Genetic Purity of Grain Sorghum
By Pat Trudeau
In the production of grain sorghum hybrids, the methods used by the seed industry to produce the crop allows for the potential of "off-types" to be present in the seed. To produce the hybrid cross, a male sterile of one genetically distinct inbred is pollinated by another inbred. If pollen is received from a source other than the intended inbred, the result is an off-type. There are five basic off-types recognized which include: mutation, grain outcross, grain forage, open head forage and Johnsongrass outcross.
The Five Groups of Off-Types
Mutation is the most prevalent off-type within the grain hybrid. Mutations occur naturally within the crossing of two inbreds. The mutation is identical to the hybrid with the exception of the number of plant-height controlling genes. The mutation will have the same head type, grain color and genetic makeup as the hybrid. However, mutations are usually 1 to 2 feet taller than the grain hybrid. Soil moisture conditions during the growing season will dictate the height expression of the mutation.
The grain out-cross off-type is caused when pollen from another grain sorghum source pollinates the sterile female parent within the seed field. The sorghum contaminant may be an off-type within the parent seed itself, or a commercial hybrid grown near the production area. The grain outcross will have a different head shape and/or color than the hybrid.
The grain forage off-type is the result of pollen from a forage sorghum (silage type) crossing with the female seed parent within the seed field. This off-type can be the same height as a mutation, but is generally 2 to 4 feet taller than the typical grain sorghum hybrid. The grain forage will typically have a compact head similar to grain sorghum, but the stalk is usually more robust. This off-type will produce very few tillers, but the plant height poses a problem during harvest of the grain sorghum. The heads of the grain forage are often unharvestable by the combine and could create a volunteer concern the following year. This potential volunteer will segregate as either a grain sorghum plant or a tall grain forage plant.
Open Head Forage
The open head forage off-type is the result of pollen from an open-headed forage plant cross-pollinating with the seed field. The pollen source is usually sudangrass, but can also include shattercane or broomcorn. This non-rhizomatous grassy type plant is usually taller than grain sorghum. It produces many slender tillers with open heads that bear large amounts of viable seed, which can remain dormant for a long period of time. The open head forage off-type is objectionable, due to a grassy appearance and volunteer concerns.
The Johnsongrass outcross occurs when pollen from Johnsongrass pollinates the male sterile plant in the seed production field. This cross-pollination does not readily take place due to the genetic differences between grain sorghum and Johnsongrass. The Johnsongrass outcross is rhizomatous, taller than grain sorghum, and has slender stalks, which produce many tillers. Johnsongrass outcrosses are almost always sterile and thus do not usually produce viable seed. However, these outcrosses produce rhizomes, which can potentially survive mild winters and infestation reoccurs in subsequent growing seasons.
The Pioneer Commitment to Quality Seed
The Pioneer commitment to produce high quality grain sorghum seed for customers entails keeping off-type contaminants at a minimum level. Pioneer works hard to ensure that sorghum producers get only top quality seed from each bag purchased. The process begins with selection of well-isolated seed fields, and continues with the inspection and maintenance of isolation for potential contamination sources during the growing season, prior to and during bloom. Pioneer rogues seed fields up to 10 times to remove off-types. A final roguing and inspection takes place before the seed field can be harvested. Once the seed field is harvested, a composite sample of each individual seed field is tested in two separate winter growouts to determine the genetic purity of the seed. If off-type plants are present in unacceptable quantities, the entire field can be discarded prior to the conditioning process.
Pat Trudeau holds a Bachelor of Science degree in microbiology and agronomy from Montana State University. He currently is Sorghum Parent Seed Manager responsible for the production of inbreds used in commercial sorghum production. He has also served as Quality Testing Technician and Quality Testing Manager and been with Pioneer Hi-Bred since 1984.