Corn Development

May 26, 2000
While there are still signs of damage in some fields from blowing soil, hail, and low temperatures, the return of warmer temperatures this week, as well as rain in many places, is quickly returning the crop to the "fast track" of development. The corn planted in early April at Urbana is at or approaching V7 (seven leaf collars visible) and is 12 to 15 inches tall. Corn planted the last week of April was slowed considerably by cool temperatures this month and is only at V3 or V4 and about 8 inches tall. In almost all fields, except where there has been storm damage, stands and stand uniformity are very good this year.

Bob Nielsen at Purdue University has suggested a useful refinement to our general statement that it takes about 65 growing degree days (GDD) to add each new leaf to the corn plant. He indicates that it takes about 85 GDD to add a leaf up to the time that the corn has about 10 leaves emerged, and then only about 50 GDD per leaf to add the remaining 8 to 10 leaves. One reason for this is that cooler soil temperatures restrict development somewhat earlier in the season. Also, the lower inter-nodes do not elongate as much or as rapidly as do internodes higher up the plant; and because it's mostly inter-node elongation that pushes leaves up and out of the whorl, we can expect that later-emerging leaves will emerge faster.

By the time the corn has seven leaf collars emerged, the lower one to three leaves (depending on weather-related damage) are already deteriorating, so they may be missed if you're counting. The bottom leaves are small and are split by the enlarging stalk, so they are making little or no contribution by the time corn is a foot tall or so. The fifth and sixth leaf collars (they're on opposite sides of the plant) will be considerably more separated than those lower on the plant, so you can often estimate the fifth leaf from looking at that distance. The fifth leaf is often the lowest leaf that stays on the plant for later counting as well. If you really need to be precise in staging, you might cut the end of the fifth or sixth leaf off square so you can readily see that leaf in later visits.

Other than some reports of plants' yellowing in southwestern and central Illinois, the crop is poised for a good recovery from what so far have been relatively minor problems. Soils vary from too wet to too dry at this point, but neither problem threatens a large area over the next several weeks. Though root restrictions due to wheel traffic are probably less likely than usual this year due to dry conditions at planting, dry and excessively wet soils may start to show restricted growth due to root restrictions, especially as the crop enters the very rapid growth phase after V7.

The old saying that corn should be "knee high by the Fourth of July" is no longer relevant, at least in Illinois. I'll suggest a modern replacement: Corn that's knee high by the first of June and showing tassels by the Fourth of July is, on average, a crop more likely to produce high yields. Taking advantage of the lengthening days of June by having a substantial crop canopy there at the beginning of the month sets the crop up for good light interception and yield-producing photosynthesis in the "yield months" of July and August. So we can feel good about this crop at this point.

The only "cloud on the horizon" in most places is the continuing possibility of lower-than-normal rainfall and subsoil water content that is lower than normal. One drawback to early development is the rapid removal of water from the soil by the crop; a full canopy early is good if water supply is adequate, but it also removes water from the soil much more quickly. Even though the canopy provides shade that reduces evaporation from a moist soil surface, plants with a full canopy are a means by which water moves from the soil around the roots up to the leaves, where it evaporates through small holes (stomata) into the atmosphere.

While longer-range forecasts continue to mention drought, there is little that dryness will do to affect this year's crop over the next few weeks. The tiny ear that will become the uppermost ear on the plant is already present (but very difficult to see) on a V7 plant. The tassel has also differentiated at the growing point, or tip of the stem, meaning that all of the leaves have been initiated. The number of kernel rows is determined very early after the ear is initiated, while the number of kernels per row takes a longer time to develop. Regardless, the number of "bumps" on a tiny developing ear in corn that's 2 feet tall does little to predict eventual yield and is very seldom if ever a factor in how many kernels will eventually fill on the ear or how large the kernels will be at maturity. While it's good that people know the small ear is developing well protected inside the plant, there is little to give us clues about what it will end up as 3 months later.

If and when dry soils start to affect the corn crop, we'll deal with that problem in a more specific way. Until then, keep an eye on pests and hope for timely rains, but don't worry too much about how the plants are doing. In more fields than usual this year, they're fine.--Emerson Nafziger

Author: Emerson Nafziger