Influence of Readily Fermentable Carbohydrate Supplementation to the Diet on Nitrogen Excretion in Lactating Cows

Three mid- or late-lactating Holstein cows were used in nitrogen (N) balance trials to examine whether readily fermentable carbohydrate (RFC) supplementation to the diet can reduce N excretion without decline in productivity of lactating cows. Additionally, to determine the reduction mechanism by RFC supplementation, changes in urine, ruminal fluid, plasma, and milk components were examined. Basal diet was mainly composed of grass silage, and purified potato starch was used as RFC supplement. The cows were fed basal diet only, basal diet supplemented with 10% starch or basal diet supplemented with 20% starch according to 3×3 Latin square design. Although it was observed that crude fat and NDF digestibility and ruminal fluid pH decreased in the 20% starch supplementation, the extent of these changes were not large. There was no significant difference in milk yield and composition between treatments. Urinary N excretion or urinary N excretory rate (N excretion/N intake) reduced significantly in response to starch supplementation, the total (urine plus feces) N excretion or excretory rate was significantly lower in 20% starch supplementation than control. This reduction of urinary N excretion was due to the decrease in urine volume and urine urea concentration with starch supplementation, and it was thought that the decline in plasma urea concentration caused the decrease inurine. Two factors might be involved in this decline in plasma urea concentration, 1) the decline of ruminal fluid ammonia concentration which would result in the decrease of N supply to liver and 2) the increase in glucose production from propionate, lactate and pyruvate which would affect the sparing glucogenic amino acids. The result in this study showed that supplementation with purified potato starch as RFC had a reducing effect on N excretion without much influence on feeding and milk performance in lactating cows and the decline in plasma urea concentration might play an important role in this reduction mechanism.