Effect of conserving annual pastures for hay on serum magnesium levels of lactating beef cows in a Mediterranean environment

Abstract

Mature Friesian crossbred cows calving in April-May either continuously grazed annual pasture and received a small hay supplement in May and June, solely to prevent starvation ketosis, or had the pasture from one-third of their area conserved as hay in spring and fed back ad libitum in April, May and June. In May, deaths of cows from hypomagnesaemia occurred on the continuous grazing treatment but not on the fodder conservation treatment. Liveweight and serum magnesium levels of cows and the availability, botanical and mineral composition of the pasture on offer were therefore monitored during June, July and August. The cows on the fodder conservation treatment had higher serum magnesium levels through June, July and August. One cow on the continuous grazing treatment died of hypomagnesaemia in July, but none died on the fodder conservation treatment. After a supplement of 30 g MgSO4.7H2O and 4 kg of hay/cow.day was provided for 3 weeks in July, serum magnesium levels increased on both treatments and no further clinical cases of hypomagnesaemia occurred. Serum magnesium levels continued to increase on the continuous grazing treatment, and remained at a satisfactory level on the fodder conservation treatment and through August on pasture alone. Cation ratios (K:[Mg + Ca]) of clovers, weeds and hay at all samplings were less than 2.2, but grass sampled in July and August had cation ratios of 2.59 and 2.95, respectively. There was always a higher percentage of grass on the continuous grazing treatment. It was concluded that conservation of fodder as hay increased the serum magnesium levels of beef cows suckling calves in early winter by providing more energy and more magnesium during the period of pasture shortage in autumn-winter. Fodder conservation also appeared to contribute to increased serum magnesium levels in winter by reducing the proportion of grass in the sward.