I think nearly every corn planter in Wisconsin was planting this past week. There are some wet areas in northeastern Wisconsin that have prevented planting, but a significant jump in planted acreage should be measured by USDA-NASS in next Monday’s progress report.
Now the magic begins when dry seed imbibes water and bare or brown fields turn greener every day across the landscape. The germination process and the success of the seed in emerging and establishing is key and the first yield component determined for the growing season.
Protected within the seed coat is an embryonic plant that remains dormant until germination is initiated by the physical process of imbibing water. The white starchy endosperm is the main energy source until the young seedling is established. After planting, water and oxygen are imbibed into the seed for 24-48 hours activating the growth hormones and enzymes. Starch is broken down supplying the embryo with energy for metabolism and cell division.
Within the embryo is a miniature corn plant that already has a primary shoot, leaves and root system protected by rigid sheaths called the coleoptile (above-ground) and coleorhiza (below-ground). The first structure to emerge from the seed is the radicle root, followed by the coleoptile and seminal roots.
Figure 1. Diagram of germinating corn. Photo and graphic by Mimi Broeske.
The coleoptile is pushed to the soil surface by the mesocotyl. When sunlight falls on the coleoptile tip, enzymes are activated that soften the tip allowing the first true leaf of the plant to break through. The growing point of corn is 3/4 of an inch below the soil surface and will remain below-ground until the plant has 5 to 6 leaves.
The germination process from dry seed to seedling emergence requires about 125 Growing Degree Units (GDUs). Normally in the beginning of May, we accumulate about 10 GDUs per day, so emergence takes about 12 to 13 days. The 2022 growing season is starting out fast with record high temperatures, and I have seen some recently planted fields already emerged. Emergence GDUs may need to be adjusted:
- If conservation tillage is implemented, add 30-60 GDUs.
- If planting date is before April 25, add 10-25 GDUs.
- If planting date is after May 15, subtract 50-70 GDUs
- If seeding depth is below 2 inches, add 15 GDUs for each inch below.
- If seed-bed condition has soil crusting or massive clods, add 30 GDUs.
- If seed-zone soil moisture is below optimum, add 30 GDUs.
There might be many reasons why a seedling does not emerge in a stand of corn. The germination process is really a race between pest pressure (diseases and insects) and the ability of the seedling to outgrow the pest. Seed treatments protect the seedling from disease and insects for the first 30 to 45 days of the growing season. Planting into cloddy/crusted or cold soils can result in seedling leaves unfurling below-ground, reducing plant stand and yield potential. Imbibitional chilling can result in plant death.
This one of my favorite times of the year in Wisconsin. I wonder what the growing season has in store for these developing plants. As you drive around the state, enjoy the landscape and all the different greens that develop over the month of May.
For the first 24-48 hours after dry corn seed is planted into the ground, all that takes place is physical imbibition of water into the seed. Water and oxygen move slowly into the kernel through the seed pericarp. Membranes re-hydrate and hormones and enzymes are activated. After the seed swells, enzymes begin to breakdown starch in the endosperm. Sugars supply the embryo with energy for metabolism and cell division.
Imbibitional chilling occurs when membrane re-hydration is disrupted by free radicals before the seed finishes swelling. Cold water is much more disruptive than warm water. Sugars and salts leak from the cells and kernel providing a food source for pathogens and other microbes. It becomes a race for the plant to emerge or death from pathogens.
One of the most dramatic examples of imbibitional chilling occurs with sweet corn. In a study conducted by Bill Tracy (2005), untreated seed of six supersweet corn varieties were exposed to three treatments. Each treatment consisted of one 24-hr period at 40° F temperature and five days at 75° F. Seed was placed in rag dolls with no soil.
Sweet corn seed has a wrinkly seed pericarp with numerous cracks, fissures and potential endosperm leakage sites. In this study, most seed death occurred within the first 24-hrs (day 1) of being exposed to 40° F. Significant variety differences for the amount of seed death were observed. Significantly less seed death was occurring when exposed to 40° F on days 2 and 3, and no seed death when exposed to 40° F on day 4.
