Through photosynthesis, the leaves of the corn plant capture sunlight and carbon dioxide to produce sugars (photosynthate). These sugars are directed to the actively growing organs of the plant. Early in plant development, sugars move to the roots, where they are converted to structural carbohydrates and proteins in the developing root tissues. As the plant continues to grow, photosynthate is directed to the stalk for temporary storage.
Upon successful pollination, ear development places a great demand on the plant for carbohydrates. When the carbohydrate demands of the developing kernels exceed the supply produced by the leaves, stalk and root storage reserves are tapped. University studies indicate that during grain fill, about 60% to 70% of the non-fiber carbohydrates in the stalk are moved to other parts of the plant, but primarily the ear (Daynard et al., 1969; Jones and Simmons, 1983). This stalk depletion begins approximately 2 to 3 weeks following silking. Environmental stresses which decrease the amount of photosynthate produced by the plant can force plants to extract even greater percentages of stalk carbohydrates, which preserves grain fill rates at the expense of the stalk.
Many environmental conditions can decrease the amount of photosynthate produced by the plant. Drought stress and low available sunlight are 2 such conditions. Factors which reduce functional leaf area, such as disease lesions, insect feeding or hail damage also reduce photosynthate production.
As carbohydrates stored in the roots and stalk are mobilized to the ear, these structures begin to decline and soon lose their resistance to soilborne pathogens. High temperatures at this time increase the rate at which the fungi invade and colonize the plant. Though pathogens play a key role in stalk rot development, it is primarily the inability of the plant to provide sufficient photosynthate to the developing ear that initiates the process.
As vascular tissues in the plant become plugged by fungal mycelial growth, water supply to the plant becomes restricted. Wilting and premature death of the plant eventually follows. External discoloration of the lower stalk becomes evident as deterioration of the inner stalk tissue progresses. The structural integrity of the stalk is diminished by this decay, and the plant is susceptible to lodging. Storms and high winds provide the forces needed to topple the weakened stalks.
The growing environment has a critical effect on the ability of the plant to provide sufficient photosynthate to the developing ear. Almost any stress applied to the plant will reduce photosynthesis and resultant sugar production in the leaves.
The decrease in photosynthetic rates due to drought stress has been well documented in research studies. Water relations within the plant and CO2 and oxygen exchange are directly affected. In addition, if leaf rolling occurs during drought, the effective leaf surface for collection of sunlight is reduced.
In research studies in which water was withheld from plants beginning at the mid-grain-fill stage, photosynthesis was eventually shut down (Westgate and Boyer, 1985). Subsequent grain development depended entirely on stalk carbohydrate reserves.
Photosynthesis is most efficient in full sunlight. Studies show that the rate of photosynthesis increases directly with intensity of sunlight. One experiment indicated that photosynthesis rates are reduced more than 50% on an overcast day compared to a day with bright sunshine (Moss et. al., 1960). Prolonged cloudy conditions during ear fill often result in severely depleted stalk reserves.
Reduction of Leaf Area
Any reduction in leaf area will limit total photosynthesis. Leaf area may be reduced due to hail, frost, disease lesions, insect feeding or mechanical injury. Whenever functional leaf area is reduced prior to completion of ear fill, stalks will be weakened.
Favorable Conditions Early, Stress During Ear Fill
Even favorable growing conditions can have a negative effect on stalks. If favorable growing conditions exist when the number of kernels per ear is being established (V10-V17), the eventual demand for photosynthate will be large. Each potential kernel represents an additional requirement for translocatable sugars from the plant. If stress conditions develop during ear fill which render the plant unable to produce enough sugars, stalks will suffer.
Research has demonstrated that the number of kernels per ear on stalk-rotted plants is often greater than that of adjacent healthy plants (Table 1). The additional demand for carbohydrates by larger ears often results in greater depletion of the stalk and eventual stalk rot.Table 1. Comparison of kernel numbers between plants with rotted stalks and adjacent plants with healthy stalks.*
Research studies have documented that soil fertility has a profound effect on stalk quality. Most notable are studies which show that a combination of high nitrogen (N) and low potassium (K) can severely reduce stalk quality. Researchers suggest that yearly applications of N and K (actual N, K as K2O) should be approximately in the ratio of 1 to 1 for favorable balance in the corn plant and to reduce the risk of stalk rots and stalk breakage.
High N is associated with greater kernel number, which increases the demand for carbohydrates to the ear. Higher N also aids the movement of these carbohydrates out of the stalk and into the ear by increasing the rate of translocation within the plant.
The role of K in preventing premature plant death has long been established. K functions in the building of leaf and stalk tissue, and in regulating water movement within the plant. Increases in potassium have been associated with increased photosynthetic rate.
Hybrid genetics are an important influence on stalk lodging potential. Some hybrids naturally partition more carbohydrates to the stalk. Though useful in a poor stalk quality year, that trait may limit yield potential in a more normal environment. In the hybrid development process, researchers must be careful to select hybrids with highest harvestable yield potential across many years and environments. Too much emphasis on stalk quality alone could result in lower yield potential most years. Many carefully selected hybrids with very good stalk quality may appear inadequate during a 1-year-in-10 stalk-lodging event.
Certain plant characteristics other than stalk strength per se can influence a hybrid's potential for lodging. Hybrid maturity determines the plant's developmental stage when environmental stresses occur, which impacts stalk quality. Hybrids prone to leaf diseases may lose significant leaf area, weakening the stalks. Susceptibility to specific stalk rot pathogens also increases the stalk-lodging risk.
Oftentimes, small differences between fields, or between areas in the same field can determine whether corn stands or lodges. Differences in soil fertility, soil moisture, plant-to-plant spacing, insect feeding, or wind gusts can push plants past the lodging threshold. These effects are difficult to predict, but scouting fields in the fall can identify problem areas, and early harvest can reduce field losses.
Multiyear research studies show that stalk lodging is increased only slightly at higher plant populations. For example, a summary of Pioneer research from 35 high-lodging environments from 2004 to 2007 showed that percent stalk lodging increased only about 0.5% for each 1,000 plant/acre increase in population (Paszkiewicz and Butzen, 2007.)
Careful scouting and harvesting fields according to crop condition can help prevent field losses due to low stalk quality. Corn loss potential should be weighed just as heavily as grain moisture in deciding which fields to harvest first. Scouting fields approximately 2 to 3 weeks prior to the expected harvest date can identify fields with weak stalks predisposed to lodging. Fields with high-lodging potential should be slated for early harvest.
Weak stalks can be detected by pinching the stalk at the first or second elongated internode above the ground. If the stalk collapses, advanced stages of stalk rot are indicated. Another technique is to push the plant sideways about 8 to 12 inches at ear level. If the stalk crimps near the base or fails to return to the vertical position, stalk rot is indicated. Check 20 plants in 5 areas of the field. If more than 10% to 15% of the stalks are rotted, that field should be considered for early harvest.
Pioneer corn breeders use aggressive techniques to weed out hybrids with poor stalk quality, including manual and mechanical push tests which mimic the forces of wind on corn plants. In addition, plants are inoculated with stalk rot organisms where appropriate to ensure that susceptible genotypes do not escape detection. Pioneer plant pathologists monitor disease incidence and assist breeders in their efforts to inoculate, screen and characterize products. Research trials conducted by Pioneer corn breeders are designed to measure product performance for all important traits across a wide range of growing conditions.
Pioneer IMPACT™ plots further test product performance, including characterization of stalk quality, thus determining proper placement of new product releases. Pioneer uses information from both breeder and IMPACT plots to develop stalk lodging ratings on all its hybrids to aid customers in selecting appropriate hybrids for their fields.