Top Crop Manager

Key factors in soybean production

Study results prove using recommended soybean seeding rates is still the best strategy and is good insurance against other factors that can impact yield. Photo courtesy of Agri Skills Inc.

Soybean growers often look to seeding rates, row spacing and equipment selection as having the biggest impact on yield. However, other key factors can have an effect on yield as well.

From 2010 to 2013, Brent VanKoughnet, with Agri Skills Inc. of Carman, Man., conducted a field scale soybean study for the Manitoba Pulse Growers Association (MPGA). The four-year field evaluation of multiple seeding rates, seeding dates and row spacing comparing planters and air drills in Manitoba came to some interesting conclusions.

The objectives of the study were to compare the effect of multiple seeding rates of soybeans with two different seeding implements and four different row spacings in a full field scale environment. The project also compared the impact of plant architecture of upright versus bushy varieties. Replicated treatments were compared for each variety at a field scale, with treatment strips approximately 1200 to 1500 ft long, or about one to 1.5 acres.

In the 2013 trial specifically, two varieties (bushy and upright) were seeded comparing a vacuum planter on 15- or 30-inch spacings, and an offset disc air drill on 10- or 20-inch spacings. The planter sowed 30 32-ft strips and the air drill sowed 44 ft strips at low (123,000 seeds/acre), medium (150,000 seeds/acre) and high (176,000/acre) seeding rates, with an extra 15,000 seeds/acre for each of the air drill treatments.

“The results from 2013 and previous years of the trial really proved soybeans are very resilient and [they] adjusted to the variables much better than we expected,” VanKoughnet says. “The yields for plant populations even below 100,000 plants were surprisingly good, with not nearly as significant an increase for higher or more normal seeding rates as might be expected. Even in years where plant populations were much thinner than targeted, the yield penalty was closer to one or two bushels per acre instead of the 10 bushels per acre we would have estimated in the spring.”

The results also proved using recommended seeding rates is still the best strategy and is good insurance against other factors that can impact yield. VanKoughnet points to the last year of the trial when, for reasons they aren’t sure about, plant emergence dropped to 65 per cent compared to the 85 per cent target that was achieved in the first three years of the trial. “In most years, we typically see 85 per cent of the plants surviving. However, whether it was seed quality, seeding depth or something else, it dropped to 65 per cent in year four,” he notes. “What it does show is you can’t bank on getting an 85 per cent plant stand every year, and if you cut your seeding rates too low, you may end up at risk with lower plant stands and lower yields. Using recommended seeding rates is still the best insurance to buffer against various risks.”

In the first three of the four years of trials, survival percentages for the planter were better than for the air drill. In year four the air drill stand percentages were slightly higher than for the planter; however, the plants were not as evenly placed and they emerged less uniformly. Even in that last year with the higher seeding rates (15,000 more seeds per acre) and the higher survival, the air drill still did not translate into higher yields than the planter.

“Although there has been some concern that 30-inch spacings are too far apart for soybeans, with both varieties, the 30-inch rows met or exceeded the yields of other spacing,” VanKoughnet says. “In only one year of the study, there was a slight negative response to 30-inch rows with the more upright variety under dry conditions. For insurance, growers using 30-inch rows may want to consider growing a more bushy variety.”

One additional consideration is frost protection, and seeding into black ground with as little trash as possible. Although this is somewhat counter to direct seeding, VanKoughnet’s experience in the trials and on his farm showed seedlings in black soil tolerated a light frost, while seedlings with straw cover died. The bit of heat in the black soil seems to make a substantial difference in tolerating spring frost.

In a different multi-year trial, it was discovered seeding date had little to no effect on plant height or pod height. However, minor plant height and pod height differences were observed between different row spacings and even between different seeding rates. Height differences at harvest seemed to be most influenced by how thin the plant stand was. Areas with thicker stands and more competition produced taller plants. Thirty-inch rows effectively produce more in-row competition.

“In the first three years, pods were typically three inches off the ground in all treatments. However, in the last year of the study, all of the pods were closer to two inches off the ground, with about 20 per cent of the pods near the knife point,” Van Koughnet notes. “This change in pod height can easily affect what the flex header was able to reach and consequently translate into yield losses. The high seeding rates on 30- or 20-inch row spacings appear to have increased pod height marginally.”

Overall, equipment is not the most important factor: a well-placed seed with any machine gives good yield potential. “If you are new to growing soybeans, use whatever equipment you have for two or three years and once you have some experience, then you will know how you want to grow them in your operation,” VanKoughnet says. “Under good conditions, an air drill can do a good job, and adding an extra 10,000 to 15,000 seeds per acre is a good idea. If conditions are variable, a planter provides more forgiveness and a better seedbed over a range of drier and wetter conditions, but you can still get a good crop with an airseeder. If you are looking to buy a planter, consider looking at used planters from the U.S. where there are more available and at a reasonable price.” In many cases, the first piece of equipment to consider may be a used flex header for harvesting rather than a new piece of seeding equipment.

VanKoughnet says from his own research and from what he’s heard from Ontario, the greatest impact on yield potential is more likely to be a rainfall at the end of July or early August at the time of pod set.

“Whether the plant stand was tall and substantial or short and sparse didn’t really matter so much. If there was adequate rainfall at the time of podding, you had a good crop,” he notes. “I’ve been monitoring this at our plots on my farm, and a rainfall at the end of July or early August when the crop is filling out pods creates the most consistently significant impact on yield. In the dry years, without a timely rainfall we can still get 35 bushels per acre, a yield that is often competitive with canola and can still make us money. In good years, with a timely rainfall, we can get 55 or 60 bushels per acre, even if the plant stand looks a little thin. In areas where rainfall is less likely at the time of podding, yield potential may not be as high.”

Based on this four-year field study, the yield difference between high and low seeding rates and row spacing were either negligible or smaller than expected every year, even with much lower than planned plant populations. Yields were also very similar across the earlier seeding dates, with a distinct yield drop for the latest date in a couple of those years.

“Under the conditions observed so far, the soybean plant has compensated remarkably well,” VanKoughnet adds. “Using recommended seeding rates, timely seeding and good agronomics will get the crop started. And with the timely rains at podding, yield potential can be optimized.” 


December 7, 2015  By Donna Fleury


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