The objective of this trial was to study the effects of increasing dietary levels of crude protein (CP) on microbial protein synthesis of lactating dairy goats. The microbial markers purines bases and purine derivatives (PD), measured in spot urine samples or in urine samples from total collection, were used to estimate microbial protein synthesis. Sixteen lactating goats (four fitted with ruminal cannula) were randomly assigned to four replicated 4 x 4 Latin squares and were fed the following dietary CP levels [dry matter (DM) basis]: 11.5, 13.5, 15.5 or 17.5%. No significant differences were observed between techniques for estimating microbial protein synthesis. Therefore, 12 non-cannulated goats were used to evaluate the effect of dietary CP level on PD excretion, microbial nitrogen flow and microbial efficiency using either total or spot collection of urine. Because creatinine excretion did not differ among treatments, the average value 26.05 mg of urinary creatinine excreted per kg of body weight was used for estimating the volume of urine using spot samples. Urinary volume estimated using spot samples was similar to that measured with total urine collection. In both cases, an increase on the amount of absorbed purines was observed when the dietary CP level increased in the diet. In brief, PD can be used for estimating microbial protein synthesis in lactating goats and spot samples were a good alternative for total collection of urine.
goat; microbial efficiency; purine derivatives
