The 2019 Illinois corn crop reached 50% planted during the first week of June, more than a month later than the average of the past five years. The soybean crop reached 50% planted a few days later than corn, and more than three weeks later than the average of the past five years. May rainfall was above normal over most of Illinois, and June brought near-normal rainfall over much of the state. Still, the late planting coupled with too much or too little rainfall after planting produced July crop condition ratings of only about 40% good + excellent for both crops, compared to an average of some 70% over the past three years.
While corn crop condition ratings haven’t improved over the past month, the warm temperatures and good sunlight have resulted in good growth of the crop in most fields, except in low, wet spots. Leaf color of the crop has also improved, as soils have dried enough to allow the roots to reach N from both fertilizer and from mineralization. This has helped the crop plants and crop canopy to recover some from the rough start.
The 20% of the corn crop planted between mid-April and mid-May has reached or completed pollination in most fields, in line with growing degree day accumulations: at Champaign, about 1,400 GDDs accumulated between May 15 and July 15, enough to get a 110-day RM hybrid to, or close to, pollination. That number is only about 1,140 at DeKalb, where little corn was planted by mid-May anyway. With little disease or insect pressure and with dark green leaves, most of the fields that have reached or finished pollination should end up with high kernel counts, which means high yield potential.
Most of the Illinois corn crop was planted at the end of May or during the first half of June, and little of this late-planted crop has reached pollination: only 19% of the Illinois crop pollinated by July 14, which is about the same percentage of the crop that was planted by mid-May. June 1 through July 15 GDD accumulations totaled about 930 at DeKalb, and about 1,060 at both Champaign and Mt. Vernon, in southern Illinois. At current GDD accumulation rates of 25 to 30 GDD per day (this will decrease some with the cooler weather next week), corn planted in early June will reach begin to pollination towards the end of next week. Corn planted after June 10 will probably not pollinate until early August, although early hybrids—we can estimate about 10-11 fewer GDD for each day earlier in RM, or about 2 days earlier for each 5-day drop in RM—may pollinate by the end of July. Objective yield estimates, which use kernel counts, will be difficult to make early this year, with so little of the crop through pollination by the end of July.
There is ongoing concern about how the season will end for the late-planted crop, including whether or not it will mature before frost. If fall frost comes at or after its 50% date, which is October 15 to 20 depending on location in Illinois, the late-planted crop should be at or close to the GDD needed to mature by that time, as long as the maturity of the hybrids planted was adjusted for very late planting. From June 1 through September 30, GDD accumulations for northern, central, and southern Illinois average about 2,400, 2,600, and 2,800, respectively. Including October 1 to 15 adds about 150 GDD to these totals, but delaying planting from June 1 to June 15 subtracts about 300 GDD, or about 20 GDD per day of delay. So in northern Illinois, corn planted on June 10 is expected to accumulate only about 2,350 GDD by the average date of first frost. That’s enough to get a 95-day RM hybrid close to maturity, but not a 110-day hybrid. While a decrease is the number of GDDs required for a hybrid when it’s planted late has been noted, we have not seen a decrease in the number of GDDs needed to get the crop to its current stage so far this year, and any decrease in the number of GDDs required from now to maturity is likely going to come at the cost of losing yield due to late-season stresses that limit grainfilling.
The main challenge facing the corn crop at this point, especially the late-planted crop, is having enough water to maintain growth and to set and fill enough kernels to reach its full yield potential after the difficult start to the season. While there is enough soil moisture to maintain the crop in most Illinois fields now, the late-planted crop especially is showing symptoms of water stress under high temperatures this week. We think this is mostly because the root system of the late-planted crop is somewhat limited, both due to compaction from planting into wet soils and also because late-planted corn that develops under high temperatures tends to favor top growth over root growth. So even though the late-planted crop likely has more water in the soil now than the early-planted crop (which has used several more inches of water up to now), access to this water is somewhat limited, and the result is leaf-curling by early or mid-afternoon. In drier areas, even the early-planted crop is showing some afternoon stress, especially on slopes where the soil holds less water.
The cylinder formed by a curled corn leaf under water stress cuts the windspeed across the leaf surface and lengthens the distance the water vapor has to move once it exits the leaf, so greatly decreases the rate of water loss. That might seem like a good thing, but when leaves aren’t losing water very fast they also aren’t taking in carbon dioxide very fast—that is, their photosynthetic rates are low. Low photosynthetic rates mean slow growth, and a prolonged period of low photosynthesis means lower yield potential. A few days of water stress before pollination won’t lower yield potential by very much, but a week or more of decreased photosynthesis will decrease kernel number, and so will lower yield potential. If such stress continues after pollination, more kernel abortion will lower yield potential even more.
Although the yield potential in early-planted fields without standing water damage appears to be high now, getting the crop to reach the potential with high kernels numbers—16 to 20 million kernels per acre—will require about 10 more inches of water this season. Depending on soil and current soil water supplies, half to three-fourths of that amount of water will need to come from rainfall over the next 50 days or so. The late-planted crop is likely to set somewhat fewer kernels, but it will likely need 11 or 12 more inches of water in order to pollinate and fill kernels successfully. Some rain soon might help to increase the size and competency of the root system and so help the crop better extract water that’s in the soil. But as pollination approaches, the plant shifts its allocation of sugars (from photosynthesis) away from roots and stalks to development of the ear and to completion of the pollination process. So in fields where the crop is under water stress now, the chances of getting good yields will continue to diminish the longer it goes before rain arrives. Cooler temperatures will slow the rate of this decrease by slowing the use of water, but in areas that are dry now, there needs to be water falling from the sky before pollination if this crop is to regain some of the yield potential lost with late planting.
