Potassium, Downer Cows and Fertility: Getting Calving Season Right: Dr Gordon Rajendram

Calving is now in full swing across New Zealand, and this is the time of year I want to bring some attention to a risk that is largely preventable: the downer cow. Every season, farms across the country deal with cows going down around calving, and while there are several contributing factors, one of the most controllable is often overlooked. It comes down to how nitrogen fertiliser has been managed on the paddocks those cows are grazing in the weeks around calving.

Pasture that is naturally high in potassium, or that has recently received nitrogen, suppresses magnesium absorption in the rumen. That sets up the classic pre-calving trap: low magnesium, or hypomagnesaemia, often develops alongside low blood calcium, or hypocalcaemia. Together, these are among the leading causes of downer cows at this time of year.

A cow’s requirement of potassium is 0.6-1.2%. Generally, dairy pasture contains around 3% and higher so there is excess potassium in their diet.

As the chart shows, subclinical hypomagnesaemia affected around 22 percent of cows at calving in one study of grazing herds, and that figure nearly doubled to almost 49 percent by seven days postpartum, exactly the window when demand on the cow is highest and her reserves are most stretched. Older cows and those in their third or later lactation are particularly vulnerable, since their ability to mobilise skeletal reserves is already declining

There is a second, less visible risk running alongside this, and it plays out later in the season. When nitrogen intake is high relative to dietary energy, blood urea nitrogen climbs. This has a real cost when mating starts. In a controlled feeding trial, first-service conception rate dropped from 48 percent on a normal-protein diet to just 31 percent on a high-protein diet, a seventeen-point swing driven largely by elevated blood urea altering the uterine environment and interfering with early embryo survival. For cows already under metabolic strain around calving, that extra nitrogen load lands at exactly the wrong moment, and the consequences don't show up until the in-calf rate comes back weeks later.

My advice for the next few weeks is straightforward. Hold off on heavy nitrogen applications to paddocks that will be grazed by close-up and freshly calved cows. Keep a close eye on the potassium, calcium and magnesium balance in the diet, and where nitrogen has recently gone on, give the pasture time for potassium levels to settle before grazing.

Proactive magnesium, and calcium supplementation through the risk window makes a measurable difference, and costs far less than treating a down cow.

Article by Dr Gordon Rajendram.