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.

Long-term undernutritional stress is often a feature of sheep and beef cattle production, but has only become a major feature of dairy cattle husbandry in the United Kingdom in recent winters when food was short and expensive. An experiment was carried out to study the effects of long-term underfeeding during pregnancy and early lactation on some blood constituents, milk yield and composition and body weight of dairy cattle. Two groups of cattle were fed at 60 and 40 % of the estimated requirements for maintenance and pregnancy or lactation for 13 weeks before and 13 weeks after calving, and one group was fed at the maintenance level only for the same period. A control group was fed at 100 % of estimated requirements for this period. All groups were subsequently fed at the control level for a further 24 weeks.The experiment showed that cows undergoing long-term nutritional deprivation were able to maintain concentrations of blood constituents within narrow limits; the concentrations of such constituents as glucose or non-esterifled fatty acid did not reflect energy deficit or surplus. The animals remained clinically healthy during the underfeeding and recovery periods. The results suggest that debility occurring under field conditions in association with reduced food supply may be due to a multiplicity of factors or to severe imbalance of specific nutrients, rather than to energy or protein deficit alone.There was a difference in efficiency of utilization of energy of 19 % between cows in the most severely underfed groups which maintained lactation and those which were not able to maintain lactation. There was evidence that this difference in efficiency was detectable within a few weeks of the start of the period of reduced nutrition. Animals which were less affected in the early stages of food deprivation were also those which maintained the advantage through the deprivation and recovery periods.

1. Six adult Friesian cows were given rsSe as either 7sSe0,'-or rsSe04*-intravenously. Five of the cows had cannulas in an hepatic vein, the portal vein and one carotid artery to enable the uptake of %e by the liver to be measured. Radioactive balance studies were carried out on two of the cows given rrSeOa*-and two given 76se0,a-. A seventh cow was given an oral dose of Tk-labelled barley and the excretion of 7sSe in faeces, urine and milk was measured for 14 d.2. After the injection of 7rSe0,'-plasma 76Seconcentration decreased during the first 30 min with a mean half-life (ts of 15.6 min. From 30 to 60 min after dosing the concentration of radioactivity increased to reach approximately 50% of the level present 2 min after dosing. Following the injection of 7sSe0,a-the rsSe was cleared with a mean ti of 28.5 min during the first 30 min and plasma radioactivity increased only slightly during the next 30 min.concentration was approximately 5% lower than portal venous 7sSe concentration. Duringthe period when plasma %e activity was increasing the activity in hepatic venous plasma was 3% greater than portal activity. Ofthe rsSe cleared from plasma after injecting Ve0,'-40% was calculated to be removed by the liver.4. After intravenous dosing with 76Se0,a-or 7sSeOt-approximately 9.5 and 17.0% respectively of the dose injected was excreted in faeces and 10% in urine within 14 d. Almost three times as much 7rSe was excreted in urine and 3.5 times as much in faeces during the 6rst 24 h after dosing with 7sSe04*-as aRer 76Se0,1-.