Genetic Purity of Grain Sorghum
Field Facts by Pat Trudeau, DuPont Pioneer Sr. Production Agronomist
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 parent is pollinated by another inbred parent. 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 grain hybrids. 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, mutant plants 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.
Figure 1. Height mutant.
The grain outcross 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) plant 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. Volunteer plants will segregate as either grain sorghum plants or tall grain forage plants.
Figure 2. Forage sorghum outcross.
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.
Figure 3. Open-headed forage.
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 allow infestation to reoccur in subsequent growing seasons.
Figure 4. True johnsongrass.
Figure 5. Johnsongrass outcross.
The Pioneer Commitment to Quality Seed
The DuPont 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 grow-outs 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.
Table 1. Texas Dept. of Agriculture standards for certified grain sorghum seed as compared to DuPont Pioneer grain sorghum standards for off-type contamination.