Pathway for the elimination of melamine in lactating dairy cows

Melamine might be degraded into cyanuric acid and some other analogs by the rumen microorganism. Thus, the metabolism of melamine in ruminants may be different from that in monogastric animals. The objective of this study was to investigate the pathway for the elimination of melamine in lactating dairy cows. Four late-lactation dairy cows (body weight = 524 ± 17 kg, days in milk = 265 ± 14 d) fitted with ruminal cannulas were dosed with melamine (purity ≥99.5%) at 800 mg/d per cow that divided into 2 equal daily doses. The trial lasted for 20 d (13-d preliminary period, followed by a 7-d sample-collecting period). The method of liquid chromatography and tandem mass spectrometry was used to determine melamine and cyanuric acid contents simultaneously. Before the trial started, no melamine or cyanuric acid was detected in samples of total mixed ration, milk, plasma, urine, and feces. The melamine concentration in rumen fluid (Y, mg/L) decreased exponentially after the morning feeding (X, h) (Y = 3.85591e−X/9.25674 + 1.35924, R2 = 0.99), but no cyanuric acid was detected. Plasma melamine concentration (0.296 ± 0.014 mg/L) was relatively stable in the 3 different sampling times. The percentages of melamine excreted through milk, urine, and feces were 0.48 ± 0.06, 44.07 ± 10.79 and 10.98 ± 3.88%, respectively. It could be inferred that 44.47 ± 7.98% of ingested melamine was degraded in the rumen, because cyanuric acid was detected in plasma, urine, and feces on the condition that no melamine was contained in the total mixed ration fed to the dairy cows. The results of the present study implied that the elimination pathway of melamine in lactating dairy cows was different from that in monogastric animals. A high percentage of melamine was degraded into cyanuric acid gradually by rumen microorganisms. Most ingested melamine was excreted in urine and feces, which are the main elimination pathways for melamine in lactating dairy cows.