Effects of phosphorus supplementation on Brahman females at Kidman Springs

Much of northern Australia is phosphorus (P) deficient (Picture 1), but supplement sales figures and beef research/extension officers tell us that the majority of properties in deficient areas are not supplementing their cattle with P in the wet season. The most likely reason for this is that they don’t think the expense is justified i.e. that they will not get a good enough return on their investment. However they don’t have very good data to base this decision on as there have been few studies that have demonstrated improvement in reproductive performance from P supplementation in northern Australia or quantified a benefit. As a result advisors have had to give estimates of the benefit that producers might expect without hard data to back them up. It seems that this has not been convincing enough for many producers. The P supplementation project at Kidman Springs aims to provide producers with information on the cost:benefit of supplementing females in deficient country.

Map of phosphorus deficient areas in northern Australia
Picture 1. Map showing general extent of phosphorus deficiency over northern Australia (adapted from McCosker and Winks 1994) Red indicates areas that are deficient to acutely deficient; yellow indicates marginal deficiency or mixed; green indicates adequate or unknown levels of phosphous.


Brahman weaner heifers were randomly allocated (stratified for weight) to either a +P (n=91) or -P (n=88) treatment (average weight: +P = 171.7 kg, -P = 171.2 kg) in June 2014. The treatments grazed separately in neighbouring paddocks that were acutely P deficient (Colwell P: 2.5 and 3.1 mg P/kg) and they swapped paddocks in May each year to minimise paddock effects. The treatments were managed in exactly the same way except that their mineral loose lick supplement either contained P (+P) or did not (-P).

Heifers were mated at 2 years old for a period of 4 months and the first calves were born over the 2016/17 wet season. Non-pregnant heifers were not culled but retained so that all heifers remain in their treatment groups for the duration of the experiment. The paddocks are mustered separately in May and September each year for data collection and calves are mothered up and weaned in May.

Preliminary results

  • +P grew 33 kg more in each of the two wet seasons after weaning (Figure 1) so that on average +P was 65 kg heavier than –P after maiden mating.
  • Pregnancy rate was 10% higher in +P maiden heifers. The pregnancy rate was lower than expected in +P and ultrasound ovary scanning found more cycling heifers in +P and if all cycling heifers in each group had become pregnant then the difference would have been 23% (+P = 87%, -P = 64%).
  • Foetal and calf loss from pregnancy test to weaning was similar in both treatments.
  • Re-conception rate was 25% higher in +P first lactation heifers.
  • The average weight of +P first lactation heifers was 120 kg higher when their calves were weaned at the first weaning round (WR1) in 2017.
  • The cumulative mortality rate over the three years from weaning to 3.5 years old was 7% lower in the +P treatment. Also at WR1 in 2017, 18 -P heifers were removed for crisis feeding as they were considered to be at risk of mortality due to low body condition score. Including these heifers in the analysis gave a mortality/morbidity rate of 28% for –P heifers.
  • The average weaning weight of calves at WR1 2017 was 34 kg heavier in the +P treatment. Fifty calves were weaned from +P and 40 from -P.
  • The total weight of calves weaned from the treatments at WR 1 2017 was 3072 kg higher in +P, which when valued at the current price ($3.50/kg) was worth $10 751. The extra cost of supplement consumed by the +P treatment to WR 1 2017 was calculated to be $3839 (compared to the cost of the -P treatment in the dry season only). Dividing the extra value of calves weaned by the extra cost gave a return on investment of 280% for the +P treatment.
  • At WR1 2018 the pregnancy rate was 37% higher in wet cows, average weaner weight was 13 kg higher and the total weight of weaners was 2806 kg more in +P. Pregnancy rates in dry cows were similar (+P = 96%, -P = 92%).
Figure 1. Growth from weaning to end of mating (at two years old)

The total value of calves weaned from each treatment by WR1 2018 was $19 169 more for +P and the extra cost of supplement was $5529. This gave a cumulative return on investment in the +P treatment of nearly 350% (see Table 2).

Table 1. Summary of results (* P<0.001)

Maiden heifer average pre-mating weight392 kg327 kg65 kg *
Maiden heifer pregnancy rate70%60%10% NS
1st lactation heifer average weight (WR1 2017)382 kg262 kg120 kg *
Average P8 fat depth (WR1 2017)3.9 mm0.1 mm3.8 mm *
Calf loss (first calf)20.6%20.4%0.2 % NS
1st lactation heifer pregnancy rate30%5%25 % *
Average weaner weight (WR1 2017)173 kg139 kg34 kg *
Total weight of weaners (WR1 2017)8616 kg5544 kg3072 kg *
Overall mortality rate to 3.5 years old1%8%7 % *
Wet cow pregnancy rate (WR1 2018)57%20%37 % *
Average weaner weight (WR1 2018)185 kg172 kg13 kg
Total weight of weaners (WR1 2018)7951 kg5145 kg2806 kg *


Table 2. Preliminary economic assessment

 Total weight of extra calves (kg)Price ($/kg)Extra $ per treatmentCost of extra supplementReturn on investment
20173072 kg$3.50$10 751$3839280%
20182806 kg$3.00$8417
Total5878 kg$19 169$5529347%

This trial of supplementing phosphorus to heifers and measuring the impacts of doing so is scheduled for completion in mid-2020 when a full economic assessment and cost-benefit analysis will be conducted.


Tim Schatz, Principal Livestock Research Officer, NT Department of Primary Industry and Resources, Tel: 08 8999 2332, email: tim.schatz@nt.gov.au

Click here for a recent project update including photographic comparisons of the two treatment groups over time.