Considerations for Selecting Corn Hybrids in Pennsylvania

Corn hybrid selection is one of the most critical decisions that corn producers make and has a large impact on the profitability of corn production systems. Differences in performance among commercially available corn hybrids of the exact same maturity often exceed 20 bushels per acre. Therefore, improving corn yields by 20 bushels per acre or more through careful attention to hybrid selection is not unrealistic.

Commercial seed companies should be able to supply the most detailed information on hybrid characteristics. Seed suppliers who have worked with these hybrids in your area should have a basic understanding of many of the hybrid traits that are important in your crop production system. They also have access to performance data collected from their own evaluation trials conducted in the region. Data provided by seed companies are a great resource for learning specific traits and other detailed information on a hybrid and are best used for comparing hybrids within a company. For comparing hybrids from different seed companies, University variety trials are a great resource. In these trials, hybrids from different seed companies are evaluated side by side and are rated based on the same scale.

This fact sheet describes important characteristics that should be considered when selecting corn hybrids, such as hybrid maturity, herbicide and insect tolerance traits, disease resistance, standability, yield, and silage quality potential, in addition to other specialty traits. To minimize risk and increase average yield across the entire farm, we recommend planting multiple hybrids each year.  

Maturity

Maturity is of primary importance in the hybrid selection process because it is critical to choose hybrids that can use as much of the available growing season and still have a minimal risk of being frosted before maturity. Hybrids that are too early will have lower yield potential and those that are too late may result in poor quality grain or silage.

Corn hybrid maturity can be rated according to two systems: the relative maturity system (RM) and the growing degree day (GDD) system. In the more popular RM system, hybrids are assigned a days-to-maturity rating based on the grain moisture at harvest compared to other hybrids. As a result, a hybrid with a lower RM rating should have lower grain moisture levels at time of harvest than one with a higher RM rating. The rating is not a measure of the actual days required for maturity, since it is not uncommon for a 105-day hybrid to require 150 days from the date it is planted to reach maturity. The ratings also vary somewhat among companies; this makes comparisons of maturity more difficult. One way to overcome this is to compare moisture contents of specific hybrids in performance trials where they are evaluated side-by-side; lower moisture contents translate into an earlier rating.

In the GDD system, the number of GDD is calculated between the date of planting and black layer (growth stage R6, which marks the end of the grain filling period). The advantage of this system is that it provides a link between hybrid maturity and local weather information. A disadvantage of this system is that GDD requirements of hybrids may change somewhat with location and season. Hybrids in Pennsylvania often do not require as many growing degree days as they do in the Midwestern states. Also, GDD requirements of specific hybrids may be lower in cool seasons and late plantings. Another disadvantage of GDD ratings is that they are not necessarily indicative of differences in grain moisture because hybrids may differ in dry-down rates after black layer. However, company seed guides often include a dry-down rating, which describes the rate of grain moisture loss after black layer within hybrids of the same maturity.

A good rule of thumb is that corn for grain should reach maturity one to two weeks before the first killing frost in Fall. Other factors, such as harvest method, marketing plans, and timeliness considerations, may also have a bearing on maturity selection. The need for a field dry-down period with ear corn and dry shelled corn means that maturity should not be pushed in these systems. Compared to dry shelled corn, slightly longer-season hybrids (2 to 5 days) can be used in high moisture corn, and even longer-season (5 to 10 days) hybrids can be utilized for silage production, provided wet soils will not interfere with harvest. Earlier hybrids may provide some advantages for early corn markets or more timely harvesting. If you consider this approach, be sure to monitor yields of these earlier maturing hybrids to make sure this strategy is economical for your system. A key to managing maturities in an area as diverse as Pennsylvania is to monitor crop development each year and use this information in selecting hybrid maturities for future years. Appropriate maturities for grain production in Pennsylvania are shown in Figure 1 and Table 1 below.

