Effects of timing of abomasal infusion of fatty acids on the daily rhythms of milk synthesis and plasma hormones and metabolites in dairy cows.

Dairy cows display daily rhythms of milk synthesis that appear to be driven by a circadian clock located in the mammary gland. These rhythms are altered by the time of feed availability. Fatty acids have been shown to entrain circadian rhythms in liver and adipose tissue in experimental models, but their role in the mammary gland has not been well investigated. Our objective was to determine the effects of the timing of fatty acid absorption on the daily rhythms of milk synthesis. Nine lactating Holstein cows were arranged in a 3 × 3 Latin square design. Treatments were abomasal infusions of 350 g/d of a free fatty acid stock enriched in cis-9 18:1 either continuously throughout the day for 22 h (CON) or for 8 h from 0900 to 1700 h (DAY) or from 2100 to 0500 h (NGT). Treatment periods were 12 d with a 5-d washout. Cows were milked every 6 h during the final 7 d of each period to determine the daily patterns of milk synthesis. A 24-h rhythm was fit to time course data using cosine analysis, and the amplitude and acrophase (time at peak) were determined. Daily milk and milk protein yields were decreased by DAY and NGT compared with CON, whereas milk fat yield was not changed. Milk yield fit a 24-h rhythm in CON and DAY but not in NGT. Furthermore, DAY delayed the peak of the daily rhythm of milk yield by 2 h compared with CON. Fat and protein concentrations exhibited daily rhythms in CON and NGT but not DAY. Fat yield only fit a 24-h rhythm in DAY. Both de novo and mixed-source fatty acid yields were reduced by DAY and NGT, suggesting that the faster infusion rates may have resulted in concentrations of fatty acids that exceeded a threshold sufficient to inhibit de novo fatty acid synthesis. Plasma glucose concentration failed to display a daily rhythm in any treatment, whereas nonesterified fatty acids showed a rhythm in CON and NGT, but this rhythm was abolished by DAY. Insulin fit a rhythm in NGT and tended to fit a rhythm with a lower amplitude in CON, but no rhythm was present in DAY. Blood urea nitrogen exhibited a daily rhythm under all treatments, and both the mean and amplitude were increased by DAY. Daily rhythms of milk synthesis were also modified by DAY, with a slight delay in the daily peak of milk yield and elimination of the rhythms of milk fat and protein concentrations. Infusion at night had little effect. Daytime infusion also modified the daily rhythms of plasma metabolites by reducing the amplitude of nonesterified fatty acid concentration and increased the amplitude of blood urea nitrogen.