PHOSPHORUS DEFICIENCY IN THE SPOTLIGHT
When I started working with farmers 40 years ago, if you asked a farmer what fertilizer he used, he would invariably tell you how much Super he was applying. Back then fertilizer 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 fertilizer being applied. As the fertilizer 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 level. 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.
WHAT ARE THE PHOSPHORUS REQUIREMENTS FOR A HIGH PRODUCING DAIRY COW?
Saliva production 30 grams (this requirement triples if calcium is deficient)
Milk production 20 grams (based on 20 litres)
Foetal development 7 grams (requirement increases towards late gestation)
Other metabolic processes 2 grams
Total 59 grams
Roughly this is equivalent to 17 kilograms of grass consumed at a level of 0.35% P. While cows can cope with lower consumption levels at certain times, I would suggest the average requirement across the season is close to the above for most high performing herds.
When you take into account that each litre of milk produced requires at least 1 gram of phosphorus it is not surprising that we are seeing more problems in our cows that are bred to produce ever higher levels of milk production.
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?
These increased requirements are coming at a time when environmental pressures are forcing large reductions in phosphate fertilizer use. Average phosphorus levels in pasture are correspondingly falling. Where we once typically saw pasture levels of 0.35 – 0.55%, we are now recording 0.24 – 0.35% with fewer samples exceeding the 0.35% level. Phosphorus levels will also change in pastures depending on soil temperature, moisture and stage of growth. Unless body storage levels (bone) are maintained at adequate levels in the animal, deficiencies in blood phosphorus levels can lead to problems.
THE CALCIUM AND PHOSPHORUS CONNECTION
Phosphorus, like calcium, stores efficiently over time primarily in the bones. The body regulates the balance between available and stored levels of both of these critical elements through the release of hormones that tightly control absorption, excretion and storage.
The interactions between phosphorus and calcium are closely tied. This should come as no surprise when you consider that they are so closely linked within the matrix of bone. To a certain extent the body can pull on either element to cover a limited number of body processes when one or the other becomes low in the blood. Science has shown that a cow that is calcium deficient can use up to 3 times the normal level of phosphorus in early lactation. When this is considered, it is no wonder phosphorus is showing up so often as a serious deficiency in early lactation.
SHOULD PHOSPHORUS BE FED TO SPRINGER COWS?
Recent research from northern hemisphere herds indicates that feeding phosphorus at this time of the year can be potentially problematic, slightly increasing the risk of milk fever and subclinical calcium deficiency.
This research out of Europe seems to have impacted much of the advice given around phosphorus use here in New Zealand. However, the situation on the average NZ dairy farm differs markedly from what is the normal situation in Europe. It must be remembered that European farms typically feed much higher levels of phosphorus than those fed here. Often phosphorus has been fed to excess in order to achieve higher production and in the hope of achieving better conception rates. This has in turn led to environmental problems (particularly in Holland) and a focus on reducing levels.
Contrasting Europe with New Zealand, herds here barely receive the minimum daily phosphorus requirement even when being fed an all grass diet. Add high energy and phosphorus deficient crops into that equation and it is common to see a gradual phosphorus deficiency across much of the herd, particularly in the older cows.
In my opinion any potential issues around the overuse of phosphorus are dwarfed by the level of the phosphorus deficiency problem that is growing year on year here in New Zealand. The combined effects of low seasonal herbage levels, coupled with modern grass cultivars delivering higher soluble sugar content, and feeding of phosphorus deficient crops such as maize and fodder beet, is setting up phosphorus issues on a large proportion of our farms.
For further advice and guidance on mineral management contact your vet, or the team at Agvance Nutrition.
Chris Balemi. Agvance Nutrition
www.agvance.co.nz
