At a Dairy Grazing Apprenticeship virtual roundtable, Dan Gard, district sales manager at S&W Seed Co., recounted a heartbreaking childhood story related to prussic acid poisoning. Prussic acid is also called hydrocyanic acid or cyanide.
Gard was 9, and it was his first year participating in 4-H. The county highway department had recently sprayed an herbicide along the roadsides. The herbicide drifted onto his family’s sorghum x sudangrass crop where eight of their calves were grazing.
“The herbicide stressed those plants and caused them to produce prussic acid. The calves got hold of it, and that was the end of it,” Gard said. All eight calves died.
Fifty years later, Gard is happy to report that the first prussic acid-free sorghum x sudangrass hybrid – S&W Seed Co.’s PF™ Trait – is now commercially available for graziers and forage producers. This is great news for those who rely on sorghum x sudangrass to feed their livestock but worry about the potential impact of prussic acid poisoning.
Sorghum x sudangrass (along with nearly 1,000 other species of plants) naturally contains a cyanogenic compound that is bound in a nontoxic form called dhurrin. When these plants are stressed by cutting, wilting, crushing or freezing, a compound also present in these plants called emulsion reacts with the dhurrin and frees it, creating prussic acid.
Prussic acid is a potent, rapidly acting poison which enters the bloodstream of affected animals. It inhibits oxygen utilization by cells – animals die from asphyxiation. Signs of poisoning usually occur 15 – 20 minutes after the toxin is consumed, and death occurs very quickly. Ruminants such as cattle and sheep are more susceptible to prussic acid poisoning than pigs or horses.
The seed company worked in close collaboration with Purdue University to develop the hybrid seed (it is not a GMO). The effort was led by Purdue’s Mitch Tuinstra, a professor of plant breeding and genetics in the College of Agriculture. The seed is available treated or untreated, but there is not yet a certified organic seed.
“Purdue was able to identify two sorghum mutants that did not produce a detectable prussic acid signature. They worked for several years to refine this, and it was done with traditional breeding. The last 10 to 12 years, we along with Purdue have refined the mutant traits to make sure there is no prussic acid whatsoever,” Gard said.
Gard has been trialing PF Trait with a small group of Amish dairy farmers in Lancaster County, PA. The seed has also been trialed widely across the U.S. In surveys, growers report that the seed provides a peace of mind that they didn’t formerly have when grazing sorghum x sudangrass.
“They don’t have to worry about running out to a field and moving cattle, especially if they’re five, 10 miles down the road in a drought or even a frost situation,” Gard said.
Growers also report that the hybrid saves time and labor by allowing for an extended grazing period in spring and autumn. A study at Kansas State estimated that the autumn grazing period in their Kansas trials could be extended by 32 days.
Research conducted by associate professor Tara Felix and her team at Penn State indicates that PF Trait competes with other sorghum x sudangrass varieties in nutrient composition, palatability and yield.
Gard recommended planting the seed at 25 – 30 lbs./acre. One of his customers has a 400-cow organic dairy in northern Pennsylvania. This grower plans to take the first two cuttings of PF Trait as baleage for winter feed, and then begin grazing the regrowth in late July when the cool season pastures are coming to their end due to the heat.
PF Trait is very aggressive, according to Gard. Each original stalk cut on the first harvest will produce between four to six tillers. Because of this, he suggested making sure there is enough nitrogen available for the plants to regrow and regenerate.
“An old industry rule of thumb is 40-40-40. Harvest it when it’s 40 inches tall, 40 days after planting and give it 40 pounds of nitrogen,” he said.
Gard also said growers need to be aware that there are other common plants and grasses (such as johnsongrass) that can produce prussic acid and to make sure they are controlled. Also, sorghums can still accumulate high nitrate levels, especially during droughts, and this can be dangerous to grazing animals.
Currently, S&W Seed Co. is asking growers planting PF Trait to sign a stewardship agreement so the company can follow up on their experiences with the variety. Gard said, “It’s the only way we can keep track of people at this point. We want to be able to call and email people with updates, so we can keep learning. We also want to make sure people are happy.”
