Corn growth is largely temperature driven, although moisture also has some effect especially extreme drought or extreme soil saturation.

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1 1. Early-season corn growth Corn growth is largely temperature driven, although moisture also has some effect especially extreme drought or extreme soil saturation. The effect of temperature on corn growth begins even while the crop is still below ground. Growing Degree Units (GDUs) can be a helpful guideline to determine about when corn should emerge. A medium-maturity hybrid requires roughly 120 GDU to emerge. Planting depth and soil moisture can modify actual time to emergence, but if more than 120 GDU have been accumulated with no emergence, it may be time to start digging up seed to see what is going on. There is actually quite a lot going on beneath the soil surface before emergence. Germination is usually defined as the time when the radicle, or seed root, breaks through the seed coat. However, before that happens, the seed must soak up enough water to initiate enzyme activity and cell growth. The coleoptile is the next structure to elongate and break through the seed coat. This leaf-like structure encloses the embryonic leaves, protecting them as they push through the soil before emergence. The last structures to appear are the lateral seminal roots, 3 to 4 seed roots that emerge from the same end of the seed as the coleoptile, and together with the radicle, make up the seminal root system. This root system is the primary source of water and nutrients until the nodal root system takes over at V3 to V5. Soil Surface Coleoptile Crown Mesocotyl Radicle Lateral Seminal Roots Photo by Kraig Roozeboom, K-State Research and Extension. 1

2 Emergence occurs when the coleoptile breaks through the soil surface. This is accomplished by elongation of the coleoptile and by elongation of the mesocotyl, the root like structure that connects the crown (base of the stem) and the seed. When the coleoptile senses light, mesocotyl elongation stops, and the depth of the crown is set. The length of the mesocotyl is much more flexible than the length of the coleoptile, so the crown depth (and the resulting position of the nodal roots) is fairly consistent, regardless of seed depth. After emergence, vegetative stages are designated by counting leaf collars. A V1 plant has one leaf collar emerged; a V2 plant has two, and so on. Sometimes it is not immediately obvious if the first leaf is still present, especially with larger plants. A helpful indicator is that the first leaf has a more rounded tip than subsequent leaves. Soon after emergence, at about V1, the nodal root system begins to grow. The first four nodes stay compressed at the base of the stem and give rise to the first sets of nodal roots. By V6 these roots have become the major supplier of water and nutrients to the corn plant and also are important for anchoring the plant to prevent lodging. Nodal roots eventually are initiated from up to 7 to 10 nodes, including several above-ground nodes. First leaf First leaf collar Coleoptile Nodal root Photo by Aaron Sindelar, K-State Research and Extension. The growing point typically stays below the soil surface until appearance of the fifth leaf collar. This is important when considering the implications of early-season leaf loss due to a freeze, insect damage, or hail. Don t assume a young corn plant is dead just because the above-ground leaf area is destroyed or severely damaged. Often, if the growing point survives, additional leaves soon emerge and yield losses can be minimal, especially if leaf loss occurs soon after emergence. 2

3 At about V5 to V6, the growing point differentiates into the tassel. All leaves have been initiated. Although the growing point is above the soil surface by V6 and becomes more vulnerable to above-ground threats, the tightly wound leaf sheaths provide significant protection. The progression of corn growth depends primarily on the rate of GDU accumulation during the season. To calculate the daily GDU accumulation, add the maximum and minimum temperatures for each day, divide by 2, and subtract a base temperature of 50 (little corn growth occurs at temperatures less than 50 F). Any temperature below 50 F is considered to be 50, and any temperature over 86 F is considered 86. Growth rate is maximized at temperatures from 86 to 97 F. At temperatures greater than 97 F, growth rate slows dramatically. Most seed suppliers provide GDU ratings for their hybrids, indicating the GDU required to reach physiological maturity (black layer). -- Kraig Roozeboom, Cropping Systems and Crop Production Specialist kraig@ksu.edu 2. Diagnosing poor corn stands and early-season growth problems What are the most common causes of emergence problems or poor seedling growth? There can be several possible causes. If there is poor seedling emergence, start by looking for patterns. Whether the emergence problems are occurring in small but regular skips across the field, uniformly throughout the field, only in localized areas, or in random scattered areas could be a clue to the cause. Here are just a few of the possible scenarios: A uniform pattern of skips suggests a clogged, jammed, or broken planter. If the field has an uneven pattern of emergence, you ll have to do some digging in the areas with emergence problems. If there is a hole where the seeds were placed, an empty seed coat at the bottom of the hole, and a little pile of soil next to the hole, you might suspect rodent damage. Soil insects can also cause patterns of non-uniform emergence. Several insects attack planted seed, destroying the germ or feeding on the geminating tissue. When cool temperatures delay germination, the risk of damage increases as the seed is exposed to a longer feeding period by insects. Possible culprits include seed corn beetles, seed corn maggots, and wireworms. If the seed has been treated with a seed-applied or plantingtime insecticide, insect damage is less likely to be the cause of the problem. Uneven patterns of emergence may also be caused by prolonged waterlogging in lowlying areas of the field. If emergence problems are relatively uniform throughout the field, you may find that soil surface crusting has prevented emergence. This can be easily verified by a little digging. If seedlings cannot push through the soil surface crust, you should find malformed seedlings just beneath the soil surface. Poor emergence or uneven emergence can result from planting corn too shallowly, and not having all the seed planted into adequate moisture for germination. This seems to occur most often in no-till, where variation in residue cover causes variable surface moisture. Planting too deeply can also result in poor emergence and stand establishment, especially if soils are crusted. 3

