Diurnal variation in concentrations of plasma urea nitrogen and amino acids in pigs given free access to feed or fed twice daily.

A repeated-measures experiment was conducted to test the effect of sampling times after feeding on concentrations of plasma urea nitrogen (PUN) and plasma free amino acids (PFAA) in pigs fed by two feeding methods. Seven castrated male pigs (mean weight of 65 kg) were randomly assigned to switchback treatments that were 3-d periods of either free access to feed or twice daily feeding. Blood was sampled at 2-h intervals (sampling points) during the day (0800 to 1800 h) and again during the night (2000 to 0600 h) on the last day of each of three 3-d feeding treatment periods. Concentrations of PUN and most PFAA were not affected by the feeding method and time of day (day and night), but they were affected by sampling points. The pigs fed twice daily had postprandial peak PUN at 4 h (32% increase over prepandial) and peak PFAA at 2 h after feeding during 12-h day and night periods. However, the pigs with free access to feed exhibited only a slight fluctuation of PUN during the day and night and almost constant concentrations of most PFAA during a 24-h period. The PFAA results suggest that concentrations of most plasma essential and nonessential amino acids varied relatively more than those of other amino acids in response to sampling times.