. Planting soybeans later than the recommended window will force plants into their reproductive growth stages when the plant is much smaller and less advanced than those planted at the optimum time and can result in reduced yield. “Generally, the more nodes on the main stem before they go into the reproductive stage the better the yield,” says Buss.

“Besides time of seeding, another important factor is deciding on a seeding rate,” says Buss. “Row crop or solid seeding, according to research conducted in Manitoba, the optimum economic seeding rate will be between 140,000-155,000 live plants/acre. Calibrate carefully. Work from your live plants/acre target back to your actual seeding rate. Seed retailers can help you with this.

“If you are a solid seeder, I strongly recommend that once the crop is up to put a hoola hoop out and do a count of live plants to find out how close you got to your target stand density. Unless you measure, you will not know for sure. For solid seeders, counting the plants in a 28.25 inch hoola hoop and multiplying by 10,000 will give you your live plants/acre count. There are also apps designed for smartphone use that will allow producers to use a hoola hoop of any diameter and will automatically calculate stand density using the data they input. Row length based stand density measuring tools and apps are also available for those row cropping.”

First-time growers likely won’t have the necessary bacteria species present in sufficient numbers in their soil for the beans to fixate nitrogen. Buss suggests, “First-time growers should double inoculate, both on the seed and in the row. The granular in the row provides a back-up population of Rhizobia, especially when the weather isn’t very conducive to inoculant survival. Fungicides can be really important as well, especially to protect seeds from disease during wet, cool weather.”

Buss also suggests that the idea soybeans don’t need fertilizer because they don’t often show a yield response to phosphorus (P) or potassium (K) applied in the year of production is incorrect. Soybeans need fertility, just in a different way. Soybeans like fertile soil, not fertilizer.

“What we’re trying now is to think about fertilizing soybeans as a crop rotation issue. Laying down P and K with other crops in the years before soybeans are grown on a field. Beans use a lot of both P and K, but they prefer it in the soil, not as applied fertilizer. Get the soil in the proper condition ahead of time to optimize soybean growth. Soil test and use proper crop rotations.”

As with most crops, Buss recommends always purchasing quality seed as the best place to start. “I also can’t stress enough how important data collection and the hoola hoop is,” states Buss. “Understanding how your decisions through the seeding process have affected the plant stand and yield is very valuable information to have for future crops.”