Cool Temperatures at Planting

Planting is off to a fast start in Illinois, with 5 percent of the corn crop and 3 percent of the soybean crop planted by April 11 (NASS). With the exception of much of western Illinois, where heavy rain fell on April 8-9, most of Illinois remains dry, and planting continues, but with some concern about low temperatures that persist.

Temperatures the first half of April were close to average, but were above average the first week and below average the second week of the month. Growing degree-day accumulations by April 15 ranged from about 95 in northern Illinois to 150 in the southern part of the state. Below-normal temperatures are expected to continue through the third week of April, with lows ranging from the 30s to the 40s and highs in the 50s and 60s in northern and southern Illinois, respectively. That means totals of only 20 to 30 GDDs for the week in northern Illinois, and 50 to 60 in southern Illinois.

Corn typically requires about 120 GDD (base 50) from planting to emergence, and soybean may require about 130 GDD; these aren’t exact for either crop, but should be close when soil temperature is around 50 degrees at planting. This means that crops planted in late March or early April in the southern half of Illinois have emerged or are close to emergence, but most of those planted several days after April 1 in central and northern Illinois are still waiting to emerge. Fields planted after the first week of April have not emerged, and may not have sprouted yet.

Daily minimum soil temperatures at the 2-inch depth have trended downward over the past week, and are in the upper 30s in northern Illinois and in the mid-40s in central Illinois. Helped along by dry soils and good sunshine, daily high soil temperatures are mostly in the 60s and 70s. These are expected to continue over the next week, although cloudy days will mean lower maximum and possibly higher minimum temperatures. We no longer use threshold soil temperatures to tell us when to start planting, in part because seed quality and treatments help with stand establishment so allow us to stretch the planting window on the early side. Still, soil temperatures in the 40s are not ideal for establishing the uniform stands we’d like to see.

Some producers are wondering if they should proceed to plant or if they should wait until temperatures return to normal. Seed planted within the next few days will do little besides start to take in water, and this water will be colder than is ideal. This increases the chances of imbibitional chilling injury, which is membrane damage that can cause abnormal growth. Such injury is not common, but there is also little potential benefit to offset the risk of planting earlier into cool soils: planting early doesn’t do much to increase yield potential when it’s too cool for growth. Delaying planting will mean that soils will begin to warm sooner after planting, but will also increase the chance that wet weather will cause further delays. One way to split the difference is to wait a few days, and to begin planting on April 19 or 20 only if the forecast still has temperatures starting to warm during the last week of April. Some may want to take this chance to apply N or do other work that would normally wait until after planting.

The possibility that wet weather will return to delay planting is of course a major incentive to plant when soil conditions allow in April, even if it’s still cool. With relatively dry soils in most areas, rainfall events of an inch or less won’t cause much delay in planting, especially once warmer temperatures arrive to speed drying rates. If the forecast includes heavy rain soon after planting, especially if that accompanies a cold front that drops temperature, the decision whether to keep planting gets more difficult. We’ll remain optimistic that we won’t have this problem in 2021, but if the forecast changes, we may want to revisit the plant-now-or-delay question.

There is some concern about uniformity of emergence of corn, and perhaps of soybean, planted into cool soils. Once GDD accumulation after planting reaches 100—with daily lows less than 50 and highs of 60 that could take three weeks—check germination and sprout length. Recent work at Ohio State University showed that it took 36 soil GDD (we can take air and soil GDD as equivalent) to move from 10 to 90% emergence in one field. That would mean an emergence window of about two days with day/night temperatures of 60/76, but as much as a week with daily high temperatures in the low 60s. This difference from first to last emergence will mostly disappear once it warms up—36 GDD represents only half a leaf stage during early vegetative growth—so is not a cause for concern as long as emergence percentage is good and seedlings are healthy. With cool soils, the claim that plants that emerge more than 48 hours later than the first ones turn into “weeds” can be ignored. Wet soils add disease or low oxygen to the list of concerns, and fields in those areas should be checked more closely.

The question about whether yield potential is affected by low temperatures that persist after planting, or that return after the crop has emerged, remains unanswered. We saw in 2018 that when temperatures began to warm at planting and stayed high after planting, stands were uniform, growth was rapid, and yield potential was high. In 2017, heavy rain and low temperatures after planting led to massive replanting of corn, and yields from May planting were usually higher than from corn planted in April that survived the deluge. We have also noted what I’ll call “failure to thrive” when plants experience wet soils and cool temperatures after emergence. In a few planting date studies, corn planted in early April that experienced low temperatures (but not frost) after emergence in May yielded substantially less than corn planted later in April. This has happened in soybean trials as well, but rarely. The hypothesis is that low temperatures, especially when soils are wet, affect the physiology of the plant in a way that limits pod or kernel numbers very early in the plant’s life. It’s a very difficult hypothesis to test in an experiment.

While more corn than soybeans have been planted to date in Illinois, many producers started planting soybeans before corn this year, as in recent years. We have found that both crops respond the same to planting date on a percentage basis, which suggest that planting order makes little difference. But some agronomists continue to promote the idea that soybeans need to be planted early enough so flowering begins before the longest day of the year (June 21), and that this means more pods on lower nodes. Soybean plants need to reach the V3 stage before they are physiologically ready to flower, and that needs to happen by early June in order for flowering to begin before the solstice. In fact, soybeans often don’t flower by June 21 because they don’t grow enough by early June, even when they are planted in April. Cooler nights also delay the start of flowering, even if plants are far enough along.

According to NASS, soybean flowering began before June 21 in six of the last 20 years in Illinois, with percentage flowering by June 21 ranging from 2% to 13% (in 2018) of the Illinois crop. That’s an average of 5% over those six years, and of 2% over 20 years. Years in which the Illinois soybean crop began to flower before June 21 were almost exclusively those with above-normal temperatures in May following timely (but not necessarily extra-early) planting. Of the six years with early flowering, three (2004, 2016, 2018) ended up with above-trendline yields, and three with below-trendline yields, including 2003, with the lowest soybean yield in the past 32 years. This means that having much of the soybean crop flower before the summer solstice is relatively rare, and also that an early start to flowering doesn’t always lead to high yields. Southern Illinois is warmer, but longer-maturity soybeans there need a longer night to begin flowering, and so need to reach V3 earlier in order to flower before days get long enough to stop the flowering process in June; this process starts up again in July when nights again get long enough.

It is certainly a relief to have planting begin on time and under better soil conditions than we have had the past two years. We’ll remain optimistic that warmer temperatures will appear soon, and that the crop will get off to a great start. While we know that a great start brings no guarantees, we’ll take it every time over a bad start.