Study On The Metabolism And Residue Of Low Dosage Of Melamine In Dairy Cattle

The objective of this study was to evaluate the metabolism and residue regularity of low dosage of melamine (MEL) in dairy cattle. These results will provide scientific basis for the establishing of MEL safety limit in dairy cattle feed. This study included three parts:The first part of this study was to investigate the transfer regularity of MEL from feed to milk through preliminary trial. Twenty-four China Holstein dairy cows were divided into 2 blocks according to milk yield (block 1: Milk yield < 20 kg/d per cow; block 2: milk yield > 20 kg/d per cow). Cows of block 1 or block 2 each were randomly assigned to 1 of 4 treatments in a randomized complete block design and each treatment had 6 cows. The cows of treatments 1 to 4 were dosed with MEL at 0 (Control), 90 (Trt1), 270 (Trt2) and 450 (Trt3) mg/d per cow, respectively. The trial lasted for 19 d (13 d feeding period, followed by 6 d clearance period). Melamine content in diets and milk was determined by gas chromatography-mass spectrometry (GC-MS). Results indicated that the levels of MEL used did not affect milk yield or composition (P > 0.05). The mean milk MEL concentration increased consecutively during the initial 3 d after MEL feeding in all the MEL-supplemented groups, and then fluctuated slightly over the remaining 10 d of MEL feeding. When melamine administration was terminated on d 14, the mean melamine concentration decreased rapidly. After 4 d, no melamine could be detected in any of the treated groups. Milk MEL concentration measured between 3 to 13 d was significantly affected by MEL feeding doses (P < 0.05) and was linearly related with MEL intake (R2 = 0.84), but was not influenced by milk yield (P > 0.05). On the contrary, transfer efficiency of MEL from feed to milk was not affected by MEL doses (P > 0.05), but was affected by and linearly related with milk yield (R2 = 0.80).The second part of this study was to investigate the residue of MEL and cyanuric acid (CYA) in milk and tissues of dairy cows through feeding trial. Forty mid-lactation dairy cows were divided into 4 groups (n = 10/group) in a completely randomized design. Cows of the four groups were dosed with MEL (purity≥99.5%) at 0 (Control), 300 (Trt1), 500 (Trt2) and 1000 (Trt3) mg/d per cow, respectively. The whole trial lasted for 19 d (12 d feeding period, followed by 7 d clearance period). Milk samples were collected at d 1, 2, 3, 4, 8, 12, 13, 14, 15 and 19. On d 13, three cows of Trt2 and Trt3 were chosen randomly to slaughter, and tissue samples (kidney, liver, mammary, bladder, gluteus medius and longissimus dorsi) were collected. Feed, milk and tissue samples were analyzed for MEL and CYA simultaneously by liquid chromatography tandem mass spectrometry (LS-MS/MS). Minor MEL was detected in concentrated feed background (6.23±1.26 mg/kg). However, no CYA was detected. In MEL treated groups, milk MEL concentration increased quickly and reached a stable concentration at d 4, 8 and 12 after the first administration of MEL. Milk MEL concentration of treated groups in steady-state condition was significantly affected by MEL feeding dose (P < 0.05), with a linearly relationship between MEL intake and milk MEL concentration (R2 = 0.91). No CYA was detected in milk of all groups. MEL residue in tissues of Trt3 was about 2-fold higher than Trt2, with the highest concentration in the kidney. The difference of CYA residue in tissues of TRT2 and TRT3 was not very obvious. Liver, kidney and bladder has higher CYA residue than other tissue. The CYA may come from the degradation of MEL in rumen.The third part this study was to investigate the pathway for the elimination of MEL in dairy cows. Four late-lactating dairy cows fitted with ruminal cannulas were dosed with MEL (purity≥99.5%) at 800 mg/d per cow, divided into two equal daily doses. The trial lasted for 20 d. The first 13 d was preliminary period, followed by a 7-d sample collecting period (Feed intake and total output of feces, urine, and milk were measured from d 14 to 16. Blood samples were collected at 0 (pre-dose), 4, and 8 h after morning feeding on d 17 and 18, and rumen fluid were collected at 0 (pre-dose) 1, 2, 4, and 8 h after morning feeding on d 19 and 20). LC-MS/MS was utilized to determine MEL and CYA simultaneously. Before the trial started, no MEL or CYA was detected in samples of feed, milk, plasma, urine and feces. In sample collecting period, MEL concentration in rumen fluid decreased exponentially after the morning feeding, but no CYA was detected. Plasma MEL concentration was relatively stable at the three different sampling times (P > 0.05). The percentage of MEL excreted through milk, urine and feces were 0.48±0.06, 10.98±3.88 and 44.07±10.79%, respectively. Therefore, 44.47±7.98% of ingested MEL may be degraded by rumen microorganism. This speculation was conformed by the fact that CYA was also detected in plasma, urine and feces. Whether ammelide and ammeline were excreted commonly with MEL needs further study. The results of the present work implied that high percentage of MEL may be degraded gradually by rumen microorganism to produce its analogues (CYA). The kidney and digestive tract are two main pathways for the excretion of undegradable MEL through feces and urine.