Beef Cattle

How much phosphorus supplementation do beef cows need?

The array of mineral supplements available is often overwhelming. With all of the choices in phosphorus levels, how do you know which one is right for you? Why is phosphorus so important anyway?

Phosphorus is extremely critical for animal nutrition — so critical, in fact, that it is found in every single cell of the body. Without adequate phosphorus in the diet, animals don’t grow, breed or produce milk. Severe deficiency can result in lethargy, gauntness, rough hair coat and brittle bones that are more prone to fracture. Even mild deficiency can result in cattle that appear normal but have reduced production.

Hay and forages grown on fields with traditionally high soil phosphorus levels are likely to contain adequate amounts for your cattle. A recent Virginia Tech study comparing soil and forage phosphorus levels gathered in the Chesapeake Bay Watershed found that over half of the soil samples contained high or very high phosphorus levels (greater than 0.3% phosphorus on a dry matter basis). Additionally, out of 168 forage samples, over 90% came back with adequate phosphorus to support all production levels for average beef cows.

But a comparison of phosphorus found in the soil and phosphorus found in forages (Figure 1) revealed that this relationship is not linear, nor is it predictable. Many forage samples collected from soils containing relatively low phosphorus came back with higher phosphorus levels than forages grown on soils with much higher phosphorus levels. The take-home message from this finding is that you can’t skimp on forage testing and solely rely on soil test results when making your feeding decisions. You must do the forage analysis!

FIGURE 1: Relationship between soil P and forage P (taken from Phosphorus Supplementation of Beef Cattle, M.A. McCann, et. al., Virginia Tech).

FIGURE 1: Relationship between soil P and forage P (taken from Phosphorus Supplementation of Beef Cattle, M.A. McCann, et. al., Virginia Tech).

Forages containing adequate levels of phosphorus are advantageous for you as cattle producers. Phosphorus is one of the most expensive components of a mineral supplement. A supplement with low phosphorus fortification will be significantly less expensive than one containing high phosphorus fortification. This brings us back to our original question: How do you know what level to choose? For the purpose of this discussion, we’re only going to talk about mature beef cows. The phosphorus needs of mature beef cows will vary from roughly 10 grams per head per day for a dry cow in the first trimester of pregnancy up to as much as 30 grams per head per day for a heavy milker in peak lactation (Figure 2). When you think about mineral supplementation, a rough rule of thumb is that the percent phosphorus in a supplement eaten at 4 ounces is the number of grams phosphorus delivered. For example, an 8% phosphorus mineral eaten at 4 ounces will provide roughly 8 grams of phosphorus, while 10% phosphorus mineral will supply 10 grams and so on.

The first step in this process is to get a chemical analysis done on your hay or feedstuff. Soil tests are great for determining fertilization levels but are poor indicators for nutrient concentrations in forage. Next, compare the dry matter phosphorus level from your test to the values in Table 1 for recommendations on mineral phosphorus levels for various production groups. Book values of phosphorus concentration for most common hay species are generally in the 0.1–0.2% range, but phosphorus levels will vary according to stage of maturity at harvest, available phosphorus in the soil and other factors. It is always a good investment to have every lot of hay chemically analyzed for nutritional content. If you are not familiar with how to do this, contact your local Cooperative Extension office or local NRCS office for more information.



Given the value of every calf born and added profit in every pound of weight gain in today’s market, it’s too risky to take a gamble with phosphorus nutrition. You may be able to utilize a mineral supplement containing lower phosphorus levels and still maintain high productivity if your forages receive adequate phosphorus fertilization (either from preexisting soil phosphorus or by applying fertilizer). The only way to confirm this is through forage testing. Remember the Virginia Tech study? While 90% of samples came back adequate, 10% did not. Don’t risk a cow not getting pregnant or a calf gaining half as much weight as it should by assuming that your forages are better than they are. Think of phosphorus supplementation like auto insurance. Sure, you may go your entire life without getting into an accident. In the same way, you may have adequate phosphorus in most lots of hay. But all it takes is one major accident or one winter of poor-quality hay to cause you to go bankrupt!

ULTRALYX® offers a wide variety of mineral supplements at various levels of phosphorus fortification so that you can customize your feeding program to match your cattle needs and fill in the gaps left by deficiencies in available forages and feedstuffs.

Visit your local ULTRALYX dealer for more information on available ULTRALYX products or call 1-888-718-3493 to speak to an ULTRALYX representative.