Early-Season Management of Soybean

If the old saying that rain on Easter means that it will rain on the next seven Sundays applies to snow, we’re in trouble – it snowed across a wide swath of Illinois on Easter Sunday (April 1) and also on April 8.

We had enough dry weather in March to allow some ammonia to go on early, but there has been little opportunity for field work over the last six weeks. Rainfall over the past month has been below normal for the northern third of Illinois and above-normal in the southern half of the state, especially along I-70. Even though it’s not sopping wet in many areas, below-normal temperatures in recent weeks means very slow drying of soils. While we know that conditions can change quickly – even as I write this the forecast has improved for the rest of this week – it’s clear that the spring of 2018 is not going to be one that allows a very early start for field operations.

Soybean following soybean

With soybean acreage in Illinois expected to increase some and corn acreage to fall this year, some soybeans in 2018 will follow soybeans. As I’ve written before, there is no particular concern in planting soybeans after soybeans, except perhaps to avoid doing this if soybean cyst nematode egg counts are high. We have no reason to expect that SCN counts are unusually high, but if this will be the third year of soybean in the same field or if there was any hint of SCN damage in the 2017 crop, it might be worth taking a count yet this spring. SCN-resistant varieties are a must in any case.

The yield penalty for soybeans that follow soybeans instead of corn varies some by site and year, but most of our research shows this penalty to be modest, usually less than 10%. Averaged over three trial sites and two years (2016 and 2017), soybean following: 1) continuous corn yielded 76.9 bushels per acre; 2) two years of corn yielded 71.4 bushels; 3) one year of corn yielded 69.2 bushels, and; one year of soybean yielded 68.0 bushels per acre. In 2017 we had soybean following two years of soybean, and averaged over three sites, these yielded about 2.5 bushels less than those following one year of soybean.

In two long-running studies in western Illinois, tillage has had either no effect on yield of soybean following soybean, or has decreased yield. If the soybean stubble was not tilled last fall, it would probably be better to plant soybeans without tillage this spring. We did see a slow start to no-till soybeans under the cool, wet conditions of 2015, and at the Monmouth site that year, no-till soybeans following soybeans yielded 5 bushels less than tilled. This differences was even larger in soybean following corn that year. Soybean following corn tends to yield a little more when tilled than with no-till, in fact, though the difference averaged over years is not enough to pay for tillage operations.

Other than normal scouting for disease and weed management, though, there are few other management considerations specific to growing soybeans after soybeans.

Cover crop management

Cereal rye planted into corn stalks last fall has made much less growth than normal, especially in comparison to 2017, when February temperature averaged nearly 10 degrees warmer than in 2018 and the cover crop grew for a couple of months before April. The slow growth this year will continue as long as soil temperatures remain in the upper 30s to low 40s as they are now. But rye is a cool-season crop, and will start to grow rapidly once it warms up.

Conditions have not been good to kill the cover crop with herbicide, so slow growth may be preferable to rapid growth for now. But a choice will need to be made in the coming weeks about how long to let the rye grow before spraying to kill it. We want enough growth to produce the benefits for which we planted the cover crop, but we also need to manage it so it doesn’t interfere with soybean establishment. If soybean seed can be placed into soil well, this shouldn’t be a big concern. But as long as the weather and soils stay cool, soils will dry slowly, especially once the rye is killed and is no longer taking up water. Lower amounts of residue due to slow growth will help some, but soybean seed placement and crop emergence could still be a challenge, especially if soils continue to dry slowly or the weather turns wetter. Clearing residue off the row will help, if that’s an option.

Planting date

In the spring of 2017, soil conditions and temperatures were so favorable early that some people planted a “test” area of soybeans in February to see how they’d do. They did well – soybean are quite tolerant of frost as long as they haven’t emerged yet or have emerged and their “neck” has straightened out to bring leaves and cotyledons to the horizontal. So the period in mid-March with temperatures in the 20s last year didn’t kill the crop, and some of these soybeans yielded as much as those planted in late April.

