Dhurrin-Free Sorghum-Sudangrass Prevents Prussic Acid Risk

Sorghum sudangrass is a hybrid, warm-season annual forage, known for its drought tolerance and inexpensive production costs. Sorghum sudangrass is ideally suited for grazing due to finer stems and improved regrowth potential relative to other sorghum varieties, like forage sorghum, discussed last month. While there are many benefits to using a sorghum sudangrass, one negative outcome is that dhurrin, a specific protein in sorghum plants, can be converted into hydrogen cyanide (HCN) and can affect grazing animals. The toxic effects of HCN in grazing animals are commonly referred to as prussic acid poisoning. Prussic acid poisoning is exceptionally devastating to ruminant animals and death can occur within minutes after consumption. Total plant HCN concentration is greatest when plants are in early development, but declines as plants mature. However, certain environmental factors, such as drought or frost, can increase the concentration of HCN to a toxic level when plants are at a grazing or harvesting height. This is why it is particularly dangerous to graze a sorghum plant following a frost, because HCN will be present at concentrations toxic to livestock. However, HCN does dissipate in the plants to safe levels again over time. A common mitigation technique employed by livestock producers is removing grazing animals from potentially toxic pastures for the recommended 10 to 14 days after an insult, like frost, until HCN levels have reduced. Although this method helps prevent prussic acid poisoning, frequently removing animals from pasture is a management challenge for livestock producers.

Recently, a cultivar of sorghum-sudangrass has been naturally selected to be devoid of a gene involved with the synthesis of dhurrin, effectively eliminating any potential for prussic acid poisoning. Due to the new development of dhurrin-free sorghum sudangrass, there is a dearth of information surrounding this variety. At Penn State, we tested this variety of dhurrin-free sorghum sudangrass and compared its nutritional composition to that of a conventional variety.

Two plots of identical size (5 acres) were established in the summer of 2023 at the Penn State Haller Farm in State College, PA, on dry-land silt loam to silty clay loam soils. One plot was planted with a dhurrin-free sorghum sudangrass (DF) and another with a near-isogenic conventional sorghum sudangrass (CON). The sorghum sudangrasses were planted into warm soils (60°F) with a no-till seed drill at a seeding rate of 25 lbs/acre. Dry N fertilizer was applied at 125.5 lbs/acre, 8 days after planting, and again after the 1^st cut at 69 days after planting, where 55 lbs/acre of N fertilizer was applied to both plots. On days 52 and 115 after planting, both plots were cut with a pull-type mower-conditioner and allowed to wilt for 72h in windrows before being baled and in-line wrapped in plastic. After a 28-day fermentation, a bale core was used to collect samples from bales from each variety and each cutting, and the samples were then analyzed to quantify dry matter (DM), crude protein (CP), neutral detergent fiber (NDF), acid detergent fiber (ADF), and ash.

The first cut was harvested at a more mature stage than the second cut, and the effect of maturity was reflected in the nutrient concentrations. The first cut had a lower concentration of CP, and greater concentrations of NDF and ADF than the second cut. This difference in forage quality based on maturity is important for livestock producers to be mindful of. The CP and fiber concentrations of the first cut make it more appropriate for mature cows, while the greater CP and reduced fiber concentrations of the second cut would meet the nutrient requirements of growing calves.

The forage type (DF or CON) had very little effect on nutritional quality. Concentrations of CP and NDF were not different between DF and CON. The DF sorghum sudangrass contained one percentage unit more ADF and one percentage unit less OM than the CON variety. However, this difference lacks biological significance and would not impact animal performance. Because these forages were allowed to wilt in the field prior to being baled, the DM percentage was different for the first cutting, attributed to a brief period of rain at the first cut. However, the percent DM was similar between varieties at the second cutting. Results from the in vitro ruminal disappearance experiment also indicate little difference in ruminal disappearance between the DF and CON varieties. The rate and extent of disappearance was not different between the DF and CON varieties, reflective of the similar nutritional composition between these two varieties.

Dhurrin in sorghum sudangrass presents a serious risk for livestock producers grazing these forages in scenarios where there is the possibility of prussic acid poisoning. Management practices such as removing cattle from pastures during a drought or before a frost reduce the risk of prussic acid poisoning. However, these practices require additional labor and do not completely eliminate the risk of prussic acid poisoning. A dhurrin-free sorghum sudangrass variety, such as reported in this experiment, effectively eliminates this risk while maintaining a similar nutritional quality to a conventional variety, making an acceptable feedstuff for beef cattle. At this time, only one variety of DF sorghum sudangrass exists.

Penn State is grateful for the support of Mitch Stephens, Assistant Beef Farm Manager at Haller Farm, where this research was conducted. In addition, we thank S&W Seed Company for the opportunity to test this novel variety. Producers interested in the variety are encouraged to reach out to S&W Seed Company for additional information.  

Published first in Lancaster Farming