Summer News 2008-09
Phosphorous is a macro mineral, and all animals need it. It is biologically involved in various metabolic, neurological and cellular functions.
The functions of phosphorous
It plays a key part, that of carrier of the energy source adenosine triphosphate for cells, hence its importance in converting energy obtained from grazing to usable body-metabolic energy.
It is also involved in blood buffer systems and with the activation of certain B vitamins.
Phosphorous is also a component of genetic material and works alongside calcium to ensure the development of healthy teeth and bones.
The consequences of a phosphorous shortage
A phosphorous shortage gives way to lower grazing intake, which in turn lowers growth rate. Subsequently, fertility, milk production and weaning mass are negatively affected. A lower growth rate can also lead to suppressed and irregular oestrous cycles.
In other words, a phosphorous shortage gives way to a general lowering of productivity in all kinds of animals, stunted weight gain in calves, lowered conception rate and anestrous, and lowered milk production. Animals that suffer from a phosphorous shortage have poor condition, are anaemic and have coarse coats. Lower reproduction is secondary and is the consequence of lower energy levels and protein intake, which, in turn, are the consequences of phosphorous shortages.
Aphosphorosus, the ilness triggered by a phosphorous shortage, also takes different forms.
The pica eating disorder - characterised by symptoms such as licking and chewing rags, tins, rocks and poles - as well as excessive salt intake, consuming sand and osteophagia (chewing or eating bones) are all typical consequences of a phosphorous shortage, but they can also be the result of sodium, potassium, energy and protein shortages. Other symptoms of phosphorous shortages include swollen joints, stiff legs, accelerated hoof growth, loss of appetite, stunted growth, serious emaciation, suppressed and irregular oestrous cycles, lowered fertility, listlessness, dull coats and lowered immunity.
Recommended phosphorous intakes
Sufficient phosphorous intake is vital for a high calving percentage and growth rate (weaning mass). Cows should consume between 4,5kg and 6,7kg of phosphorous per year.
Phosphorous shortages are common in South African pastures and veld. When plants are green and growing, phosphorous comprises an average 0,3%. This level is lowered to 0,15% as the grass matures.
The same can be said for hay - the later it is cut, the lower its phosphorous content. During the winter months and times of drought, when hay is the main food source, animals don't consume sufficient amounts of phosphorous.
Fertilising hay with phosphorous does improve the situation somewhat, but additions to fertiliser (fluoride and aluminium carriers) make phosphorous less bio-available to animals. As the phosphorous content of grains - maize, oats, wheat and rye - is much higher than the calcium content, you can increase phosphorous intake by supplementing your animals' usual diet with these.
The bio-availability of phosphorous should be considered when this macro-mineral is supplemented. Highly available sources include dicalciumphosphate (Dical, CaHPO4-2H2O) ammonium phosphate (NH3PO4) and sodium phosphate (NaPO4). Ruminant uptake of anhydrous phosphorous sources or compounds with metals such as iron, aluminium and so forth is not so effective.
The calcium: phosphorous (Ca:P) ratio is also important as the two elements need to work together to ensure good bone formation. A ratio of between 1,5:1 and 2:1 should be maintained for the process to run optimally. Should the ratio exceed 5:1, it will cause symptoms of phosphorous shortage, including lowered growth, weak fodder effectiveness and suppressed reproduction. If the ratio is lowered to 1:1, it causes kidney stones (urinary calculi) in male animals.
As the optimum falls in such a narrow tolerance band, the Ca:P supplement should be balanced with the roughage found in the veld or in the complete feed. Leguminous crops, such as lucern, contain high levels of Ca (1,5% to 2,5%) and low levels of P (0,3%). Should this crop be supplemented, the supplement needs to contain more phosphorous than calcium. In turn, grains, grain crops and oil cakes contain higher levels of phosphorous (0,3 to 0,9%), and the mineral supplement should contain more calcium to restore the balance.
The animal's production stage, age and level of milk production all impact the amount of phosphorous required. As heifers are still growing, they need significantly higher levels of phosphorous for bone growth. As cows age (between five and six years), their ability to absorb phosphorous dwindles - for this reason they need more phosphorous (up to 50%). Dairy cows with high production expectations also need extra phosphorous for milk production.
The phosphorous needs of animals differ, and the availability of phosphorous in roughage also fluctuates depending on the time of year. As a producer you need to manage the situation and schedule interventions where required. For optimal results, a well-planned supplementation programme should be adhered to in order to ensure that the productivity of your animals is not suppressed by a feeding regime.
Ideal phosphorous intake for beef cattle, from Beef NRC 2000
| Stage of production | Daily phosphorous intake |
| First heifer calve | |
| Last trimester | 24 g/per day |
| Post-calving - 5l milk/day | 23 g/per day |
| Mature cow | |
| Mid trimester | 18 g/per day |
| Last trimester | 13 g/per day |
| After calving - 5l milk/day | 19 g/per day |
| After calving - 10l milk/day | 26 g/per day |
Various tests have been undertaken to determine the effect of phosphorous supplementation.
To follow are the general findings:
No phosphorous: 52,9% calving percentage (some cows calve every second year)
With phosphorous supplementation:
75,6% calving percentage
Grazing intake increases by 25%
Body mass increases by 15%
Average daily gain increases by 130 to 320g
The better the veld quality, the better response to be gained from phosphorous supplementation.
Change in phosphorous and calcium content of grass and legumes throughout the life cycle
| Roughage | Calcium % | Phosphorous % |
| Lusern | ||
| Green feed | 1,50 | 0,33 |
| Hay: early flowering phase | 1,37 | 0,22 |
| Hay: full flowering phase | 1,19 | 0,24 |
| Veld grass | ||
| Green feed | 0,32 | 0,37 |
| Hay: sundried, harvested at a young growth stage | 0,29 | 0,28 |
| Hay: mature | 0,26 | 0,22 |
For an adult cow's total phosphorous needs:
| Phosphorous intake | P6 intake (g) | |||
| Green season | ||||
| Phosphorous obtained from grazing on the veld | 12 | 0,37 | 4,44 | |
| Phosphorous need | ||||
| In-calf | 18 | 13,56 | 0,226 | |
| Post-calving, low milk production | 19 | 14,56 | 0,243 | |
| Post-calving, high milk production | 26 | 21,56 | 0,359 | |
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Summer News - 2007/08
