Biofuel co-products as livestock feed - Opportunities and challenges

Hydrogen sulphide in cattle fed co-products of the ethanol industry 103TABLE 2Maximum recommended water sulphate concentrations for cattleWater sulphate level(ppm (mg/L))CommentLess than 600Safe600–1 000 Generally safe. Slight performance reductions in confined cattle may occur with high water intakes.1 000–2 000 Grazing cattle not likely to be affected. Performance may be decreased, particularly in confined cattle consumingdry feed. May result in diarrhoea. May cause slight decrease in copper availability.2 000–3 000 Performance likely to be decreased, particularly in confined cattle consuming dry feed. Grazing cattle may alsobe affected. Likely to result in diarrhoea. May cause substantial decrease in copper availability. Sporadic cases ofS-induced PEM possible.3 000–4 000 Performance will likely be reduced in all classes of cattle. Likely to result in diarrhoea. May cause substantialdecrease in copper availability. Sporadic cases of S-induced PEM likely.Greater than 4 000 Potentially toxic. Should be avoided.Source: Adapted from Wright, 2007, with modifications based on NRC (2005) recommendations.A 1999 survey of 263 United States feedlots in 10 stateswith greater than 1000 animal capacities (NAHMS, 2000)demonstrated that approximately 77 percent of watersamples contained less than 300 ppm sulphate, 15 percentof water samples contained 300 to 999 ppm sulphateand 8 percent of water samples registered greater than1000 ppm sulphate. Effects of different concentrationsof water sulphate on animal performance are reported inTable 2. NRC (2005) recommends that water for feedlotcattle should contain less than 600 ppm sulphate, althoughWright (2007) reported that water sulphate concentrationsless than 1000 ppm are generally safe. Water sulphateconcentrations between 1000 and 2000 ppm will probablyhave no effect on grazing cattle growth and reproductiveperformance, but may decrease growth performance inconfined cattle. In addition, these water sulphate concentrationsmay result in diarrhoea and a slight reduction incopper bio-availability (Wright, 2007). Water sulphate andS concentrations should be assessed in combination withdietary S levels to determine total S intake. The consumptionof water containing 1000 ppm of sulphate can contribute0.10 to 0.27 percent S to the diet. Thus, even withmoderately elevated S content in water, the practical rationfor ruminants may easily exceed 0.40 percent total dietaryS (Olkowski, 1997).MECHANISM OF ACTION OF EXCESS DIETARYSULPHURHigh S intake can adversely affect ruminants in two ways:decreased bio-availability of other trace minerals; andproduction of H 2 S, that can reach toxic concentrations.High dietary S can decrease the bio-availability of traceminerals through formation of insoluble complexes withinthe rumen. One such interaction is that of copper, S andmolybdenum, which combine to form copper tetra thiomolybdate.This complex renders copper unavailableto the animal (NRC, 2005). Suttle (1991) reported a50 percent decrease in copper absorption when dietaryS concentration increased from 0.2 to 0.4 percent of thediet DM. This secondary copper deficiency can result inimpaired reproduction and performance (NRC, 1996).Gould (1998) also reported that the bio-availability ofother minerals, particularly iron and zinc, may be limitedbecause of the formation of insoluble salts with sulphide.Availability of selenium also may be limited due to S,because Ivancic and Weiss (2001) reported decreased truedigestibility of selenium as dietary S content increased,and Ganther and Bauman (1962) reported increasedurinary excretion of selenium with excess dietary Sconcentrations.More extreme effects of excess S involve reductionof sulphate and other non-toxic forms of S by ruminalmicrobes to H 2 S and its ionic forms, which are highly toxicsubstances that interfere with cell respiration (Beauchamp,Bus and Popp, 1984; Bray, 1969; Kandylis, 1984) and maylead to the central nervous system disorder known as PEM.Hydrogen sulphide is a colourless, flammable, water-soluble(0.25 g/100 mL) gas. Sulphide is also soluble in plasma (1 gin 242 ml at 20 °C) and it can penetrate cells of all types bysimple diffusion (Pietri, Roman-Morales and Lopez-Garriga,2010). It is this property that makes H 2 S a broad-spectrumtoxicant. Sulphide is lipophilic (5 times more soluble inlipophilic solvents than in aqueous solvents) and can passplasma membranes. A typical concentration of H 2 S in bloodplasma is 50 µM and may be three times higher in brain(Olson, 2011).SOURCES OF HYDROGEN SULPHIDEEndogenous synthesis of hydrogen sulphide bymammalian cellsThe amino acid cysteine is central to the endogenousproduction of most H 2 S (Figure 1; Olson, 2011). Cysteinemay be catabolized by several biochemical pathwaysinvolving trans sulphuration or oxidation reactions togenerate H 2 S. As shown in Figure 1, the cysteinemay be derived from methionine as a donor of theS. The biogenesis of H 2 S has been proposed to be apromiscuous by-product of three pyridoxal phosphatedependentenzymes (Kabil and Banerjee, 2010):cystathionine β-synthase (CBS), cystathionine γ-ligase