June 22, 2012

This time of year the growth rate of corn plants always amazes me. In all but replanted, and very late planted fields, we have already determined the number of kernel rows. (This usually occurs at about the 6- to 8-leaf stage). This year most areas had sufficient moisture, although in some parts of variable fields, you see corn plants "rolling" and looking gray in the afternoon. These stressed plants will likely produce ears with 12-14 rows, while the same hybrid with better moisture will produce 16-20 rows—a yield constraint by the time it is knee high.

Most fields are now at the 12- 16-leaf stage and 10 or so days to tassel emergence. The plants in these fields are in the process of determining number of kernels per row, and/or ear length. The more stress the plants feel, the fewer florets (potential seeds) will form lengthwise on the ear, resulting in fewer kernels. What happens to the developing florets now and until 10 days after pollination is absolutely critical to yield. Photosynthesis (sugar production) and making that sugar available to the florets of the ears at this stage will keep potential kernels from aborting. Drought stress will dramatically slow or even shut down photosynthesis. If you see severe leaf rolling, those leaves are not contributing to sugar production.

Estimating Soil Water Use and Irrigation Needs

A 60 mph view tells us that the plants are growing extremely rapidly—and that that rapid growth is using an amazing amount of water, if it is available. (Some areas of the state didn't accumulate much subsoil moisture over the winter/spring.) You can see daily estimates of crop water use, including estimates for the previous three and future three days, at /gdd-etdata .

On extreme days, we are using 0.4 to 0.5 inches per day in many locations. While we seldom have many consecutive days of extreme water use, we may have three to four extreme days before temperatures and wind speeds decrease, bringing ET rates back to more normal levels.

Remember that as the plants grow above the ground, the root system is expanding below the soil. When corn is at the 16-leaf stage, we recommend using a rooting depth of 2.5 ft to schedule irrigations. At the silk stage, use a rooting depth of 3 ft and at the blister stage, use 3.5 ft. With varying stored soil moisture, varying growth stages, and varying ET rates across the state, it is imperative to monitor soil moisture to schedule your irrigations.

When figuring your well's capacity, a good conversion factor to remember is that 450 gallons per minute applies 1 acre-inch of water every hour. If your well pumps 900 gpm, you are applying 2 acre-inches every hour. This equates to 144 acre-inches every three days. For a 133-acre pivot, you can apply about 1.1 inches of water every three days. Calculate your application capacity and keep in mind how much stored soil moisture you have when scheduling your next irrigation.

Looking for that Million Dollar Rain

On dryland acres in Nebraska, we are already seeing crop damage in areas that missed out on the big rains last week. Corn in these areas is showing reduced growth, both in height and leaf area. The worst spots are already turning brown and dying. Even if we get significant rain soon, we already have yield loss accumulating. If we see rains in these areas by tasseling, we can still get good silk emergence and yields. If not, yields will deteriorate rapidly.

I have seen "head high" corn make over 150 bushels per acre if rainfall occurs before pollen shed. I have seen total losses where it doesn't.

The "drought tolerant" hybrids may prove their worth, and all the breeding and improvement in silking vigor and pollen-silk interval apparent in most corn hybrids (compared to those available 20 years ago) will be invaluable this year. No-till and minimum tillage techniques save a couple of inches of water, and may make the difference between a crop and no crop in these areas. (See G2000 Tillage and Crop Residue Affect Irrigation Requirements.)

In most areas of Nebraska it has been several years since we've had this level of stress. Moisture conservation practices, residue, crop rotation, weed control, and good long-term soil management may really pay off this year.

Thomas Hoegemeyer, Professor of Practice, Department of Agronomy and Horticulture
With Chuck Burr, Extension Educator in Phelps County
Gary Zoubek, Extension Educator in York County