Abstract
The influence of purified, protein-free feed with urea and ammonium salts as nitrogen sources (0-feed) and of non-purified, urea-rich, low-protein feeds (ULP-feed) on the sulfur metabolism of cows has been studied by determining the contents of sulfur fractions in faeces, urine, milk, blood and rumen fluid. The sulfur of 0-feed was composed entirely of inorganic sulfate. During balance trials the N:S ratio in the feed varied from 6.1 to 9.5, and the sulfur content from 0.22 to 0.31 % of the dry matter. In every trial (seven with 0-feed and two with ULP-feed), of five or seven days duration, the cows were in high-positive sulfur balance. The 0-cows excreted a greater proportion of their total sulfur output via urine than the ULP-cows. The excretion of inorganic sulfate sulfur, as a proportion of the urinary and faecal sulfur, was greater for 0-cows than for ULP- or NorP-cows (cows on normal, protein-rich feed); the opposite was the case with regard to the excretion of ester sulfate sulfur and neutral sulfur. The sulfur contents of milk and blood showed only minor inter-feed differences. The sulfate content in the rumen fluid of the 0-cow rose rapidly after the commencement of feeding and then fell quite rapidly. We conclude tentatively that in the rumen of the 0-cow hydrogen sulfide is generated so quickly that the whole of it cannot be used for the synthesis of sulfur-containing compounds, a considerable proportion of it being lost in eructations or excreted as inorganic sulfates in the urine.
Highlights
Long-term feeding experiments with dairy cows, which Virtanen started in 1961, have shown that cows on purified, protein-free feed with urea and ammonium salts as the sole sources of nitrogen (0-cows) consume less feed during the first three months of the lactation, and yield less milk than cows on low-protein, urea-rich feed (ULP-cows) or on normal, protein-rich feed (NorP-cows) (Virtanen et ai. 1972, Ettala and Kreula 1976)
The amounts of certain free amino acids in the blood plasma of 0- and ULP-cows are much lower than the corresponding values in NorP-cows
The differences in milk production between 0- and ULP-cows are difficult to explain on the basis of the lack of certain free amino acids in the plasma (Virtanen et ai. 1972)
Summary
Long-term feeding experiments with dairy cows, which Virtanen started in 1961, have shown that cows on purified, protein-free feed with urea and ammonium salts as the sole sources of nitrogen (0-cows) consume less feed during the first three months of the lactation, and yield less milk than cows on low-protein, urea-rich feed (ULP-cows) or on normal, protein-rich feed (NorP-cows) (Virtanen et ai. 1972, Ettala and Kreula 1976). The amounts of certain free amino acids in the blood plasma of 0- and ULP-cows are much lower than the corresponding values in NorP-cows. The differences in milk production between 0- and ULP-cows are difficult to explain on the basis of the lack of certain free amino acids in the plasma We have measured the excretion of sulfur and its contents in milk, blood, plasma and rumen fluid of 0-, ULP- and to some extent of NorP-cows. By these means we have tried to indicate possible anomalies in the utilization of sulfur in 0-cows
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