How to identify a phosphorus deficiency
It’s not uncommon for beef producers to be unaware that a phosphorus problem exists in their herd. If stock are gaining weight and body condition is maintained once the wet season has started, there can be a misconception that stock are getting the nutrients they need from pasture – which isn’t always the case.
Here are some ways to identify a phosphorus deficiency:
While clinical signs such as ‘peg leg’ – where cattle develop an arched body, staggering gait and brittle bones – are a sure indicator of an acute P deficiency, serious production issues could be occurring long before the deficiency is identified in the herd.
Soil maps can provide a general indication of phosphorus deficiency. Soil P is difficult to use for diagnosis at the paddock level where there can be marked differences in soil types within a paddock.
Careful soil sampling that captures the variability across large paddocks is required to obtain a full appreciation of soil P status (Table 1). This may be expensive if multiple samples are needed. Soil Colwell P levels of >8 mg/kg suggest that the pastures contain sufficient P.
Measuring the ‘plasma inorganic phosphorus’ (PiP) levels of cattle that have not been fed P supplements for at least two weeks over the wet season is the best method to determine if the pastures are providing enough P (the P-Screen test).
To collect a sample, randomly select 20–25 head (growing animals, heifers in early-mid pregnancy or lactating first-calf cows) grazing the test paddock. Sample late in the growing season when both dietary protein and energy are considered adequate.
In continuously mated herds, introduction of growing steers (prior to the wet season) or pregnant heifers can improve the confidence in the diagnosis. Because the PiP test is the most accurate indicator of dietary P, it can also be used to assess the effectiveness of supplementation programs. However, testing supplemented animals will not establish whether or not you need to supplement in the first place.
Table 2 below shows the best estimates of P status in growing and breeder cattle for a range of plasma inorganic phosphorus levels and diet dry matter digestibility.