This year, soils in some areas were dry enough to plant by mid-March, and some people again planted soybean then. [Some corn got planted as well, but that no longer attracts the attention that super-early soybean planting does.] Conditions since have been much less favorable than they were a year ago, and this has kept the early-planted crop from emerging, at least in most fields. Only about 30 growing degree days (base 50) have accumulated over the past month at Champaign – that’s maybe a fourth of the number of GDD needed for the crop to emerge. It will be surprising if soybean seed that has been in cold, wet soils for the past month is still viable, but we won’t know for sure until soils warm up. For the curious, digging up seeds and putting them in a damp paper towel in a warm room for a few days will show whether they’re still alive. Even if they’re alive there’s no guarantee that they’ll be able to emerge and become healthy plants.

Lost in the attention given to the survival of soybeans planted very early is the question about such early planting – provided the crop survives – affects soybean yield. Given how rare it is that soybeans can be planted by or before mid-March, we have not done trials on this. We mostly have anecdotes, and those may be skewed towards those times when the crop survived. We have seen a few cases, especially in the very dry spring of 2012, when planting in early April was followed by stressful (cool or dry) conditions that limited plant height and yield compared to soybeans planted later. Even if soil conditions allow a March-planted crop to emerge, there is virtually no chance that it will yield more than a crop planted in the same field in late April, and some chance (if it survives) that it will yield less.

Overall, our data across 26 soybean planting date trials show that soybeans produce full yield if planted anytime between the second week of April and the end of April. The rate of yield loss with planting delay accelerates into May, reaching about 2/10ths of a bushel per day by the end of the first week of May, a quarter of a bushel per day by mid-May, a third of a bushel by the end of May, and 4/10ths of a bushel per day by June 10. These are lower loss rates than we often see presented elsewhere, most of which are based on a limited number of trials. That doesn’t mean we shouldn’t try to plant as early as we can to get full yields, but it does show what most farmers know from experience – that high soybean yields depend more on what happens during the season than on when the crop gets planted, although planting by mid-May increases the chance that the crop will be able to respond to favorable conditions later. A corollary to that observation is that planting soybean into poor soil conditions just to get them planted early can decrease the ability of the crop to respond to favorable conditions later, and thereby end up costing yield.

Seeding rate

While 100,000 or even fewer plants per acre will maximize yield in many cases, our research shows that this is not always enough plants. Trying to minimize the seeding rate can end up costing yield and losing money, especially in those cases when emergence and stand establishment are lowered by conditions at or after planting. While responses to plant stand do vary across trials, we have found that 115,000 to 120,000 plants (not seeds) per acre are often needed to produce the highest dollar return on the seed investment. If we plant good seed into good conditions we can expect 85% stand establishment, in which case we should plant about 140,000 seeds per acre, which for most seed companies today is one unit of seed.

Nitrogen

Despite that fact that most trials in Corn Belt states in recent years – including our trials in Illinois – have shown little or no yield increase from applications of 45 to 90 pounds of N (100 to 200 pounds of urea) during the growing season, this practice continues to draw a lot of interest. In a set of trials we just finished, applying 45 or 100 pounds of N at planting time produced large increases in yield two years in a row on an irrigated loam soil near the Illinois River at Chillicothe, Illinois. Planting-time N had no effect on yield in most other trials in heavier, higher-organic-matter soils. We did find yield increases in a number of trials when we applied the same amount of N four different times, from planting through early podfilling. While repeated use of N may help explain some “contest” yields, the yield increase from four applications was not enough to pay even half the cost of these applications. Putting that much N on also means a lot of N left in the soil at the end of the season, so more N loss through tile.

