Phosphorus deficit in the spotlight

When I started working with farmers 40 years ago, if you asked a farmer what fertiliser he used, he would invariably tell you how much super he was applying. 

Back then, fertiliser was either straight super or potassic super. Most farmers could readily rattle off the units of P being applied per acre. It was common to see soil ‘Olsen P’ levels in excess of 60 on loam soils and as high as 110 on some pumice soils. Pasture commonly came back at levels in excess of 0.55% P on a dry matter basis. 

At the time, if you had the courage to suggest to a farmer that he consider cutting back the phosphorus he used, he would give you a very nervous look.

Jump forward to the present day, farmers haven’t just been weaned off their P addiction, many have gone cold turkey. Because of environmental constraints around feed brought onto the farm, many farms see little or no P fertiliser being applied. As the fertiliser budget is reduced, this situation has seriously decreased plant P levels. Adding to the issue is that NZ cows used to be fed mainly grass. They are now fed a variety of feeds, many being P deficient yet supplying high soluble sugar or starch levels. Where it used to be rare to see a case of phosphorus deficiency it is now becoming a common problem.

A developing concern

There is hardly a day goes by when we don’t hear from a farmer describing what they think are normal calcium deficient downer cows. When questioned, the symptoms often don’t quite fit – these cows don’t always respond so well to normal treatment. Sometimes a bottle of calcium in the vein will get them up, only for many to go back down again later. Often these cows will only respond to a calcium phosphorus combination into the vein, or if given calcium borogluconate, can tend to go down again.

Phosphorus deficient cows are commonly called crawler cows, for good reason. Their symptoms are different to those of the classic sleepy milk fever cow. Crawler cows are bright in the eye, they will fight to get back on their feet, they simply lack the hind end coordination that allows them to stand up and stay up, hence the tendency to crawl.

Why is phosphorus so important?

Phosphorus is key to carbohydrate metabolism - it controls insulin release and excretion.

It forms ATP, which is the primary energy source for body cells. 

Phosphorous is important in saliva production.

Along with calcium, phosphorus forms the matrix of bones.

Phosphorus is essential to conception and required at high levels for foetal growth.

Phosphorus can be identified as a key element in over 400 metabolic processes.

While this list is a very simple description of just a few of the processes, you can see why this element is so key to health, production and reproduction in all living creatures.

Phosphorus demand also increases with higher levels of carbohydrates and soluble sugar consumption, as these energy sources require higher levels of phosphorus to utilise the energy. Given we are feeding grasses that supply higher levels of soluble sugar, as well as crops with the same attributes, is it any wonder that phosphorus requirements are increasing?

• Chris Balemi is managing director of Agvance Nutrition