The Magic of Corn Seed Germination and Emergence
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.
Seeding Depth Affects Corn Plant Emergence Uniformity and Grain Yield
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.
Further Reading
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
Back to the Basics for Weed Control
The saying ‘all good things must come to an end’ is proving true for weed control in soybeans. Gone are the years of simply using Roundup to take care of everything (if they weren’t already for some acres). I confess that I was skeptical of weeds working around Roundup but should have known better since nature is geared to find a way to endure and overcome. Just like in the animal kingdom, survival depends on self-preservation. The same has happened with weeds, shifting their genetics to survive being sprayed with Roundup.
The ‘Easy Button’ for weed control is over, not that we should ever have relied on straight Roundup as our sole option. We must now return to solid practices and ensure we use multiple modes to control problem weeds, which will help to stack the deck against further weed resistance and keep our herbicide options viable.
Statistically, it is highly unlikely for any weed to simultaneously develop resistance to multiple herbicides. For example, say there is a 1 in 1,000,000 chance (much higher than the real world) for resistance to Roundup with a similar probability for resistance to 2-4D. The odds of finding both resistances in the same plant is 1/1,000,000 x 1/1,000,000 = 1/1,000,000,000,000, far worse than the minuscule odds of winning the lottery.
The more logical path to multiple resistance is the weed population becoming resistant to one herbicide until the trait is locked in, and then using a non-resistant herbicide until that weed population develops resistance to that as well.
In reflecting on the path to multiple resistance, we should hopefully see the error of single or even double modes of action. Using Xtend or Enlist soybeans on weeds that are already resistant to Roundup increases the danger of developing multiple resistance. With essentially one mode of control (the Roundup is doing nothing), you risk selecting plants resistant to Roundup and 2-4D or Dicamba.
You should attack these weeds aggressively. Multiple modes of action will hopefully drive the population to nothingness, while using ineffective treatments will only allow them to spread to other fields. Even if you have no resistant weeds, it is still a strong policy to use multiple modes of action as a preventative measure in your fight against weeds. Remember that this is not a one-year fight; weed seed can sit in the soil for multiple years and come back to haunt you.
The basic program for weed control is starting with pre-emergence herbicide(s). This package can have several modes of action and prevent weeds from establishing in the first place. This will help the soybeans to get ahead of the weeds and allow post emerge spraying on a timely basis which eliminates having to come in when many of the weeds have grown beyond the size of the label.
The University of Wisconsin has a very nice table of herbicides and their modes of action. It can be found at https://ipcm.wisc.edu/download/pubsPM/Herbicide-Mode-of-Action.pdf. It is important to match modes of action to the weeds you are trying to control. A good example is that group 2 (ALS Inhibitors) are not effective at controlling water hemp.
After emergence, there are now several soybean options to control problem weeds: Xtend (Dicamba), Enlist (2-4D, Liberty), Liberty GT27 (Liberty) and coming next year XtendFlex (Dicamba, Liberty). Dicamba is probably the most effective and delivers some residual control and also has a great respray program for weed escapes. Unfortunately Dicamba has restrictions placed on its use due to some offsite movement during unfavorable conditions. 2-4D is a good spray and more forgiving but, just like Dicamba, can have the same offsite movement if the label is not followed properly. Liberty is my third choice as it is expensive and somewhat finicky when spraying.
Though all three can be very effective, I lean towards Xtend as they have the clear lead in genetics and overall weed control. Enlist is next with its three modes of action that can be sprayed. I do like Liberty GT27 because they are genetically strong and allow for a mode of action beyond GT (Roundup).
I cannot stress enough to follow the label on all three. These herbicides and soybean platforms are key weapons in our fight against weeds. There are many misinformed people that want to take them off the market in their zeal to ‘protect the environment’ and we don’t want to give them any reason to cite misuse.
The easy days of Roundup being the magic bullet are gone but, with very minimal changes in our spray program, we can maintain the level of weed control we are accustomed to.
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