When it comes to cattle supplementation, knowing what to feed, when to feed it and why can be a challenge.
To help producers make informed decisions about supplementing phosphorus, Northern Territory Department of Agriculture and Fisheries Director of Livestock Industries, Tim Schatz answered some key questions on every producer’s mind.
Q1. Why is phosphorus important for northern beef cattle?
Much of the pasture in northern Australia is deficient in phosphorus (P), and P is a critical nutrient that’s required for:
- Growth and repair of body tissues
- Bone and organ structure and function
- Conversion of feed into energy
- Development of the foetus and milk production.
When animals are deficient in P their growth and reproductive performance can be greatly reduced. Research has found large responses in production when deficient cattle receive P supplementation. There are reports of P supplementation increasing liveweight gain by 30-70kg in growing cattle and resulting in large increases in reproductive rates in breeders (P supplementation was found to have a return on investment of over 700% in one study).
Q2. How can producers tell if phosphorus is limiting their cattle?
They can watch their cattle closely for signs of P deficiency which include:
- Poor growth
- Reduced weaning rates
- Brittle bones (eg. broken bones or collapsing in the crush)
- Depraved appetite (eg. bone chewing, or eating dirt)
- Stiff/painful gait when walking (peg leg) and high mortality rate.
However, most of these things are symptoms of severe P deficiency, and it can still be profitable to supplement P at lesser levels of P deficiency.
The Plasma inorganic Phosphorus (PiP) blood test is currently considered to be the best way to test for P deficiency. Good responses to supplementation should occur when PiP test results are < 1.3 mmol/L in breeders and <1.5 mmol/L in growing animals. The blood samples should be taken from animals that are not being supplemented as P from the supplement will elevate blood P levels. Talk to your local extension officer for more info on PiP blood testing
Soil can be tested for P deficiency, but there are usually several different soil types within a paddock and cattle may be able to get sufficient P through selective grazing on soil types where P is adequate. Soil Colwell P test results of <6 mg/kg are considered to indicate that the soil is deficient in P, and <4 mg/kg indicates acute deficiency.
Running a small trial to see if there is a response to P supplementation is another good way to work out if P supplementation will be beneficial. The easiest way to do this is to use “indicator steers” to compare the growth of steers over a wet season in a paddock that is being supplemented to one that is not.
Comparing the growth of about 20 steers per paddock should give a fair idea if there is a response to supplementation (although you need to be aware of potential stocking rate and paddock differences – if one paddock has better pasture than the other). Breeder trials are more difficult and take much longer than steer growth trials, but if you find a growth response to supplementation in steers then it is likely that there will also be a response in breeder performance.
Q3. When is the right time to feed phosphorus?
P is usually the main limiting nutrient in the wet season when pasture is green, and this is usually when the biggest responses to P supplementation are seen. P supplementation during the dry season usually does not improve growth, but it may be helpful for lactating cows as a lot of P leaves their body in milk.
Q4. How much phosphorus should producers be providing?
General rules of thumb for supplementation on P deficient country are that breeders should be given 9 g/day of P and growing animals 6 g/day of P. Note that these figures are for actual grams of P, so producers need to calculate how much P is in the supplement they are feeding. For example, if a supplement contains 12% P then cattle need to eat 75 g of supplement per day to get 9 g of P per day. Your local extension officer can help you with these calculations if you are not sure.
Q5. What are some common mistakes you see with phosphorus supplementation?
Probably the most common one is leaving it too late to order supplement before the wet season, which can result in it not being able to be delivered in time.
Also, some paddocks don’t get supplemented because they are inaccessible during the wet season, but the recently completed Easy P project has shown that delivering P supplement in bulk to a paddock before the wet season is just as good as delivering it regularly throughout the wet season. The amount of supplement to put out before the wet season (eg. to provide cows with 9 g of P per head per day) can be estimated for the number of animals in the paddock, the number of days to be supplemented and the percentage of P in the supplement.
Q6. If a producer isn’t sure where to start, what’s the first thing you’d recommend they do?
Keep an eye out for symptoms of P deficiency, do some blood PiP testing, and find out whether your annual steer growth is what you would expect for the region.
If possible, run a small trial to see if there is a response in steer growth to P supplementation.
References
Schatz TJ, McCosker KD, Heeb C (2023) Phosphorus supplementation improves the growth and reproductive performance of female Brahman cattle grazing phosphorus-deficient pastures in the Victoria River District, Northern Territory, Australia. Animal Production Science, 63(6), 544–559. doi:10.1071/AN21565
Additional resources
Phosphorus supplementation of cattle in northern Australia →
Improving beef business performance with phosphorus supplementation →
