MILK-FAT DEPRESSION, THE GLUCOGENIC THEORY, AND TRANS-C-18 1 FATTY-ACIDS/

Metabolic and endocrinological characteristics were compared for cows that differed in the extent of milk fat depression. Forty-one multipar ous Holstein cows were fed control (40% concentrate and 60% forage) an d high concentrate (80% concentrate and 20% forage) diets in a doubale -reversal design. Cows showing one or more percentage units of depress ion in milk fat were arbitrarily classified as responders (n = 26); th ose remaining were classified as nonresponders (n = 15). Compared with non-responders, responders had greater increases in DMI, estimated NE (L) intake and balance, BW, milk yield, protein and lactose yields in milk, weight percentage of trans-C-18:1 fatty acids in milk, and conce ntrations of triiodothyronine and thyroxine in serum when switched fro m the control diet to the 80% concentrate diet. Lack of an increase in concentrations of glucose and insulin in serum of cows with the great est decline in percentage of milk fat casts doubt on the ability of th e glucogenic theory to explain milk fat depression completely. However , higher concentration of transC(18:1) fatty acids in milk fat for res ponders than for nonresponders provides correlative support for the th eory that trans-C-18:1 fatty acids may contribute to milk fat depressi on. The greater increases in concentrations of thyroid hormones in ser um for responders than for nonresponders introduces an additional endo crinological factor that may influence secretion of fat in milk when d iets contain high percentages of concentrate.