Aspects of the metabolism of 14C-labelled compounds by cows on a protein-free feed with urea and ammonium salts as the sole source of nitrogen

In the feeding experiments performed at the Biochemical Research Institute with test cows (the so-called 0-cows) the biosynthesis of milk component from different energy sources and from urea used as the nitrogen source was studied. The basic idea was to elucidate the effect of various feed components and substances formed in rumen fermentation on the biosynthesis of milk components. In the studies preparations labelled with 15N and 14C were used. The feed of the test cows did not contain protein at all, the carbohydrates were hexose-based and fat in the form of oil was used very scantily. All the proteins required were synthesised by test cows in symbiosis with their rumen microbes from ammonium nitrogen which they obtained from urea and ammonium salts. Protozoa disappeared gradually from the rumen and the number of bacteria increased, becoming many tens of times the number in normally-fed cows. Of the substances labelled with 14C, stearic acid and acetic acid had the highest incorporation into the different milk components. Stearic acid is transferred to milk fat almost solely as such, but apparently is used for the formation of oleic, linoleic and linolenic acids as well. Acetic acid also is incorporated mainly into fat, though it is transferred in considerable amounts also to the other milk components. Propionic acid is by nature gluconeogenetic and butyric acid lipogenic. The carbon of sucrose and lactic acid is incorporated fairly evenly into the various milk components. The studies suggest that there are only very small amounts of aromatic compounds in 0-cow tissues. According to the relative retention times the components of milk are synthesised from the different energy sources at various rates. The syntheses of citric acid and lactose are the most rapid, those of protein and fat the slowest. The feeding has a marked effect on the composition of the milk fat. On the basis of these experiments, the far-developed urea feeding does not seem to have any pronounced effect on the participation of the substances studied in the biosynthesis of milk.