with Phil Rennie
During the dry period a cow’s metabolism gradually builds and then increases quickly once lactation starts. This is most notably seen with the demand for two key ‘macro’ minerals – calcium (Ca) and Magnesium (Mg).
Calcium is an essential component of the skeleton and serves a role in muscle contractions, blood coagulation, enzyme activity and hormonal secretion. Demands rise rapidly when lactation starts and can result in milk fever cases as well as subclinical problems. Subclinical hypocalcaemia, or low calcium, has been linked to ‘sad cow syndrome’, retained membranes and infertility.
Magnesium is required for production of hormones that are important for the absorption of calcium from the gut and the mobilisation of calcium from bone. Low magnesium levels can suppress a cow’s appetite as well as cause irritability in the herd and reduce milk let-down.
Fortunately, magnesium supplementation prevents grass staggers and helps the cow to mobilise her calcium stores to prevent milk fever. Cows don’t store magnesium therefore daily dosing is required. This means during a patch of adverse weather and the cows miss out on their daily dose, you can expect some clinical cases. Lush, fast-growing spring pasture is often very low in magnesium. Calcium and magnesium demands are exceptionally high in the weeks surrounding calving for calf growth and lactation. High-producing, older cows are often most susceptible to deficiency. It is best to avoid supplementing calcium in the three weeks before calving as the cow needs to prime her body to mobilise her own calcium stores. So, additional lime flour should be stopped.
Magnesium supplementation should begin about one month prior to the start of calving and continue until peak milk is achieved, which is normally up to four-six weeks after the last cow has calved.
Supplementation can be achieved in a variety of ways. Dusting pasture with magnesium oxide at a rate of around 100g per cow per day. Or drenching with magnesium oxide at a rate of 30-40g per cow per day.
Maize silage is naturally very low in magnesium, calcium and salt, so needs to be balanced by adding these elements. If cows are eating a lot of supplement, then it is possible to add magnesium to this feed at a rate of 60g per cow per day. Note the 60g is additional to the base magnesium that is added to the maize silage to help balance the ration value.
Magnesium sulphate or chloride may also be added to the water supply, but care must be taken not to make the water unpalatable, as magnesium tastes bitter. Flavour enhancers may assist.
Slow-release magnesium capsules are also available, but these alone will not provide enough. They may be indicated for cows at grazing where dusting and water supplementation are impractical.
The options above should allow for typically no more than two-three per cent of your cows succumbing to metabolic disease. If there’s more than that, talk to your local vet for specific management around calving to prevent problems getting out of hand.