05/06/2026
Again .. consideration of all the things !
Interestingly I was thinking about phosphorus today โฆ I didnโt speak it (phone listening and all)
๐ด Phosphorus: an important mineral requiring greater attention in WA horse diets
๐ถ โDonโt you forget about Pโฆโ
Yes, to the tune of Simple Mindsโ Donโt You Forget About Me.
Phosphorus is not often considered a particularly exciting topic, nor does it usually attract the same attention as sugar, starch, protein or โbig headโ disease.
But in WA horse diets, phosphorus is one of those minerals we really do need to talk about because it can quietly become an issue over time, particularly in forage-based diets.
This is not about blaming hay or pasture. ๐พ
Hay and pasture simply reflect the soil, rainfall, growing season, fertiliser history, plant species, maturity at cutting and weather conditions they came from.
In WA, many horses are fed forage grown on sandy, acidic or highly weathered soils, under very different growing conditions compared with other parts of Australia. This means Rhodes hay, meadow hay, ryegrass-based hay, lucerne and C3 grass hays grown in WA should not automatically be assumed to have the same calcium, phosphorus or trace mineral profile as forage grown elsewhere.
The issue is not that the hay is โbadโ.
๐ฑ A note on C4 grasses and oxalates
This post is mainly about low phosphorus relative to calcium in WA forage-based diets.
If your horse is grazing or eating higher-oxalate C4 grasses such as kikuyu, buffalo or setaria-type pastures, there is an extra layer to consider. Oxalates can bind calcium, so calcium's availability and the calcium-to-oxalate relationship need to be assessed, as well as the Ca ratio.
Teff can be variable and still needs oxalates to be considered.
Rhodes hay and Rhodes grass are generally low in oxalates. The low-phosphorus/high-calcium WA pattern is still very relevant.
โ
The key message is the same: test the forage and balance the whole diet, not just one mineral in isolation. Your horseโs diet needs to be balanced for the actual forage being fed.
Over the last 20 years of reviewing hay tests and balancing WA horse diets, one repeated pattern I see is phosphorus sitting low relative to calcium. In some Rhodes hay, meadow hay and mixed grass hay diets, the total diet may sit at 5:1, 7:1 or even 8:1 calcium to phosphorus.
Over a short period, this imbalance may not produce obvious clinical signs. Horses have physiological reserves, and mineral deficiencies rarely develop immediately. However, over months or years, chronically low phosphorus intake, especially when calcium intake is substantially higher, may negatively affect bone mineralisation, muscle function, appetite, growth, recovery and overall musculoskeletal soundness.
Phosphorus is not just a โbone mineral.โ ๐ฆด
It is also involved in:
โก ATP energy transfer
๐งฌ cell membrane structure
๐งฌ nucleic acids
๐ด normal metabolic function
๐ช muscle function and recovery
๐ฑ growth and skeletal development
(National Research Council [NRC], 2007; Toribio, 2011; Maier & Kienzle, 2024)
โ๏ธ Why the calcium-to-phosphorus ratio matters
Calcium and phosphorus both need to be supplied in adequate amounts, but the ratio between them also matters.
The first step is not just looking at the ratio. The first step is calculating whether the horse is actually receiving enough calcium and enough phosphorus in grams per day for its body weight, age, workload and life stage.
Only after those daily requirements are met should the whole-diet Ca ratio be assessed.
For many adult horses and young stock, a practical Ca target often sits around 1.6:1 to 2:1, adjusted for age, growth, pregnancy, lactation, workload and clinical status.
A ratio can look โacceptableโ on paper, while the horse is still short in total phosphorus grams per day. Likewise, a diet can look โmineral-richโ but still be functionally poor in phosphorus if calcium is much higher than phosphorus.
This is one area where WA feeding conditions can differ considerably.
Many feed and supplement formulations are developed around more common cereal-grain or eastern-state feeding patterns, where excessive phosphorus and insufficient calcium are often the primary concerns, especially in diets containing grains, bran, pollard or high-oxalate pastures associated with nutritional secondary hyperparathyroidism, commonly called โbig headโ disease (Krook, 1968; Toribio, 2011; Lacitignola et al., 2018).
However, in many WA forage-based diets, I often see the opposite pattern:
๐พ low phosphorus forage
๐พ calcium much higher than phosphorus
๐พ Lucerne, Rhodes or meadow hay values estimated from non-WA averages
๐พ supplements contributing more calcium but insufficient phosphorus
๐พ diets that appear โbalancedโ on paper but are not balanced for the actual WA forage being fed
This is why I prefer to use real hay analysis results rather than relying on generic values from another state, another season or another growing environment.
๐ What can low phosphorus look like over time?
Clinical signs may be subtle and gradual. They can include:
๐ด reduced appetite or selective eating
๐ด pica: chewing dirt, wood, manure or unusual materials
๐ด Poor growth in young horses
๐ด inadequate bone mineralisation
๐ด weakness or reduced performance
๐ด muscle fatigue
๐ด shifting or unexplained lameness
๐ด bone pain or soreness
๐ด increased risk of bone weakness in more severe or prolonged cases
In growing horses, inadequate phosphorus intake may contribute to rickets-like changes. In mature horses, long-term deficiency may contribute to osteomalacic bone changes, resulting in poorer mineralisation of mature bone (NRC, 2007; Toribio, 2011).
