The Impact Of Dietary Factors, Parity, DIM And Milk Composition On Milk Urea Nitrogen Content Of Holstein Cows

Milk urea nitrogen(MUN) could be used as an indicator of the level of dietary protein, the balance between energy and nitrogen, and the nitrogen utilization.This study was conducted to evaluate the pretreatment methods of determination of MUN(Experiment 1), the sampling methods of determination of MUN(Experiment 2), the impact of dietary factors on MUN (Experiment 3), the impact of dietary factors on MUN(Experiment 4), the relationship between MUN and plasma urea nitrogen(PUN), glucose(GLU) and free fatty acids(FFA) (Experiment 5).The first part was to discuss the pretreatment methods of determination of MUN with diacetyl-oxime kit. The result showed that centrifuged at 1,500 r/min for 10 min to degrease and at 3,500 r/min for 20 min to deprotein had a significant difference with not degrease and centrifuged at 3,500 r/min for 20 min to deprotein(P＜0.01), but there was no significant difference with not degrease and centrifuged at 5,000 r/min for 15 min to deprotein (P＞0.05), and the CV% was 4.68%, the recovery rate was 94.91%. These results indicated that the determination of the content of MUN could be centrifuged at 5,000 r/min for 15 min and not degreased.The second part used thirty lactating Holstein Cows to analyze the impact of sampling time during and between milkings on milk composition, including sampling before milking, post milking and standard method with flowmeter during milking, and morning sampling, middLe sampling and night sampling with flowmeter in a day. The results showed that there was extremely significant difference in the content of milk fat, protein, dry matter, lactose, somatic cell count(SCC) and MUN between standard method and before milking and post milking (P＜0.01). There was extremely significant difference in the content of milk fat, dry matter, SCC and MUN content among milkings with flowmeter(P＜0.01), and no significant difference in the congtent of milk protein and lactose (P＞0.05). The results indicated that the sample should be sampled with flowmeter.The third part used 10,316 Dairy Herd Improvement(DHI) records to analyze the impact of parities and days in milk(DIM) on MUN, and the relationship between MUN and milk composition. The results showed that MUN had extremely significant difference among parities (P＜0.01). MUN peaked at 90 to 120 DIM, rose until 90 to 120 DIM from calving, and declined from 90 to 120 DIM. There was curve correlation between MUN and FCM (y = -0.001x~2+0.132x + 16.88, R = 0.963, P＜0.001), MUN content peaked at 33.3 kg/d of FCM There was curve correlation between MUN and milk protein content (y = -0.932x~2 + 5.854x + 10.02, R = 0.819, P = 0.012), MUN content peaked at 3.14% of milk protein content. MUN content declined gradually with the increase of milk fat content (y = -0.249x~2 + 0.827x + 19.5.9, R = 0.974, P＜0.001), and rose gradually with the increase of SCC (y = 0.001x + 18.94, R = 0.838, P = 0.005).The fourth part used 21 trials with 92 treatments and 551 lactating cows to analyze the impact of dietary factors on MUN of Chinese Holstein Cows. The results showed that, Single factors yielded extremely significant regressions on MUN including dietary crude protein(CP), net energy(NE_L),rumen degradable rotein(RDP), net energy to protein ratio(N:P), rumen undegradable protein(RUP)(P＜0.01). CP had the strongest relationship with MUN content, followed by NE_L, RDP, N:P, RUP, ADF, RDP:RUP, DMI and NDF. About the mixed effects models, every combination of dietary factors yielded extremely significant regressions on MUN(P＜0.01), the R~2 of the combination of CP and RDP.RUP was the highest. The results indicated that dietary CP is the the major factor.The fifth part used thirty lactating Holstein Cows to analyze the relationship between MUN and PUN, GLU and FFA. The results showed that there was a extremely significant linear correlation between MUN and PUN(y = 0.607x + 3.626, R=0.88, P＜0.001), and negative correlation between MUN and GLU, positive correlation between MUN and FFA.In conclusion, MUN content is impacted by unnutrition factors including sampling method, pretreatment methods of determination, parity, DIM, and nutrition factors including CP, NE_L, N:P, RDP and RUP, and there is strong relation between MUN content and the content of milk protein, fat and SCC. DIM and the level of dietary CP are the major factors.