Weighing up Phosphorus Supplementation

AUSTRALIA - Northern beef producers recognise the impact of phosphorus (P) on their productivity. It's easy to measure the response of adding P to cattle’s diet in severely P-deficient country. However, determining its economics in not-so-deficient areas is a bit trickier.
calendar icon 19 October 2012
clock icon 3 minute read
Meat & Livestock Australia

Here MLA’s Northern Beef R&D Research Coordinator, Geoff Niethe, outlines some of the options for northern cattle producers.

In an attempt to provide more certainty into the P supplementation debate, MLA is funding several research projects to establish a more reliable and practical P test and to determine the responses in growth rates and fertility that occur at various levels of P in the diet.

Challenges include accurately defining the marginal phosphorus deficient areas and the classes of cattle and the seasonal conditions when a positive response will occur.

In the interim, a new P manual is in production using all the knowledge that has been accumulated to date and will address the key principles to consider when applying strategies and practices to get the best economic returns from feeding supplementary phosphorus.

Some of the important facts on P are:

Which animals most need P?

  • The animals that need P the most are growing animals, late-pregnant breeders and wet cows.
  • Soil P levels is an easy way to determine if P supplementation should occur but it can prove problematic where various soil/ land types exist in the same paddock. In general, where soil P levels are deficient (5mg/kg or less), feed P supplements to all classes of stock; are marginal (6–8mg/kg), feed P to young breeders and test older breeders; or exceed 8 mg/kg, the economic benefits from feeding cattle are marginal.
  • Responses to P supplement will be variable if animals on P deficient country have access to adjacent high P soils such as frontage country.
  • Animals showing signs of severe phosphorous deficiency including bone chewing, broken bones, peg-leg, poor body condition of breeders and botulism.
  • There are no simple diagnostic tests for the P status of cattle. Blood tests on growing steers immediately after the wet season are still the best indicator of P status, while faecal P is a more practical and readily obtainable procedure.
When should P be fed?
  • Deficient animals respond best to P supplement when their diet has adequate protein and energy. This is why P supplementation is most effective during the wet season.
  • On deficient country, feeding P over the wet season to young growing stock can increase their growth by 40–60kg/year or lactating breeder cows can increase conception rates by 15–20 per cent.
  • Where the native pasture on deficient country contains at least 30 per cent stylo, cattle may respond significantly to P supplement during the dry season.
  • A typical wet season P supplement will contain 10–12% P; a typical dry season supplement will contain 2–4 per cent P and also non-protein nitrogen (eg urea).
  • Supplements should be compared on the cost of their P content, on the practicality of feeding out and on whether the animals will be able or willing to eat target amounts.
How do P supplements affect the stocking rate?
  • As cattle eat more pasture when P supplements are fed, the stocking rate should be reduced to avoid overgrazing.
  • On deficient country, lowering the stocking rate will not reduce the need to feed P.

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