【作者】 翟少伟；

【导师】 刘建新；

【作者基本信息】 浙江大学 ， 动物营养与饲料科学， 2006， 博士

【摘要】 乳尿素氮(Milk urea nitrogen,MUN)有可能成为评价奶牛蛋白质营养状况和减少氮污染的一个重要指标。中国荷斯坦牛与国外的荷斯坦牛品种在培育过程、管理方式以及日粮组成方面有着很大的差异。中国荷斯坦牛MUN方面的相关研究还鲜见报道。 本文以中国荷斯坦牛为对象,系统研究了MUN测定方法(试验一);日粮蛋白质水平(试验二)、降解率(试验三)对MUN的影响;中国荷斯坦牛MUN与氮排泄的关系(试验四)等内容。 试验一 MUN的测定方法 探讨用二乙酰一肟法对测定MUN的技术以及奶样保存温度、防腐剂(重铬酸钾)添加对测定结果的影响,研究了中国荷斯坦牛MUN浓度昼夜变化情况。结果发现: 二乙酰一肟法测定MUN时波长为525nm,反应液煮沸时间13-16分钟测定结果较好,批内变异系数(Coefficient of variation,CV)为2.32%,批间CV%为2.40%,回收率为98%,用此法测定MUN精确度和准确度高、操作简单、所需试剂和仪器容易获得。 奶样中不加防腐剂的条件下,室温下第二天MUN浓度有所下降;冷藏条件下四天内可以保持MUN不变;而冷冻或添加防腐剂的奶样保存两周MUN浓度基本不变。MUN浓度随着防腐剂的添加水平(0‰、0.3‰、0.6‰、0.9‰和1.2‰)而逐渐上升,1.2‰的防腐剂添加奶样MUN值显著高于未加防腐剂或添加0.3‰防腐剂组(P<0.05),其它水平的防腐剂添加组之间以及与未加防腐剂组之间的差异均不显著(P>0.05),说明添加常规剂量的防腐剂不会对测定结果造成明显的影响。 日饲喂3次的中国荷斯坦牛MUN和尿液尿素氮(Urinary urea nitrogen,UUN)浓度存在着明显的昼夜变化。一天中,早晨的MUN浓度最低,晚上MUN浓度最高,中午MUN浓度居中,提示了收集全天样品的重要性;此外,MUN浓度与UUN浓度和排泄量密切相关(P<0.01),为MUN预测UN排泄量提供了理论依据。更多还原

【Abstract】 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.更多还原