The urinary excretion of allantoin and uric acid (collectively purine derivatives, or PD) provides an estimate of the yield of microbial protein (MP) from the rumen. The technique is useful for estimating rumen efficiency of dairy cows, but due to the problems associated with urine collection, its use is limited on the large scale. Measuring PD in milk would allow more routine assessments to be carried out on a practical level.

Twelve multiparous mid-lactation Holstein/Friesian cows were used in a 3x3 Latin square changeover experiment with 3 four-week periods. Cows were allocated to squares on the basis of their mean milk yield of the 7 days prior to the trial. Within squares, treatments were randomly allocated to each cow. The first 25 days of each experimental period were used for adjustment, and the last 3 for the collection of milk and urine samples.