Corn is quite unique because much of its potential productivity is determined during the planting process. Thus, corn is often much more responsive or dependent on variables implemented during the planting than other Southern crops. Corn’s determinate growth habit and fixed plant characteristics mean it possesses tremendous potential productivity that is greatest when it germinates. However, any stress endured during the season permanently reduces productivity. In other words, every mistake going to cost you performance, so you can never attain your full potential.
Early planting is a well-known component of successful corn production, since environmental stress normally increases during the summer, reducing yield potential of late-plantings. However, rushing the process often instigates major problems that overwhelm the benefits of early planting. We often hurry to get our crop planted in the South, because rainfall restricts days suitable for fieldwork during prime planting time. Although corn is an amazingly productive plant, it will not tolerate problems at any growth stage without suffering some level of yield reduction. Many troubles related to ill-timed or haphazard corn planting cause serious irreparable yield reduction. It is your responsibility to make sensible cropping decisions, execute timely and precise practices and implement many inputs designed to enhance or preserve corn productivity, setting the table for this season.
One of the most significant inputs affecting crop productivity is implementing a rotation system. Corn growers in the Mid-south have traditionally planted corn in rotation with other primary crops, such as cotton and soybeans. Thus, the value of rotation to our corn production systems may be under-appreciated. Research consistently shows corn productivity at least 15% lower when grown continuously, compared to a crop rotation. Furthermore, continuous cropping also substantially increases the likelihood of disease infection, weed competition and insect infestation which will cut yield level or increase management expenses. Thus, I strongly discourage growing corn in the same field for more than two consecutive years in Mississippi. If you chose to plant continuous corn, my primary suggestion is to select hybrids with resistance to specific foliar diseases which thrive when corn follows corn. One expression which grossly over-simplifies this issue is “disease package.” There are a number of specific diseases which survive on corn residue, including Northern corn leaf blight, Southern corn leaf blight and Gray leaf spot that could be problematic in continuous corn. Hybrid resistance normally varies for each of these specific diseases. Thus, you need to closely analyze as much data as you can to select suited hybrids and spread your risk.
Historically, the primary factor limiting corn planting in Mississippi is wet soils. Wet soils not only mechanically restrict planting, but also restrict root development, which is critical for nutrient uptake and plant growth later in the season. However, you should also be aware that soil temperature is also very critical to successful corn germination, emergence and early growth. Soil temperature is the primary factor regulating germination rate, which can affect stand success and plant growth uniformity, both of which are paramount to high corn productivity. Corn seed germination requires a minimal temperature of about 50 degrees F and germination rate increases substantially as soil temperature rises. Thus, the standard guideline for determining earliest planting date is when morning soil temperature at a 2-inch soil depth is 55 degrees F and/or 50 degrees F at a 6-inch soil depth. These levels generally ensure emergence within two weeks. Although early planting is a critical component of successful corn production, planting corn exceptionally early will not generally produce the highest yields, particularly when grown with irrigation. This is not only due to the problems noted previously, but extraordinarily early planting enhances maturity very little and is not generally favorable for optimal vegetative development, because corn growth rate is correlated to temperature, and heat unit accumulation (GDD 50) is historically very low during early March. Our rainy springs not only encourage you to plant quickly, but also tempt you to plant marginally wet fields, particularly when planting intentions are high. This may compact soil, especially in the seed-furrow, which severely restricts early root growth and limit productivity from the onset of the season. Excessive planter unit down-force may also promote undesirable seed-furrow compaction. These hard, compacted seed-furrow walls restrict and/or even prohibit nodal root penetration in some cases, causing rootless corn syndrome, poor nutrient and water uptake and exacerbate root lodging at maturity. Also, clay soils are often prone to shrink when they dry, causing the seed furrow to open and expose nodal roots – just like shallow planting. This limitation is critical because newly emerged plants are extremely sensitive to their intimate environment and will permanently reduce their potential productivity. Thankfully, you have tremendous ability to control these limitations mainly by choosing when you plant.
