Phosphorus nutrition of cattle in northern Australia

Désirée Jackson
Formally Queensland Government

Approximately 70% of soils in northern Australia are phosphorus-deficient. Cattle grazing phosphorus-deficient pastures need additional phosphorus, particularly during the wet season when energy and protein levels are at their peak. Less phosphorus is required in the dry season because protein and energy are more limiting in the diet.

Unfortunately, few producers in northern Australia feed phosphorus supplements during the wet season, citing reasons such as: their cattle are gaining weight already, the difficulty in supplementing cattle spread across paddocks and cattle are unlikely to eat phosphorus supplements during the wet.

The consequence of not supplementing cattle with phosphorus in phosphorus-deficient areas is a lost opportunity for significant productivity gains. Many cattle properties in northern Australia are breeder-based, selling off progeny when they are quite young. Therefore, maximising breeder efficiency is critical, through improving weaning rates and turning off heavier weaners at a younger age. Using wet season phosphorus supplementation is an essential part of an effective breeder management program in achieving these aims.

How is phosphorus used by the animal?

Phosphorus is involved in almost every metabolic reaction in the body, including:

  • formation of bones and teeth, and a structural component of skeletal tissues
  • absorption of carbohydrates such as glucose through intestinal tissues
  • transport of fatty acids throughout the body
  • metabolism of energy
  • facilitation of fat, carbohydrate and protein utilization
  • improving efficiency of feed utilization
  • energy transfer reactions
  • normal milk secretion
  • buffering of body fluids.

Even rumen bugs need phosphorus

The rumen microbes require nutrients (energy, protein and minerals) to digest pasture, particularly the fibre fraction. A deficiency of phosphorus in the diet suppresses appetite, resulting in a net decrease in feed intake.

Consequences of a phosphorus deficiency

Some of the more obvious signs of phosphorus deficiency are easily recognised, such as:

  • stiffened gait or peg leg
  • bone chewing (increasing the risk of botulism)
  • depraved appetite (pica) which includes chewing of rocks, dirt, wood, bones or hair (pica will also occur if an animal is sodium or potassium-deficient)
  • increased mortality.

The less obvious signs of phosphorus deficiency that have serious productivity and economic consequences are:

  • reduced feed intake
  • reduced milk production
  • bone fragility leading to bone breakages
  • reduced fertility.

If animals don’t have a high reserve of phosphorus in their bones and they are phosphorus-deficient, there will be a rapid reduction in feed intake. This can occur as early as three weeks after they become phosphorus-deficient.

Why feed phosphorus in the wet season?

During the wet season when feed is green and growing, energy and protein levels are at their peak.  Generally, phosphorus is higher in green feed, but is much lower relative to energy and protein in phosphorus-deficient or marginally-deficient country. Low levels of phosphorus restrict the animal’s ability to utilise protein and energy in the diet. If sufficient phosphorus is not provided during the wet season, microbial crude protein production will decrease, reducing the amount of protein that can be utilised by the animal.

Supplementing phosphorus during the wet season provides the animal with the phosphorus it requires for production (e.g. growth, pregnancy and lactation). If sufficient phosphorus is fed, phosphorus will be restored to the bones, which is critical to increasing phosphorus reserves in the body and reducing the risk of bone breakages.

During the dry season, protein, energy and phosphorus levels all decline. Early in the dry season, protein is usually the first most limiting nutrient, followed by energy later in the dry season. Unless additional protein and energy are supplemented to the animal, there is no benefit in supplementing a high level of phosphorus.

How much phosphorus is needed by the animal?

Phosphorus stored in the bones can be mobilized by cattle for a short-term diet deficiency, when cattle aren’t receiving enough phosphorus in the diet, such as in early lactation. Up to 30–40% of these phosphorus stores can be mobilized by an animal that is in severe phosphorus deficiency. Bone turnover (mobilization and deposition) is much lower in mature cattle than young animals therefore it is critical that young animals receive as much phosphorus as possible for productivity and skeletal growth, but also to ensure maximum bone accretion.

The amount of phosphorus absorbed in the body is also determined by the balance between phosphorus and other nutrients such as energy, protein and calcium. Table 1 shows the approximate amounts of phosphorus needed by steers or breeders under different production circumstances. Under higher levels of production more phosphorus is required by the animal.

Table 1. Approximate amounts of phosphorus needed by steers or breeders under different production circumstances

Class of animal

Liveweight gain
(kg/day)

Milk production
(kg/day)

Phosphorus needed
(g P/day)

Amount of phosphorus needed
(% in the forage as usable phosphorus)

Steer (300kg) 0

6

0.09

Steer (300kg) 0.6

13

0.14

Dry breeder (400kg) 0

0

7

0.09

Wet breeder (400kg) 0

5

17

0.16

Wet breeder (400kg) 0.6

5

24

0.18

 

Testing for a phosphorus deficiency

A number of tests and observations collectively give a reliable indication of whether cattle are likely to respond to a phosphorus supplement.

