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Assessing soybean seeding dates

Use soil temperature as your guide to seeding dates.

November 22, 2007  By Bruce Barker

During the last decade, soybeans have established a respectable acreage in Manitoba, moving past a specialty niche crop to one grown on hundreds of thousands of acres. One of the primary challenges, though, is growing the warm season soybean crop in the relatively short growing season of Manitoba. This has led growers to try to plant as early as possible, in order to gain the most from the limited heat units in Manitoba.

“My concern when working with seeding dates is that when soybeans are planted in late April or early May, we don’t have much solid research, but we know from other parts of the country that if you seed too early, you get reduced plant stand and vigour,” explains Manitoba Agriculture, Food and Rural Initiatives’ (MAFRI) Bruce Brolley. “I remember one field that was seeded April 21 where only 30 percent of the plants emerged and the field was eventually reseeded.”

The rule of thumb that MAFRI has been using for seeding soybeans in Manitoba has been that if there was an early spring, when seeding for other crops started in April, that soybeans could be planted around May 8. An average spring meant crops were planted around mid May, and a late spring with soybeans going in around May 20 to 24.


“Try to wait as long as you can after you finish seeding your canola. The issue with stand establishment in the spring of 2007, where some growers stopped seeding canola because the soil was so dry and they switched to soybeans because they could seed deeper into moisture, was that the weather turned wet and cold,” says Brolley.

On the other hand, a problem with seeding too late is the weather in August the last number of years has been so dry that there has been no podding in August and yields have been poor. Unless the area got an August rain, only pod set during July contributed towards yield.
Seeding by May 15 tends to ensure that the plants are mature enough by the summer solstice to begin flowering in early July, being able to take advantage of available soil moisture.

Jane Froese with the Department of Plant Studies at the University of Manitoba explains another difficulty is assessing soybean maturity with Corn Heat Units, calling them ‘Company Heat Units’, since the heat unit values assigned to varieties are provided by the company that produced the variety. While these heat units provide farmers some direction for selecting varieties, the heat units do not always reflect actual maturity in Manitoba. That makes assessing seeding dates even more difficult.

In order to assess whether seeding early is a good strategy, and to develop some ‘made in Manitoba’ guidelines, researchers at the University of Manitoba and MAFRI started a research program in 2006 to determine the effect of four seeding dates (mid May to mid June) on soybean height (reflecting pod clearance at harvest), yield, seed weight, as well as seed protein and oil content.

This study was conducted in four southern Manitoba locations in 2006: Carman, Morden, Arborg and Rosebank. Two glyphosate resistant varieties were examined at each site: 90M01 (a 2575 heat unit, large sized soybean) and 25-02R (a 2500 heat unit, medium/large sized soybean). Due to the shorter growing season in Arborg, Apollo (a 2450 heat unit, large sized
soybean) was substituted for 90M01. Each of the two soybean varieties were sown at four approximately evenly spaced seeding dates ranging from the second week of May to the third week of June. Data were collected on soybean height, seed yield and weight, days to maturity, as well as seed protein and oil content.

2007 data are still being analyzed and there is hope that the trial will be carried on in 2008.
Based on the one year of analyzed data, Froese observed some trends in seeding dates and their impact on growth, although the data only reflects the response of two soybean varieties
at three locations over only one field season. It should also be noted that 2006 saw an unusually hot and dry summer growing season. In terms of plant height, Froese reported that although only one variety (25-02R) at one location (Carman) displayed a reduction of height with delayed seeding date, the observed difference of 16 centimetres would likely have a meaningful effect on pod clearance at harvest.

Froese also observed that soybeans sown in the last week of May or later required fewer days to mature, but had a reduced seed yield and weight. The amount of this reduction is a function of the growing season, particularly weather conditions between flowering and onset of frost. “In fact, research from other studies suggests that the actual amount of yield loss, if any, is related to the amount of stress in the growing season in late July to early September,” she explains. This observation correlates with what Brolley has seen in the field over the past two summers, where dry conditions in August resulted in little podding.

Neither protein nor oil content was affected by seeding date in the 2006 data.

Soil temperature a good guide
Brolley says that until the research is completed, growers should use soil temperature to help determine seeding dates. He explains that soybeans, as a warm season crop, germinate at 10 degrees C soil temperature at seeding depth. Germination occurs in three stages. The first is when the seed takes in moisture (imbibes) and the seed coat swells. The second is when enzyme activity starts to convert starch in the seed into sugars for the embryo to use. The third is when the root emerges from the seed.

“The first stage can happen at low soil temperatures, but if the temperatures are too cold for the next two stages, then the seed is at risk of rotting or suffering seedling disease,” explains Brolley.

If pushing the soil temperature envelope in the early spring, Brolley says that growers should try to plant during the warmest part of the day. Because soil temperature fluctuates throughout the day, he suggests planting in the early afternoon to take advantage of the warmer soil temperatures. That way, the seed has a better chance of initiating enzyme
activity. “As long as the weather doesn’t turn really cold for a few days, that can help, but if you’re planting right after canola when the soil temperature is only four to five degrees C, that is taking a big risk,” he says.

Of course, seeding dates are always a balancing act. Growers want to go as early as possible to avoid weather delays at seeding. But going too late can result in a shorter plant and reduced yield – how much lost yield depends on the growing conditions during the reproductive stages. Based on the 2006 trends for the Manitoba study and trends from other studies in other areas, farmers are encouraged to focus on the first half of the seeding window, that is, the second and third weeks of May.

In Manitoba, there is another issue. A combination of the heavy soil (gumbo) and the poor drainage in the Red River Valley means that after a large rainfall it can sometimes be seven to 10 days before farmers can get back into the field. Since soybeans are one of the last crops to be seeded, farmers have become caught seeding in early June due to rains that began May 20. The issue was that farmers were waiting another day or two before seeding and were left having to seed in June. This is the incentive to push the seeding envelope and to get the soybean seed into the ground.

For those wanting to seed earlier, Brolley encourages them to seed as shallow as possible, while still getting the seed a half inch into moisture, because in the spring the soil temperature is cooler deeper down. “Why seed deeper than you have to, which can delay emergence and possibly affect vigour?” questions Brolley. Growers seeding soybeans
early are also encouraged to use a seed treatment. For the earliest fields, wait until the afternoon before starting to seed when the soil has warmed up.


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