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

The corn plant during July transitions from developing vegetative structures to reproductive structures. It is significant for yield in that two of the three components of yield are set up during this month. In the first half of the month, the number of potential ovules that could develop into kernels is determined. In the second half, the number of potential cells in the kernel endosperm, which ultimately affects kernel weight, is determined. However, everything is predicated on the success of pollination and fertilization of the ovules on the topmost ear from pollen released by the tassel (see "Methods for determining corn pollination success").

During early July ear development is rapid and prior to tasseling (V18). The upper ear shoot is developing faster than other shoots on the stalk. Brace roots are now growing from nodes above the soil surface. They will scavenge the upper soil layers for water and nutrients during reproductive stages. Moisture deficiency will cause lag between pollen shed and beginning silk ("nick"). Usually the largest yield reductions will result from this stress. The plant is using 0.30 inches of water per day. Lodging will cause 12-31% yield reduction. Frost (<28 F) will cause 100% yield loss due to plant death (see "Frost"). Hail will cause 100% yield loss when completely defoliated (see "Hail damage on corn"). Drought will cause 4% yield loss per day due to drought or heat when leaf rolling occurs by mid-morning (see "Drought"). Flooding (<48 h) will not affect yield, however, other management options need to be considered (see "Flooding effects on corn").

At the silking (R1) stage the actual kernel number and potential kernel size is determined. R1 begins when any silks are visible outside the husks. Pollen shed begins and lasts 5-8 days per individual plant. Silk emergence takes 5 days. Silks elongate from base of ear to tip of ear. Silks elongate until pollinated. Silks outside husks turn brown. The plant has now reached its maximum height. First 7-10 days after fertilization cell division occurs within kernel after which kernels begin to fill with starch.

The plant must have a healthy root system because proper uptake of moisture and nutrients are critical at this time. Hot and dry weather results in poor pollination and seed set. Drought dehydrates silks (delaying silking) and hastens pollen shed causing plants to miss window nick for pollination. Drought decreases yield 7% per day (leaf rolling by mid-morning). Rootworm beetle clips silks which prevents pollination if less than a half-inch of silk is showing

Nitrogen applied through irrigation water, should be applied by V18. Rootworm beetle control should be implemented if 4-5 beetles are observed feeding near ear tip. Stresses that reduce pollination result in a "nubbin" (an ear with a barren tip).

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

Corn planting was nearly complete by the end of last week. As we head into the month of June, the corn plant changes from a juvenile to more of an adult. The seminal roots that originated in the seed are dying and the nodal roots are becoming the dominant root system which will eventually occupy a cylindrical volume roughly 5-6 feet in diameter and 5-7 feet deep depending upon soil characteristics.

Another change is occurring on the leaves. Juvenile leaves have cuticular and epicuticular wax on the surface giving the leaf a bluish cast. The V5-V7 leaves have decreasing amounts of epicuticular wax leaving only the glossy green cuticular wax commonly seen on adult leaves. By V8 the transition from juvenile blueish cast to adult glossy green wax is complete.

By V6 (about 24-30 days after emergence - 475 GDU) all plant structures have developed on the growing point. All plant parts are present. The growing point and tassel, differentiated in V5, are above the soil surface. The stalk is beginning a period of rapid elongation getting taller. The determination of kernel rows per ear begins and is complete by about V10-V12. This yield component is strongly influenced by hybrid genetics. Tillers (suckers) begin to emerge at this time. Lower leaves degenerate and are torn from the stalk as it expands. During early June there is a new leaf emerging (V-stage) about every 3 days.

      

June is the time to apply nitrogen (up to V8) before rapid uptake period in corn. Precise fertilizer placement is less critical. Lodging can often occur during this time since brace roots have not appeared. Rootworm eggs will soon hatch and larvae begin feeding on root systems. Defoliation from hail ,wind, and leaf feeding corn borers may decrease row number on the ear. If a frost would occur during June there would be 100% yield loss caused from plant death and killing of the growing point. Hail can cause up to 53% yield loss when completely defoliated. Short-term flooding can cause severe yield loss if the growing point is below the water surface.

By V12 ( 42-46 days after emergence - 815 GDU) potential kernel rows are determined. The number of kernel rows is set. The number of ovules (potential kernels) on each ear and size of ear is being determined and is strongly affected by environmental stresses. During late-June there is a new leaf emerging every 2 days and brace root formation begins stabilizing the upper part of the plant. The plant is utilizing 0.25 inches of water per day. Nutrient deficiencies, will reduce the potential number of kernels and ear size. Large amounts of nitrogen, phosphorous, and potassium are being utilized at this stage. Early hybrids- progress faster through growth stages and usually have fewer leaves and smaller ears than late hybrids.

For most of Wisconsin hybrids (~100 day), each plant typically develops 20-21 leaves. The rate of plant development for any hybrid is directly related to temperature, so the length of time between the different stages will vary as the temperature varies.  Environmental stress may lengthen or shorten the time between vegetative and reproductive stages. The length of time required for the yield components of ear density, kernel number, kernel weight varies between hybrids and environmental conditions.

Ears per unit area, kernel number per ear and kernel weight all contribute to yield. These yield components of corn are determined early in the life cycle of the corn plant with some established by the end of June.