It is usually easy to identify situations where cattle are very phosphorus-deficient. It is more difficult to determine whether animals are marginally deficient to deficient.

Factors to consider include: soil and vegetation, the class of stock, cattle management (including grazing management) weaning, and any improved pastures (such as stylos).

A blood phosphorus analysis is likely to give the best indication of the phosphorus status of the growing animal, although it cannot be applied to the lactating cow.

If cattle haven’t been on a phosphorus supplement, then faecal NIRS in conjunction with a faecal phosphorus analysis, can be used to determine phosphorus status. This can then be used to determine the ratio between phosphorus and protein, and the ratio between phosphorus and digestibility to ensure that the diet is balanced.

Feeding phosphorus – what to look for in a supplement

Key considerations in selecting a phosphorus supplement are:

1. An animal’s dietary phosphorus status as well as protein and energy
It is important to balance the supplement with what the animal is receiving in its diet. This can only be achieved by assessing the animal’s diet quality through faecal NIRS analysis. For more information see Assessing pasture diet quality (NIRS).

2. Percentage of phosphorus in the lick
If animals are consuming small quantities of lick then a larger concentration of phosphorus in the lick is required. If they are consuming copious amounts of lick then the phosphorus concentration should be decreased to save wastage and reduce the overall cost of the lick.

3. The quantity of lick consumed
Meeting daily phosphorus requirements is important. This depends on daily intakes, which are influenced by the type of lick fed (palatability and hardness) and access to the lick. Delivery of adequate amounts of phosphorus during the wet season remains a key industry issue.If a lick is not being consumed at adequate rates, it is likely that it is unpalatable. Look firstly as reducing or removing substances such as sulphate of ammonia and lime, and salt, if the salt is not being used as an attractant. Lime is generally not needed in a lick. Most northern Australian pastures have a high calcium:phosphorus ratio, meaning there is sufficient calcium in the diet relative to phosphorus. If additional calcium is required, it can be provided through the phosphorus source (MDCP or DCP) that is being fed.If urea is limiting intake, it may also need to be reduced.

4. Ensuring phosphorus is balanced with other nutrients
It is important that the phosphorus and nitrogen (e.g. urea and/or protein meal) in the lick are balanced with the protein and phosphorus the animal is receiving from the pasture. Providing too much phosphorus in the lick relative to nitrogen will result in wastage of phosphorus. Providing too much nitrogen (e.g. urea) relative to phosphorus means that the nitrogen ingredient is wasted.

5. Ensuring the lick is cost-effective.
The most cost-effective lick will have the lowest cost per unit of phosphorus. These licks generally tend to have the highest phosphorus concentration. These licks may appear to be more expensive on a cost per tonne basis however, once the cost per unit of phosphorus is calculated, if there is a high concentration of phosphorus in the lick, these licks might be the cheaper licks to feed.

6. Use only phosphorus sources that are registered for stock feeding
Other sources of phosphorus, such as rock phosphates, are high in cadmium and/or fluoride. Cattle are most at risk of consuming cadmium through phosphate fertilisers that are not registered for livestock feeding. These fertilisers contain very high heavy metal levels that can result in unacceptable residue levels.

7. Monitor the feeding program
It is critical that the supplement intakes are monitored to ensure cattle are consuming an adequate amount of phosphorus. If this is difficult to implement in practice, try calculating the amount of lick put out at a watering point over a month, then estimate the number of cattle drinking at this watering point on average, to calculate daily intake. If they are not consuming enough phosphorus, change the recipe or try a different product.

More information

Phosphorus management of beef cattle in northern Australia (PDF, 2MB), Désirée Jackson et al. 2012. This book outlines general principles that can be applied to strategies and practices when feeding phosphorus to beef cattle.

You can also get more information from the Meat & Livestock Australia NBRUC conference paper ‘Phosphorus nutrition and management – overcoming constraints to wider adoption’ (Dixon et al., 2011, pages 102-109). Download a copy of the NBRUC August 2011 proceedings.


Phosphorus management of beef cattle in northern Australia

The recently published ‘Phosphorus management of beef cattle in northern Australia’ is a compilation of the latest research, demonstration and practical knowledge available in northern Australia.

In this webinar, principal author Désirée Jackson discusses management decisions surrounding:

  • classes of stock that are a priority for phosphorus feeding
  • when to feed phosphours supplements
  • formulating phosphorus supplements
  • how much phosphorus to feed
  • how to effectively feed phosphorus.

Additional topics include:

  • benefits of phosphorus feeding
  • signs of phosphorus deficiency
  • diagnostic tests for phosphorus status of cattle
  • economic responses to phosphorus feeding.

Join Désirée as she discusses these topics in this webinar recording. For your convenience, here are the webinar presentation slides (PDF, 2MB).