Phosphorus, Need it or Leave it?

By Jeff Lehmkuhler, UK Extension Beef Specialist

Dramatic increases in the components used to formulate complete mineral supplements resulted in sticker shock for many beef cow-calf producers. Knee-jerk responses to these increased prices have resulted in some cutting mineral supplements with white salt or discontinuing feeding them all together. Mineral supplements prices, like many inputs, have come down in cost, but not to levels of a few years ago. Sound familiar? How many other items can you think of that have increased in price and not returned their "normal" price? Rather than dwell on these increased input costs, why not consider how to possibly trim these costs?

If the total annual costs to maintain a beef cow are in the $400-$500 range, $40-$50 mineral supplementation expenses are a small slice of the expense pie. Reality sets in, however, when feeder calf prices are low resulting in low revenues. We naturally look for strategies to cut expenditures, especially those that are top of mind. Certainly, mineral supplements are top of mind for many producers that are purchasing frequently. The question becomes, can we cut mineral supplement expenditures and if so, how?

Phosphorus is an essential macromineral involved in bone structure and integrity. It is pivotal in cellular energy metabolism as well. Deficiencies in phosphorus can result in poor performance, reduced fertility and other side effects. Because of its importance in fertility, phosphorus supplementation has been common in dairy and beef production. With greater environmental pressure with respect to surface water quality due to phosphorus's potential to increase algal blooms in bodies of water, phosphorus supplementation to livestock has been revisited. The use of phytase in monogastric diets to increase phosphorus availability with reduced dietary phosphorus supplementation is common today. Dairy nutritionists have reduced dietary phosphorus levels to better match supply of phosphorus to the animal's requirements. The majority of feedlot nutritionists no longer recommend the use of supplemental dietary phosphorus in corn- and corn coproduct-based diets as these diets often meet or exceed the requirements of growing and finishing cattle.

However, many beef cow-calf and stocker operators do not closely monitor mineral status of their forages. Using forage analysis for minerals, we can better match diet supply to the animal's requirements. According to the recent National Research Council's Recommended Nutrient Requirements for Beef Cattle publication (NRC), the phosphorus requirement for growing beef calves is 16 mg/kg of body weight for maintenance plus 3.9 gm of phosphorus per 100 gm of retained protein. To put this in perspective, a 550 lb steer gaining 1.75 lb/d would have an estimated 11 gm of available phosphorus daily. Because a portion of the phosphorus is tied up in the plant, the NRC utilizes a factor of 68% to account for availability in forages. Thus, the needed intake of total phosphorus is equal to the available phosphorus need divided by the percent availability or 11 gm/0.68 = 16 gm of total phosphorus consumed. A calf consuming forage at a rate of 2.6% of body weight would essentially require a forage phosphorus concentration of 0.25% on a dry matter basis.

In Table 1 forage analysis information from pasture samples from West Virginia and Tennessee along with hay samples from Wisconsin is presented. The mean forage phosphorus concentration would exceed the requirement for these calves growing at the rate of 1.75 lb/d and would not require additional dietary phosphorus supplementation. We found no differences in performance for stocker Holstein steers when offered a trace mineralized salt or a 67:33 blend of trace mineralized salt and dicalcium phosphate to provide a 6% phosphorus mineral supplement. There are forages in each state, however, that would require supplementation and forage testing will assist in preventing clinical and subclinical deficiencies.

Table 1. Minimum, mean and maximum forage phosphorus concentration (% of Dry Matter) from Tennessee, West Virginia and Wisconsin.

State	Min	Mean	Max
Tennessee1	0.13	0.36	0.55
West Virginia2	0.10	0.34	0.59
Wisconsin3	0.08	0.25	0.48

¹ Samples collected in 2003 & 2004 fescue and fescue / legume only; 292 samples

² Average of 607 samples collected in WV

³ Average of 56 samples of hay from beef operations from 17 counties in WI

In the southeast, the use of soybean hulls and corn gluten feed as supplements for forage-based beef systems is common with distillers grains supplementation increasing. The phosphorus concentration for 615 samples of corn gluten feed and 3,519 samples of dried distillers grains analyzed by Dairy One laboratory was 1.04% and 0.88%, respectively. Assuming an intake of 3 lbs of corn gluten or distillers grains, the intake of phosphorus would be 14 gm and 12 gm from each of these sources, respectively. At a substitution rate of 0.5 lb of forage for each 1 lb of supplement, forage intake would be reduced only 1.5 lbs. Following the scenario above, a 2.6% intake level for a 550 lb calf would be near 15 lbs or 13.5 lbs of forage and 1.5 lbs of supplement resulting in 10 gm of available phosphorus from the forage. Total available phosphorus intake is calculated to be 22 to 24 gm or 200% to 218% the recommended phosphorus intake.

Consider that often the same 6% phosphorus mineral supplement would continue to be offered free choice at a rate of 4 ounces daily adding another 6 gm of available phosphorus. Instances in which corn gluten or distillers grains would be offered at approximately 2 lb daily would provide the same amount of available phosphorus as a 6% phosphorus mineral supplement consumed at 4 ounces daily. In many forage-based backgrounding or stockering operations feeding corn gluten feed or dried distillers at 2 lbs daily would not require a mineral supplement containing phosphorus. Reducing the phosphorus levels in mineral supplements will lower the cost of the mineral supplementation. Rather than cutting mineral supplements with white salt or removing them completely from the program, consider forage testing and developing a strategic supplementation program.