Influence of time of feeding a protein meal on diurnal patterns of amino acids in duodenal digesta protein of lactating dairy cows

Four lactating cows were offered a mixed ration for ad libitum intake from 08:00 to 24:00 h and 2.5 kg (dry matter basis) of a protein supplement (470 g kg−1 crude protein; from barley, canola meal, soybean meal, fishmeal and corn gluten meal) at either 08:30 h (day) or 00:30 h (night). Diurnal patterns in amino acid proportions of duodenal digesta protein were measured in cows fitted with duodenal cannulae by sampling digesta at 3 h intervals over three alternate days to obtain samples at each hour of the day. Time of feeding the protein supplement did not affect mean daily proportions of any amino acid in duodenal protein but all, except tyrosine and phenylalanine, varied with time of the day (P<0.05). Aspartic acid, threonine, glutamic acid, glycine, alanine, valine, isoleucine, leucine, lysine and arginine proportions also had treatment by time of day interactions (P<0.05), although their form differed among amino acids. Glutamic and aspartic acids, which differed most markedly in their proportions in the protein supplement, as well as in isolated rumen bacterial protein, were used to conclude that the most likely reason for diurnal amino acid patterns was different diurnal patterns in duodenal flow of rumen bacterial protein and feed protein that escaped degradation in the rumen. A substantial rumen passage time lag, of 6–7 h, before the amino acid profile of the protein supplement was reflected by changes in the profile of duodenal digesta protein, indicates that the amount of feed protein which escapes the rumen may not be accurately predicted using first-order kinetic models, unless rumen passage and digestion time lags are utilized. Thus, presenting evaluations of CP, and perhaps other, fractions of feeds on the basis of first order models with only a digestion time lag may have little interpretive value.