What is happening in the corn plant during the month of May?

Good progress has been made planting corn in Wisconsin. Corn planted was at 56 percent complete, one day behind last year, but 10 days ahead of the five-year average.  Corn emerged was at 6 percent, the same as last year, and 4 days ahead of the five-year average. There was some concern that corn planted before April 20 might have experienced some imbibitional chilling due to cool weather towards the end of April. However, the crop has emerged well and there is currently more concern about black cutworm damage.

Corn seed is usually planted between 1.5 and 2 inches deep. For the first 24-72 hours after dry 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 pericarp. Membranes re-hydrate and hormones and enzymes are activated. Imbibitional chilling occurs when membrane re-hydration is disrupted by free radicals. 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.

After the seed swells, enzymes begin to breakdown starch in the endosperm. Sugars supply the embryo with energy for metabolism and cell division. The first structure to emerge is the root radicle. The plumule, which consists of the coleoptile and first leaves emerges from the seed and then from the soil. Elongation of the coleoptile ceases at the soil surface (VE) and elongation of mesocotyl ceases. The first leaves rupture the coleoptile tip. It takes about 7-10 days (125 Growing Degree Units - GDUs) for the VE stage to occur.

Emergence (VE) is affected by a number of factors. If conservation tillage is implemented add 30-60 GDU to VE. If the planting date is before April 25 add 10-25 GDU to VE. If planting date is  after May 15 subtract 50-70 GDU to VE. If the seeding depth is below 2-inches then add 15 GDU to VE for each inch below 2 inches. If the seedbed condition is crusted or cloddy then add 30 GDU to VE. If the seed-zone soil moisture is below optimum, add 30 GDU to VE.

Shortly after the radicle emerges from the seed, seminal roots emerge and supply water and nutrients to the developing seedling. These roots are also important for nutrient uptake of pop-up and starter fertilizer. Seminal roots begin to die between V2 and V4. Nodal roots become the primary root system of the plant and arise from below-ground nodes near the growing point around V2.

Banding small amounts of starter fertilizer to the side and slightly below the seed can improve early vigor, especially when soils are cool. However, little evidence supports use of pop-up or starter fertilizer for increasing grain yield in many soils in Wisconsin.

Figure 1. Corn plant parts at V2. Photo from Iowa State University publication
("How a corn plant develops"-Special Report No. 48-Ritchie, Hanway and Benson, 1996)

Once the leaves emerge from the coleoptile, the seedling begins photosynthesis. The growing point is below the soil surface. All of the growth and development for above-ground plant parts is taking place on the growing point. This continues until V5-V6 (about the end of May). Frost at this time will not affect yield (<28 F). Hail will not affect yield. Severe yield losses can occur from flooding when plants are under water for more than 48 hours.

Troubleshooting

1. Watch for seed attacking insects
2. Germination and emergence is delayed when there is inadequate moisture and /or cool soil temperatures (<50°F) 
3. If the first leaves unfurl below-ground, the seed was planted too deep, and/or the soil is cloddy or crusted.
4. Herbicide injury: coleoptile will be corkscrew shaped, and have swollen mesocotyl.