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The efficacy of a feed supplement for ruminants depends mainly on its effect on the ecology and nutrition of the micro- organisms inhabiting the rumen. Sulphur (S) has long been recognized as an essential element for ruminal micro-organisms, and its metabolism is closely related to nitrogen metabolism.

Sulphur is a constituent of several organic nutrients required by the ruminant and is essential for rumen microbial synthesis of certain amino acids (cysteine, cystine and methionine), vitamins (thiamin and biotin) and enzymes. In order for these nutrients to be synthesized in the rumen, sulphur must be present in the diet.

Sulphur requirements of rumen micro-organisms
Rumen microflora may alter the dietary form of both sulphur and nitrogen by either breaking down dietary protein to yield ammonia and sulphide, or synthesis of microbial protein from dietary urea or inorganic sulphate. In sulphur deficient diets, S-supplementation improves performance by enhancing bacterial protein synthesis in the rumen and improving the amino acid balance (Morrison et al., 1990).

Species of ruminal bacteria differ in their content of methionine and cystine. The nitrogen:sulphur (N:S) ratio of protein in mixed ruminal bacteria may therefore alter with change in diet, since the composition of bacterial species changes with diet changes (Bird, 1973). Gutierrez et al. (1996) found that the N:S ratio of rumen bacteria ranged from 8:1 to 31:1 (mean of 21.6:1) and concluded that a 20:1 ratio between available nitrogen and sulphur should be adequate to supply the requirements of the rumen microbes.

It is known that enhanced fungal activity can cause a significant decrease in the resistance of plant particles to size reduction and therefore enhance the rate of fibre degradation. Ruminal fungi concentrations and activity may be increased by supplementation with a variety of sulphur sources (Morrison et al., 1990; Gutierrez et al., 1996).


Factors affecting sulphur requirements
Sulphur requirements differ between ruminant species. In woolled sheep, the high sulphur content of wool affects the sulphur requirements. Other factors that affect sulphur requirement are age, physiological state, and nitrogen and sulphur source.

In order to maximize the efficiency of utilization of dietary nitrogen by rumen micro-organisms, the ratio of N:S should not be greater than the ratio of those elements in bacterial cells. The ratio of the increment of N:S stored by sheep was 13.5 when sulphate was added to a sulphur deficient diet. Therefore, in order to maximize the efficiency of utilization of dietary nitrogen by sheep the N:S ratio in the feed should not be greater than 13.5.

When comparing the N:S requirement of the ruminant system with that of the ruminal bacteria it should be noted that the ruminant tissues have a definite requirement for sulphur per se. Ruminants, particularly sheep, also produce substantial quantities of wool or hair keratin which have a high sulphur content with a N:S ratio of 4 between 4:1 and 6:1. The overall dietary N:S ratio required by the ruminant system must therefore be narrower than that required by ruminal bacteria (Bird, 1973).

The N:S ratio of most body tissue protein is approximately 15:1 or less. Thus it follows that microbial protein is deficient in sulphur amino acids for body tissue synthesis and some deamination of non-sulphur amino acids must occur. The difference between N:S ratio of bacterial protein (20:1) and wool (5:1) must accentuate this loss, since half of the absorbed sulphur amino acids may be used for wool synthesis, thereby leaving an excess of non-sulphur amino acids.






Sulphur sources
Sulphur sources differ in their ability to enhance ruminant microbial activities. Sodium sulphate and methionine have been shown to stimulate riboflavin and B12 vitamin synthesis by rumen micro-organisms to a greater extent than cysteine or elemental sulphur (Fron et al., 1990). Kahlon et al. (1975) ranked sodium sulfate and elemental sulphur about equal in their ability to furnish available sulphur. Jesudasan (1972) as reported by Tisdale (1977) demonstrated that protein synthesis from addition of the following inorganic sulphur sources decreased in the following order:
(NH4)2SO4 > S > Na2SO4 > K2SO4 > CaSO4 > MgSO4. Ammonium sulphate and elemental sulphur were therefore the most effective in promoting the synthesis of rumen microbial protein and certain volatile fatty acids. These two sources are also relatively inexpensive and freely available.

Production responses with sulphur supplementation
A considerable amount of work has been done showing that increased dietary sulphur leads to increased meat-, wool- and milk production. The higher production is usually the result of a combination of the following:

• increased dry matter, fibre and cellulose digestibility,
• increased feed intake and
• improved nitrogen balance.