Herbicide and Insect Tolerance Traits

With some exceptions, such as hybrids developed for specialty markets (non-GMO, organic), all modern corn hybrids are genetically modified to tolerate specific insect pests and over-the-top application of specific herbicides. Almost all modern corn hybrids are tolerant to glyphosate, many are also tolerant to glufosinate, and several are also tolerant to 2,4-D choline and FOP herbicides. The wide availability of herbicide-tolerant traits in both corn and soybean varieties means that the same herbicides can be used year after year in a corn-soybean rotation. While convenient, be aware that the recurrent application of the same herbicide, or even another herbicide of the same mode of action, leads to the development of herbicide-resistant weeds. Therefore, when assessing the benefits of an herbicide tolerance trait as part of your corn hybrid selection, consider which weeds are typically problematic in your cropping system and how valuable it is to have the option of using each specific herbicide. For current information on herbicide-resistant weeds, see the International Herbicide-Resistance Weeds Database: www.weedscience.org/Home.aspx

In addition to herbicide tolerance, most hybrids are genetically modified to express specific toxins that provide tolerance to some aboveground (caterpillars) and belowground (rootworms) insects. There are many different toxins, and each may target a different insect species. Further, widespread cases of insect resistance have been reported for some toxins. When assessing the benefit of an insect tolerance trait, it is important to identify which insects are targeted by the toxin trait package, and whether insect resistance has been reported in your region. For current information on commercially available insect tolerance packages, target insect species, and resistance cases, check out the Handy Bt Trait Table for US Corn Production: www.texasinsects.org/bt-corn-trait-table.html

Disease Resistance

Hybrid selection is the main avenue for control of the most predominant corn diseases in Pennsylvania. Stalk rot, one of the most serious and common corn diseases, is favored by environments where stress occurs during August. If stalk rot appears to be a persistent problem in your system, consider placing more importance on standability and stalk rot resistance in your hybrid selection. Gray leaf spot, northern leaf spot, northern leaf blight, and tar spot are also common in Pennsylvania. Gray leaf spot is most severe in continuous no-till corn fields. Fields that are along creeks and rivers are particularly vulnerable to gray leaf spot because of the extended periods of dew. Disease symptoms include light gray, rectangular-shaped lesions that are restricted by the leaf veins. Under high disease pressure, the lesions coalesce and form large areas of dead tissue. Where this disease is serious, gray leaf spot resistance should be a primary consideration in your hybrid selection. Both corn northern leaf spot and corn northern leaf blight occur most frequently in valley areas where heavy dews and early morning fog or mist are common. Northern leaf spot (race 3) symptoms are tan to brownish, linear, chainlike lesions. The northern leaf blight symptoms are elliptical, grayish-green streaks that develop on the leaves and may extend the length of the leaves. Under severe infection, all leaves may die. Both diseases can cause early plant death. Resistant hybrids are the best means of control. Tar spot is a relatively new disease in Pennsylvania. Signs of the pathogen include small, raised black spots (called stromata) on corn leaves and husks. The stromata may or may not be surrounded by a tan halo. Cool temperatures and high relative humidity favor disease development.  Tar spot can accelerate leaf senescence and plant death. The use of hybrids with some level of resistance and early detection before corn reaches reproductive stages is crucial to manage the disease. Fungicide applications should be based on the severity of the disease at critical growth stages and the weather forecast to assess the risk of disease development. A crop disease forecasting tool can be found at the Crop Protection Network website to help assess the risk of all major diseases of corn: www.cropprotectionnetwork.org

Learn to identify the major corn diseases in your area and select hybrids with specific resistance to these diseases. Be wary of hybrids advertised as having "good disease resistance"—instead ask your dealer about specific resistance to diseases that you know are common on your farm.

Standability

Standability is an important hybrid characteristic in Pennsylvania, where corn harvest often continues late into the fall. Typically, the later the harvest, the higher the chance of lodging. Poor standability is primarily due to stalk rot but can also be caused by weak stalks or root systems as well as insect damage. Standability ratings can be easily found in seed catalogues. Plant height, a similarly common characteristic found in seed catalogues, can also be used as an approximate measure of plant standability. Everything else being equal, a shorter hybrid will have better standability than a taller one. Standability can vary significantly across hybrids. Short-stature hybrids represent the most recent breeding effort to improve plant standability, and are discussed below under specialty characteristics.  

