Determination of milk urea by flow injection analysis

Performance evaluation of an enzymatic spectrophotometric method for milk urea nitrogen

Our objective was to determine the within and between laboratory performance of an enzymatic spectrophotometric method for milk urea nitrogen (MUN) determination. This method first uses urease to hydrolyze urea into ammonia and carbon dioxide. Next, ammonia (as ammonium ions) reacts with 2-oxoglutarate, in the presence of reduced nicotinamide-adenine dinucleotide phosphate (NADPH) and the enzyme glutamate dehydrogenase (GlDH), to form l-glutamic acid, water, and NADP⁺. The change in light absorption at 340 nm due to the conversion of NADPH to NADP+ is stoichiometrically a function of the MUN content of a milk sample. The relative within (RSD_r) and between (RSD_R) laboratory method performance values for the MUN enzymatic spectrophotometric method were 0.57% and 0.85%, respectively, when testing individual farm milks. The spectrophotometric MUN method demonstrated better within and between laboratory performance than the International Dairy Federation differential pH MUN method with a much lower RSD_r (0.57 vs. 1.40%) and RSD_R (0.85 vs. 4.64%). The spectrophotometric MUN method also had similar method performance statistics as other AOAC International official validated chemical methods for primary milk component determinations, with the average of all RSD_r and RSD_R values being <1%. An official collaborative study of the enzymatic spectrophotometric MUN method is needed to achieve International Dairy Federation, AOAC International, and International Organization for Standardization official method status.

Evaluation of flow injection analysis for determination of urea in sheep's and cow's milk

Difficulties in measuring the urea content in sheep's milk often occur with spectral photometry due to the high protein and fat concentrations of the milk. In this study an enzymatic flow procedure (QuickChem 8000 Ion Analyser, Lachat Instruments, Milwaukee, USA) to determine the urea content in ovine and bovine milk was evaluated. Urea content is determined by the Berthelot reaction after splitting it enzymatically with urease. The free ammonia diffuses through a teflon membrane into a stream of reagent solutions. Detection takes place by means of a reaction between the ammonium ions with hypochlorite and salicylate producing a green colour, which is measured spectrometrically in a flow meter at 660 nm. By using a diffusion cell chemical deproteinisation of milk is not necessary and capacity is high. The assessed procedure exhibited high accuracy and precision and reached a sample capacity of 55 samples an hour. Storage of the milk samples for several days as well as chemical preservation with bronopol had no effect on the measurement procedure. Due to the complexity of the apparatus and the costs associated therewith, the device proves less suitable for routine diagnostics but rather serves as a reference method for the measurement of urea concentration in milk.

Effects of management, feeding, and treatment on clinical and biochemical variables in cattle with displaced abomasum

OBJECTIVE

To determine effects of management, feeding, and treatment on clinical and biochemical variables in cows with displaced abomasum (DA).

ANIMALS

374 cows that received 470 treatments for DA.

PROCEDURE

Blood and milk samples were obtained from 139 affected cows for analysis; for all cows, clinical data, management, feeding, and treatments were evaluated.

RESULTS

Multiparous cows were more predisposed to DA than primiparous cows were, and Swedish Friesians were more predisposed than Swedish Red and Whites were. Eighty percent of cows had left-sided DA, and 20% had right-sided DA. In > 50% of affected cows, clinical signs appeared just before calving to 2 weeks after calving. Incidence of twin calves and periparturient diseases was significantly higher in affected cows than in the overall Swedish cow population. Content of neutral detergent fiber in the silage was low in herds with DA. Feeding a total mixed ration was a risk factor for DA. Treatment by surgical methods gave a significantly higher recovery rate than nonsurgical methods.

CONCLUSIONS AND CLINICAL RELEVANCE

Displaced abomasum is a periparturient nutritional disease. Feeding roughage with low neutral detergent content is a more important causative factor than the amount of concentrates fed at the time of calving. The basic principle for prevention of DA is to maintain good ruminal filling before and at calving. The amount of high-quality roughage fed before and at calving should be kept to a minimum. By changing routines for periparturient feeding, it should be possible to reduce the incidence of DA.

