Urea concentrations in milk and blood as influenced by feeding varying amounts of protein and energy to dairy cows

Abstract

Eight multiparous cows of the Swedish Red and White Breed were used to study the effect of diet on concentrations of urea in milk and plasma as well as on glucose concentrations in plasma. The cows were divided into four pairs and fed a ration containing varying amounts (low = L; standard = S; high = H) of crude protein (P) and metabolizable energy (E). When fed according to the Swedish standard (P SE S), all cows had a milk urea content around 5 mmol l −1. Concentrations altered only slightly when the amount of protein ingested was decreased or increased - as long as the ratio between protein and energy was held constant (P LE L; P HE H). When the ratio was changed, however, the milk urea concentrations either decreased to around 4 mmol l −1 (P SE H; P LE S) or increased to between 6.5 and 7.6 mmol l −1 (P SE L; P HE L; P HE S). It was found that the main factor influencing the milk urea content was not the amount of protein ingested in relation to requirements, but the relationship between protein and energy in the ration. Concentrations of urea in morning and afternoon milk were similar and the day-to-day variations were small. The correlation between urea concentrations in plasma and milk was 0.98, with concentrations in plasma always slightly higher. Diet had no effect on blood plasma glucose concentrations.