In most dairies, forages make up 40 percent to 60 percent of a ration.

At such a high inclusion rate, forages can have a tremendous impact on your cows’ diet. That’s why many of you pay close attention to the nutrients — particularly fiber, protein and energy — that your forages contribute to the diet.

However, one nutrient isn’t given its fair share of attention. That nutrient is phosphorus. It is of particular concern because an excess of phosphorus in the diet can result in significant environmental consequences. That’s why many of you must account for it in your nutrient-management plans.

Forages may contribute more phosphorus to the diet than you realize. But, you don’t know just how much phosphorus they bring to the ration until you have them analyzed for it on a regular basis. Here’s why you need to take a good look at the phosphorus content of your forages.

So much variation

If you have always relied on “book values” to tell you the phosphorus content of your forages, you may be sharply under- or over-estimating the true amount of phosphorus those forages contribute to the diet. That’s because the phosphorus content of forages varies significantly.

Forage-testing data from Dairyland Laboratories, Inc., headquartered in Arcadia, Wis., can attest to that. Last year, the phosphorus content of 1,254 legume haylage samples analyzed at Dairyland Labs ranged from 0.22 percent to 0.42 percent of forage dry matter. (For more details, please see “Forage phosphorus varies” on page 46.)

In contrast, the book value for phosphorus in legume haylage is 0.32 percent, according to the 2001 “Nutrient Requirements of Dairy Cattle,” more commonly referred to as the “dairy NRC.”

With that type of variation, you can’t rely on a book value to tell you the phosphorus content of legume haylage — or any forage, for that matter. A phosphorus analysis, preferably obtained from a wet-chemistry test, is the only way to know the actual amount of phosphorus your forages bring to the diet.

Enough to meet her needs

According to the dairy NRC, lactating cows require about 90 grams of dietary phosphorus per day.

If a cow eats 13 pounds of alfalfa haylage with a phosphorus content of 0.30 percent, she consumes 18 grams of phosphorus, says Kurt Cotanch, forage lab manager at the W.H. Miner Agricultural Research Institute in Chazy, N.Y. However, if a forage-phosphorus analysis shows that same amount of forage contains 0.40 percent phosphorus, her daily phosphorus intake from alfalfa haylage is actually 24 grams, or more than 25 percent of her phosphorus needs.

Now, add to that the 18 grams of phosphorus supplied by 13 pounds of corn silage in her diet, and you meet more than 45 percent of the cow’s phosphorus requirement with just these two forage sources alone. (For more details, please see the tables below.)

The point is, the forages in the diet — in combination with grains and byproduct feeds — can easily meet, or even exceed, the cow’s phosphorus needs, Cotanch says. 

The opposite scenario also can occur if forages supply too little phosphorus. If that happens, the diet may be somewhat deficient in phosphorus, Cotanch adds. In general, though, excessive phosphorus in the diet is a more common problem today.

Knowing the phosphorus content of your forages can help you keep dietary phosphorus at appropriate levels.

Get rewarded
Great strides have been made toward the goal of reducing excess phosphorus in dairy rations.

In Wisconsin, for example, dairy producers have taken the message about lowering dietary phosphorus to heart. Since early 2002, the phosphorus content of lactating-cow rations submitted for analysis to the University of Wisconsin Soil and Forage Analysis Laboratory in Marshfield, Wis., has steadily decreased from 0.50 percent to 0.40 percent of diet dry matter. That’s getting close to the current dairy NRC recommendation of 0.32 percent to 0.38 percent for lactating Holstein cows that produce between 55 and 120 pounds of milk per day. Although this is a step in the right direction, there is still room for improvement.

Knowing the phosphorus level of your forages is a step in the right direction — a step that may yield monetary rewards, as well.

As early as next year, Wisconsin producers may get paid to lower phosphorus in their rations. The Wisconsin Natural Resources Conservation Service (NRCS) is expected to approve the “Feed Management Incentive Payment” sometime next year, says Don Baloun, assistant state conservationist with the NRCS. According to preliminary details, Wisconsin producers who reduce phosphorus in their rations to 0.40 percent or less will qualify for an incentive payment funded by the Environmental Quality Incentives Program. 

Someday you, too, may be rewarded for trimming excess phosphorus levels in your rations. Testing your forages for phosphorus is a good place to start.


Forage-phosphorus testing goals

  • Test all forages. test each new load of baled hay that pulls into the driveway, each new cutting of alfalfa or grass haylage, every bunker or bag of corn silage.
  • Test regularly. An analysis done at least twice per month is a good place to start.
  • Request a wet-chemistry analysis. It’s more accurate for minerals than near-infrared spectroscopy (NIR).