This growing season is one of disturbing contrasts – north and south, and early on versus now. Much of northern Wisconsin has had ample rain – some counties too much – while growers in the southern four tiers of counties struggle with drought. After an early start at planting, some growers in southern Wisconsin are now worried whether they’ll get a crop at all. Even growers in parts of central Wisconsin fret whether Mother Nature might turn off the spigot on corn that’s ahead of other years.
For a summary of the growing season (April 1 to July 4) at the UW experiment stations at Arlington and Marshfield, see http://corn.agronomy.wisc.edu/Season/. Precipitation in northern Wisconsin is comparable to the 30-year normal, but, in southern Wisconsin, accumulated precipitation is below normal and approaching some of driest years recorded (1976, 1988, 1989 and 2005). The southern four tiers of Wisconsin counties are most affected.
Corn is heading into pollination and potential grain yield will be determined soon after fertilization takes place following pollen shed and silk emergence. UW-Madison corn agronomist Joe Lauer says corn management decisions during drought depend on what happens during pollination. Each potential kernel has a silk attached. If fertilization is successful, within 1 to 3 days the silk will detach from the developing kernel. Silks will remain attached to unfertilized ovules and be receptive to pollen up to seven days after emergence. Silks eventually turn brown and dry up after pollination is over.
Two techniques assess pollination success or failure. The quickest is the “shake test.” Carefully unwrap ear husk leaves and gently shake the ear. Silks from fertilized ovules will drop off. The proportion of silks dropping off the ear indicates the proportion of future kernels on an ear. Lauer says to randomly sample several ears in a field to estimate the success of pollination. The second is to wait until 10 days after fertilization. Developing kernels will appear as watery blisters – “the ‘blister’ R2 stage of kernel development,” he reminds.
“If pollination is unsuccessful, we are usually trying to make the best of a bad situation,” he says. If pollination is poor yet some kernels are developing, the plant can gain dry matter and farmers should wait with harvest and decide whether or not to take the crop as corn silage. If there is no pollination, then the best quality forage will be found as close to flowering as possible. Quality decreases after flowering. The challenge is to make sure that no potential pollination occurs and that the forage moisture is correct for the storage structure.
Lauer says drought-stressed corn can be grazed or used for forage, either as green chop or silage. Because of the potential for nitrate toxicity, grazing or green chopping should be done only when emergency feed is needed.
The decision to chop corn for silage should be made when you’re sure pollination and fertilization of kernels will not or did not occur and whole-plant moisture is in the proper range for the storage structure so fermentation can occur without seepage or spoilage losses. If there’s no grain now, florets on the ear were either not pollinated or have not started to grow due to moisture stress, and the plant will continue to be barren. If the plant is dead, harvest should occur when whole plant moisture is appropriate for preservation and storage, Lauer reports.
If pollination and fertilization of kernels did occur but it was poor, don’t chop until you’re sure that there is no further potential to increase grain dry matter and whole plant moisture is in the proper range for the storage structure. “These kernels may grow some, if the plant is not dead. If kernels are growing, dry matter is accumulating and yield and quality of the forage is improving,” he mentions.
Green, barren stalks will contain 75 percent to 90 percent water. If weather remains hot and dry, moisture content drops, but if rain occurs before plants lose green color, plants can remain green until frost, says the UW agronomist. Drought stressed corn has increased sugar content, higher crude protein, higher crude fiber and more digestible fiber than normal corn silage. Drought generally reduces yield and grain content resulting in increased fiber content, but this is often accompanied by lower lignin production that increases fiber digestibility.
Lauer says evapotranspiration demand of corn varies during its life cycle and peaks around canopy closure. Estimates of peak evapotranspiration in corn range between 0.20 and 0.39 inches per day. Corn yield is most sensitive to water stress during flowering and pollination, followed by grain-filling, and finally vegetative growth stages. (See Table on page 3.)
During vegetative development, water stress results in shorter plants and less leaf area, while leaf number generally isn’t affected. Corn roots can grow 5 to 8 feet deep, and soil can hold 1.5 to 2.5 inches of available soil water per foot of soil, depending upon soil texture. Ear size may be smaller. Kernel number (rows) is reduced. Early drought stress doesn’t usually affect yield in Wisconsin through the V10-V12 stages, explains Lauer. Beyond these stages water stress begins to have an increasing effect on corn yield.