Evaluation of bulk milk analyses of acetone, urea, and the fat-lactose-quotient as diagnostic aids in preventive veterinary medicine

The purpose of this study was to evaluate the use of additional acetone and urea analyses to the established milk quality system (protein, fat, lactose, and somatic cell count) in bulk milk samples twice a month. Samples were obtained from 44/93 herds in a lowland/highland area. When ranging herds with most acetone values over 0.24mmol/1 (90th percentile), only two out of 10 herds would be on a corresponding list when ranged for highest ketosis treatment incidence. Most high acetone values occur after periods with concentrated calvings, but also in periods without calvings, presumably because of feeding with inferior silage high in butyric acid. Milk fat concentration, milk lactose concentration and the interaction term between these, the fat-lactose-quotient, show the largest absolute values in all three models with ketosis treatment incidence, milk protein concentration, and milk yield as dependent variables in general linear analyses on standardized variables. Acetone and urea concentrations in bulk milk show only small absolute values or are omitted from the models. The practical conclusion from these analyses on bulk milk samples to be used by advisory personnel is small, but it seems that high bulk milk fat concentrations and low lactose concentrations are unfavourable when it comes to ketosis treatment incidence, but favourable when it comes to milk protein concentration. Bulk milk acetone concentrations as a measure of the energy supply in herds must only be interpreted after calving intensity and silage quality are assessed.

Evaluation of additional acetone and urea analyses, and of the fat-lactose-quotient in cow milk samples in the herd recording system in Norway

The purpose of this study was to evaluate the use of additional acetone and urea analyses to the established herd recording system (protein, fat, lactose, and somatic cell count) in cow milk samples every second month. Samples were obtained from 43/82 herds (800/1368 cows) in a lowland/highland area. Acetone concentration (mmol/l) were classified into acetone classes 1 (<0.7); 2 (0.7-1.4); and 3 (>1.4). There was an association between increasing acetone class and increasing ketosis incidence in herd (Kendall's Tau-b = 0.13). The highest percentage frequencies of acetone classes 2 and 3 were in weeks 4-6 postpartum. Milk yield, urea, and lactose were significantly lower; milk protein not different, and milk fat was significantly higher in herds with high ketosis incidence and in cows in acetone classes 2 and 3. General linear models, obtained by backward elimination of non-significant variables, showed that milk protein was negatively associated to milk yield and positively associated to milk fat. Milk yield decreased, and milk fat concentration increased with increasing acetone level. Milk fat and milk lactose were negatively associated, and a fat-lactose-quotient might be promising in a model to rule out subclinical ketosis cows.

Variations with breed, age, season, yield, stage of lactation and herd in the concentration of urea in bulk milk and individual cow's milk

The concentration of urea in the milk of 510 dairy cows in 10 herds was determined at regular intervals for a year. The herds contained approximately equal numbers of Swedish Red and White, and Swedish Holstein cows. The mean +/- sd concentration in the samples from individual cows was 5.32 +/- 1.13 mmol/l, and the mean concentration in bulk milk was 5.39 +/- 0.96 mmol/l. These values indicated that on average the herds were fed too much protein relative to their intake of energy throughout the year. Herd factors had a strong influence on the milk urea concentration. The concentration was lower during the first month of lactation than later in the lactation, and lower when the cows were housed during the winter than when they were grazing. There was a weak positive relationship between the daily milk yield and urea concentration, particularly during late lactation, but there was no relationship with either breed or age. Bulk milk urea was a reliable guide to the average urea concentration of a herd.

