Milk fat depression, the glucogenic theory, and trans-C18:1 fatty acids

Metabolic and endocrinological characteristics were compared for cows that differed in the extent of milk fat depression. Forty-one multiparous Holstein cows were fed control (40% concentrate and 60% forage) and high concentrate (80% concentrate and 20% forage) diets in a double-reversal design. Cows showing one or more percentage units of depression in milk fat were arbitrarily classified as responders (n = 26); those remaining were classified as nonresponders (n = 15). Compared with nonresponders, 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 concentrations of triiodothyronine and thyroxine in serum when switched from the control diet to the 80% concentrate diet. Lack of an increase in concentrations of glucose and insulin in serum of cows with the greatest decline in percentage of milk fat casts doubt on the ability of the glucogenic theory to explain milk fat depression completely. However, higher concentration of trans C(18:1) fatty acids in milk fat for responders than for nonresponders provides correlative support for the theory that trans-C(18:1) fatty acids may contribute to milk fat depression. The greater increases in concentrations of thyroid hormones in serum for responders than for nonresponders introduces an additional endocrinological factor that may influence secretion of fat in milk when diets contain high percentages of concentrate