Diurnal Variation of Rumen Ammonia, Serum Urea, and Milk Urea in Dairy Cows at High and Low Yields

Abstract

Milk urea content as an indicator of nutritional status may be a useful tool if major sources of variation are considered. Blood and milk samples were collected frequently during 16 to 19 h from four Holstein cows to study diurnal variation of urea content. Corn silage, alfalfa hay, and concentrates were fed. Rumen ammonia, VFA, and pH were measured in three of the cows. A clear serum urea peak, 70 to 85% higher than the lowest concentration, was observed in the higher yielding cows. The serum urea peak occurred 1.5 to 2.0 h after the rumen ammonia peak. Urea in milk equilibrated with serum with a time lag of 1 to 2 h when the rate of change in serum was .5 to 1.0 mM/h. At this rate, the average difference between serum and milk urea content was .8 mM. Urea in total milk tended to be more closely correlated to serum than samples from the gland cistern, but deviations were minor. Our results indicate a relatively rapid equilibration between blood serum and milk urea, also in the gland cistern. Equilibration may be explained by diffusion of urea along the mammary ducts and through the mucosa in the alveoli. If urea is to be used as an indicator of nutritional status, diurnal variations of serum and milk urea should be considered; time of sampling versus time of feeding is crucial. A small milk sample from a healthy quarter may give information on urea that is as good as that of a sample from regular milking.

Effect of rumen-degradable protein balance deficit on voluntary intake, microbial protein synthesis, and nitrogen metabolism in growing double-muscled Belgian Blue bulls fed corn silage-based diet1,2,3

“…According to our results, the maximum NH 3 -N concentration in the rumen was followed 3 h later by the maximum urea-N concentration in the plasma. This delay between these 2 peaks was in agreement with those observed by Gustafsson and Palmquist (1993).…”

Slow-release urea in supplement fed to beef steers

“…Butler (1998) reported low fluctuation of PUN during the day (between 0.02 and 0.03 mg/dL). However, this fluctuation occurred in lower intensity in the animals fed TMR diets, likewise animals in the present experiment, when compared with animals fed forage and concentrate separately, according to Gustafsson and Palmquist (1993). Also, despite both the feeds (hay and concentrate) were offered together, manual mixture could allow selection of the feeds by the animals (Forbes 1995) and, by consequence, different dynamics of intake, which could affect the PUN levels.…”

Low degradable protein supply to increase nitrogen efficiency in lactating dairy cows and reduce environmental impacts at barn level

“…In order to depict urea variations in relation to the diet, uraemia was assessed just before and 3 h after the morning ration distribution. Even if urea concentrations in blood and milk are correlated when measured at short time intervals (Gustafsson and Palmquist, 1993), blood was preferred to milk as it was easier to sample at both times. Blood samples were collected from the tail vein once per period (day 10) at 0800 h and at 1100 h to assess plasmatic uraemia (KONE Instruments 200 Corporation).…”