Corn and soybean after a slow start to the 2024 season

Although April-May rainfall exceeded normal amounts in Illinois by up to 50 percent in some areas, and average temperatures have exceeded normal by several degrees, the weather record fails to capture what the 2024 planting season actually looked like. Planting in parts of the state was nearly complete by early May, while in other places, many acres remain to be planted going into June.

Planting began early, with 1 percent of both corn and soybeans planted in Illinois by the end of March. Statewide, planting progress was slow in April, with around 30% of both crops planted by the end of the month. Progress in May was unsteady, but by June 2, 89 percent of corn and 81 percent of soybean crops had been planted. Those percentages are ahead of the 5-year (2019-2023) averages, but only because less than half the corn crop and less than one-fourth of the soybean crop had been planted by early June in 2019. Averaged over the last four years (leaving out 2019), about 95 percent of corn and 86 percent of soybeans were planted by June 1 in Illinois. Both crops this year trail those percentages by about 5 points.

Corn or soybean first?

Although much has been said and written about the need for planting soybeans early—even earlier than corn, according to some—the percentage yield loss from planting the two crops in early June is similar, at about 15 percent for corn and 13 percent for soybean. As planting is delayed further, corn yields decline a little faster than soybean yields, but we have also found that the range of yields (as percent of maximum) is wider for late-planted corn than for late-planted soybean. In other words, percent yield loss from planting corn late can be smaller—but can also be greater—than loss from planting soybeans late.

While corn may be able to overcome a late start a little better than soybean, this happens only if the weather is favorable the rest of the season. We think that is in part because late-planted soybeans have their critical yield-forming growth periods (such as flowering and podsetting) shortened by daylength effects. Because corn tends to develop according to growing degree days, growth periods needed for high yields can be maintained as long as stress levels remain low. In four of the past ten years of a corn planting date study at Monmouth, planting date has had no effect on yields; over those four years, corn planted between April 16 and May 5 averaged 246 bu/acre, while corn planted between May 25 and June 4 averaged 245 bu/acre.

While one year will not answer the “do I start planting corn or soybean first?” question, the sporadic heavy rainfall, storms with wind and hail, and occasional soil crusting in 2024 seems to have favored corn over soybean so far. Figure 1 below shows soybeans planted on April 17 and corn planted on April 22, in nearby fields at the UI South Farms near Champaign. More than three inches of rain fell in the week after planting corn, and corn emergence was delayed some by crusting and wet soils. Stands are good, and corn responded to warm temperatures, with corn crop conditions, especially leaf area, visibly better than those of soybeans. These early-planted soybeans will probably show some flowers over the next week, but plants are small, and the main flowering period will come in July. This year might be one in which the advantage to early planting goes to corn over soybean. But a lot of season remains, and we know only that the severity and timing of stress is unlikely to affect the two crops differently, regardless of when they were planted.

Managing late-planted crops

While a lot of advice is being offered with regard to how we can manage corn and soybeans to lower the penalties following a tough start to the season, previous experience with such seasons, and some research, has shown that our eagerness to “do something” to help the crops often fails to help, especially when we rush to supply more inputs without paying enough attention to how likely such inputs are to help. If the weather turns dry in July, for example, yields may be limited by lack of adequate water, and that can be worse when soils have been compacted by planting (late) into compacted soils in order to speed emergence and give the crop a few more days to develop. With that in mind, we offer a few points below, along with caution that weather will be the big factor in determining how well crops respond to management, and what final yields will be.

1. Getting the rest of the crop planted: The last full-insurance date for corn is June 5 in nearly all of Illinois, and for soybean is June 15 in the northern 1/3^rd of Illinois and June 20 in the rest of the state. For doublecrop soybeans, July 5 is the last date. Crops can be planted after these dates, but maximum insurance coverage will decline with each day of delay. These are also the earliest dates to file for prevented planting. We generally avoid switching to earlier-maturity soybeans when planting late, because early-maturing varieties tend to have shorter critical growth periods and so end up with fewer seeds to fill. We would generally not want to switch to earlier-maturing corn hybrids unless the seed on hand is later than mid-season – about 111-day RM for central IL; 109-day RM for northern IL.
2. Nitrogen for corn continues to be an issue, especially in areas that received above-normal rainfall after most, or all, of the N was applied. Most of the fertilizer N that was applied by early May is now in the nitrate form, so subject to possible loss by leaching (in lighter-textured soils) or denitrification (in heavy-textured soils and where water has stood during warm periods.) On the other hand, microbial mineralization of N from soil organic matter is occurring at high rates in warm soils now, and this N is initially in the ammonium form, not subject to loss. Ammonium will convert to nitrate quickly in warm soils, but the steady N supply will help plant access to N as they begin to grow. If all of the N for the crop was applied last fall and/or in early spring, whether the crop will need additional N depends a great deal on whether soils dry out to allow roots to develop and to reach the N in the soil over the next few weeks. If that happens, and if the crop maintains good leaf color, supplemental N may not be needed. The corn plants shown in Figure 1 are in an N trial, and plots without N are not showing deficiency at stage V6-7. In another part of the field where corn is following corn, zero-N plots are beginning to show signs of N deficiency. But if fields with normal (MRTN) N rates of 175-200 lb/acre for corn following soybean remain free from standing water and with healthy green color, applying supplemental N is unlikely to be necessary.
3. Row spacing and seeding rate for soybeans: We have not seen much evidence that late-planted soybeans consistently yield more in 15-inch than in 30-inch rows, but having rows closer together can help canopy development when plant growth is limited by stress, so using 15-inch rows makes sense. Late planted-soybeans tend not to need higher seeding rates than early-planted ones, but they may respond to seeding rates higher than some would call “normal” today. Conditions this spring have not been great for soybean emergence and plant establishment, so early seeding rates of about 100 thousand per acre may have resulted in stands too low to maximize yield. Inadequate soybean stands should be supplemented by planting more seed without destroying the existing stand. We suggest adding enough seed to bring stands to at least 110,000 plants per acre.
4. Corn plant stands are mostly good where water hasn’t stood in fields. Low spots that held water when air temperatures were high typically have no stand; these can be filled in once they dry out, as long as this can be done without too much damage to existing stands. Such damage can include overpopulating in places with existing stands. Despite what some have claimed, a corn plant that emerges later than its neighbors hardly ever turns into a “weed.” Plants with small or missing ears lower overall yield, of course, but tend not to compete much with their neighbors. This means that ears on neighboring plants tend to get a little larger than they would have gotten if that plant were normal. If the “runt” plant were a weed, its presence would lower yields of neighboring, early-emerging plants.
5. Using in-season fungicide before fungal diseases are found has not proven to be more likely to increase yield in late-planted compared to early-planted corn or soybean. Fungicides should be used when disease levels are high enough to make a response likely. This principle can be difficult to apply in the case of tar spot of corn, which can develop so quickly that fungicide applied after scouting may not prevent yield loss. So it pays to stay tuned to reports of diseases showing up, and to act once a disease seems likely to lower yield by at least the amount needed to cover the cost of the treatment.

Does the start of the season predict yields?

The quick answer is no: the earliest start to the growing season ever in Illinois was in 2012, when rains returned too late for any yield in a lot of fields. In contrast, a major rainfall event in early May in 2017 caused very large amounts of replanting, yet that was the first year that Illinois corn yields exceeded 200 bushels per acre. Although weather in July and August is more important for yields than planting date or early-season conditions, late planting means that crops miss out of some favorable growing conditions early in the season, and getting trendline yields in such years requires that weather be unusually good. There is nothing to suggest that this won’t happen in 2024, but no guarantee that it will.