Yield Performance from University Variety Trials

Yield potential can vary greatly among hybrids, and this is demonstrated by the range in yield observed in variety trials, which is often greater than 50 bushels per acre between hybrids of comparable maturities (Table 2). Similarly, differences in fresh silage yield from hybrids of comparable maturity often exceed 3 tons per acre (Table 3). Hybrid performance trials are conducted annually by the Penn State Official Variety Testing Program. Corn hybrids of different companies are evaluated at three sites in each of the three corn maturity zones in Pennsylvania. On average, 20 to 50 corn grain hybrids and 20 to 40 silage hybrids are evaluated per zone. In addition to yield, plant population, lodging, and grain moisture data are collected for grain corn, and silage quality data are collected for silage corn. These trials provide a source of independent information on hybrid performance. Annual reports can be accessed online.

When interpreting yield performance tests grown over a wide region, consider only those hybrids with a maturity adapted to your farm, even though later hybrids in these tests may have higher yields. When grown under your local conditions, these later hybrids may not yield as well, and they will have higher moisture levels and an increased risk of frost before maturity. Further, it is essential that performance information be based on the results of several trials and not just one test. Average performance from a number of sites in your region is often a better indicator of future performance than the results of a single test on your farm. Therefore, when using the Penn State Corn Variety Testing Report, we recommend referring to data summaries by adaptation zone, and not using data from a single trial to make hybrid selection choices. In addition to yield, compare the grain moisture at harvest between hybrids of similar maturity. High grain moisture at harvest may indicate the need for drying, which incur in drying costs, and may be undesirable.

Special Considerations

Several special hybrid characteristics can be considered in specific situations. The importance of these characteristics depends on your specific operation. Generally, these characteristics should only have a major impact in hybrid selection if they have an impact on the yield and quality of the final product.

BMR

Brown midrib (BMR) corn hybrids contain a genetic mutation that limits and alters the production of lignin within the stover portion of the plant. As lignin reduces the digestibility of cellulose and other fiber fractions, reducing the amount of lignin present will increase the fiber digestibility of BMR corn silage hybrids. BMR corn hybrids contain the bm3 mutation, which leads to greater neutral detergent fiber digestibility (NDFD) as compared to conventional corn silage hybrids.

University studies suggest that milk production increases when BMR hybrids are fed to the highest-producing cows on a dairy, because increased NDFD leads to increased daily dry matter intake, and ultimately, greater milk production. BMR hybrids offer little advantage to low-producing dairy cows or beef cattle whose animal production is not typically limited by total dry matter intake.

BMR hybrids typically produce 10 to 40% less yield per acre than conventional corn hybrids, with increased seed costs compared to conventional corn hybrids. Additionally, the biosynthetic pathway that produces lignin also produces molecules that serve as plant defense mechanisms; when lignin production is reduced, plant disease resistance is also reduced. Though the bm3 mutation offers the greatest animal performance increases as compared to conventional or other bm mutations, the bm3 mutation is also the most susceptible to diseases such as Anthracnose stalk rot, Gibberella ear rot, grey leaf spot, Northern corn leaf blight, and Anthracnose leaf blight. Producers who choose to plant BMR hybrids must be aware of the increased agronomic and nutritional management requirements associated with BMR corn silage production.

Short-stature trait

Short-stature corn hybrids are an emerging option that is still in the initial phase of commercialization. These hybrids are approximately 30% shorter than traditional hybrids but have comparable leaf and ear development, thus the difference lies in the stalk internodal distance being shorter in the short-stature hybrids. They offer several advantages such as improved standability, which is attributed both to its shorter height but also to its thicker stalk, season-long access for in-season applications of nutrients and crop protection using standard ground equipment, and potential for higher planting densities, which may lead to increased yield. Initial research trials conducted by Penn State on pre-commercial hybrids indicate that grain yield of short-stature hybrids was comparable to that of traditional hybrids, but silage yield was sometimes lower, even up to 10% lower. However, silage quality was generally superior for short-stature hybrids, which is attributed to the proportionately lower stalk biomass compared to grain and leaf biomass in short-stature hybrids. Continued research efforts are being made to evaluate these pre-commercial hybrids as they are being developed by the industry.

