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Researchers take second look at phosphorus requirements for cattle

Necessary for bone growth and reproduction, the phosphorous requirements of beef cattle were believed to have been determined in the late 1800s.

Necessary for bone growth and reproduction, the phosphorous requirements of beef cattle were believed to have been determined in the late 1800s. But, current economic and environmental affects of this essential mineral are sending researchers back to study whether cattle really need as much as was previously decided.

Primarily found in forages, phosphorous is one of the most costly minerals to supplement, said Dr. John Paterson, extension beef specialist at Montana State University in Bozeman, Mont.

Producers primarily use dicalcium phosphate, which is a mined product.

Current phosphorous requirements are based on research conducted in the late 1950s when it was discovered the mineral was needed for reproduction. Producers generally supplement a mix of 12 percent each of calcium, phosphate and salt.

What Paterson and other researchers want to know is could they take the phosphate level down to 9 percent and still get the same results for less money?

“There's the argument,” he said. “How low can we go to try to save the ranchers? It depends.”

Research has shown that when phosphorous was supplemented, the percent calf crop weaned increased, weaning weights increased and calf weight weaned per cow exposed increased.

Based on these studies, producers have been trying to supplement about six pounds of phosphorous per cow per year. Although that likely is reasonable for cows grazing on native, unfertilized pastures with little or no protein or energy supplementation, it may not be right for cattle that graze on phosphorous-fertilized, cool season grass pastures.

In a recent study, Paterson said it was found that for much of the year the animals just about met their phosphorous requirements without supplementation. But, during the winter and prior to calving, it was too low. So now the question is, should producers supplement in front of the winter versus all year?

“If you give it just once a year, they are going to eat like crazy,” said Paterson. “They are so phosphorous starved.”

Excess phosphorous runoff can pollute surface water by causing algae populations to grow rapidly. Algae decomposition consumes dissolved oxygen in the water affecting the growth and reproduction of fish, clams, crabs, oysters and other aquatic life.

Paterson said one can tell when cattle are deficient in phosphorous because the animals begin to eat dry, bleached bones. This is not a good thing because if cattle eat fresh bones, they stand a chance of contracting botulism.

Other symptoms include stiffness in the front quarters, weak and broken bones, reduced feed intake, impaired reproductive rates, lowered milk production and reduced calf weaning weights.

“Cows just won't cycle,” said Paterson. “The number of calves weaned in the fall goes way down. If you do get a calf, that calf will be lighter.”

According to Paterson, nutritionists will continue to recommend some level of supplemental phosphorous for free ranging animals.

Wayne Greene from Texas A & M University recommends 8 to 12 percent phosphorous for unfertilized grasses, 4 to 8 percent for fertilized warm-season forages and 0 to 4 percent for cool-season perennial and annual forages when designing a free-choice mineral supplement containing 15 to 30 percent salt as the base.

However, in a paper recently published by the Council for Agriculture Science and Technology it was recommended that beef producers should discontinue supplementation of phosphorous in feedlot diets. Some nutritionists are changing formulations because recent research indicates that phosphorous requirements may be lower than previously accepted and they also have concerns about the environmental consequences of over feeding phosphorous.

Paterson said he agrees that there is a case for lowering the amount of supplemental phosphorous fed to beef cattle. Yet, before that amount can be determined, extension specialists like himself need to know what producers are feeding.

These studies are important to producers because phosphorous is a biological necessity, but perhaps they can feed less than they have in the past and maintain healthy cattle.

It is important to have forage analyzed to determine how much should be supplemented, Paterson concluded.