4 Corn seed planted too deeply. The coleoptile could not emerge through the soil surface. Surface compaction was also a problem in this case. Photo by Stu Duncan, K-State Research and Extension. The pre-emergence seedling is protected by the surrounding soil from temperature extremes and other above-ground threats, but is susceptible to desiccation if the soil dries sufficiently after germination and before the root system is large enough to access water from deeper in the profile. Research has shown that a 25% loss of stand at emergence results in less than 10% yield loss. Replanting may not be necessary if the remaining stand is relatively uniform (few large gaps in adjoining rows) and is within 20% of the target population. If the plants emerged in good fashion, but the seedlings then have problems maintaining adequate growth and development or leaf color, there may be several possible reasons. A few of the most likely causes include: Compacted soil or waterlogging. Wet soils and unusually cool temperatures can inhibit root growth especially, slowing plant development. This can cause yellowed, wilting plants due to poor root growth, drowning, or a seedling blight infection. Seedling blight is often characterized by stem tissue near ground level that is discolored or water-soaked in appearance. Also, planting in wet soil can compact the seed furrow, inhibiting root growth. A shallow compaction layer can slow early root growth, resulting in stunted, nutrient deficient plants. Sidewall and seed zone compaction in heavy clay soil. Photo by Stu Duncan, K-State Research and Extension. 4

5 Early-season lodging. This is usually associated with hot, dry weather during V1 to V6, which prevents adequate development and penetration of nodal roots. Plants can survive for a time on just the seminal root system, but they will have little mechanical support. Reasons for poor nodal root development and an elevated crown include sidewall compaction, erosion after emergence but before nodal root development, and sinking of the seedbed due to pounding rains. Often a good soaking rain is enough to allow nodal roots to establish and plants can recover. Inter-row cultivation can be used to push soil against plants with exposed crowns. Corn seedling lodging caused by shallow planting and poor nodal root development (plant on left). Photos by Doug Shoup, K-State Research and Extension. White grubs or wireworms. These soil insects may be eating the roots, which will cause the plants to wilt. Black cutworms. These insects, which can be found in the soil or on the surface, cause window paning of the leaves on young plants. Cutworms may also cut off seedling plants at the soil surface. Flea beetles. These tiny leaf-chewing insects can cause scratches on leaves. Eventually, the leaves may shrivel, turn gray, and die. Plants are more susceptible to flea beetle injury when temperatures are cold and seedling growth is slow. Seedling plants are often able to recover from flea beetle injury because the growing point remains below ground level until the fifth leaf emerges. 5

6 Chilling injury (cold weather crown rot). When this occurs, plants are stunted and may display nutrient deficiency symptoms. Root development is usually normal, but the crown will have dark brown or black discoloration, which can be seen by splitting the stem. This kind of injury is associated with unusually cool temperatures from emergence to V4 not freezing, but close to it. Symptoms are similar to Stewart s Wilt, so check for flea beetle feeding, which is the vector for this disease. Freezing temperatures. A freeze that occurs after emergence can cause leaves to first appear watersoaked, then turn white within a few days. A freeze can kill leaves. Plants will recover from this if the freeze occurs before the fifth leaf emerges because the growing point is still underground. Plant roots are undamaged by a freeze. If the weather warms back up sufficiently after the freeze injury occurs, chances of plant survival are increased. If it stays unusually cool and wet, crown rot can occur. In rare cases, such as the Easter freeze of 2007, temperatures can get low enough to damage the crowns below the soil and kill the plants. Corn after a hard freeze caused 25% stand loss in this case. Photo by Stu Duncan, K-State Research and Extension. Free ammonia from an anhydrous ammonia application. This can injure roots and kill germinating seed if the ammonia was applied too shallowly (especially in coarser soils), too close to the time of planting, or if dry soil conditions slowed the conversion of ammonia to ammonium. One way to minimize damage is to apply the ammonia at a 10 to 15 degree angle from the direction of planting. If injury occurs then it is more randomly distributed, reducing the multi-plant skips, and allowing the unaffected plants to compensate. Ammonia injury can also occur when sidedressing anhydrous ammonia under dry soil conditions. Root injury can occur if the plants get too big or the knives run too close to the row. Ammonia injury resulting from poor soil sealing can cause leaves to appear watersoaked or have dead margins. Roots may appear sheared off, or burned off. Plants will normally recover from this injury, but yields can be reduced. Putting a urea-based N fertilizer in contact with the seed. Urea will hydrolyze into ammonia and injure the seedling. 6

