Prussic acid & nitrates: making sorghum safe for grazing and harvesting

Why keep C4s in your forage portfolio?

Pearl millet, sorghum, Sudan grass and its hybrids add volume when perennials lift their feet in the heat. Thanks to their drought tolerance and ability to regrow, they become useful partners to get through the hottest summer months. What's more, several Canadian trials have shown that they can extend the grazing season and reduce feed costs when integrated into a rotational grazing strategy.

There are several advantages to using C4 as forage:

• High yields in the hot season
• Better water use efficiency (drought tolerance)
• More attractive nitrogen:biomass ratio than C3
• Sowing in early to late June
• Several species and cultivars (forage, sweet and BMR types)
• Reduce herd feed costs
• Prolongs the grazing period

Figure 1: Dry matter yield curve for different forage groups according to growth period (Quebec/Eastern Canada)( McCartney et al., 2009).

Chart 2: Dry matter yield of different forage species at first and second cuts (Darby, 2019).

On the other hand, the use of C4 also has disadvantages :

• Minimum soil temperature of 14-16°C required for sowing
• Limited use in warm regions of Quebec (> 2300 UTM)
• No registered herbicide
• Production of prussic acid (HCN)

However, not all C4s produce the same HCN concentrations.

Table 1: Prussic acid concentration as a function of species (adapted from Richards et al., 2017).

HCN versus nitrates: understanding the basics

HCN : In sorghum and related species, the cyanogenic glucosides found mainly in young leaves release HCN when the tissue is chewed, frozen or chopped. Cyanide blocks oxygen utilization at the cellular level. The evolution can be dazzling Symptoms appear within 5 minutes of ingestion, and death can occur within 15 minutes.

Nitrates: Accumulated mainly in the lower third of stemthey can be reduced to nitrites in the rumen, which transform haemoglobin into methaemoglobin the animal lacks oxygen, with a general evolution slower The onset of toxicity: half an hour to a few hours after ingestion (Stichler and Reagor, 2001).

Recognizing symptoms

Symptoms of prussic acid poisoning are :

• Difficulty breathing (rapid, weak, irregular pulse)
• Anxiety and agitation followed by depression
• Trip
• Muscle tremors
• Groan
• Dilated pupils
• Decubitus (animal lying on its side)
• Bloating
• Sometimes marked salivation and vomiting
• Terminal convulsions
• Bright red mucous membranes

Table 2: Comparison between nitrates and prussic acid (adapted from Stichler and Reagor, 2021)

Risk factors

• Weather one gel breaks down the cells and quickly releases HCN; the risk goes down in 5-6 days complete drying (Barnhart & Dewell, 2011).
• Fertilization a contribution in high nitrogen unbalanced phosphorus and potassium increases the risks. Spraying of poultry manure call for ceilings and uniform distribution (Al-Beiruty et al., 2020).
• HerbicidesSome treatments, such as the application of 2,4-D, increase HCN concentrations in plants (Llewellyn & Norberg, 2014).
• Plant stage : The leaves concentrate more HCN, young plants and regrowth → HCN and nitrates higher.

Table 3: HCN content of sorghum hybrids at different growth stages and in different parts of the plant (vegetative part and root system).
(adapted from Muthuswamy et al., 1976).

Prevention in the field: rules that save lives

Animal sensitivity :

• Ruminants are more sensitive to HCN than monogastric animals.
• Cows are more sensitive than sheep.
• Hungry animals are more at risk (rapid, heavy feed intake).

Table 2: Animal tolerance to prussic acid (adapted from Gensa, 2019)

Entering the pasture :

• Sorghum x Sudan grass : wait 24-30 in. (60-75 cm) before entering (Barnhart & Dewell, 2011).
• Sudan grass : to aim 18-20 in. (45-50 cm) (Barnhart & Dewell, 2011).
• Simple rule if you manage several hybrids: at least 18-24 in. (45-60 cm) before entry (Barnhart & Dewell, 2011).
• In all cases, don't put hungry animals on these plots: let them in full to avoid over-consumption of the most toxic parts (Llewellyn & Norberg, 2014).