For growers who frequently experience serious losses from bird damage when corn is in the milk stage, husk tightness and coverage may be important considerations.

Other Special Characteristics

These characteristics include harvestability as ear corn, seedling vigor, test weight, tight husk cover for bird resistance, stay-green potential, and grain or silage quality.

Harvestability of a hybrid can be particularly important for ear corn producers. Some hybrids have the tendency to lose kernels from the ear during the picking process, lowering the yield and quality of the ear corn. On other hybrids, husk removal is sometimes difficult, which contributes to reduced airflow in the bin and increases the potential for spoilage. Differences in seedling vigor do exist between hybrids and can have an effect on stand establishment in stressful situations, such as early planted no-till corn or corn planted into heavier, wetter soils. However, under most other situations, hybrid differences in seedling vigor become less important. Test weight differences also exist between hybrids and these should be considered if higher test weight is an advantage from a marketing standpoint. Studies have shown that test weight is not a good indicator of feed value, so 

there is likely to be little advantage in feeding high test weight hybrids. For growers who frequently experience serious losses from bird damage when corn is in the milk stage, husk tightness and coverage may be important considerations.

Many of the new hybrids being developed possess increased levels of what is known as stay-green, or the ability to retain leaf color past physiological maturity. This characteristic appears to improve late-season plant health and to increase the grain dry-down rate. This characteristic may also help to extend the harvest window for silage production by reducing the whole-plant dry-down rate.

Grain and silage quality differences should also be considered since feed quality differences do exist among hybrids. For grain, higher protein concentration is desirable. Silage quality can be assessed using the Organic Matter Digestibility Index (OMD), which combines the potential digestibility of protein, fat, NDF, and starch. Several studies have shown that forage organic matter digestibility is directly related to the lactation performance of dairy cows. However, you should monitor the economics of growing hybrids with improved quality traits. Frequently, the quality differences among hybrids are small and the yield differences among comparative hybrids are much greater. In most cases, it is not worth trading off yield for silage quality. In silage hybrid testing trials, quality differences among groups of hybrids typically range up to 1 percentage point in grain protein and up to about 10 percentage points in organic matter digestibility.

Management x Hybrid Interactions

Hybrids can differ in their adaptation to management practices such as tillage systems, nitrogen rates, populations, row spacing and soil productivity levels.  Therefore, the idea of fine-tuning hybrid selection based on the existing management practices or soil productivity levels has long been a topic of research. However, most research so far has shown that hybrid differences under these practices are difficult to predict and they are often not large enough to override the individual effects of hybrid and management practices. Therefore, most researchers recommend using hybrids that perform well under a wide range of conditions. For example, studies in Iowa, Wisconsin, and Ohio, have indicated that optimum populations do not differ greatly among commercial hybrids. These studies have also shown that the best hybrids at low populations are generally the best at high populations. Similarly, studies have shown that the best hybrids in conventional tillage are also usually the best in reduced tillage systems. Row spacing is another important factor in corn production, as reducing the spacing from the standard 30 inches to narrower spacing like 20 or 15 inches can alter seasonal light capture and canopy closure, potentially impacting yields. Research conducted in Ohio and Pennsylvania has demonstrated that narrowing row spacing can increase forage yields by approximately 8% and grain yields by about 4%, but this response is not dependent on hybrid. These yield improvements are attributed to enhanced light interception and more efficient resource utilization, leading to faster canopy closure and reduced weed competition. In summary, while interactions exist between hybrids and management practices, selecting hybrids that perform consistently well across diverse management scenarios is still considered the safest strategy for most growers. However, with the advancement of precision technology enabling us to obtain high resolution data for field characterization, and the advancement of modeling techniques which improve our ability to predict these interactions, it will become increasingly feasible to leverage these interactions to optimize overall farm productivity.

Authors:  Daniela Carrijo, Assistant Professor of Grain Crop Production; Adriana Murilo-Williams, Agronomy Extension Educator; Zachary Curtis, Agronomy Extension Educator; Greg Roth, Professor Emeritus of Agronomy