Net absorption of glucose and nitrogenous compounds by lactating Holstein cows

Net absorptions from portal-drained viscera of glucose, ammonia nitrogen, alpha-amino nitrogen, and urea nitrogen were measured in four Holstein cows in first lactations. Cows were fed for ad libitum consumption a completely mixed, 60:40 corn silage:supplement diet (dry basis) in two equal feedings at 12-h intervals daily. Portal blood flow (dye dilution) and net absorption (portal-arterial difference times portal blood flow) were measured at hourly intervals for 12 h for all cows 4 and 8 wk postpartum and for two cows 4, 8, 12, 16, and 20 wk postpartum. Apparent digestibilities of dry matter and nitrogen were measured immediately after net absorption. Milk yield and composition were measured with absorption and digestion. Mean daily dry matter intake and milk yield were 14.1 +/- .6 and 23.8 +/- 1.8 kg. Portal blood flow was not affected by time of day or weeks postpartum; mean was 1371 +/- 123 liters/h. Net absorption of glucose was negative, indicating no uptake of glucose from dietary sources. Means (g/day) for nitrogenous sources were: nitrogen intake 387, fecal nitrogen 121, urinary nitrogen 146, milk nitrogen 116, net ammonia nitrogen absorption 149, net alpha-amino nitrogen absorption 137, and net loss of urea nitrogen from blood to the gut 93. Net absorption of amino nitrogen and alpha-amino nitrogen in blood was greater than in plasma; corresponding losses of urea nitrogen and glucose to the gut were greater from blood than plasma.