In the animal body, about 80 percent of P isfound in the skeleton. Its major role is as aconstituent of bones and teeth. The remain-

der is widely distributed throughout the body incombination with proteins and fats and as inor-ganic salts.

Phosphorus constitutesabout 22 percent of the miner-al ash in an animal’s body, alittle less than one percent oftotal body weight. It is essen-tial in transfer and utilizationof energy. Phosphorus is pre-sent in every living cell in thenucleic acid fraction.

Calcium and P are close-ly associated with each otherin animal metabolism. Ade-quate Ca and P nutritiondepends on three factors: a sufficient supply ofeach nutrient, a suitable ratio between them,and the presence of vitamin D. These factorsare interrelated. The desirable Ca:P ratio isoften between 2:1 and 1:1.

Vitamin D3 is essential for Ca utilization.Inadequacies in the vitamin will imbalance theavailable Ca:P.

A liberal supply of Ca and P is essentialfor lactation. Calcium and P make up about 50percent of the ash of milk.

Earliest symptoms of P deficiency aredecreased appetite, lowered blood P, reducedrate of gain, and “pica”, in which the animalshave a craving for unusual foods such as woodor other materials. If severe deficiency occurs,there will be skeletal problems.

Milk production decreases with P defi-ciency, and efficiency of feed utilization isdepressed. Long-term P deficiency results inbone changes, lameness, and stiff joints.

Cattle: Young and growing animalsrequire relatively more P than do mature ones.Gestating and lactating animals need more P

than other classes of mature animals. Specific Prequirements for maintenance, growth, lactation, and pregnancy depend on many factors. Recommendations in NationalResearch Council (NRC) publications arebased on complex models that consider body

size, breed, milk productionlevels, and environmentalconditions.

For dairy cattle, the Ca:Pratio should be at least 2.4:1for cows when lactating, butshould be less than 1.6:1 fordry cows to minimize Caintake during that period. Forbeef cows and feedlot cattle,the ratio is not so critical,although normally it would notbe allowed to exceed 4:1.

Supplemental dietary P is needed undermost practical feeding situations. Deficiency ofP is the most widespread and economicallyimportant of all the mineral deficiencies affect-ing grazing livestock.

On grazed pasture, where soils are low inP, fertilizing with P can reduce risk of grasstetany. Research in Missouri showed thatadding about 60 lb/A of P2O5 increased themagnesium (Mg) content of tall fescue leaves.

Phosphorus enhances reproductive perfor-mance at several stages in the reproductivecycle. In Arizona tests, P increased rebreedingefficiency for beef cows. Irregular estrus peri-ods have been associated with moderate P defi-ciency, infertility with marginal P levels, andanestrus with low P levels in Australia. In Texastests, 64 percent of the control cows produced acalf on range alone compared to 85 percent ofthe cows on range plus P supplement.

Phosphorus has been shown to increasefertility, calving rates, calf growth rates, and,when applied to pastures, carrying capacity.

Swine: A suggested ratio of total Ca to

32 Better Crops/Vol. 83 (1999, No. 1)

Phosphorus in Animal Nutrition

Phosphorus (P) is an essen-tial nutrient for all animals.Deficiency of P is the mostwidespread of all the mineraldeficiencies affecting live-stock. Phosphorus must bebalanced in the animal dietwith adequate calcium (Ca)and vitamin D for growth,reproduction, gestation, andlactation.

total P is between 1:1 and 1.25:1. When basedon available P, the ratio between total Ca anddigestible P should be 2.8:1 to 3.3:1. Adequatevitamin D is needed for Ca and metabolism, buta very high level of vitamin D may mobilizeexcessive amounts of Ca and P from bones.

The biological availability of P in cerealgrains is variable, ranging from less than 15percent in corn to as much as 46 percent inwheat (see Table 1). The greater availability ofwheat P is due to a naturally occurring phytaseenzyme. Estimates of availability of P differsomewhat between European and NorthAmerican sources. In particular, the Europeanfeed industry is tending to use monocalciumphosphate in preference to dicalcium phos-phate. Low-phytate varieties of corn and barleyare being developed. These will likely havenormal P content with much higher bioavail-ability to both swine and poultry (non-rumi-nants).

Microbial phytase can be added to cerealgrain-oilseed meal diets to make grain P moredigestible. The P in a typical corn-soybeanmeal diet is only about 20 percent digestible,but adding phytase can increase the digestibil-ity to as much as 46 percent.

The use of phytase can reduce dietary Prequirements and lower P excretion by as muchas 30 percent. Recent studies in Europe sug-gest that adding phytase can also improve feedconversion slightly, by 1 to 2 percent. One dif-ficulty with phytase is its sensitivity to heat dur-ing feed processing.

Poultry: Hens use most of their P in bod-ily functions other than egg production. Butadequate P is important to achieve a high rateof egg production.

Phosphorus deficiency causes lower bodyweight, reduced feed efficiency, skeletal prob-

lems, and reduced eggshell quality. Low diet Pcan depress egg hatchability, but P content ofthe egg is not altered.

Caged layer hens require high P, morethan hens on litter. “Cage layer fatigue syn-drome” is caused by low P levels in diet. Thereis a high death rate.

Much work on recommended P levels hasfound NRC recommendations to be sound andadequate.

Horses: The Ca and P requirements ofhorses have received considerable attention.Both nutrients are essential for strong bonedevelopment, proper mineralization of osteoidtissue, and adequate energy utilization.

The Ca:P ratio should be monitored whenP intake is greater than Ca and when low Cautilization from feedstuffs occurs. Calcium: Pratios of 6:1 do not appear detrimental tomature horses if P intake is adequate. Foals andyearlings have been fed Ca:P ratios of 3:1 withno problems.

Goats: Phosphorus is required for tissueand bone development. A deficiency will resultin slow growth, “pica” appetite, and unthriftyappearance. Low levels of P in the blood oftenaccompany it.

Dogs and Cats: Low P diets seldomoccur in properly fed pets. However, animalsrequire adequate Ca and P in their diets toensure strong bones and teeth and good muscledevelopment. A P deficiency in puppies causesrickets and poor growth. In cats, a high meatdiet can cause an imbalance of Ca to P, becausemeat is high in P.

Appreciation is expressed to Dr. Larry Chase of CornellUniversity and to Dr. Jock Buchanan-Smith, Dr. SteveLeeson and Dr. C.F.M. de Lange of the University ofGuelph for their assistance in reviewing this chapter.

Better Crops/Vol. 83 (1999, No. 1) 33

TABLE 1. Amounts and estimated availability of P in selected feed materials for swine.

Soybean Dehulled Meat and Dicalcium MonocalciumCorn Barley Wheat meal canola meal bone meal phosphate phosphate

%

P content 0.28 0.36 0.37 0.65 1.01 4.98 18 25(NRC-U.S.)P availability 13 27 45 28 19 81 87 90(NRC-U.S.)P availability 16 37 46 38 30 80 65 80(CVB-Europe)