1. Three adult dairy cows were fitted with cannulas in a mesenteric, portal, hepatic and jugular vein and a carotid artery. They received infusions of step-wise increasing amounts of ammonia as ammonium acetate via a mesenteric vein until NH, intoxication occui.red. Sodium acetate was used in control infusions. The maximum rate of uptake of NH, by the liver and the conccntrations of glucose, urea, lactate, acetate and bilirubin in blood were measured.2. During the infusions of ammonium acetate the liver extracted almost all the NH, qresent in the portal vein until an infusion rate of approximately 15.0mmol/min was reached. The maximum capacity of the liver to remove NH, during its first pass was on average 1.84mmol/min per kg wet weight. The cows became intoxicated when arterial plasma ammonia concentrations reached 0.8 mmol/l. Concentrations of NH, in jugular venous blood were between 66 and 74'7; of those in the carotid.In ruminants much of the protein and non-protein nitrogen in the diet is normally metabolized to ammonia 1)y rumenal micro-organisms. Some of this NH, is incorporated into microbial protein but a proportion of it (depending upon rumen pH) is absorbed through the rumen wall. If this NH, reaches the systemic circulation it is very toxic. In most physiological conditions it is converted in the liver to the less toxic urea thus keeping the NH, concentration in the systemic circulation low. However, under certain circumstances, for example when cattle are fed rations containing large amounts of either readily degradable protein or nou-protein nitrogen the amount of NH, which enters the portal circulation may exceed the rate at which the liver can metabolize it. The concentration of NH, in the systemic circclation will then increase until, when arterial NH, concentrations are approximately 1 .O mmol/l, the cow shows clinical signs of NH, intoxication (Davidovitch et al. !977).One factor which will affect the susceptibility of the dairy cow to NH, intoxication is, therefore, the capacity 01' its liver to metabolize NH,. When systemic NH, concentrations increase there is also an increase in blood concentrations of glucose, lactate, pyruvate and free fatty acids (Visek, 1972;Barej et al. 1974;Garwacki et al. 1979). The capacity of the bovine liver to metabolixe NH, has not been measured in vivo.This report describes an experiment in which NH,, as ammonium acetate (NH,Ac), was infused in increasing amounts into a mesenteric vein of three adult cows until they became intoxicated. The uptake of NH, by the liver was measured by determining the NH, concentration in portal and hepatic venous blood.

Weekly blood samples were collected from 42 Friesian cows between 5 weeks before and 10 weeks after calving, and analysed for serum cholesterol, albumin, total protein, glutamate-dehydrogenase, sorbitol-dehydrogenase and ornithine-carbamyl-transferase and blood glucose.Cholesterol concentrations increased 2^-fold during the first 8 weeks of lactation. There was also a transient decrease in glucose concentration at calving, but there was no relationship between conception rate and the concentrations of these two constituents.Albumin concentrations decreased at calving in some but not all cows and remained low for up to 2 weeks. Average concentrations of albumin determined by two analytical methods, a HABA dye-binding and a single radial diffusion assay method, were significantly lower (P < 0-05) between 0 and 2 weeks post-partum in eight cows which required four or more services than in 32 cows which conceived after fewer services. Similarly, the average decrease in concentration over calving was greater in those cows which conceived only after four or more services (P < 001).Globulin concentrations (total protein minus albumin) decreased during the 5 weeks before, and increased in the 3 weeks following calving. The net change over calving was significantly related to conception rate, and the albumin/globulin ratio was reduced (P < 0-001) in the eight cows requiring four or more services.Activities of all three enzymes doubled after calving, but there was no correlation between these increases and the decreases in albumin concentration, suggesting that, if liver insufficiency is a factor, it is probably due to malfunction rather than tissue damage. o r c l o s e t o calving, and have shown that concentrations may not return to pre-calving values until In a study of 351 dairy cows sampled between 2 or 3 months later. Little (1974) showed that the 40 and 100 days post-calving, Rowlands, Little & decrease does not occur in all cows, and concluded Kitchenham (1977) found a negative association that those affected synthesized less albumin. The between serum albumin concentration and both cows in which albumin concentrations decrease milk yield and the number of services required for least at calving, or recover most quickly, are likely conception.to be those with the highest concentrations in early Several authors (Hojovcova, 1966;Little, 1974; lactation. Manston et al. 1975;Rowlands et al. 1975; Szulc, The objectives of the study described Treacher et al. 1976) have noted decreases paper were to ascertain the degree to which indiin serum albumin concentration in dairy cows at vidual cows show such characteristic changes in • Present address: Nuffield Department of Clinical albumin concentration, to test the hypothesis Medicine, John Radcliffe Hospital, Oxford.(Rowlands, 1978) that cows which are better able

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