Nitrogen Concentration in Blood and Rumen Liquor of Cattle Fed Low Protein Diets

Abstract

Crude protein determination of a grazing animal's diet is difficult and expensive. Traditional methods include forage sampling (usually not representative of the diet selection process) and the use of fistulated animals for direct diet collections. Indirect methods were tested to provide a rapid estimate of diet protein at less cost. Concentration of blood serum urea N (BUN) and the concentration of total nitrogen (N), protein N, microbial protein N, and non-protein N (NPN) in rumen liquor were determined in 4 cows and 4 steers fed diets at maintenance (7.1%) and 3 sub-maintenance levels of crude protein (CP) (4.3, 5.2, and 6.2%). Cottonseed hulls constituted the basal diet, with cottonseed cubes added to vary the CP content and molasses added to provide isocaloric diets. However, diet CP affected the in vivo digestibility of the diets and hence their caloric values. Concentrations of BUN did not differ (P<.05) with changes in dietary CP. The concentration of total N, protein N, microbial protein N, and nonprotein N (NPN) in the rumen liquor (P<.05) increased as diet CP increased. The percentage of NPN in the total N was reduced when diet CP was below 5.2%, but it did not differ significantly when diet CP was between 5.2 and 7.1%. The ratio of microbial nitrogen to total protein nitrogen was not affected by level of dietary crude protein. Total N was a sensitive indicator of the CP content of the diet and was the easiest and quickest method tested.