Jumbled Kernel Symptom in Corn & Late Glyphosate Applications
RL (Bob) Nielsen, Extension Agronomist
The "jumbled kernel" symptom in corn results when pollination/fertilization or initial kernel development fails throughout the ear for some percentage of the ovules. The surviving kernels expand to fill the empty spaces left by the absent kernels and the resulting pattern of kernels on the ear appears to be a jumbled collection of kernels. The problem can be caused by typical stresses such as excessive heat and drought near the time of pollination or early kernel set. Extensive damage to exposed silks during pollination due to insect feeding can also cause incomplete kernel set. However, the problem can also occur when the herbicide glyphosate is applied off-label to RoundupReady(tm) corn that is shoulder high or closer to pollination.
Some RR hybrids are not homozygous for the glyphosate resistant gene. Rather they are heterozygous for the trait; one resistant gene contributed from one inbred parent and one susceptible gene from the other. The resistant gene is dominant and thus the hybrid plant itself is resistant to glyphosate. Following pollination among the heterozygous "parents" in the hybrid field, the "next generation", represented by the kernels (fertilized ovules) of the ears, segregates genetically such that approximately 1/4 of the developing kernels contain two copies of the susceptible gene (in other words, these kernels are homozygous for the susceptible genes), 1/2 of the kernels remain heterozygous (and thus resistant), and 1/4 contain two copies of the resistant gene (and thus are also resistant). Glyphosate applied late can be lethal to the 25% of the developing kernels that are homozygous susceptible if they come in contact with the glyphosate during their development. Thus, up to 25% of the potential kernels on an ear may be damaged by late (off-label) applications of glyphosate and, thus, up to 25% yield loss may occur from this damage.
The ears shown in the accompanying photos were taken from a field that had been treated with glyphosate when the plants were between shoulder- and head-high. The late application was not intentional, but earlier applications were not possible due to frequent rainfall and soggy soils. Giant ragweed threatened to take over the field and the decision was made to go in with a high-clearance applicator with drop nozzles on corn to control the weeds. All of the ears in the field (yes, 100%) suffered kernel loss and subsequently exhibited the jumbled kernel symptom.
Remember: There are physiological reasons for crop growth stage limits listed on herbicide labels!