7 Seedlings damaged after starter fertilizer containing urea-n was placed in direct seed contact. Photos by Dorivar Ruiz Diaz, K-State Research and Extension. Nitrogen (N) deficiency. This does not usually occur until a later stage of growth in conventional tillage systems. But in no-till corn, especially in high residue situations, N deficiency is common where producers haven t applied nitrogen as a starter, or broadcast a significant amount of N prior to or at planting. In early planting in very cold soils where no N was applied close to the seed as a starter, seedlings may be N deficient in conventional-till also. Nitrogen deficient corn seedlings will be spindly, with pale yellowgreen foliage. As the plants grow, the lower leaves will fire, with yellowing starting from the tip of the leaf and progressing back toward the stalk. Phosphorus deficiency. This can result in stunted growth and purple leaves early in the growing season. Phosphorus deficiency is often enhanced by cool, wet growing conditions. Iron deficiency. This can cause upper leaves to be pale green between the veins. Iron deficiency is more common on high ph and calcareous soils. Sulfur deficiency. This can result in stunted plants having pale green leaves, with no distinct pattern on the leaves. Herbicide injury. This is not as common now as in the past, but can still occur. Corn is very susceptible to injury from carryover sulfonylurea herbicides which may have been applied to a previous crop, such as wheat. Carryover depends on soil ph, soil texture, application rates, rainfall, and other factors listed on the herbicide labels. Symptoms include stunting, chlorosis, and an overall sickly appearance. Corn will not grow out of this type of injury. 7

8 ALS herbicide carryover injury to corn. Photos by Stu Duncan, K-State Research and Extension. For more details, see Diagnosing Corn Production Problems in Kansas, K-State publication S- 54, at: Also, see Corn Production Handbook, K-State publication C-560, at: -- Kraig Roozeboom, Cropping Systems and Crop Production Specialist -- Dorivar Ruiz Diaz, Nutrient Management Specialist -- Jeff Whitworth, Extension Entomology -- Doug Jardine, Extension Plant Pathology -- Stu Duncan, Northeast Area Crops and Soils Specialist -- Doug Shoup, Southeast Area Crops and Soils Specialist 3. Nutrient deficiency symptoms in corn The following are the classic diagnostic symptoms of some of the most common nutrient deficiencies in corn. Keep in mind that most field nutrient deficiencies are not as severe as those pictured and may not appear exactly as classic deficiencies normally presented in photos. Marginal deficiencies are difficult to differentiate among various nutrient disorders, diseases, and genetic variability. 8

9 * Nitrogen. Young plants will generally have pale, yellow-green foliage. Deficiency affects lower leaves first, with V-shaped yellowing or firing down the midribs of the leaves. The pattern of firing starts from the tip of the leaf and progresses back toward the stalk. Stalks are often thin and spindly. Nitrogen deficiency symptoms in corn. * Phosphorus. Relatively severe deficiencies will result in purpling of plant leaves (although other factors may also result in purpling of leaves). Plants will be stunted and delayed in maturity. Delayed maturity will often result in higher grain moisture at harvest. Phosphorus deficiency symptoms in corn. Photo on left by Dorivar Ruiz Diaz, K-State Research and Extension. 9

10 * Potassium. Deficiency affects lower leaves first and is displayed as firing of leaf margins. Oldest leaves have the greatest amount of firing. Potassium deficiency symptoms in corn. Photos by Dorivar Ruiz Diaz, K-State Research and Extension. * Sulfur. Sulfur deficiency somewhat resembles nitrogen deficiency, especially on young plants. Plants are stunted with pale, chlorotic leaves. Sulfur deficiency symptoms appear in the younger leaves first and show an interveinal chlorosis. Sulfur deficiency symptoms in corn. 10

11 * Zinc. Zinc deficiency results in stunted plants with bronzing discoloration between leaf margins and midrib. Internodes of the stalk are shortened. Mild deficiencies are difficult to diagnose. Most commonly found on high ph soils, and sandy soils. Problems are often enhanced by erosion or soil leveling. Early-season zinc deficiency. Photo by Stu Duncan, K-State Research and Extension. * Iron. Plants are stunted. Mild deficiencies display interveinal chlorosis on new leaves, while more severely deficient plants display completely yellow to almost white upper leaves. Although corn is a relatively tolerant crop to iron chlorosis, this deficiency can be found on high ph, calcareous soils. Iron deficiency symptoms in corn. For more details visit the Nutrient Deficiency section in the K-State Soil Fertility website -- Dorivar Ruiz Diaz, Nutrient Management Specialist ruizdiaz@ksu.edu 11