Should you supplement with phosphorus?

Low phosphorus levels are a major problem for cattle in northern Australia.

Department of Agriculture and Fisheries principal beef extension officer Mick Sullivan said the principal effect of phosphorus deficiency was reduced pasture intake – and consequently nutrient intake.

“The reduction in nutrient intake affects the breeder’s ability to maintain body condition, resulting in lower weaning rates and increased mortality,” Mr Sullivan said.

“Milk production is also reduced, leading to lower weaning weights. In growing cattle, the lower nutrient intake produces lower growth rates. All of these impacts have serious consequences for production and profitability.”

Other symptoms of phosphorus deficiency include:

  • bone chewing, which increases the risk of botulism
  • chewing of other objects (rocks, sticks, wire etc.)
  • peg leg
  • bone breakages.

“Fortunately, we have effective vaccines that can prevent losses from botulism (progressive paralysis caused by the ingestion of a toxin found in bones and carcases),” Mr Sullivan said.

“Botulism vaccination is critical wherever cattle chew bones or botulism has been known to occur.”

How to diagnose phosphorus deficiency

A range of tools are available to assess the phosphorus status of paddocks.

“A history of bone chewing, poor cattle performance and in extreme cases peg leg indicates phosphorus deficiency is a problem,” Mr Sullivan said.

“Forage indicative land type and soil phosphorus maps (available from The Long Paddock website) can provide a guide to the land types on a property and likely soil phosphorus status.

“However, the broad scale of this mapping must be considered. Soil tests determine the phosphorus status of soil at a particular site, but the challenge in using soil tests is the range of soil types commonly found in grazing paddocks and the grazing patterns of cattle. Cattle favour the better land types and if they have access to areas of high phosphorus soils their overall phosphorus status can be okay.

“The best test for phosphorus status is blood phosphorus in conjunction with faecal NIRS to assess diet quality. Blood testing must be undertaken at the end of the growing season when dry cattle can still gain weight (March to April).

“Only dry growing animals should be tested and the cattle must not have been fed phosphorus over the growing season. The test is based on the mean blood phosphorus level of 20 animals so 25 should be bled. Beef extension staff can provide further information on testing and help producers plan for testing.”

Breeder management

Lactation significantly increases the phosphorus requirements of breeders.

“Calving at the most suitable time for the region and avoiding cows lactating for long periods in the dry season will also mitigate the effects of low diet quality and improve drought resilience,” Mr Sullivan said.

“Timely weaning of breeders and, if appropriate, early weaning enables breeders to better cope with low phosphorus pastures.

“The reduced requirements for energy, protein and phosphorus after weaning can markedly improve body condition. Weaned breeders have the opportunity to replenish their body phosphorus reserves in preparation for the next lactation.

“Where controlled mating is not feasible, breeder segregation systems can enable out-of-season calving cows to be better managed.”

More information

For more information about managing phosphorus deficiency, visit