Field investigations into hypomagnesaemia in dairy cattle, with particular reference to changes in the concentration of blood constituents during the early grazing period

Abstract

1. In five experiments, groups of milking cows were changed abruptly from winter-stall feeding to graze a number of different swards, and changes in the concentration of blood-serum magnesium and of other blood constituents have been studied.2. Wide variations in the severity of hypomagnesaemia in individual cattle were found in all experimental groups, due possibly to such factors as individual variations in the intake of herbage, the requirement and body reserves of magnesium and the capacity to absorb magnesium from the gut.3. The degree of hypomagnesaemia observed was independent of the level of milk production, but was generally less severe in Friesian than in Shorthorn and Guernsey cattle. The onset of hypomagnesaemia was delayed for a few days in cattle with a high serum-magnesium concentration at the commencement of grazing.4. In two out of three experiments in which the effect of fertilizer treatment was studied, the incidence and severity of hypomagnesaemia was increased by the application to the sward of heavy dressings of nitrogenous fertilizer. In the third experiment, severe hypomagnesaemia occurred on a plot which had received no nitrogen fertilizer, due, it is thought, to a restricted intake of herbage magnesium, since the sward was extremely sparse. The feeding of supplements of flaked maize, crushed barley, crushed dredge corn, molassed sugar-beet pulp or a concentrate mixture balanced for milk production, to grazing cattle did not reduce the incidence of hypomagnesaemia.5. The cutting of grass and feeding it in the stall did not prevent the development of hypomagnesaemia.6. Blood pH and the concentrations of bloodserum calcium, sodium, potassium and blood glucose in cattle were unaffected by a change from winter feed to spring grazing, but a marked change in blood-serum non-protein and urea nitrogen and blood ammonia nitrogen, but not in any other nitrogenous constituents of the serum, was observed. The concentrations of serum non-protein and urea nitrogen and blood ammonia nitrogen were highest in groups of cattle grazing swards which had received a heavy dressing of nitrogen fertilizer and had a high nitrogen content.7. In the two experiments in which the severity of hypomagnesaemia was increased by the application of nitrogenous fertilizer, there was a close group association between high concentrations of blood serum non-protein and urea nitrogen and blood ammonia and low concentrations of blood-serum magnesium. High levels of serum urea and blood ammonia during grazing are thought to reflect a high ammonia production in the gut, which may be responsible for the disturbance in magnesium metabolism which produces hypomagnesaemia.

Chemical composition of pastures in relation to grass tetany in beef breeding cows

“…) is assumed to continue, it can be seen by extrapolation of Kemp's data that the average availability of Mg in high-protein New Zealand spring pastures may be very low indeed. The variations in the apparent availability of magnesium between animals, and for the same animal at different times, have been remarked upon by all workers who have attempted such measurements (e.g., Bartlett et al 1957;Kemp et al, 1961;Hutton et at. 1965), which suggests an explanation of the variations in susceptibility to hypomagnesaemia and tetany observed in the field.…”

“…Similar effects on the serum-Mg level could be produced by the introduction of ammonium salts into the rumen. Since then several workers have confirmed the rapid build up in ruminal ammonia that occurs when highly digestible, highly nitrogenous feeds are rapidly consumed (Johns 1955b;Bartlett et al 1957;Head and Rook 1957;Hogan 1964).…”

“…It may be pertinent here to recall that Bartlett et al (1957) considered that serum-Mg values do not necessarily provide a good index of the magnesium metabolism of the animal; they suggested, in particular. that a state of apparentlv mild hypornagnesaemia (15-2.0 mg per 100 ml) might represent a condition of acute shortage of magnesium to the animal.…”

“…There it is found that serum-Mg levels fall suddenly, and very low values are almost invariably associated with the onset of tetany (Kemp 1960). This lowered blood Mg is preceded by a fall in the urinary excretion, indicating a deficiency to the animal (Head and Rook 1957;Kemp, Deiis, Hemkes. and van Es 1961).…”

“…High potassium in the herbage is perhaps the more frequently implicated, although Bartlett et al, (1957) reported severe hypomagnesaemia induced by heavy dressings of ammonium sulphate. This condition was associated with high concentrations of non-protein and urea nitrogen in the blood serum, and high levels of blood ammonia.…”