Grain sorghum plant height is a measurable trait that is controlled by four major genes. Grain sorghum products utilize recessive dwarfing alleles (a version of a gene) at three of the four major genes which promote short plants and one in a form that promotes taller plants.2 Therefore, typical US grain sorghum products are called 3-dwarf products. However, one of those recessive genes is somewhat unstable and can revert to the tall form at a relatively low frequency. The result is a taller plant that is called a height mutation. The height difference can be about 1 to 2 feet (Figure 1), depending somewhat on growing conditions. Using the same terminology as above, these would be considered 2-dwarf plants. On average, it is expressed in about 1 to 2% of the crop and only affects the visual aspects of the field.1 A sorghum plant with zero dwarfing genes could be about 9 to 12 feet tall or more.
Managing Height Mutation in Sorghum Seed Production
There are differences between grain sorghum products in the frequency of height mutations that occur. Products that produce very high frequencies of height mutations are eliminated during research and testing. Rouging of tall mutant off-types in a seed production field is often required to help manage the frequency of height mutations; however, it does add to the cost of goods of that product. Mutations can be reduced, but not eliminated by rouging. Growing conditions in the production field can influence the effectiveness of the rouging operation since good growing conditions such as adequate water and moderate temperatures, promote good height expression and allow for accurate identification and removal of tall plants. Conversely, more stressful conditions, water limitations and high heat, lead to poorer expression of height and less accurate removal of tall plants.
1McClure, A. University of Tennessee Extension. 2015. Tall plants scattered in your grain sorghum? Tall Plants Scattered in Your Grain Sorghum? - UT Crops News
2Tuinstra, M. New stable-dwarf sorghum varieties. Purdue University, College of Agriculture. Stable-Dwarf Sorghum (purdue.edu)
Web sources verified 4/3/2023. 2114_224001