Pioneer uses a variety of ratings to assist with sorghum hybrid selection and management. This article provides an in-depth explanation some of the most critical hybrid ratings and how each score can be used as a management tool.
Comparative Relative Maturity (CRM)
CRM scores the approximate length of time from emergence to physiological maturity, which will vary depending on planting date, environment and growing conditions. CRM is the sum of three stages in sorghum development: number of days from emergence to flowering, number of days to color, and number of days to drydown. It is very important when evaluating yield results to match hybrids of similar maturity.
Relative Maturity (RM)
Relative maturity is the approximate length of time in days from emergence until flowering. Environmental conditions can affect maturity measurement; therefore, relative maturity is an approximate rating. The RM score can be used to predict flowering dates. On average, one RM difference represents two to three days difference in flowering. This allows producers to choose a planting date and hybrid combination that will avoid summer stress factors (e.g. heat or midge).
Yield for Maturity
By harvesting sorghum immediately at maturity, it is possible to capture the highest yield possible by reducing risk of grain loss due to environmental factors. As a general rule, maximum yield potential increases as maturity increases. Hybrids that mature early but have the same or higher yield than fuller-season hybrids will receive a higher yield for maturity score.
Yield Under Stress
Hybrid performance is evaluated by comparing stress environments (dryland) to non-stress environments (irrigated). High ratings indicate stability over multiple locations and environments. A high yield under stress rating also indicates how well a hybrid performs against other hybrids of similar maturity under stress. A score of 5 is considered average. A hybrid with a yield under stress score of 9 would attain higher yields than most hybrids under adverse conditions. It is important to note that this rating is based on actual yield data from stress environments, not visual appearance. Grain sorghum hybrids react differently to environmental conditions. The characteristics used to describe this environmental reaction are heat response and color score. Both heat response and color score are desirable traits when growing sorghum in stressful environments. Environmental fluctuations dictate which of these characteristics has the most impact from one year to the next.
A heat response score of 1 indicates a hybrid will dramatically delay heading under stress. A score of 9 is indicative of a responsive hybrid that readily heads under stress. These scores are for northern growing areas only (not Delta and south/central Texas environments.) Heat response indicates the hybrid’s ability to head under severe moisture stress. Under high temperatures, if sorghum does not exert a head, “blasting” can occur. Head blasting is the physical evidence of sterilization of the male flowering parts.
A color score of 1 is assigned to hybrids that slow grain coloring and drydown under cool fall conditions. A score of 9 is indicative of a hybrid that matures faster than average under cool conditions. Grain coloring in sorghum hybrids is closely associated with physiological maturity. Hybrids early to color are more responsive to cool temperatures than other sorghum hybrids. Under cool fall conditions, hybrids with a high color score will mature and drydown rather than delaying maturity.
Drydown is a score that indicates how soon a hybrid will reach harvest moisture after physiological maturity. Flowering notes, color score and harvest moisture are used to determine this score. A low score indicates the grain dries slowly after physiological maturity and will take longer to harvest under cooler conditions. Higher scores indicate the hybrid will continue to dry down under cool conditions. The grain may reabsorb moisture from the air if the relative humidity is high.
The root strength score reflects how well the plant remains standing based on the integrity of the root system. Root lodging is often observed after a high wind event, which may occur many weeks prior to harvest moisture. Root lodging is a problem in wet soil conditions, after wind events, where the plant leans over the entire length of the stalk. Root lodging is not related to stalk breakage. A root strength score of 5 or 6 is acceptable in areas where lodging seldom occurs. A score of 8 or 9 would indicate exceptional root lodging resistance, even under the most extreme conditions.
The stalk strength rating reflects how well the plant withstands stalk breakage or crimping. Insects and disease can weaken the stalk significantly and cause reduced standability. Under adverse conditions, some hybrids have the ability to stand better than others. A stalk strength score of 6 is acceptable in most cases, but higher scores are suggested in areas where stalk lodging is a persistent problem.
