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.

Method

Brahman weaner heifers were randomly allocated (stratified for weight) to either a P supplementation (+P) (n=91) or no P supplementation (-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). The paddocks were set stocked and the treatments swapped paddocks in May each year to minimise any paddock effect. The loose lick supplement was distributed twice-weekly and fed year-round in troughs that were shelted from rain by sheds. The wet and dry season recipes of the supplements fed to each treatment group are shown in Table 1. Salt was substituted for Biofos^® in the P- supplement, however, limestone was added to the P- wet season supplement to make the ratio of calcium to P the same in both licks.

Table 1. The recipes for the loose lick supplements used in the Kidman Springs phosphorus supplementation study

All other aspects of management of the treatment groups was the same.

Heifers were first mated at two years old for a period of four months from late December 2015, with the first calf crop weaned in May 2017. Two more calf crops were weaned before the study concluded at the weaning muster in May 2019. 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

• Heifers in the P+ treatment grew 33 kg more in each of the two wet seasons after weaning. As a result P+ heifers averaged 65 kg heavier than P- heifers after mating.
• Pregnancy rate was 10% higher in P+ maiden heifers. The pregnancy rate was lower than expected in P+ treatment. Ultrasound ovary scanning found more cycling heifers in P+. If all cycling heifers in each group had become pregnant, then the difference would have been 23% between treatment groups (P+ = 87%, P- =64%).
• The average weight of P+ first lactation heifers was 120 kg higher when their calves were weaned in May 2017.
• Foetal and calf loss from pregnancy testing to weaning was similar in both treatments for first calf heifers, but higher in P- in subsequent years.
• Reconception rate in lactating cows has been consistently higher in the P+ treatment cows (2017 P+=30%, P-=5%; 2018 P+=57%, P-=20%; 2019 P+=70%, P-=8%).
• Average weaner weight from P+ cows was consistently higher than weaners born to P- cows (see Table 2).
• The study ceased in May 2019 (12 months ahead of schedule) due to the poor body condition of the lactating cows in the P- treatment.
• A very basic economic comparison of the profitability of the P+ treatment was conducted by dividing the extra value of the weaners produced from the P+ treatment in each year by the cumulative extra cost of the P+ supplement to that point (see Table 3).
• Over the first three calf crops, an extra 11.8 tonnes of calves were weaned from the P+ treatment and this was worth about $379 per heifer originally allocated to the treatments (the study started with 88 heifers in P- and 91 in P+).

Table 2. Details of the first three calf crops weaned from the P+ and P- treatments in the Kidman Springs P supplementation study.

Table 3. Assessment of the return on investment in the P+ treatment in the Kidman Springs P supplementation study

Discussion

The crude economic analysis conducted to date only tells part of the return-on-investment story, as it doesn’t account for the greater mortality rate in P- and the heavier weight of cows in P+, (meaning that the P+ cows are likely to be more valuable at the time of culling for age). The cumulative mortality rate from the start of the study to May 2019 was 13% higher in P- (P+=2%, P-=15%) and the average weight of cows was 96 kg heavier in P+. When the greater salvage value of cows in the P+ treatment (calculated to be $26,026 in May 2019) is added to the extra value of weaners produced, the return on investment by May 2019 was 880%. This sort of a return on investment is quite staggering and although it is not a proper economic evaluation, it indicates how great an effect that P supplementation can have on the profitability of properties in P deficient areas (note – a proper economic evaluation will be done in future).

Also, it is likely that the mortality rate in P- would have been even higher if the study had not been stopped in May 2019 and efforts made to improve the body condition of P- cows that had been lactating and were weak and in very poor condition. Twenty-nine P- cows (33% of the total) were segregated into a small holding paddock and provided with hay, molasses, high energy and protein pellets and P supplement until their body condition improved. If this had not been done then it is likely that the mortality rate in P- would have been higher.

Mortalities are also likely to have been higher if the study had been done with larger mobs in bigger paddocks, as occurs on commercial properties, where uneven grazing around water points means that cows have to walk further between water and pasture each day (hence, use more energy).

The reason for the dramatic deterioration in P- cows during their second lactation is likely to be because so much P had been drained from their bodies during their first and second lactations. If cows don’t get enough P from their diet when they are lactating, they mobilise P from their bones and tissues to put into milk. This was confirmed by blood testing which found that blood P levels were adequate in calves from P- cows but were very low in the P- cows. This drain of P over two lactations without P stocks being replenished by supplementation seems to have resulted in the P- treatment crashing.

This study has documented the large effects on productivity and profitability that providing P supplements to heifers and cows can have when they are grazing P deficient country. It has shown very large returns on investment from P supplementation and is already resulting in increased adoption of P supplementation.  However, it should be noted that the response will vary depending on the level of P deficiency and benefits of this scale may not be seen where P deficiency is not as severe.

Project articles

• December 2019 project update including interview with project leader and Principal Livestock Research Officer, Tim Schatz.
• January 2019 project update including photographic comparisons of the two treatment groups over time.

This project is complete, and the following paper is expected to be published in Animal Production Science in 2022: Schatz TJ, McCosker KD, Heeb C (in press) The effect of phosphorus supplementation on the growth and reproductive performance of Brahman females grazing phosphorus deficient pastures in the Victoria River District, NT.

Contact:

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