In order to achieve these responses, N:S ratios should be kept within acceptable limits by adding sulphur to the diet. Total sulphur content is therefore a function of the nitrogen content and should vary between 0.18 and 0.25% of diet dry matter.

Sulphur fertilization of pastures has several positive effects on the amount and quality of the grass. This leads to higher intakes, improved gains and higher stocking rates. The question whether it is more efficient from the standpoint of animal performance and economy to fertilize forage with sulphur as opposed to dietary sulphur supplementation, has not been fully answered.


Dairy cattle
Tisdale (1977) quotes several papers in which the addition of adequate amounts of sulphur improved dairy cattle performance. These improvements included a higher production of milk solids, milk fat, milk protein and milk casein. The higher casein content raised cheese yields. Higher milk production is also reported in various other studies.

Sulphur plays an important role in the dietary cation-anion balance of dairy cows. Tucker et al. (1991) used supplemental sulphur to manipulate dietary cation-anion balance of dairy cows receiving corn silage diets. They reported lower blood pH, lower urine pH, lower blood HCO3 levels and increased plasma and urinary calcium excretion as the (S) anion was added to the diet.

Beef Cattle
Under conditions of a sulphur deficiency, elemental sulphur supplementation of beef cattle rations did not only improve average daily weight gain, but also decreased feed costs per kg of gain and increased the carcass grading (Table 1; Hill et al. 1984).

Table 1: Feedlot performance of steers fed high energy, urea treated corn silage with or without added sulphur (Hill et al. 1984).

Sheep
There have been numerous reports of increases, both in wool and meat production, related to an increase in the dietary intake of sulphur by sheep and lambs. In a review article, Tisdale (1977) reports higher wool clips (up to 33%), improved wool strength, higher lamb survival rates and increased body weight gain. Because of the additional need of sulphur for the synthesis of wool, the sulphur requirements of sheep (per kg of body weight) are higher than that of cattle. The N:S ratio of the diets had a pronounced effect on production performances. Figure 1 indicates the relation between the N:S ratio and the average daily weight gain of lambs.

Adding sulphur to supplemental feeds for sheep will effectively prevent prussic acid poisoning (where these animals graze on wilted pastures). The sulphur aids in the conversion of prussic acid to harmless thiocyanide, which is excreted by the kidneys.


Over supply and toxicity
Excess sulphur supplementation (" 0.38 % of DM) of sheep increased insulin synthesis, which caused a descending glycemia curve. Alkaline phosphatase and serum transaminase levels were very high due to certain hepatic troubles of a toxic nature (Statov et al., 1994). Where levels of sodium sulphate exceed 0.4 % in the diet of lambs, rumen motility and dry matter intake decrease (NRC, 1985).

Zinn et al. (1997) evaluated three levels of dietary sulphur in the diet of crossbred heifers in a feedlot. They concluded that dietary sulphur levels in excess of 0.20% of dietary dry matter decreased average daily gain (P <0.1), dry matter intake (P < 0.1), feed efficiency (P < 0.1) and the longissimus muscle area (P < 0.05). A high dietary content of sulphur (0.39% of diet DM) influenced zinc, phosphorus and magnesium in the digestive tract of steers in the following way: Solubility of zinc was lower in duodenal digesta. Magnesium absorption in the stomach region was lowered but absorption distal to the proximal duodenum increased. The net absorption of phosphorus from the small and large intestine increased (Golfman and Boila, 1990).

High sulphur concentrations also affect dairy cows neatively. Concentrations of 0.35% sulphur and above in diet dry matter were associated with diminished dry matter intakes and a concomitant drop in milk production (Tisdale, 1977).

Combining sulphur in the form of sulphates with a high proportion of highly fermentable feed appear to decrease the amount of sulphur required to produce toxic signs in sheep and cattle (Bulgin et al. 1996).


