Relation Between Milk Urea Nitrogen Content And Protein Nutrition In Chinese Holstein Cows

Milk urea nitrogen (MUN) could be used to monitor the change of dietary protein nutrition and output of nitrogen to environment. The applications of MUN in dairy industry abroad have been reported, but there are many differences between Chinese and foreign Holstein cows in breeding, management and diets. Little information is available about MUN in Chinese dairy cows. This study was conducted to evaluate the method to determine MUN content (Experiment 1), investigate effect of dietary protein level (Experiment 2), degradability on MUN content (Experiment 3), and relation between MUN and N excretion (Experiment 4) in order to provide the basic theory of MUN utilization in Chinese lactating dairy cows. Experiment 1 Study of determination method for milk urea nitrogenThe trials in this part were conducted to evaluate effeciency of diacetyl monoxime method to determine MUN, effects of reserved condition for milk sample and preservative addition on MUN values, and diurnal variations of MUN content in lactating Chinese Holstein cows.The results appeared to be optimal when the wavelength for determination was 525 nm and reaction liquid was boiled for 13-16 min. The CV (Coefficient of variation, CV) of within-run and between-run were 2.32 and 3.40%, respectively. The average recovery was 98%. The reagents and apparatus in this method were easy to achieve. It seemed that this method would be suitable for routine analysis with higher accuracy and reliability.The MUN content declined rapidly after the second day if the milk samples stored under room temperature without addition of preservative(0.6‰ potassium dichromate), the MUN content in sample stored in refrigerator was not changed within four days. While the MUN content in samples stored in freezer or with additon of preservative were not changed significantly during period of two weeks. The MUN content increased with the increasing preservative levels (0, 0.3,0.6, 0.9 and 1.2‰) . The MUN values with addition of 1.2‰ potassium dichromate were significantly higher than those without addition of preservative or with addition of 0.3‰ preservative (P<0.05) There were no significant differences in MUN values between samples stored without and with ordinary level of preservative (P>0.05).It indicated that the MUN value might not be influenceded by adding normal dosage of preservative.There were significant diurnal variations when lactating Chinese Holstein cows were fed three times per day (P<0.05). The MUN content in morning sample was lower than the samples in noon and evening with higher value than those in morning and noon, indicating that it would be essential to collect milk samples of whole day. There was significant correlation between MUN content and urinary urea nitrogen (UUN). Experiment 2 Effect of dietary protein levels on MUN content, production performance and nitrogen excretion in lactating Chinese Holstein cowsTwelve multiparous lactating cows were divided into four groups by days in milk and milk yield. The cows were fed diets with varying crude protein levels (13.0, 14.0, 15.0 and 16.0 %, DM basis) with isocarloric. Experiment was conducted according to a multiple 4 x 4 Latin Square design. Experimental periods were 56 d, each period consisting of 14 days with d 1 to 9 for adjustment and d 10 to 14 for data collection. The objective of this study was to investigate the effect of dietary protein levels on MUN content in Chinese Holstein cows.Crude protein levies had significant effects on plasma urea nitrogen (PUN), MUN, and urinary nitrogen (UN) excretion (P < 0.01), but little effects were found on milk yield and composition (P > 0.05). There was a closely linear relationship between MUN (mg/dl) and PUN (mg/dl) (P<0.001), MUN = 0.78 x PUN +1.22 (R2=0.90).These results indicated that MUN might be available to reflect the change of dietary protein levels.Experiment 3 Effect of dietary protein degradability on MUN content and lactating performance in Chinese Holstein cowsFifteen multiparous lactating cows were divided into three groups by days in milk and milk yield. The animals were fed three diets with levels of protein degradability (Rumen undegradable protein, RUP) being 30.8, 36.2, and 41.6%, respectively (DM basis) with being isocarloric and isonitrogenous. Experiment was conducted according to a multiple 3 x 3 Latin square design. Experimental periods were 63 d, each period consisted of 21 days with d 1 to 14 for adjustment and d 15 to 21 for data collection of urine and feces. The objective of this study was to investigate the effect of dietary protein degradability on MUN content in Chinese Holstein cows.Crude protein degradability had no significant effects on MUN content, milk yield and composition (P > 0.05). There was a close correlation between MUN (mg/dl) and PUN(mg/dl) (PO.001), MUN=0.81xPUN+1.52 (R2=0.88). Little differenc was found between this equation and that in experiment 2.These results suggested that the dietary rumen degradable protein (RDP) or RUP level had similar effect on MUN content and higher RUP levels might not improve milking perforamce. Experiment 4 Relation between MUN and nitrogen excretion in Chinese dairy cowsThirty-two Chinese Holstein lactating cows were used to investigate the relationship of MUN and N excretion loading to the environment. Cows were fed the similar amount of forage and varying amount of concentrates according to milk production. Total collection of urine and feces were conducted continuously for three days. The MUN was significantly correlated to total N excretion (TNE) (R2=0.70), UN excretion (R2=0.85), and fecal nitrogen (FN) (R2=0.22). The following equation was proposed to predict TNE (g/d) based on MUN (mg/dl): TNE =15.46 (±1.83) xMUN +193.40 (±28.79). The results obtained in this study suggested that MUN might be used to predict TNE from lactating cows.In conclusion, diacetyl monoxime method might be used as a suitable method to analyze MUN content. The change of above 1% protein level in diets would be reflected by MUN content, but the change of degradability in dietary protein could not be detected by MUN content. The MUN could be used to predict not only UN excretion, but also predict TNE excretion.