by Christine Gelley

Sorghum-sudangrass is a hybrid summer annual grass that can be harvested repeatedly throughout the summer months, up until the first frost.

It is a cross of two sorghum species — sudangrass, which is a fine-stemmed, leafy, annual forage; and grain sorghum, a stalky, upright, annual forage that is commonly used for silage or grain.

When hybridized, you get sorghum-sudangrass, which embodies the beneficial characteristics of both forages. It is typically ready to graze at about 30 inches in height. When managed well, sorghum-sudangrass is a high-yielding and high-quality forage. There are many varieties of sorghum-sudangrass on the market today.

When shopping for a variety to meet your needs, here are some key words to look for on the seed tags:

“BMR,” or brown midrib. Brown midrib sorghum-sudangrass has a valuable trait for forage producers. It has less lignin in the plant tissue, which increases the digestibility of the forage by grazing animals in comparison to traditional varieties of sorghum.

With greater digestibility, animals can more effectively allocate the nutrients from the forage into meat and milk. While forages with the BMR trait will not grow as tall as other types of sorghum, the reduction in total yield is compensated for in quality. The BMR trait is visible when you look at the midvein of the leaf.

Highly tillering. When growing sorghum species for grazing purposes, plants that create many tillers are desirable. More tillers means you will have more leaf tissue. Leaf tissue is the most high-quality part of the plant and the easiest to digest, so more tillers and more leaves is what you want when grazing.

Plants prone to actively tiller after a harvest will regenerate with lush growth that is ready to graze again quickly (given that adequate inputs of water and nutrients are supplied to the growing crop).

While sorghum-sudangrass can be an excellent option for many producers to extend the grazing season, it is not the best choice for every operation (not suitable for equine), and it is important to be aware of potential issues associated with sorghum-sudangrass.

Prussic acid. Sorghum-sudangrass does have some concerns with prussic acid poisoning if managed improperly. The plant contains a compound called dhurrin that is converted into hydrogen cyanide in the rumen and is then absorbed into the bloodstream.

Only in situations where the plants are very young or after freezing temperatures does this become a concerning issue. Young tillers (less than 20 inches) contain high levels of dhurrin, and when plant cells are disrupted by frost events, they release dhurrin that can accumulate in threatening levels.

To avoid these issues, wait to graze sorghum-sudangrass until it is about 30 inches tall, pull animals off pasture with sorghum-sudangrass prior to a frost, then allow 10 days rest before allowing animals to return to graze sorghum-sudangrass after a freeze. Do not confuse grain sorghums for forage sorghums or sorghum-sudangrass. Grain sorghums have greater concentrations of dhurrin in their leaf tissue than forage types.

Nitrates. In scenarios where fertilizer has been applied and then subsequent drought has occurred, nitrates can accumulate in leaf tissue at dangerous levels. Nitrogen application is intended to assist in the development of healthy and plentiful leaf tissue, but if the plants are subjected to high-stress scenarios shortly after fertilization, the nitrates can be suspended in the leaf tissue before being converted into amino acids and proteins.

Nitrates are converted to nitrites in the rumen, and rumen microbia convert the nitrite into ammonia. If these levels exceed the threshold for the rumen microbes to convert, then the animal can experience nitrate poisoning, which slows the flow of oxygen through the bloodstream.

Careful consideration must be taken to apply fertilizer when plant stress concerns are minimal to avoid potential issues with nitrates.

Forage growers looking to extend the grazing season with a fast-growing, high-yielding, high-quality forage may find the desired results in a bag of sorghum-sudangrass.

Gelley is the Ohio State University Agriculture and Natural Resources Extension educator for Noble County. Her specialization is forage and pasture management, and she is also a member of the OSU Extension Beef Team that publishes the weekly Ohio BEEF Cattle letter, which can be found at beef.osu.edu.