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MILK UREA CONCENTRATIONS IN DAIRY CATTLE: There has been increased use of milk urea concentration as an indicator of dietary protein intake and protein metabolism in dairy cattle over recent years. The value of milk urea content data in predicting dietary composition, particularly for pasture-fed cattle, has not been well described. PROTEIN METABOLISM AND UREA SYNTHESIS: Many factors influence the degradation of dietary proteins in the rumen, post-ruminal protein metabolism and urea synthesis in cattle. Strong positive correlations between nitrogenous fertiliser use and the crude protein content of pastures were identified by use of meta-analysis. Similar strong positive correlations were noted between dietary protein intake, rumen ammonia and plasma urea concentrations. The costs of urea synthesis include energy losses, and importantly, the loss of endogenous amino acids, which are deaminated in the synthesis of urea. MILK UREA AS AN INDICATOR OF PROTEIN METABOLISM: Urea concentrations in blood, plasma and milk are strongly correlated. Milk is an adequate indicator of blood and plasma urea content, but non-nutritional factors may significantly influence milk urea concentrations. Recommendations for dietary protein management based on milk urea concentrations must be undertaken with care.

The possibility of using hair to help diagnose hypo-cuprosis was investigated. Diets containing different levels of molybdenum and sulphur were fed to nine Hereford steers. Liver reserves of copper were depleted rapidly when dietary levels of molybdenum and sulphur exceeded 2.7 mg/kg and 3.8 g/kg respectively. Following the onset of diarrhoea, liver reserves of copper were repleted by injecting copper glycinate. Samples of liver, blood and hair were collected at regular intervals and analysed for copper. Progressive changes in liver levels of copper were closely paralleled by changes in copper levels in hair and plasma. Relationships between copper levels in liver and plasma, and liver and hair were both shown to be asymptotic. Plasma and hair levels were sensitive to changes in liver reserves below about 20 μg/g, but changed little above this level. Sampling variation was shown to be less with hair than with plasma. It was concluded that copper levels in hair are a useful diagnostic aid in detecting hypocuprosis. Hair provides an integrated record of copper availability during its period of growth, and thus may be preferred to blood plasma which represents only a spot sample in time.

The paper describes a package of computer programs known as ‘CAMDAIRY’, which is written for CP/M80 and MS-DOS operating systems. The core program is a bio-mathematical model of a lactating cow, which incorporates functions to predict nutrient requirements, feed intake, substitution effects when feeding concentrates, tissue mobilisation and partition of nutrient utilisation between milk production and growth. Nutrient partitioning is described by a series of asymptotic curves relating energy intake to milk production, such that energy requirements per litre increase progressively with level of milk production. This model is incorporated into an econometric model, ‘Maximum Profit’, which uses linear programming procedures to formulate rations for up to two groups of cows in a herd in a way which maximises income above feed costs, whilst meeting nutrient requirements and satisfying constraints on feed supply and milk production requirements. Other programs in the CAMDAIRY package are ‘Least Cost’, a program which calculates a leastcost ration using fixed energy requirements for milk production, and ‘Analysis’, a program which predicts likely milk production given characteristics of the cows, feed intake and feed composition.