These signs are not specific to phosphorus deficiency. Lameness, poor condition, selective eating and pica can have many causes. However, they are a good reminder phosphorus should not be ignored when formulating and balancing a diet properly.
๐งช The solution is not to add minerals indiscriminately
The appropriate approach is not to throw in a generic supplement and hope for the best.
Instead:
โ
Test your horseโs hay; wet chemistry is preferred, while NIR can be useful as a screening tool
โ
Use local WA forage analysis results wherever possible
โ
Calculate calcium and phosphorus intake in grams per day
โ
Meet the horseโs calcium and phosphorus requirements first
โ
then assess the actual whole-diet Ca ratio
โ
Aim for a practical Ca ratio of around 1.6:1 to 2:1, where appropriate
โ
adjust for age, growth, pregnancy, lactation, workload and clinical status
โ
Evaluate what balancers and supplements are adding
โ
Avoid adding more calcium when calcium is already excessive
โ
Select an appropriate phosphorus source only when phosphorus is genuinely deficient
๐พ Phosphorus sources also need careful selection
There are feed-based phosphorus contributors, and then there are more targeted mineral phosphorus sources.
They are not all the same.
Feed-based phosphorus contributors may include:
๐พ wheat bran/mill mix
๐พ lupins
๐พ linseeds
๐พ sunflower seeds
๐พ copra meal
๐พ oats
๐พ barley
Targeted mineral phosphorus sources may include:
โ๏ธ dicalcium phosphate
โ๏ธ monosodium phosphate
Wheat bran, mill mix, lupins, linseeds, sunflower seeds, copra meal, oats, barley, dicalcium phosphate and monosodium phosphate can all contribute phosphorus, but they do not all bring the same nutrient profile, mineral profile or phosphorus availability.
Wheat bran and mill mix can be useful phosphorus contributors in some situations. But, phosphorus in bran is largely phytate-bound and may be less available than phosphorus from mineral sources e.g., dicalcium phosphate or sodium phosphate salts (Hintz et al., 1973; van Doorn et al., 2004).
Lupins can contribute phosphorus, but they also bring calories. Depending on the lupin product, protein and starch/NSC % still need to be considered as part of the whole diet.
Linseeds can also contribute phosphorus, but they also bring fat, omega-3 fatty acids and calories. They should be used for the whole nutritional picture & not simply added to โfix phosphorus.โ
Sunflower seeds can contribute phosphorus, but they also add fat, calories and omega-6 fatty acids. The omega-3 to omega-6 balance of the total diet still needs to be considered.
Copra meal can contribute phosphorus too, but again, it brings calories & fats with it, so it needs to be fitted into the diet properly.
Oats and barley can contribute phosphorus, but they also add starch. This means they may not be suitable for EMS, IR, laminitis-prone, PPID, easy-keeper or high-risk metabolic horses. Barley also needs appropriate processing for better digestibility of starches.
MSP can add extra sodium. Sodium intake need to be assessed carefully in WA diets, especially where salt is already being added separately. This applies to sensitive or ulcer prone horses.
Again, the answer is not just โadd phosphorus.โ
The answer is to balance:
โ๏ธ calcium
โ๏ธ phosphorus
โ๏ธ magnesium
โ๏ธ sodium
โ๏ธ protein
โ๏ธ calories
โ๏ธ fat profile
โ๏ธ starch and sugar load
โ๏ธ the whole forage base
โ๏ธ the horseโs actual requirements
This is especially important for:
๐ด forage-only or low-hard-feed diets
๐ด senior horses
๐ด growing horses
๐ด lactating mares
๐ด poor doers
๐ด horses in work
๐ด EMS, IR or laminitis-prone horses
๐ด horses with soundness, bone or metabolic concerns
Phosphorus may not be exciting.
But it is essential.
And in WA, I believe it deserves far more attention because balancing a horseโs diet should be based on the forage actually being fed, not assumptions from another state, another season or another soil type.
๐ถ Soโฆ donโt you forget about P.
๐พ๐ด Balance the horse to the forage, not to generic eastern-state averages.
References
Hintz, H. F., Williams, A. J., Rogoff, J., & Schryver, H. F. (1973). Availability of phosphorus in wheat bran when fed to ponies. Journal of Animal Science, 36(3), 522โ525. https://doi.org/10.2527/jas1973.363522x
Krook, L. (1968). Dietary calcium-phosphorus and lameness in the horse. Cornell Veterinarian, 58, 58โ73.
Lacitignola, L., Rossi, G., Sica, E., & Crovace, A. (2018). Nutritional secondary hyperparathyroidism in two ponies. Open Veterinary Journal, 8(2), 149โ153.
Maier, I., & Kienzle, E. (2024). A meta-analysis on quantitative calcium, phosphorus and magnesium metabolism in horses and ponies. Animals, 14(19), 2765. https://doi.org/10.3390/ani14192765
National Research Council. (2007). Nutrient requirements of horses (6th rev. ed.). National Academies Press.
Toribio, R. E. (2011). Disorders of calcium and phosphate metabolism in horses. Veterinary Clinics of North America: Equine Practice, 27(1), 129โ147. https://doi.org/10.1016/j.cveq.2010.12.010
van Doorn, D. A., Everts, H., Wouterse, H., & Beynen, A. C. (2004). The apparent digestibility of phytate phosphorus and the influence of supplemental phytase in horses. Journal of Animal Science, 82(6), 1756โ1763.