Especially in the late-planted crop, there is some talk about needing to apply foliar fungicide and, in some cases, perhaps more N fertilizer. If enough N was applied earlier, there should be no need to apply more, especially if the upper leaves have a dark green color in the morning before stress begins to show. Some have suggested that late-planted corn will more likely need foliar fungicide, with the idea that protecting the canopy is more important when (with late-planted corn) there is less leaf area, or perhaps with the expectation that fungal diseases will more likely attack corn that pollinates late. The planting date studies that we’ve conducted in Illinois have, since 2010, included foliar fungicide applied at pollination as a treatment. Results do not support the idea that fungicide increases yield of late-planted corn more than it does of early-planted corn. Averaged across 21 trials in the northern half of Illinois, foliar fungicide increased the yield of corn planted in early April by 7 bushels per acre, and increased yield of corn planted in late May by 6 bushels per acre. Across 12 trials in the southern half of the state, fungicide increased the yield of corn planted in mid-April by 7 bushels, and increased the yield of corn planted in early June by only 2 bushels per acre. With corn crop prospects so closely tied to the water supply for the rest of this season, spending money on additional inputs (besides irrigation) may not be very helpful to the bottom line.
Most of what I mentioned about corn crop condition and planting above applies to soybean as well, with even fewer soybean acres planted early (in April). May was not a great month for early-planted soybeans in any case this year, even if they had good enough stands and didn’t need to be replanted. Greg Steckel and Marty Johnson were able to plant soybeans in a planting date study on April 9 at Monmouth. Those plants are now about 18 inches tall, and, in 15-inch rows, they have formed a near-complete canopy. Most of the soybeans in Illinois, though, are “canopy-challenged” now, and it’s likely that some of the 30-inch rows will not form a complete canopy at all. Late planting also resulted in a slow start to flowering—only 12 percent of the crop flowered by July 14 this year, compared to 77% on the same date in 2018, and a 5-year average of 54%.
Like corn, late-planted soybeans in the drier parts of Illinois are showing symptoms of water stress. These aren’t as noticeable as they are in corn, but are mostly seen as a slight drooping of leaves in the afternoon as the stress intensifies. The result is the same, though—plants under stress for much of the day do not do photosynthesize very well, and so they do not make very fast growth. This is compounded by the fact that late-planted soybeans have small root systems that can’t get access to very much soil water, and as a result, leaf area and plant height are increasing slowly. Root damage from wet soil conditions after planting also contributed to this in some places, and growth has been slow in these fields or parts of fields as well. As we know from past experience, soybeans are likely to “come around” and make good growth once they get past this period of slow growth, but it’s certainly better when that happens in late June than in late July.
While we’ve historically said that soybeans are less sensitive to late planting than corn, that was more true when soybeans yielded 40 to 50 bushels per acre than when they yield 70 or 80 bushels per acre, as they have done in recent years. Rapid early growth and canopy formation is key to high soybean yields, and this has not happened—and will not happen—in Illinois in 2019. With only about 20% of the Illinois soybean crop emerged by two weeks before the longest day of the year (June 21), the hours of sunshine on that day did little for the crop. The only path to good yield potential this year will be through a turn to unusually favorable weather, with good rainfall and temperatures in August and good rainfall and above-normal temperatures in September. We know from doublecrop soybeans that yields of 50 bushels per acre or more are possible when planting is late, but that does not tend to happen when growth through mid-July is as limited as it is in many field this year. We don’t have much information on how soybeans planted in late June or early July will do in central Illinois, but the slow growth of the crop since planting is a cause for concern.
Along with symptoms of water stress in some soybean fields is a lightening of canopy color that may indicate lack of adequate nitrogen. In fact, the N fixation process is sensitive to plant stress, and this could be a factor. It’s also the case that soybean leaves tend to give up their N (that is, break down proteins and export the N to the rest of the plant) as stress continues, in some cases even dropping leaves like they normally do at the end of the season. There are two reason why loss of leaf color is a concern. One is that soybean plants store significant amounts of N in their leaves as podsetting starts, and how much N is stored this way is a major factor how many pods set and how well seeds fill. The other concern is that paler leaves means lower current photosynthesis, and so slower growth. The “dark green blanket” of plants and leaves that we saw in nearly every field in mid-July in 2018 is not present in many fields this year, and this makes it hard to be optimistic about this year’s crop.
As with late-planted corn, some have proposed adding N or using foliar fungicides as a way to lower the stress on soybean plants and to increase yields this year. But when growth is limited by the amount of water available, either because soils are dry or because roots don’t have access to the water in the soil, then adding other inputs is unlikely to add much yield. In the soybean planting date studies we conducted since 2012, we included a treatment comparing foliar fungicide (usually with an insecticide as well) with no fungicide at each planting date. Over six trials in southern Illinois, foliar fungicide produced an average yield increase of 4.1 bushels per acre when planting was in mid-May, and of 3.7 bushels per acre when planting was in mid-June. Across 17 trials in central and northern Illinois, foliar fungicide increased yield by 2.5 bushels per acre when planting was in late April, and by 1.7 bushels per acre when planting was in early June. We have not done similar studies using fertilizer N, but have found such limited response to using N on soybeans that it’s unlikely that it will ever pay on soybeans, especially on soybeans that undergo stress that limits early growth.
As I’ve often stated, the crop canopy will “tell the tale” of this year’s corn and soybean crops better than any other thing we can look at. If in mid-August we see fields with dark green leaves, we can have hope for good—if not great—yields.
Source: Emerson Nafziger, University of Illinois