Livestock producers have used sorghum-sudangrass as pasture, either on an emergency basis or as a planned crop, for five decades. Yet they know it comes with a warning. If it frosts or if severe drought sets in, you need to pull animals off pasture for a while. Otherwise, it could be deadly once prussic acid forms in plants.

Pulling animals off new sorghum-sudangrass varieties in the future may no longer be necessary, thanks to work by Mitch Tuinstra, a professor of plant breeding and genetics at Purdue University. He has developed a sorghum that does not contain the compound dhurrin. Dhurrin is the substance that combines with plant enzymes to create hydrogen cyanide, known in the livestock industry as prussic acid. Once Tuinstra’s achievements translate into varieties without dhurrin, the risk of cyanide poisoning in grazing animals will be reduced, no matter the weather conditions.

Achieving the goal

Tuinstra thought it might be possible to modify sorghum’s DNA to disrupt the biochemical pathway that produces dhurrin. He treated sorghum seeds with ethyl methanesulfonate to produce random mutations within sorghum DNA.

Students helped screen 50,000 plants from 12,000 mutant lines to identify genotypes that don’t produce dhurrin. Genomic sequencing identified the gene that was altered. Tuinstra then created experimental hybrids for field testing.  

Field tests show that dhurrin-free sorghum yields as well as conventional varieties, with similar pest resistance. Sheep on field trials at Purdue preferred eating the dhurrin-free varieties compared to conventional sorghum-sudangrass.

Keith Johnson, Purdue University’s longtime forage specialist, sees it as one of the greatest advances in forage and pasture management he’s seen during his career. “Every year as freezing temperatures arrive, I receive phone calls and emails from livestock producers concerned about the potential of prussic acid poisoning,” he explains. “Prussic acid release occurs with an enzymatic reaction associated with dhurrin. The release of prussic acid occurs when sorghum tissue is damaged by animal grazing or freezing.

“Sorghum-sudangrass and sudangrass is a viable double-crop forage crop following winter wheat harvest. When I hear angst about its use in this cropping system from livestock producers because of the potential of prussic acid poisoning, I suggest pearl millet as the crop of choice. Pearl millet does not have any dhurrin.”

Yet in preference trials conducted at Purdue University, beef cattle preferred sorghum-sudangrass over pearl millet. The yield potential of sorghum-sudangrass is also greater than pearl millet.

“Dr. Mitch Tuinstra’s research group at Purdue University mutagenized sorghum seed. There was a selection that produced no dhurrin; therefore, no concern of prussic acid poisoning when utilized after a freeze!” Johnson says. “That is why it is such a major step forward.”

From testing to product

The opportunity to purchase dhurrin-free sorghum-sudangrass is not here yet, Johnson acknowledges, but the business plan is in place for livestock producers to buy this new development within several years.

Ag Alumni Seed, Indiana’s foundation seed company and a nonprofit Purdue affiliate, provided some funding to Tuinstra to continue his research and connected him with S&W Seed, which eventually licensed the technology and plans to bring it to market in the U.S. and Australia initially.

Currently, S&W is continuing field trials with dhurrin-free sorghum and envisions adding other traits to improve the appeal. These new hybrids could reach fields within a few years.

Information from Purdue Ag Communication contributed to this article.