Phosphorus Inclusion in Free-Choice Mineral Supplements

by Dr. John Arthington, University of Florida/IFAS

published in The Florida Cattleman and Livestock Journal

As phosphorus costs have increased over the past couple years, I have received several questions regarding high-salt, low-phosphorus trace mineral supplements for our Florida cows. As little as 8 to 10 years ago, it was common to see free-choice, salt-based mineral supplements fortified with 9 to 15% phosphorus. Indeed, in 1998 our own blend at the Range Cattle Research and Education Center contained 12% phosphorus. Since that time we have gradually decreased this level to approximately 5% phosphorus. Although free-choice mineral supplements with greater than 10% phosphorus are still widely available in the marketplace, we are now seeing many products offered with low-phosphorus inclusions at a greatly reduced cost. It is important to understand that a low-phosphorus mineral supplement is not appropriate for all production circumstances, but in many cases these alternatives could provide significant savings without a reduction in performance. 

The phosphorus requirement of beef cows varies with stage of production. For example, most of our Florida beef cows have a phosphorus requirement ranging from 0.18 to 0.24% of the total diet dry matter. Assuming a dry matter intake of 2.75% of cow body weight, this would equate to 25 to 33 grams of phosphorus daily. The majority of this intake will come from pasture or stored forage, thus forage phosphorus concentration is the most important variable when determining the suitability of a low-phosphorus mineral mixture. A common range for the phosphorus content of improved perennial grass pastures is 0.18 to 0.22%, although examples of samples above and below this range are seen. In this scenario, cows will be phosphorus deficient in only a few months of the year which corresponds to times of peak lactation. For most Florida production systems, peak lactation occurs in the late fall or early winter, a time when many Cattlemen are also providing winter supplements. These supplements serve as another source of phosphorus for the cow, complementing the phosphorus consumed in the forage. For example, molasses-based liquid supplements typically contain supplemental phosphoric acid, an excellent source of dietary phosphorus for cows. A product, such as this, containing 0.70% phosphorus and consumed at 5 to 6 lb daily will supply 16 g of supplemental phosphorus leaving a deficiency of 16 grams (at most), which the cow would need to overcome from the forage consumed. In this scenario, the forage would only need to contain 0.14% phosphorus to meet the cow’s full requirement. This concentration would be considered quite low and would not be commonly witnessed in improved perennial grass pastures in Florida. To assess how your winter supplement contributes to your cow’s total phosphorus requirement, multiply the daily amount of supplement consumed by the phosphorus content of the supplement (contained on feed tag) and subtract this from her requirement. With this information, producers can estimate the amount of additional phosphorus needed (if any) to avoid deficiency. 

During the spring and early summer months, when cows are consuming forage with no supplemental feeds, they are solely dependent upon the phosphorus content of forage to meet their requirements. Fortunately, spring forage regrowth typically contains the greatest phosphorus concentrations of the year. In addition, cows have a unique ability to select forage of greater quality than the average of the forage mass presented in a pasture. Thus, provided there is adequate forage for selection, cows will select forage with approximately 20% more phosphorus than the total plant mass available. This is only possible, however, if there is sufficient pasture forage available, which is a common condition during the late spring and summer in Florida. A phosphorus deficiency, if any, should then be addressed through the consumption of phosphorus-containing mineral supplement. 

In closing, it is important to mention that cows do not have the nutritional wisdom to consume free-choice mineral supplements at the level needed to meet their phosphorus requirement. In fact, during the summer months, cows will often consume significantly more free-choice mineral than the tag’s targeted range. This is due to their craving for salt and has no association with their phosphorus need. During these times, producers can realize significant cost savings by hand mixing their free-choice mineral supplements with straight stock salt. This additional salt will help satisfy the cow’s craving at a significantly reduced supplement cost. 

To better assess the total phosphorus intake and requirement of your cows, please contact your local IFAS County Extension Agent or the Faculty at the IFAS Range Cattle Research and Education Center in Ona.