Three experiments were conducted to determine the relationship of milk fat secretion to concentration and ratio of rumen fermentation products, blood metabolic parameters, ration characteristics, and certain enzymic activity involved in fatty acid synthesis.

In Experiment I, 21 cows in mid-lactation were randomized according to milk production to (1) roughage ration (~22% CF) restricted to NRC; (2) concentrate ration (~12% CF) ad libitum; or (3) concentrate ration (~12% CF) restricted to NRC and fed for 30 days.

Ad libitum concentrate showed higher dry matter intake (DMI), body weight (BW), milk production than restricted concentrate. The opposite was true for fat test. Concentrate rations (ad libitum and restricted) when compared to roughage showed higher values for DMI, BW, milk production, serum glucose, molar proportion propionate and valerate and lower values for fat test and molar proportion acetate.

In Experiment II, 24 cows 180 d. post parturition, were assigned to four rations based on anticipated parturition. Rations were: (1) roughage ad libitum (~20% CF); (2) concentrate ad libitum (~10% CF); (3) concentrate restricted (~10% CF) to NRC; and (4) normal ration (~14% CF) ad libitum. Cows on ad libitum and restricted concentrate consumed less dry matter, acid detergent fiber, neutral detergent fiber and crude protein than control cows (roughage and normal). Cows fed control rations gained more weight than concentrate fed cows. No difference was found for milk production. Milk protein was higher for control cows. Concentrate cows had lower fat test than control cows. Cows on all rations decreased fat test up to week three or four but by week eight or nine, concentrate fed cows had decreased to their lowest fat test. Energy restriction improved milk fat secretion compared to ad libitum intake after the eighth week. Molar proportion VFA favored propionate for concentrate cows and was similar for restricted and ad libitum concentrate. Total ruminal VFA concentration was higher for ad libitum concentrate cows and those cows showed slightly higher glucose and smaller blood acetate compared to control.

Twelve cows in mid-lactation were fed the rations of Experiment II to determine the effect of available energy intake on activity of key enzymes of fatty acid synthesis. Activity of 6-phosphogluconate dehydrogenase was not different among rations. Mammary tissue showed from five to six times higher activity than adipose tissue. Fat tissue activity tended to be higher for concentrate rations compared to roughage. No differences among rations were found for glucose 6-phosphate dehydrogenase nor isocitrate dehydrogenase. Mammary tissue was 21 to 28 times more active than adipose tissue for latter enzyme. Fatty acid synthetase showed no difference in activity due to rations, but mammary tissue was seven to ten times more active than adipose tissue.