With so many voices today claiming that N application on soybean “can” increase yield and others saying that it still won’t increase profits, what should producers do? In an ideal world, 500 Illinois farmers would put out a set of N strips (next to strips without N) in a field or two each year, and results would be brought together to produce data-backed expectations of how profitable this practice is on different soil types and across years. One of the reasons that’s difficult today is that so many soybean fields are harvested on an angle to the rows, making yield data collection difficult or impossible. There is also no one to organize such work and little noticeable interest by those who might fund such a project. “Trying” N by applying it to a field or two is sometimes suggested by those who feel that this is a profitable practice. This approach, of course, provides no information on whether or not applying N did anything.

For those interested in a “lite” N trial on soybean

Both times that we’ve seen a large yield increase from N on soybeans were on lighter-textured soil with N applied at planting. Applying N at planting typically makes leaves and cotyledons of small plants darker green in color compared to plants without N. In cases where N ends up increasing yield, this darker green color persists into vegetative stages, and plants tend to show increased growth and more green through most of the rest of the season. Where planting-time N doesn’t increase yield, the difference in green color between plants with N and those without N disappears as the plants make vegetative growth, and as their roots get more access to N from the soil and from N fixation in nodules. By the time plants are 6 to 8 inches tall, the effects of planting–time N are often no longer visible.

Based on what we’ve seen, I’m suggesting a low-cost alternative to large-scale application of N as a way to see where and how often N might have the potential increase soybean yields. Here’s the outline:

  1. After planting and before emergence, choose a uniform spot at least 20 feet away from endrows or edge of the field, and put flags in the corners of an area 15 feet x 15 feet square. We expect to see N effects more often on soils that are lighter in texture and lower in organic matter, so place this accordingly, in two or three fields or parts of fields with contrasting soil types if that’s an option. If possible record GPS coordinates for each site.
  2. Weigh out enough N fertilizer – urea or lawn fertilizer (without herbicide) – to provide 50 pounds of N per acre on the 225 square feet you flagged out. Calculate this by dividing the number 25.83 by the percentage of N in the product (urea may be 46-0-0, or 46% N; lawn fertilizer might be something like 27-0-4, or 27% N) to give the amount of product needed. As an example, if using urea (46% N), you would need 25.83/46 = 0.56 lb. of product. Multiply this by 16 to get number of ounces, or by 453.6 to give number of grams, if you have a gram scale. If you have a measuring cup but not a scale, urea weighs roughly 3/4ths as much as the same volume of water, so a cup (8 fluid ounces) of urea weighs about 6 ounces.
  3. Apply the fertilizer carefully, by hand or using a hand spin-spreader, uniformly over the soil inside the square.
  4. The “data” to be taken can, for most people, just be a photograph with the image split between the area with N and an area (outside the square) without N. I suggest taking a photo at about the V2 stage (two full trifoliolate leaves present), and another one about a month later, at perhaps V6-V7, when plants may be a foot tall or so. Find the side of the square that gives the best contrast under existing light conditions. Feel free to supplement the photo by noting what you can see (or not see) by eye.
  5. If the second photo shows no difference in greenness of plant size between those that received N and those that didn’t, the experiment could end there, with the conclusion that N probably made no lasting difference on growth, and so is unlikely to increase yield. If the plants with N are still greener and/or larger than those without N, that would be a signal to come back once or twice more, to see if the differences persists to the podfilling stages in mid- to late August, and again before leaves drop.
  6. If plants inside the treated square are visibly different than those outside, and there’s enough ambition and curiosity, you could harvest 15 or 20 randomly-selected plants inside the treated area and outside the treated area, and take a photo with the two sets of plants next to one another to show any visual effects on height or pod number. Those interested could count the number of pods, or even thresh the plants (in burlap bags works best) and weigh seed to estimate yield. Calculating yield would require an estimate of number of plants per foot of row. Yield estimated this way are highly variable, so they may not line up with what we thought we’d see based on plant size and appearance.

I’d be happy to look at photos from such comparisons; if there’s enough interest I could also develop a small reporting form to make a record for each trial. I’ll also be glad to send a layout for anyone interested in doing a strip trial with and without N. I can be contacted by email (link below, on my name) or my cellphone number is (217) 369-1997.