Post-Freeze Lodging Score (PFL)
The post-freeze lodging score is based on lodging that occurs as a result of a freeze before maturity, with subsequent wind damage. This rating is different from lodging that results from stalk rot. Stalk breakage usually occurs close to ground level under post-freeze lodging conditions. A score of 9 indicates a very low probability of lodging after a freeze. A score of 6 would indicate the possibility of up to 20 percent lodging under post-freeze conditions. A score of 5 or less indicates over 50 percent of the plants could lodge after freezing conditions.
The plant height score is an indicator of the average plant height from the soil surface to the top of the head. Plant height can vary from field to field and from one season to the next due to environmental conditions, time of planting, soil fertility, rainfall, region planted, etc. Tall hybrids are not necessarily more susceptible to lodging; however, if lodging does occur, tall hybrids may be more difficult to harvest.
Head exsertion is a relative measure of the distance from the flag leaf (first leaf under the head) to the base of the head. Hybrids with a score of 4 to 6 have an acceptable head exsertion rating. When the head does not exert beyond the flag leaf, a score of less than 4 is given. When the head grows past the flag leaf far enough to allow harvest without processing excess leaf material, a score of greater than 5 is given. Extreme head exsertion (9) can cause the head to lean and contribute to a poor lodging score.
A height uniformity score greater than 5 is used when a hybrid heads at a uniform height. When there is variation in head height, forcing harvest equipment to take more leaves during harvest, a score of less than 5 will be assigned.
The staygreen score is an indicator of a hybrid’s capacity to stay green and healthy until harvest, especially under adverse conditions such as drought stress or high heat conditions. A score of 6 or greater usually means a hybrid can withstand above average stress until harvest. A high staygreen score (good plant health at maturity) usually improves standability.
Greenbug resistance scores reflect the amount of observed damage from biotype-E or biotype-I greenbugs on hybrid seedlings, under greenhouse conditions. These scores do not reflect how well the plant can withstand damage from any other aphid species. Most hybrids either score high (9) or low (1-3). Low scoring hybrids need to be treated sooner than higher scoring hybrids when damaging populations of greenbugs occur. Biotype I greenbugs are the predominant biotype across Kansas, Nebraska and the High Plains region.
The charcoal rot fungus weakens plant tissue supporting the stalks. When this occurs, stalk tissue eventually collapses and lodging can easily occur under high wind conditions. Charcoal rot is observed by splitting the stalk lengthwise near the base of the plant and looking for shredded black tissue. Charcoal rot is most likely to occur under late season moisture stress conditions; therefore, lowering seeding rates or applying irrigation can reduce charcoal rot potential. Insect damages weaken the sorghum plant, resulting in stalks that are more prone to fungal infection. Proper insecticide use may reduce the incidence of charcoal stalk rot.
Fusarium rot can occur in the sorghum head or stalk. Several other fungal organisms can form a disease complex that includes Fusarium. This disease complex can make it difficult to identify Fusarium as the definitive damaging pathogen. Fusarium stalk rots are more likely to occur under cool, wet conditions. Crop rotation, residue management and balanced fertilization are recommended where Fusarium rot is prevalent.
Head Smut Resistance
Pioneer® brand hybrids are rated for resistance to several head smut races. Head smut fungus causes sterility of the plant but does not kill the plant. The infected plant continues to grow and sometimes tillers, taking up more nutrients and space. Yield losses are directly related to the infection percentage, since infected plants produce no grain. A head smut score of 5 indicates 10 to 15 percent of the plants could be infected if the soil environment is right. Lower scores mean higher incidence is possible. Susceptible hybrids may be planted in areas where this disease does not occur, but should be strictly avoided in endemic areas.
Maize Dwarf Mosaic Virus Resistance (MDMV)
MDMV may cause severe stunting of infected plants depending on stage of growth and hybrid. A reduction in number of heads, tillering, head length, number of seeds per head and seed size can occur. Leaf mottling with dark green on a yellow background is evidence of MDMV infection. Cool evening temperatures may cause a red discoloration of the plant. A score of less than 5 (susceptible) is given to hybrids that can transition from foliar mottling to necrosis. Most hybrids will exhibit some leaf mottling when infected. Hybrids showing only minor leaf mottling when infected with MDMV are given a score of 6 to 8. Eradication of Johnsongrass in areas around sorghum fields will greatly reduce the likelihood of MDMV infection because the virus is vectored from Johnsongrass to the sorghum crop via aphids, hoppers, and other insects that feed on both. In areas where MDMV-MCDV virus complex is a concern, plant only hybrids with high levels of MDMV resistance.
