SULFUR, AN INVISIBLE THREAT

Sulfur is an essential element in animal nutrition, present in many feed ingredients and additives. It is a key component of amino acids like methionine and cysteine and is necessary for protein synthesis and metabolic functions. However, excessive sulfur intake, whether from organic or inorganic sources, can cause significant toxicity issues in both swine and poultry.

In animal feeding practices, several key contributors to sulfur intake consistently stand out: DDGS, lysine sulfate, copper sulfate and high-sulfur protein meals such as canola and soybean meals can increase the total sulfur load.

Canola meal, soybean meal, and other protein-rich by-products contain significant amounts of organic sulfur, primarily in the form of sulfur-containing amino acids like methionine and cysteine. Canola meal, for instance, contains around 0.6-0.8% total sulfur. Although this sulfur is organic rather than inorganic sulfate, excessive inclusion of these meals can still increase the overall sulfur load in the diet.

DDGS meal

Lysine Sulfate

Lysine sulfate, used as a cost-effective lysine source, contains 20-25% sulfate. In typical diets, lysine sulfate is included at 0.5-1%. Since sulfate itself contains about 33% sulfur, we can expect approximately 330-825 grams of sulfur per ton of feed, making lysine sulfate one of the main sources of sulfur, particularly in swine and poultry rations.

Copper Sulfate

Copper sulfate (CuSO₄·5H₂O) contains about 40% sulfate. When copper sulfate is used as a growth promoter in swine and poultry at inclusion rates of 1 kg per ton (1,000 ppm), it contributes about 132 grams of sulfur per ton of feed, significantly impacting the total sulfur load when used at high doses.

Research has shown that excessive sulfur levels can have severe health and performance impacts on both swine and poultry. A study by Spears et al. (2003) demonstrated that total sulfur levels exceeding 4 g/kg (4,000 mg/kg) of dry matter can significantly reduce feed intake and cause diarrhea in swine. In Poultry, we start to observe a reduction of daily gain from 500ppm of sulfur. NRC suggests maximum tolerance at 4,000 ppm while University of Guelph data would recommend to set the limits at 3,000 ppm. Levels exceeding this threshold have been linked to gastrointestinal distress, reduced growth rates, and mineral imbalances.

The primary toxicity mechanism of sulfur in animal nutrition arises from its conversion to hydrogen sulfide (H₂S) by sulfur-reducing bacteria (SRB) in the gut. When sulfur reaches the large intestine or cecum, anaerobic bacteria such as Desulfovibrio convert it into H₂S, a toxic gas that can impair cellular respiration and damage the intestinal lining.

Hydrogen sulfide is not only toxic to the intestinal epithelium but can also be absorbed into the bloodstream, where it acts as a potent respiratory toxin, inhibiting cytochrome oxidase and leading to cellular hypoxia. Additionally, high sulfur levels increase osmotic pressure within the gut, drawing water into the intestinal lumen and causing osmotic diarrhea. This can lead to severe dehydration and nutrient loss, ultimately reducing animal growth and performance.

Another critical impact of sulfur toxicity is the disruption of mineral absorption. High sulfur levels interfere with calcium and magnesium uptake, increasing the risk of deficiencies and skeletal disorders. This can be particularly problematic in rapidly growing animals, where calcium and phosphorus balance is crucial.

Impact on Microbiota

High levels of dietary sulfur can alter the gut microbiota by favoring the growth of sulfate-reducing bacteria (SRB) over beneficial microbes. This microbial imbalance can impair nutrient digestion and absorption, reduce gut health, and increase the risk of gastrointestinal disturbances. Moreover, the production of H₂S by SRB can inhibit beneficial bacteria, further disrupting the microbial ecosystem.

Respiratory Toxicity

Besides intestinal damage, H₂S gas released from manure and slurry can accumulate in confined animal housing. At high concentrations, it poses a significant respiratory hazard for both animals and farm workers. Chronic exposure can cause respiratory distress, coughing, eye irritation, and in severe cases, even death of animals. Proper ventilation and manure management are crucial to minimize H₂S emissions and protect respiratory health.

Swine and poultry exhibit different levels of sensitivity to sulfur toxicity. Generally, swine are more susceptible than poultry due to higher water consumption and more substantial gastrointestinal fermentation. Among swine, young piglets are more vulnerable than adults, as their gut microbiota and digestive systems are not fully developed, making them more prone to diarrhea and nutrient loss. In poultry, younger birds are also more sensitive than older ones, as rapid growth rates increase their nutritional demands and vulnerability to digestive disturbances.

Basic formulation practices should enable you to prevent exceeding contamination threshold

 1. Monitor Feed Ingredients: Regularly assess the sulfur content of raw materials, especially DDGS, Canola and sulfates content in premixes

2. Use Lysine Hydrochloride: Prefer lysine hydrochloride over lysine sulfate to minimize sulfur intake.

3. Careful Use of Growth Promoters: when copper sulfate is used as a growth promoter in pig growers and chicks diets, replace copper sulfate by dicopper oxide to limit sulfur contamination while maintaining superior growth promoting effect

4. Balanced Feed Formulation: Follow sulfur level in formulation matrix to ensure that total dietary sulfur does not exceed recommended limits.

Sulfur toxicity in swine and poultry nutrition is a multifaceted problem that requires vigilant management. By understanding the key sources of sulfur and their mechanisms of toxicity, nutritionists can make informed decisions to minimize risks. Regular monitoring, balanced formulation, and the strategic use of proven alternative additives are essential to maintaining optimal animal performance and health.

David SERÈNE

NutriSPICES Director