The triad of metabolic diseases commonly referred to as 'milk fever', 'grass staggers' and 'ketosis' are essentially a perinatal imbalance, or lack of calcium, magnesium and glucose respectively in the blood of cows and sheep.
Some of the key points to minimise occurrence of these diseases in the spring months is often lost when the nuances of recent scientific research are published and new nutritional products and/or therapies are launched. We also acknowledge a huge divergence in types of farm management systems run and the nutrition offered. In these ruminants, the incidence of clinical and subclinical disease in a population of cows can be considerably high. A recent study of NZ Dairy herds showed there was a high prevalence of subclinical milk fever in the pasture-fed, spring-calving dairy herds sampled of around 52 per cent and a large between-herd variation. A similar survey conducted previously showed ketosis had a prevalence of upwards of 17 per cent seven to 10 days post calving.
What do we know for our pasture feed herds or flocks? The role of magnesium is pivotal in achieving a target Body Condition Score (BCS) and adequate energy intakes prior to and post birthing.
BCS is determined by good planning in the autumn — the extended dry period has created challenges in the Western Bay. Largely it is now predetermined. The supply of energy in the form of dry matter does determine the wholesale supply of magnesium. Restrict dry matter, you restrict magnesium also — a double hit. Magnesium not only prevents grass staggers, it assists in the hormonal regulation of calcium — reducing milk fever almost five-fold. As a minimum, magnesium for cows should be supplied at least two weeks pre-calving. Calcium supply is only of benefit in the immediate days following birth.
The balance of feeding to requirements pre-natally is the key to ensure adequate feed supplies are available to the two to three-fold increase in nutrient requirements post-natally.