Conclusion
An adequate supply of sulphur to grazing crops is associated with several factors that determine the quality. These include Vitamin A content, chlorophyll content, amount and nature of plant protein, nitrate level and the content of non-protein nitrogen. The sulphur level of the forage, and its N:S ratio, are related to cellulose and dry matter digestibility, dry matter intake, microbial protein synthesis, and nitrogen recycling and utilization in the ruminant. In animal performance studies, adequate sulphur levels and low N:S ratios have been associated with high performance of sheep, cattle and dairy cows. If dietary sulphur levels are adequate (0.20-0.25% S and N:S ratios of 10:1 to 12:1), no improvement in performance can generally be expected. On the other hand, if dietary sulphur levels are inadequate (0.15-0.18% S or less, and N:S ratios wider than 13:1), improvements in ruminant performance may frequently be observed on supplementation.

Analysis of crude protein (total N x 6.25) is based on the content of all nitrogen compounds (including non-protein nitrogen and nitrates), with no estimate of the quality of the protein. If a sulphur analysis is included, calculating the N:S ratio in the diet can make some estimate of the protein quality. If this ratio is in the order of 10:1 to 13:1, and the total sulphur content is 0.20 to 0.25%, it can generally be assumed that the protein is of high quality and that the animal will make maximum utilization of the feed.

Optimum N:S ratios for growth in plants is usually between 14:1 and 16:1. Ruminants seem to perform most satisfactory when the ratio of N to S in their feed is between 10:1 and 12:1. This would suggest the supplementation of sulphur in order to achieve the above ratio.

by Stephan Breytenbach, Kynoch Feeds, Randburg; AFMA Matrix March 1999

References

· Bird, P.R., 1973. Sulphur metabolism and excretion studies in ruminants: XII. Nitrogen and sulphur composition of ruminal bacteria. Aust. J. Biol. Sci. 26: 1429-1434.

· Bulgin, M.S., Lincoln, S.D. and Mather, G., 1996. Elemental sulfur toxicosis in a flock of sheep. J. American Vet. Med. Assoc. 208: 1063-1065.

· Fron, M.J., Boling, J.A., Bush, L.P. and Dawson, K.A., 1990. Sulfur and nitrogen metabolism in the bovine fed different forms of supplemental sulfur. J. Anim. Sci. 68: 543-552.

· Golfman, L.S. and Boila, R.J., 1990. Effects of molybdenum and sulfur on minerals in the digestive tract of steers. Can. J. Anim. Sci. 70: 905?920.

· Gutierrez, C.L., Contreras, L.D., Ramirez, C.J.T., Sanchez, F. and Gonzalez, C.H., 1996. Sulphur supplementation improves rumen activity. Feed Mix 4.2: 18?19.

· Hill, G.M., Hembbry, F.G., McMillin, K.W., Harpel, R.A. and Blanchard, J.P., 1984. Effect of Sulphur levels in Urea-treated corn silage diets. Sulphur in Agriculture. 8: 8-10.

· Kahlon, T.S., Meiske, J.C. and Goodrich, R.D., 1975. Sulfur metabolism in Ruminants. II. In vivo availability of various chemical forma of sulfur. J. Anim. Sci. 41; 1154-1160.

· Morrison, M., Murray, R.M. and Boniface, A.N., 1990. Nutrient metabolism and rumen micro-organisms in sheep fed a poor quality tropical grass hay supplemented with sulphate. J. Agri. Sci. Camb. 115: 269-275.

· National Research Council, 1985. Nutrient requirements of sheep. 6th ed. Washington DC: National Academy Press, 1985: 15

· Statov, C., Soran, V., Ognean, L., Florescu, S. and Statov, D., 1994. Physiological aspects of the use of sulphur in sheep. Biological abstracts, Vol. 103, Iss. 002, Ref. 015029.

· Tisdale, S.L., 1977. Sulphur in forage quality and ruminant nutrition.. Technical bulletin no 22, The Sulphur Institute, 1725 K Street, N.W., Washington, D.C.

· Tucker, W.B., Hogue, J.F., Waterman, D.F., Swenson, T.S., Xin, Z. and Hemken, R.W., 1991. Role of sulfur and chloride in dietary cation-anion balance equation for lactating dairy cattle. J. Anim. Sci. 69: 1205-1213.

· Zinn, R.A., Alvarez, E., Mendez, M., Montano, M., Ramirez, E. and Shen, Y., 1997. Influence of dietary sulfur level on growth performance and digestive function in feedlot cattle. J. Anim. Sci. 75: 1723-1728.




Author: Stephan Breytenbach

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