Dan Undersander and Pat Hoffman
Agronomy and Dairy Science Departments, University of Wisconsin
1575 Linden Drive, Madison, WI 53706
Rumen Undegraded Protein (RUP) or by-pass protein provides certain amino acids to the high
producing dairy animals in quantities not available from microbial protein. Therefore
measurement of this constituent is critical to balancing rations and maximizing milk
production. The problem is that the actual RUP of a forage is dependent on the forage,
animal, and level of feed intake (or rate of passage). Numerous chemical and enzymatic
techniques have been developed but all show significant deviations from in situ
measurements. However, in situ measurements are not feasible for commercial measurement
of RUP due to analysis time and cost of animal maintenance and measuring microbial protein
in the sample after in situ digestion. Further, in situ results from a single animal will
vary greatly from in situ results of other animals.
We have developed a Near Infrared Reflectance Spectroscopy (NIRS) equation to estimate RUP
of a legume or grass-legume silage based on 24-hour in situ digestion of the sample.
Silage samples for equation development were from those submitted to forage testing
laboratories. They were oven-dried and ground through 1 mm cyclone mill. A technique was
developed to 'standardize' cows and all samples were digested in duplicate in 8 dairy
cows. An NIRS equation was developed to measure microbial proteins with a reference
procedure based on purine content (Zinne and Owens with modification by Aharoni and Tagari
(1991). Crude protein was determined on the digested residue by Kjeldahl. Samples were
read on Perstorp Model 6500 before digestion (for equation development) and after
digestion (to determine microbial protein in the sample). The equation was developed
using ISI version 4.0 software. The RSQ of the equation was 0.84 and SEC was 1.55.
Results of the in situ measurements also indicated the problems of other common measurements of RUP. The correlation (RSQ) of in situ measurements with acid detergent insoluble nitrogen, neutral detergent insoluble nitrogen, and soluble protein were all low at 0.25, 0.30, and 0.50, respectively.