Effects of feeding magnesium sulfate to dry pregnant dairy cows with different body condition scores on intake in late gestation, periparturient blood calcium concentrations and production in early lactation

Abstract

An experiment was conducted to investigate the interaction between the responses of cows fed magnesium sulfate during late gestation and body condition score (BCS). Cows were fed a total mixed ration before calving and a pasture-based diet in early lactation. Seventy-two cows were fed differentially to achieve target BCS, on an 8-point scale, of about 4, 5 and 6 units, 1 month before calving. Cows were individually fed a total mixed ration of maize silage, crushed barley, canola meal and barley straw at about 1.6 kg dry matter (DM)/100 kg liveweight during the 27 days, on average, before parturition. In each BCS treatment, half the cows were fed 105–155 g magnesium sulfate (depending on liveweight) in the last 3 weeks while the remaining cows were provided with 18–27 g magnesium oxide, maintaining similar levels of magnesium in both diets.

The dietary cation–anion difference of the control and magnesium sulfate diets averaged +5.1 and –5.5 cmol(+)/kg, respectively, while urine pH values were 6.4 and 5.4, respectively. The cows fed the control diet, on average, calved in better condition (P<0.05) than those fed magnesium sulfate (BCS at calving were 5.1 and 4.7 units for the control and the magnesium sulfate treatments, respectively). The feed intake of cows that did not receive magnesium sulfate was maintained until the day before calving whereas the feed intake of cows fed magnesium sulfate declined sharply from about a week before calving, and was always lower (P<0.05) than the control group. During the 4 weeks before parturition, the cows in the medium BCS group generally consumed more DM than did those in the other 2 BCS treatments (8.9 v. 7.4 and 8.2 kg DM/cow.day; P<0.05). During the 2 days before calving, all cows had similar intakes (7.7 kg DM/cow.day; P>0.05). The control diet resulted in lower concentrations of total blood serum calcium but only immediately after calving (1.90 v. 2.02 mmol/L; P<0.05). The production data indicated that magnesium sulfate had no effect (P>0.05) on either milk yield or milk composition even though the use of magnesium sulfate reduced the incidence of subclinical hypocalcaemia from 64 to 46%. However, there was a positive relationship between BCS at calving and milk fat concentration (P<0.05). There were no significant (P>0.05) interactions between dietary additives and the BCS of the cows. In conclusion, the use of magnesium sulfate resulted in the relative maintenance of blood calcium concentrations, reducing the incidence of subclinical hypocalcaemia at calving, while having a negative effect on intake before calving. However, these effects were not translated into benefits in animal production.