Stock Sense with Paul McKee Ravensdown Technical Manager

Every year some five to 10 per cent of New Zealand’s dairy cows will fail to adequately maintain their calcium levels within the 24-48 hours after calving.

This crucial period can see them develop metabolic disease, known as ‘milk fever’, if not supplemented post-calving. So why does the cow calcium system fail? Simply put, the cow’s hormone system regulates their calcium status, which fails under the pressure of calving.

Cows obtain calcium from two sources, dietary calcium absorbed across the gut membranes and resorption or ‘mobilisation’ of calcium from body stores, such as bones. On top of the calcium required for normal metabolic function, calcium is required to develop the calf and produce milk.

Calf requirements

Normally, the cow copes with the five to eight grams of calcium the calf requires in late pregnancy. However, some cows struggle to meet the increased calcium demands for colostrum and milk production at calving, which is up to 50 grams of calcium a day.

The obvious solution to prevent milk fever is to supplement the cows with calcium in the period before calving. But it’s near impossible to get enough calcium into the cows through supplementation.

The cow needs to mobilise calcium from her bones pre-calving to reach the desired levels. This is best done by providing the cow with a low-calcium diet pre-calving. After calving the issues with calcium mobilisation no longer exist, and we can supplement the cow with considerable amounts of calcium to ensure she gets the calcium required for milk production and avoids milk fever.

Lime flour

The most routinely used method for post-calving calcium supplementation for cows is to give them a quality lime flour while they are in the colostrum mob. Lime flour can either be dusted onto pastures or included in supplementary feeds at the rate of 100-300 grams per cow a day. It contains 37-39 per cent elemental calcium and is ground to an ultra-fine powder for easy absorption.

With the low milk payout, farmers may be tempted to use a lower quality, coarser, agricultural lime as the calcium supplement for their cows. While both forms of lime may chemically be the same, there are two clear advantages in using Lime Four over agricultural lime.

Finer particle size – a smaller particle is more likely to be dissolved and the calcium absorbed while passing through the digestive system. Larger particles may not be sufficiently dissolved before passing out of the cow.

Calcium content – the lower the quality of calcium carbonate the less calcium the cow can absorb.

Using a quality lime flour post-calving will help reduce the number of cows with milk fever, and more importantly the effects milk fever has on cow health, reproduction and milk production. Combining the use of lime flour with a low calcium diet pre-calving means your cows will bounce back from calving a lot quicker.