After freezing :

• Remove animals immediately after the episode (Barnhart & Dewell, 2011).
• Wait 5-6 days complete drying before returning, and avoid any grassing on nights with risk of frost (Barnhart & Dewell, 2011).
• Nights when frost is likely are avoid grazing. After a killing frost, it's best to wait then silage if regrowth no longer allows safe grazing (Barnhart & Dewell, 2011).
• If any regrowth appear, wait until they reach 18-20 in. (sudan) or 24-30 in. (sorghum × sudan) before any grazing, or even better, harvest (hay/silage) and analyze before serving (Barnhart & Dewell, 2011).

Cutting and storage :

• Fanning and silage dissipate HCN; well-dried or well-fermented forage is generally safesubject to analysis (Barnhart & Dewell, 2011; Llewellyn & Norberg, 2014).
• Wait 3 weeks before opening a silo from a suspect HCN plot, then analyze before serving (Robson, 2007).

Fertilization :

• Respect doses and fractionate nitrogen.
• Balance phosphorus and potassium to support the plant's use of nitrogen.

In case of suspicion or accident

• Stop access by plot or by forage.
• Call your vet and store samples (chilled).
• Document the incident to adjust practices (entry heights, fertilization, post-freeze windows, dry choice, silage or pasture).

In brief

C4 grasses are more productive, more efficient in their use of water and have relatively low production costs, making them an attractive choice for increasing forage yields on the farm. However, this potential can only be fully exploited if the necessary precautions are taken.

Sources consulted

Darby, H., Malone, R., Bruce, J., Krezinski, I., and Ziegler, S. (2019) Summer Annual Variaty Trial. Northwest Crops & Soils Program. 344. https://scholarworks.uvm.edu/nwcsp/344

McCartney, D., Fraser, J., and Ohama, A. 2009. Potential of warm-season annual forages and Brassica crops for grazing: A Canadian Review. Canadian Journal of Animal Science. 19(84):431-440

Stichler, C., and Reagor, J.C. 2021. Nitrate and Prussic Acid Poisoning. Texas Agricultural Extension Service. https://brazos.agrilife.org/files/2013/10/Nitrate-and-Prussic-Acid-Poisoning.pdf

Barnhart, S.K., and Dewell, G. 2011. Prussic Acid Poisoning Potential in Frosted Forages. Integrated Crop Management. https://crops.extension.iastate.edu/cropnews/2011/09/prussic-acid-poisoning-potential-frosted-forages

Al-Beiruty, R.Z.A., Cheyed, A.H., and Hashim, M. 2020. Hazards of Toxic Hydrocyanic Acid (HCN) in Sorghum and Ways to Control it: A Review. Plant Archibes. 20 (1): 2726-2731.

Llewellyn, D. and Norberg, S. 2014. Prussic Acid Poisoning in Livestock. Washington State University. https://wpcdn.web.wsu.edu/wp-extension/uploads/sites/2071/2024/04/Prussic-Acid-Poisoning-in-Livestock-Copy.pdf

Muthuswamy, P., Govindaswam, M., & Krishnamurthy, K. K. (1976). Effect of stage of cutting on crude protein and prussic acid content of CSH 5 sorghum. Madras Agronomy Journal, 63(3), 200-204.

Richard, C., Zhang, H., Step, D.L., and Stricklnd, G. 2017. Prussic Acid Poisoning. Oklahoma cooperative Extension Service. https://extension.okstate.edu/fact-sheets/print-publications/pss/prussic-acid-poisoning-pss-2904.pdf

Robson, S. 2007. Primefact 417, Prussic Acid Poisoning in Livestock. Profitable & Sustainable Primary Industries. www.dpi.nsw.gov.au/__data/assets/pdf_file/0013/111190/prussic-acid-poisoning-in-livestock.pdf

Gensa, U. 2019. Review on Cyanid Poisoning in Ruminants. Journal of Biology, Agriculture and Healthcare. 9(6).