Lauer says water stress around flowering and pollination delays silking, reduces silk elongation and inhibits embryo development after pollination. Moisture stress during this time reduces corn grain yield 3 to 8 percent for each day of stress (See Table 1 again). Moisture or heat stress interferes with synchronization between pollen shed and silk emergence. Drought stress may delay silk emergence until pollen shed is nearly or completely finished. During periods of high temperatures, low relative humidity and inadequate soil moisture level, exposed silks may desiccate and become non-receptive to pollen germination.
“Silk elongation begins near the butt of the ear and progresses up toward the tip. The tip silks are typically the last to emerge from the husk leaves. If ears are unusually long (i.e. many kernels per row), the final silks from the tip of the ear may emerge after all the pollen has been shed. Another cause of incomplete kernel set is abortion of fertilized ovules. Aborted kernels are distinguished from unfertilized ovules in that aborted kernels had actually begun development. Aborted kernels will be shrunken and mostly white,” says Lauer.
Water stress during grain-fill increases leaf dying, shortens the grain-filling period, increases lodging and lowers kernel weight. Water stress during grain-filling reduces yield 2.5 to 5.8 percent with each day of stress. Kernels are most susceptible to abortion during the first two weeks following pollination, particularly kernels near the tip of the ear. Tip kernels are generally last to be fertilized, less vigorous than the rest and are most susceptible to abortion. “Once kernels have reached the dough stage of development, further yield losses will occur mainly from reductions in kernel dry weight accumulation,” he notes.
“Severe drought stress that continues into the early stages of kernel development – blister and milk stages – can easily abort developing kernels. Severe stress during dough and dent stages of grain fill decreases grain yield primarily due to decreased kernel weights and is often caused by premature black layer formation in the kernels,” Lauer continues. “Once grain has reached physiological maturity, stress will have no further physiological effect on final yield.” Stalk and ear rots, however, can continue to develop after corn has reached physiological maturity and indirectly reduce grain yield through lodging. Stalk rots are seen more often when ears have high kernel numbers and have been predisposed to stress, especially drought stress, he adds.
Premature death of leaves results in yield losses because the photosynthetic “factory” output is greatly reduced, this expert continues. The plant may remobilize stored carbohydrates from the leaves or stalk tissue to the developing ears, but yield potential will still be lost. Death of all plant tissue prevents any further remobilization of stored carbohydrates to the developing ear. Whole plant death that occurs before normal black layer formation will cause premature black layer development, resulting in incomplete grain fill and lightweight, chaffy grain. Grain moisture will be greater than 35 percent, requiring substantial field drydown before harvest.
“With the onset of tasseling, the corn crop is in a critical growth and development stage for grain yield. The tasseling, silking and pollination stages…are extremely critical because the yield components of ear and kernel number can no longer be increased by the plant and the potential size of the kernel is being determined,” Lauer reports.
“For example, the potential number of ears per unit area is largely determined by number of seeds planted, how many germinate and eventually emerge. Attrition of plants through disease, unfurling underground, insects, mammal, bird damage, chemical damage, mechanical damage and lodging all will decrease the actual number of ears that are eventually harvested. The plant often can compensate for early losses by producing a second or third ear, but the capacity to compensate ear number is largely lost by R1 and from then on no new ears can be formed,” says Lauer.
“Likewise, kernel number is at its greatest potential slightly before R1; the actual number of kernels formed is determined by pollination of the kernel ovule. The yield component of kernel number is actually set by pollination and fertilization of the kernel ovule. If the ovule is not pollinated, the kernel cannot continue development and eventually dies. No new kernels form after the pollination phase is past,” he reiterates.
The only yield component remaining after pollination that has some flexibility is kernel weight. For the first 7 to 10 days after pollination of an individual kernel, cell division occurs in the endosperm. The potential number of cells that can accumulate starch is determined. At black layer formation (R6) no more material can be transported into the kernel and yield is determined, Lauer concludes.