Circadian Intake Timing in Ruminants: Nitrogen Metabolism and Milk Fat Properties

The objective was to establish effects of providing a Total Mixed Ration (TMR) at either 0900 h or 2100 h on nitrogen partitioning and milk fatty acids profiles in lactating cows. Four multiparous and four primiparous Holstein cows were used in a cross-over design study with two 6-week periods, each with 3-week adaptation. Total fecal and urine were collected during a sampling week in each period to measure nutrient digestibility and nitrogen partitioning. Milk proportions of total short, medium, and long chain fatty acids were not significantly affected by eating time. Feeding at 2100 h vs. 0900 h decreased (P?0.05) milk proportions of C10:0, C12:0, C12:1, C13:0, C13:1 and C18:3 n-3, and tended to decrease (P?0.10) proportions of C8:0 and C18:1 trans-9, while increasing that of C18:0. Evening fed cows tended to realize a greater rumen volume than morning fed cows (107 vs. 90 L, P<0.01). Evening vs. morning feeding reduced the proportion of the apparently digested N that was excreted in urine (480 vs. 550 g/g, P<0.05). Therefore, feed delivery at 2100 h vs. 0900 h improved nitrogen dynamics and milk energy output while to some extent manipulating milk fatty acids profile. Future studies are required to establish other aspects of the multiscience of eating time in ruminant ecology.