Serious crop stress during the grain filling period of corn increases the risk of stalk rots and stalk lodging (breakage) prior to grain harvest. Among the more common serious stresses that can occur during grain fill are nitrogen deficiency, foliar diseases (e.g., gray leaf spot, northern corn leaf blight), defoliation by hail, excessively wet soils due to heavy rains, excessively dry soils due to drought, excessive heat and lengthy periods of cloudy conditions.
What these crop stresses share in common is that they can significantly reduce photosynthesis and the resulting carbohydrates necessary for kernel development. During the grain filling period of corn, the developing kernels become a significant photosynthetic "sink" for the products of photosynthesis and respiration. Corn plants prioritize the movement of these photosynthates to the kernels, even at the expense of not maintaining the cellular health of the stalk, leaves and roots.
When photosynthetic capacity decreases significantly during grain fill as a result of serious photosynthetic stress, plants often respond by remobilizing stored carbohydrates from stalk and leaf tissues to supply the intense physiological demand by the developing grain on the ears. In addition to physically weakening the stalks of plants, remobilization of stored carbohydrates and/or the consequent lower cellular maintenance of root and stalk tissues increases the susceptibility of the plant to root and stalk rot diseases.
Fields at high risk for weakened stalks and stalk rot development are those whose plants have "set" fairly decent ears (e.g., ears with a lot of kernels), but then experience severe stress during grain fill. Common photosynthetic stresses that occur during grain filling include drought stress, nitrogen deficiency, defoliation by hail and foliar leaf diseases. The effects of dry weather during August on corn stalk health are accentuated in fields where root development and depth were restricted earlier in the season or, obviously, in fields with sandy soils and minimal water-holding capacity. Early-season root restriction can occur in response to saturated soils and/or shallow layers of compacted soil.
Severely stressed fields should be scouted in late August through early September for compromised stalk strength or the development of severe stalk rots. In years where crop development is delayed, stalk quality problems may not appear until mid- to late September. Recognize that hybrids can vary greatly for late-season stalk quality even if grown in the same field due to inherent differences for late-season plant health or resistance against carbohydrate remobilization when stressed during grain fill.
Stalk breakage itself is obviously easy to spot when scouting a field. However, compromised stalks may stand unnoticed until that October storm front passes through and brings them to their proverbial knees. The simplest techniques for assessing stalk integrity involve either pushing on stalks to see whether they will collapse or bending down and pinching the lower stalk internodes to see whether they collapse easily between your fingers. Sometimes the mere act of pushing stalks out of your way as you walk from one row of corn to another is enough force to collapse weakened stalks.
If possible, fields at high risk for stalk lodging should be harvested earlier than fields with lower risk of stalk lodging. This will minimize the risk of significant mechanical harvest losses resulting from downed corn.
Another side-effect of late-season stress during grain fill is the fact that plants may begin to simply "shut down" and mature prematurely, eventually evidenced by premature formation of kernel black layer (i.e., the visual indication of physiological maturity). The consequences of premature kernel black layer include not only lower grain yield, but also the likelihood of lower test weight grain.