The foundation for “plant health” and resultant high corn yields is uniform stands and extensive root systems, where plants have plenty of resources and equal ability to utilize them. These variables are primarily dependent upon the performance, timeliness and execution of the planting process. Variable plant spacing and plant emergence are common stand problems that can affect corn yield potential as much, or more than actual plant population and offer you tremendous potential to enhance corn productivity. Corn plants are extremely sensitive to variable plant spacing because they do not tiller or produce branches to adjust their plant size, and only produce one fruit-bearing organ per plant, unlike most other crops. Crowded plants produce small, variable-sized ears due to intense competition for light, water and nutrients with adjacent plants. Also, late-emerging corn plants experience permanent developmental disparity which drastically reduces grain yield because these runts poorly utilize resources. Planter meter system tune-up and proper calibration can certainly improve planter performance, but performance also depends a lot upon operator input in the field. One potential cause of poor seed distribution is excessive planter speed. Corn yield champions are fanatical regarding planter precision and normally plant their contest at 4 mph or less. I suggest keeping speed near this level and never exceed 5.0 mph. Speeds exceeding these values may create variable seed spacing and seed depth because seeds may roll and/or bounce in the seed furrow. These factors reduce yield potential by increasing plant competition for available resources or by causing permanent physiological disparity.
Many corn growers use starter fertilizer when planting to supplement their corn fertility program. Starter fertilizer promotes earlier maturity, enhances plant vigor, and often improves grain yield, especially in minimum or no-tillage systems. Starter fertilizer works by providing a concentrated phosphorus supply directly in the root zone of young plants, which is particularly beneficial when soils are cold and wet. Phosphorus placement is very important to young plants with small root systems because phosphorus doesn’t move in the soil. Even though nitrogen is an important part of starter fertilizer, it can move in the soil. That’s why nitrogen placement is not as important to corn uptake, especially since corn has a fibrous root system with lots of lateral growth. Thus, nitrogen fertilizers alone are not very valuable as starter fertilizers. The research-proven benchmark starter fertilizer source is ammonium polyphosphate (10-34-0 or 11-37-0). Many brands of orthophosphate fertilizers are readily available. But they are more expensive, have lower nutrient analyses, and routinely show no yield difference compared to polyphosphate fertilizers in field trials. When you apply starter fertilizer in the seed furrow, use no more than 4 gallons of ammonium polyphosphate per acre in 38 to 40-inch rows or 5 gallons per acre in 30-inch rows. Otherwise, you may cause salting injury to seedlings. Corn Belt growers often use coulter rigs that band starter fertilizer to the side and below the seed. These systems are efficient, safe for the plant, and effective. Corn may also be very responsive to Zinc applied as a starter in-furrow or banded in a 2×2 method, particularly at high production levels. Like phosphorus, zinc is relatively immobile in soils. Therefore, corn may respond very well to a starter fertilizer including zinc, because young plants with small roots have difficulty accessing it, particularly when soils are cool and wet. However, zinc will not be utilized very efficiently when sidedressed with nitrogen fertilizer. Broadcast zinc should be incorporated into the soil with tillage prior to planting.
Corn growers should generally strive for a goal of 24,000 to 34,000 plants per acre. Seeding rates should exceed the desired plant population about 5 to 10% depending upon planting conditions, seedbed preparation, and seed germination. However, your optimum plant population may vary considerably from these guidelines, depending upon several factors. Dryland producers should moderate their goals, because you are dependent upon mid-season rainfall to realize potential. Therefore, I generally recommend 24,000-28,000 plants per acre for dryland production. Obviously, irrigation helps alleviate potential water deficit allowing you to maximize plant population where the “plant factory” controls productivity. However, when light interception is nearly complete, corn response to higher population generally levels off. Many factors affect corn light interception, including hybrid characteristics, planting date and planting configuration, so you should integrate these factors into planting decisions. Early-maturing hybrids often have less leaf canopy than later hybrids, meaning they may be more responsive to higher seeding rates. Ultra-early planted corn (soil temperature 50-55 degrees F) should be seeded about 10% thicker than normal because cool spring conditions usually promote higher seedling mortality and smaller plants with less leaf area at tassel, meaning more plants are needed to intercept available light. Conversely, growers should reduce seeding rate at later planting dates since warm temperatures enhance seedling establishment and produce taller, leafier plants, but are more likely to expose the crop to late-season stress, decreasing grain yield potential. Narrow rows substantially improve plant spacing geometry which improves light interception and decreases plant competition for nutrients and water, so potential productivity and corresponding plant population is higher. Thus, optimum plant population in wide rows is generally around 2,000 – 4,000 plants/acre less than narrow rows. Twin wide rows should be planted at rates similar to 30-inch rows.