Downy Mildew Pathotype 3
Sorghum downy mildew pathotype 3 is a fungus present in the soil and evident in infected susceptible sorghum seedlings at about six inches in height. Downy mildew scores are taken at this early stage and again after flowering. Infected plants produce striped leaves with a downy growth on the lower leaf surface. Infected plants may die. At the heading stage, surviving infected plants are usually sterile and have shredded leaves. A score of 8 or 9 means the hybrid is resistant and a score of 1 or 2 indicates the hybrid is very susceptible. Systemic fungicide seed treatments (mefanoxam/metalaxyl) have been commonly used in endemic areas along the Texas Gulf Coast for two decades, which has significantly reduced the incidence of this disease. However, a new fungicide-resistant variant has become prevalent in the upper Texas Gulf Coast in recent years. Selecting hybrids with strong genetic resistance to downy mildew is critical for endemic areas, especially when the possibility of fungicide resistance exists.
Anthracnose is a fungal disease that destroys the sorghum plant’s vascular tissue. If anthracnose occurs at an early stage in a susceptible hybrid, significant yield loss will result. Anthracnose occurs in high moisture and humid regions of the southern United States. Scores of 8 or 9 indicate the stalk and leaf tissue is free of lesions and discoloration. Small lesions and discolored specks are present in the stalk tissue in hybrids scoring 6 or 7 for anthracnose resistance. A rating of 1 to 5 indicates severe damage can occur in heavy anthracnose environments, which may result in significant yield loss. The timing and intensity of infection is a factor when assigning hybrid scores. Anthracnose is prevalent in coastal Texas and areas of the Deep South and is most damaging on non-rotated sorghum fields. Crop rotation is an effective measure to minimize anthracnose in endemic areas.
The sooty stripe fungus causes lesions on leaves in warm, moist conditions. Sooty stripe lesions produce black or sooty reproductive structures that easily rub off. When sooty stripe lesions are prevalent on all leaves, a hybrid score of 2 is given. Scores of 5 or less are assigned to hybrids incurring yield losses as a result of sooty stripe infection. A score of 7 is assigned to hybrids where lesions are found only on the lower leaves. A score of 9 indicates no significant leaf damage.
The grain color score indicates the color of grain the hybrid produces (BRZ = Bronze; BWN = Brown; RED = Red; WHT/W = White; YEL/Y = Yellow).The apparent grain color is affected by the pigments found in the pericarp (outer layer), and the testa (true seed coat) and the endosperm. When the pericarp and the testa are both colorless, the grain color is that of the endosperm, which can be either yellow or white. The amount of pigment present in the pericarp and/or the testa determines the degree of redness, resulting in bronze or red grain color.
The endosperm is located inside the seed coat layers and is the carbohydrate storage structure for the embryo. The endosperm is not visible when looking at the grain, since it is encased in the seed coat. The endosperm color is either yellow (Y) or white (W). Yellow endosperm is usually dense and white endosperm can be floury and soft.
Test weight is also known as bushel weight. Test weight (bushel weight) is a measure of bulk density. A score of 5 would indicate an acceptable bushel weight. Heavier test weights receive a score of 6 or greater.
The weathering score is an indicator of how much the grain quality degrades and color changes under high moisture conditions near or after maturity. Hybrids with a score of 5 will discolor under average weathering conditions. A score greater than 7 indicates the grain will retain seed color and integrity under wet conditions, for a longer period of time.
John Jaster holds a Master of Science degree in plant breeding from Texas A&M University. He currently serves as Senior Research Associate working with sorghum diseases and medium maturity hybrid breeding for Texas and northeastern Mexico. He has been with Pioneer Hi-Bred since 1989.
Home, Wendell C., Robert W. Berry. "Sorghum Diseases Atlas." Texas Agricultural Extension Service Publication. Richard A. Frederiksen. 1986. Compendium of Sorghum Diseases. : The American Phytopathological Society.