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0207 PD: The influence of forage quality on phosphorus balance on the dairy farm

Charles C. Stallings Published on 06 February 2007

The feeds selected for use in the ration have an influence on overall dietary phosphorus concentration. If no inorganic mineral phosphorus source is added, the only way to change phosphorus content is to change the amounts and proportions of feeds used. Even when all inorganic phosphorus has been removed, rations are still sometimes over the recommended amounts.

Selection of feeds with low phosphorus content is possible. Since forages are typically low in phosphorus relative to protein meals and certain byproduct feeds (cottonseeds, wheat bran and midds, brewer’s grains, distillers grains), it is possible to sometimes reduce phosphorus by feeding more forage. For this to occur the forage must be of good to excellent quality.

Phosphorus content of feeds
The 2001 National Research Council’s nutrient requirements for dairy cattle contained new guidelines for phosphorus requirements and also availabilities of phosphorus in feeds. Before, it was assumed there were large variations in phosphorus available between forages and other feeds.

The 2001 NRC assumed forages to have 64 percent of phosphorus available versus 70 percent for most other feeds. The exception was corn silage which was assumed to have 70 percent available, probably because corn silage has a high concentration of corn grain.

Phosphorus content of forages does vary with maturity. More mature legumes and grasses tend to have less phosphorus. Also, rye silage and intensively managed pastures have higher concentrations than corn or barley silages. However, overall forages do have lower concentrations of phosphorus than many other feeds used in dairy rations.

When we bring feeds into the ration to supply protein, many times we bring phosphorus with it. This is true for the protein meals, whole seeds and byproduct feeds. Grains such as barley and corn do not have elevated concentrations.

The question arises as to what can be done when the phosphorus content of the ration is excessive and no inorganic sources are included in the ration. Below are some feeds I have identified that are low in phosphorus. Some of these are readily available such as citrus pulp, cottonseed hulls and soybean hulls. Others may have application under certain circumstances and limitations.

The value of feeding forages
We all know forages are usually considered to be desirable feeds for lactating dairy cows. A great deal of discussion has occurred as to the optimal ratio of forage to concentrate in the ration. This will vary with type of forage fed (alfalfa versus corn silage) and use of other feeds such as fibrous byproduct feeds such as cottonseeds and brewers grains.

The ration fed at the Virginia Tech dairy in 2005 contained 17.8 percent protein, 0.77 Mcal net energy per pound, 21 percent ADF, 33.4 percent NDF and 6.2 percent fat. The ratio of forage to other feeds is 40-to-60. The amount of phosphorus supplied could support a herd average of approximately 100 pounds of milk per cow per day. It did contain some inorganic phosphorus in the protein and mineral mix which was mainly soybean meal.

By reducing the phosphorus in the mix from 0.8 percent to 0.6 percent we could reduce the phosphorus intake to 92.2 grams or 0.4 percent of the ration dry matter (DM). A rebalanced ration with less cottonseeds and brewers grains and more forage allows us to reduce the phosphorus further.

This revised ration contains forage to other feeds ratio of 49-to-51. The protein and energy are slightly lower at 17.4 percent and 0.77 Mcal net energy. ADF was 21.3 percent, NDF 33.8 percent and fat 4.9 percent. The reduced phosphorus was still adequate for the level of production of the herd. In this revised ration, more of the phosphorus comes from the forages (35 percent versus 25 percent).

To be able to increase forage in rations and reduce other feeds, high quality must be maintained. Is it possible to feed a ration with greater than 50 percent forage? A field study by Tylutki et al. actually did this over a five-year period. In the initial year, 43 percent of the ration DM came from home-grown forage versus 57 percent from purchased ingredients.

By changing crop rotations, fertility practices and harvest strategies the farm was able to improve forage quality so that by year five, 59 percent of ration DM came from forage and 41 percent came from purchased ingredients. During this time the herd increased from approximately 400 milking cows to more than 500.

Their goals for quality were to have NDF levels of 50 to 52 percent for grasses, 37 to 40 percent for alfalfa and 37 to 42 percent for corn silage. Their overall goal was to have NDF intake at 1 percent of bodyweight. During this five-year period, manure N excretion was reduced by 17 percent and manure P excretion 28 percent.

Phosphorus availability of inorganic sources
If an inorganic phosphorus source is needed, the availability of phosphorus should be considered. For instance, less monosodium phosphate (90 percent available) would be needed to meet a certain absorbed phosphorus requirement (similar to the NRC requirement) than defluorinated phosphate (65 percent). These should be considered in ration formulation; however, in most situations no or little extra phosphorus is needed with the feeds available.

Phosphorus content of feeds will vary by feed type. Forages are typically lower in phosphorus than byproduct feeds and are usually produced on the farm using recycled nutrients. There are indications phosphorus can be increased in forages from fields that have high levels of fertilization, but this increase is not extreme. There are some readily available byproduct-type feeds low in phosphorus such as soy hulls and citrus pulp. High-quality forages can be fed at greater percent of ration DM than low-quality forages without loss of milk, resulting in better cow health and nutrient recycling.  PD

References are available upon request.

—From 2006 Virginia State Feed Association and Nutritional Management Conference Proceedings

Charles Stallings
Professor and Extension Dairy Scientist

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