Notes as Fall Harvest Continues
After an early start to fall harvest in 2021, widespread rainfall over most of Illinois in the second week of October slowed harvest progress; by October 17, 62% of the corn crop and 51% of the soybean crop were harvested, compared to 5-year averages of 59 and 62% for the two crops, respectively. With rainfall totals of 2-3 inches over most of northern and central Illinois, many fields remain wet as harvest activities resume this week.
Harvest losses
September was warm and dry, and grain dried down quickly, with a lot of corn harvested at grain moistures of less than 20% and soybeans at less than 12%. One “bonus” of rainfall following harvest during the dry weather is that grain left on the surface at harvest tends to germinate. We know that harvesting dry grain can increase harvest loss, but seeing fields turn green shows the extent of the problem. Based on 2 corn seeds or 4 soybean seeds per square foot representing one bushel per acre, I estimate that as much as 5 to 10 bushels of corn, and maybe 3 to 5 bushels of soybean, were left on the ground in some fields.
Other than making combine adjustments, there isn’t much we can do to decrease losses at harvest of dry grain. Beginning harvest with corn at 20 to 25% moisture helps, but the grain dried so quickly this fall that little could be harvested above 20% even if the first fields started out there. After the rain, grain moistures may be a little higher now than they were the first week of October, and cooler weather will slow drying some. But once grain has been dry, weathering from rain followed by drying is likely to loosen kernel attachment on the cob and to lower pod integrity, so the crop will remain subject to harvest loss.
Looking at the distribution of seedlings after rainfall can tell us some about how grain was lost during harvest. If soybean seedlings are concentrated between the combine wheel tracks, seeds either came over the separator out the back of the combine or fell out of the header during the gathering process. If they are more uniformly distributed, they were likely knocked out of pods at the cutterbar, possibly by the reel.
Most corn is typically lost as (dry) ears hit the stripper plate, but some may have come through the machine. There are some innovations including brushes to bring loose grain into the feeder house and spring-loaded stripper plates. But as long as grain separates easily from cobs, some loss is probably unavoidable. Adding to the harvest challenges is severe lodging caused by winds in some areas during the recent period of rain; there are fields with virtually every plant broken over. Getting all of the grain safely into storage will require patience this year.
One positive effect of rain that causes seeds lying on the ground to germinate after harvest is that they are not likely to turn into volunteer plants in next year’s crop. Patterns of volunteer corn distribution suggest that they often come from ears, not individual seeds, so if ears drop (in lodged corn, for example), they could still cause problems next year.
It may occur to some that stands of corn or soybean that emerge from unharvested seed might be “unintentional” cover crops. They will certainly take up more N and produce more dry weight than a crop like spring oats seeded recently, before freezing temperature stop growth. We can use volunteer corn plants to see how green they turn after they are established, which will indicate how much N was left in the soil. Some of this may be from fertilizer, but it has now been nearly two months since N uptake stopped as the corn crop approached maturity, and some is also from mineralization. Any N that plants take up this fall is unlikely to reach tile lines by next spring.
Nitrogen application
As I wrote in a recent Crop Central article, warm fall temperatures along with uncertain N availability and higher prices have increased the level of anxiety about getting fall N applied. Fields in most areas are too wet now to consider applying N, and in many areas, finishing harvest will be a priority over the next week. This will help push anhydrous ammonia application into November, which will be helpful in terms of applying when soil temperatures are low enough to minimize nitrification.
Below-normal temperatures this week have raised hopes that soil temperatures are finally beginning their decline. Figure 1 shows soil temperature (4 inches deep under bare soil at 10 AM) trends during October in central and northern Illinois. Although soil temperatures fell by about 13-15 degrees between October 14 and 17, they have since started upward, and with the current forecast of above-normal temperatures during the last week of October, it is unlikely that they will drop to less than 50 degrees by November 1.
I plan to update this trend before the end of October. The Illinois Farm Bureau has daily maps of soil temperatures; note that these are 4-inch soil temperatures at 7:00 AM, which may be a degree or two lower than those given in Figure 1. The Illinois Fertilizer & Chemical Association website refers users to the WARM network (address also in the Figure 1 caption), which gives soil temperatures for any hour of the day, plus daily minimum and maximum values, available a day later.
If, as expected, soil temperatures continue to rise over the next week, it makes sense to hold off on applying NH3 until thy fall back to 50. We hope that the forecast by that time is for temperatures to continue to drop further. We know that soil at 50 degrees is not a very safe place for keeping the N in ammonium form, but it’s reasonable to expect it to keep most of the N safe from loss. With N prices continuing to rise, protecting applied N from loss becomes more valuable.
Tillage
There’s a strong urge to do fall tillage when soils are dry. This fall, grain left behind by the combine may have provided some additional incentive to bury seeds so not as many germinated and emerged. But it can be difficult to justify the expense of tillage, at least in terms of yield. In a study this year, Dan Schaefer and I found that fall tillage did not increase yield compared to tilling only in the spring, and neither was significantly better than strip-tillage, in either corn or soybeans. We noted that fall-tilled plots had wetter soil at planting than plots without fall tillage.
We’ll report more on this study later, but we think one lesson is that yields may respond less to fall tillage than we expect. Fall-tilling cornstalks while leaving a good amount of residue on the surface may protect the soil and help with water infiltration and soil warming next spring, but it is a cost that might not always be covered, especially if next year’s crop is soybean. There is not enough soybean residue to provide good cover after most forms of fall tillage (deep ripping with MRD standards might be an exception), but we can generally expect little yield response to fall tillage of soybean stubble, and this practice remains hard to justify from agronomic, economic, and conservation standpoints.
One motivation for tilling this fall may be to repair ruts formed during harvest in fields or parts of fields that remain wet. Such repairs probably won’t be easy to complete this fall, and it may make sense to wait until next spring to do this. Saturated soils where ruts commonly appear have water in most of their pore space, so are not very “compressible.” This means that compaction, which results from having air in pore space squeezed out, isn’t as big a problem as the uneven surface with ruts. Driving heavy equipment over fields when they’re wet but not saturated (which is common when harvesting or tilling with heavy equipment) does cause compaction, however. It is rare that soils that are wet in mid-October get dry enough to allow compaction to be relieved with deep tillage in the fall, at least without replacing that compaction with new compaction.