NUTRITION ▶▶▶


Sulfur: The dose makes the poison


Sulfur (S) is an essential nutrient for ruminants, required for normal growth and development of both rumen micro-organisms and the host animal. Defi ciency of S can lead to problems ranging from poor performance to death, and excess S can equally harm the animals. The toxicity of S-compounds leads to alterations in the rumen environment, neurological disturbances and even death.


BY DR DENISE CARDOSO, PRODUCT MANAGER, AND DR ARTURO PIÑÓN, R&D DIRECTOR, ANIMINE


Essentiality of S S is a constituent of several amino acids such as cysteine, cys- tine and methionine. It is also present in other S-containing biomolecules such as biotin, taurine, co-enzyme-A, fibrino- gen, heparin, thiamine and glutathione, playing an important role in cartilage integrity. S is especially essential for wool-producing animals as the fur is rich in S-containing ami- no acids, and its deficiency is associated with decreased quantity and quality of the fibres. It is interesting to note that the utilisation of non-protein nitrogen (NPN), such as urea, is dependent on sufficient levels of sulfur in the rumen fluid. The nitrogen:sulphur (N:S) ratio of protein in mixed ruminal


Figure 1 - Schematic of polioencephalomalacia caused by excess S in the diet. Sulfur sources, when ingested, are reduced in the


rumen by rumen reducing bacteria into H2S. The inhalation of eructated gases represent 60% in ruminants, and when in high concentrations of H2S, it can lead to brain damage.


Brain damage PEM


bacteria is not constant, but a 20:1 ratio between available nitrogen and sulphur should be adequate to supply the re- quirements of the rumen microbes. A marginal deficiency of S has an impact on microbial synthesis and fibre degradation in the rumen leading to reduced dry matter intake (DMI), growth rate or milk production. Severe deficiency symptoms are excessive salivation, weight loss, dullness and death. The maximum tolerable levels of S vary according to con- centrate levels in the diet. According to the NRC (2005), 3,000 ppm S is the limit for lactating cows and ruminants fed high concentrate levels, and 4,000 ppm is the limit for rumi- nants fed high forage diets.


The importance of assessing total S supply Ruminal bacteria can metabolise elemental, inorganic and or- ganic S; therefore, the total content has to be considered for assessing dietary S intake levels. Table 1 shows the S content of some feedstuffs used in ruminant nutrition. Water can also be a source of S in some regions and can become problematic if not taken into consideration.


S toxicity and impact on DCAD In the rumen, S sources serve as a substrate to some bacteria that will convert it into sulfide to be further incorporated into microbial and animal metabolism through S-aminoacids or as sulfide. The excess of sulfur in the diet leads to an accumula- tion of sulfides in the rumen environment, increasing the risk of acute toxic and polioencephalomalacia (PEM) in those ani- mals. It is worth mentioning that sulfur is an important anion and can reduce the dietary cation-anion difference (DCAD). In lactation rations, DCAD between +25 to +30 meq/100g of dietary DM is required to achieve maximum feed intake and milk yield. Reduced DCAD not only compromises the DMI and milk production but also decreases the neutral detergent fibre (NDF) digestibility.


60%


Eructation Inhaled


40% Eliminated


Dietary sulfate source


Rumen SO4 S2- H2 Liquid phase S


Formation of toxic gases by rumen bacteria Sulfates and organic S in the rumen can be reduced by two main bacteria categories: (1) assimilatory bacteria, which will reduce the sulfate for its own metabolic needs and, as an out- come, will incorporate sulfide into sulfur amino acids, thiamine or biotin. The production of sulfide by these bacteria is lower than dissimilatory bacteria as its production is limited by or- ganic sulfur compounds in the rumen. Dissimilatory bacteria (2) are the group of bacteria that also utilise sulfate for their


26 ▶ DAIRY GLOBAL | Volume 8, No. 2, 2021


PHOTO: MARK PASVEER


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