The influence of the dietary balance between energy and protein on milk urea concentration. Experimental trials assessed by two different protein evaluation systems

Twenty-three dairy cows were fed rations with different proportions of energy and digestible crude protein (DCP). When the ration was balanced for energy and DCP according to Swedish standard the cows' milk urea concentration was 4.66-4.92 mmol/l (95% CI of mean). With increasing intakes of DCP, fed together with standard levels of energy, the mean milk urea concentration increased in proportion to the surplus of DCP. In contrast, the concentration of urea decreased when the cows were overfed with energy at the same time as they were underfed with protein. When the rations were recalculated in accordance with the AAT/PBV system for dietary protein evaluation the 95% CI for the mean milk urea concentration of the cows receiving a balanced ration was 3.76-4.56 mmol/l. The concentration of urea was dependent primarily on the PBV. When the 2 protein evaluation systems were compared there was a strong correlation between PBV and DCP. Ammonia was the only constituent of the rumen whose concentration was strongly correlated with the milk urea concentration. Taken together with earlier data the present results suggest that a milk urea concentration between 4.0 and 5.5 mmol/l should be regarded as normal at least when cows are fed conventional feedstuffs.

The relationships between seasonal variations in the concentration of urea in bulk milk and the production and fertility of dairy herds

The concentration of urea was measured in bulk milk samples taken on seven occasions during a period of 13 months from 1,658-1,815 of the 2,100 suppliers of milk to a dairy. The mean concentration of urea was significantly higher when the cows were grazing than when they were housed, but there were no significant variations within either of these periods. In herds with very low milk urea concentrations while they were housed (< 2.4 mmol/l) the mean increase during the grazing season was approximately 2.0 mmol/l, whereas in herds with very high urea concentrations while they were housed (> 6.0 mmol/l) there was a slight decrease in urea concentration during the grazing season. The annual milk yield of herds with low urea concentrations was significantly lower than the yield of herds with intermediate or high concentrations. The interval between calving and first insemination was significantly longer in herds with low milk urea concentrations, but these herds also had a higher 56-day non-return rate.

The effect of additional feeding on the fertility of high-yielding dairy cows

One hundred and ninety three pairs of cows were selected from 38 herds, and the numbers of each pair were assigned at random to experimental and control groups. Each control cow was fed according to its yield with the feedstuffs normally used by the herd. Each experimental cow was fed in the same way and, in addition, received 500 g of a feed additive twice daily from the 40th day of lactation until first insemination or to the 75th day of lactation if no visible oestrus had been observed between days 40 and 75. One kg of the feed additive contained glucogenic substances (glycerol and Ca-propionate) equivalent to 1.90 moles of glucose, 13.6 MJ metabolizable energy, and 180 g digestible crude protein. There were no significant differences between the groups with respect to their mean daily milk yield, their plasma glucose concentrations at first insemination, or the interval between calving and first insemination. However, the experimental cows had a significantly shorter interval between calving and last insemination (mean difference 11.0 days). The pregnancy rate was significantly higher among cows with high than among cows with low plasma glucose concentrations. There was also a tendency towards a reduced pregnancy rate among cows with either high (greater than 7.0 mmol/litre) or low (less than 4.0 mmol/litre) milk urea concentrations.

The relationships between the fertility of dairy cows and clinical and biochemical measurements, with special reference to plasma glucose and milk acetone

Blood and milk samples were taken at first insemination in 352 dairy cows from 18 herds for charting the relation between clinico-chemical parameters and fertility rate. Neither total protein, albumin, globulin, AST, bilirubin, bile acids, calcium, magnesium and phosphorus in blood nor urea in milk was significantly related to the rate of pregnancy. On the other hand, there was a significant difference for plasma glucose between cows that became pregnant and those that did not. Cows with low plasma glucose concentrations at first insemination also had low values four and seven weeks after calving, indicating that it is primarily cows with chronically low blood glucose which are likely to have reduced fertility. At first insemination there was no increase in the concentration of acetone in milk of cows with reduced fertility. However, they had had increased milk acetone concentrations three to five weeks after calving. It therefore seems possible to evaluate the risk of reduced fertility by measuring either blood glucose or milk acetone some weeks after calving. Owing to the difficulties associated with the sampling and laboratory techniques for glucose, analyses of milk acetone are more suitable under field conditions.