One current recommendation is to begin planting corn when soil temperatures are in the high 40s and the short-term forecast calls for warm days that will continue pushing soil temperatures higher. If soil temperatures are in the high 40s and the weather forecast calls for cold wet conditions within the next 48 hours, soil temperatures will likely drop and planting should be delayed until temperatures warm.
That recommendation is simply not our experience in Wisconsin and likely the northern tier of U.S. states. If we waited until soil temperatures were above 40° F we would need to wait until May in many years (Figure 2). It only gets later as we move north. Yet, our highest yielding planting date are in late April and early May.
Figure 2. Last date when the minimum soil temperature at the two-inch depth was above 40° F at Arlington, WI.
The only time I thought our UW Corn Hybrid Trial plots had been affected by imbibitional chilling was during 2006 at Seymour, WI (Figure 3). I happened to be along on that planting trip. It began to snow after we had finished planting and continued to be cool and wet for the next 72 hrs. We went back to the field a few weeks later and unbeknownst to us at planting, found that the field had shallow swales and a drainfield. Corn emergence was perfect over the drainfield and on the ridges of the swales. However, much plant death occurred between the drainfield spurs and swale depressions. We did not observe standing water, although it could have been another possible reason for plant death. We abandoned the trial due to stand variability.
Our current recommendation for beginning to plant corn seed is, “In southern Wisconsin, plant corn anytime after April 20, if the field is ‘fit’, and after April 30 in northern Wisconsin.” If the short-term forecast is for cold temperatures and snow/rain, then the prudent thing to do is hold-off planting. We have available excellent seed treatments that can protect the seed for the first 30 to 45 days after planting.
This advice we use to plant and establish the UW hybrid trials at 14 locations around the state. We have often had snow on our plots with no establishment and emergence issues. For the last 5 years we have planted a few hundred feet of four hybrids beginning in March and then every 2-3 weeks – we do this to get the planters out and tuned. We do see hybrid differences and in only one case did we see emergence issues for all four hybrids. Remember that insurance coverage does not begin until planting dates after April 10.
Corn planters will soon be rolling throughout Wisconsin and the Midwest Corn Belt. The annual struggle between field conditions being “just right” and not too wet versus delaying planting to another day will start to weigh on farmer’s minds. In addition, planting delays in the northern tier of U.S. states have greater impact on yield due to a shorter growing season and the added dimension (“double-whammy”) of drying costs at harvest that can occur during cool, wet growing seasons.
Figure 1 shows the impact of planting date on relative grain yield at Arlington. If all corn could be planted on one date, ideally it would be on May 1 or slightly earlier to decrease drying costs. Planting delays to June 1 will lower yields about 30%. However, in some growing seasons, 100% of the maximum grain yield can be achieved planting into late May. Grain yield decreases 0.5 bu/A per day on May 15 and accelerates to 2.5 bu/A per day on June 1.
Rarely do we see a paper published on corn seeding depth and the subsequent impact on grain yield. Precision technologies have allowed for capabilities of variable rate seeding, multi-hybrid planting on the go, and the ability to vary planting depth in real time in response to real-time soil moisture data. In a paper published by Nemergut et al. (2021), corn seed was planted at 1-, 2-, and 3-inch depths on two soil types in Ohio over three growing season (2017 to 2019). Shallow planting resulted in less uniform more extended emergence periods than 2- and 3-inch planting depths. If a plant emerged within 3 days of the first emerged neighboring plants, then there was no effect on plant grain yield. Any plant that emerged more than 3 days after the first emerged plant had a 5% decrease in kernel weight per day. Grain yield per plant increased as planting depth increased. Grain yield per acre was significantly increased by planting depth with seed planted at 2- and 3-inches yielding 8 or 10% more than the 1-inch seeding depth on one of the two soils. Other researchers have also shown improving emergence uniformity can positively increase yield, and that optimum planting depth may vary by field.
Nemergut KT, Thomison PR, Carter PR, Lindsey AJ. Planting depth affects corn emergence, growth and development, and yield. Agronomy Journal. 2021;113:3351–3360. https://doi.org/10.1002/agj2.2070