Issue No. 8, Article 2/May 15, 2009
Problems with Corn Stands
We finally managed to reach double digits for the percentage of corn planted in Illinois, with the May 10 number at 10%. Some planting took place this week, often in fields that were not really dry enough to work or plant very well. As mid-May approaches and we are at a point of losing an expected 1.5 bushels of yield for each day of planting delay, such actions are understandable. We recognize that soil compaction done now will not be relieved this year, so our only real hope for avoiding problems as a result of planting into wet soils is that it never turns dry or hot for very long during the rest of the season.
I've had several reports this past week of poor and uneven emergence in fields that were planted during the brief window that opened in late April. In fields with variable soil, the problems seem worse in the heavier, lower-lying soils. This probably means that the soils in those areas were wetter at the time of planting, but more importantly, they likely have suffered more from wet soils since they were planted. Once seeds germinate, they need oxygen to stay alive and to develop healthy plants. This need for oxygen is greater if the soil is warm, because the rate of growth activity is higher in the seedling, so it uses up oxygen faster.
How long does it take for such damage to occur? If soil temperatures are 60 or above, seeds without oxygen could die within three or four days, and seedlings may not last even that long. Seeds have to take up water before the germination process can start, but they need oxygen immediately after the enzymes that start the process are activated. Rainwater carries some dissolved oxygen, but other microbes in the soil also use up oxygen, so it doesn't last long. Of course, once soils are saturated they do not dry out very quickly to allow oxygen in, so seeds do not have to be in standing water to be damaged or killed.
Soils are still not very warm, but being in the upper 50s or lower 60s, they are warm enough to allow germination and emergence. Corn planted very early this year seemed to take more than the 120 or so growing degree-days that it normally takes for the crop to emerge. Corn planted in the last half of April is emerging at about the normal GDD accumulation. Some have noticed plants coming up in some parts of a field and not others, or some plants emerged down the row and not others, even after several days.
The first thing to check when emergence is poor or uneven is whether crusting is keeping plants from emerging. Even though working and planting in too-wet soils tend to increase the potential for crusting, frequent rainfall has kept crust formation from becoming much of a factor so far in 2009. If there is little or no crusting, then dig up seeds of unemerged plants and note their appearance. Here are some diagnostic guidelines:
- If the tissue appears whitish and healthy, but the coleoptile is thickened, then it is likely that there was mechanical interference, such as a crust or clod atop the seedling. Some herbicides act by slowing growth of seedlings, but these are not used as much as they were in the past. Prospects for these plants range from none, if they have leafed out underground or the damage is from herbicide, to moderate, if a crusting problem has been relieved and they can resume growth. If they have been restricted physically for some time, however, they may be incapable of resuming growth.
- If the seed is discolored and mushy, disease has likely invaded it and used up much of the reserves that would normally have supplied the seedling. This slows the rate of coleoptile growth, and if such growth stops before emergence, the seedling will die.
- Insects such as seedcorn maggots can eat the seed reserves as well, with results similar to that of disease infection of the kernel. Wireworms can eat parts of the seedling, including the mesocotyl--the stretch of tissue between the seed and the base of the stem, where the nodal roots will appear--and result in failure to emerge.
- The most common injury in many fields is reduced growth due to restricted oxygen supply, as described above. This will often appear as a seedlings that has seminal roots emerged but whose plumule (shoot) is smaller than normal in size and doesn't have normal color. It may have grown to some length but be small in diameter and show general "wimpiness." This comes from oxygen starvation, which can restrict the mobilization and movement of reserves from the seed and can damage the shoot tissue directly. In either case, prospects for such seedlings are not good, and even if they make it out of the ground, they might fail to thrive due to restricted growth of roots and possibly other, less-understood problems. If they are next to healthy plants in the row, their prospects are even dimmer.
- At some point soilborne diseases will invade tissue that is growing slowly or not at all. The plant cells in such tissue start to lose their integrity if they don't have access to sugars, and this makes them easy targets for fungi and bacteria to invade. Seedlings with disease will turn soft and mushy and will often discolor as the tissue dies.
If soils warm and dry and leaves of seedlings can get out into the sunlight and start to photosynthesize, the damage to seedlings might not permanently hurt their yield potential by much. But if such recovered plants are next to healthy plants, they will often never recover full yield potential.
Next week I will talk about replant decisions and techniques. This is no one's favorite topic, but unfortunately it will be an issue this year for some fields in some places where the rain keeps coming down. Getting planted the first time will be the bigger issue for most producers. Both will need some drying weather before they are even possibilities.--Emerson Nafziger