AVAILABILITY AND METABOLISM OF VARIOUS SUBSTRATES IN RUMINANTS .V. ENTRY RATE INTO BODY AND INCORPORATION INTO MILK FAT OF D(-)BETA)HYDROXYBUTYRATE

Acetate-1-14C or d(−)β-hydroxybutyrate-1,3-14C (BHBA) was infused into the left front quarters of the udders of cows fed either a normal (control) or a high-grain, low-fiber diet. Approximately twice as much 14C was recovered in the milk fat during 20 hr after acetate-14C infusion as after BHBA-14C infusion. Also, about twice as much 14C was recovered in the fat of control cows as in those fed a high-grain diet. Specific radioactivities indicated that 50% of the four carbon atoms at the methyl-terminal end of the C4-C12 milk fatty acids arose from BHBA and that only one molecule of BHBA was incorporated into any fatty acid molecule. It was calculated that BHBA contributes a maximum of 8% of the milk fatty acid carbon. Labeling patterns after acetate-14C infusion indicated an orderly condensation of C2 units in the formation of milk fatty acids from C4 to C16; however, about 40% of the milk fat palmitate and all of the C18 milk fatty acids arose from a source other than mammary synthesis. No differences were observed between dietary treatments in the plasma concentrations of BHBA, the entry rate of BHBA, its oxidation to carbon dioxide, or its contribution to the total expired carbon dioxide. About one-half of the total entry of BHBA appeared in milk fatty acids and expired carbon dioxide; the fate of the remainder was unaccountable. It is concluded that in cows fed high-grain, low-roughage diets, a deficiency of BHBA for milk fatty acid synthesis is not a causative factor in depressed production of milk fat. © 1969, American Dairy Science Association. All rights reserved.