Phosphorus (P) is a vital nutrient for many physiological processes in the body and instrumental in milk production. In fact, nutritionists and producers often say that P is crucial for reproductive performance, although there is no literature to date that corroborates this statement. However, P is well known for its negative environmental implications when not properly handled in manure.

Jamie Jarrett

The problem is the industry has begun to feed P beyond a lactating cow’s requirement. The National Research Council (NRC) (2001) states that the P concentration in the ration of a lactating cow should be between 0.32 and 0.42 percent of dry matter.

However, most dairy rations are formulated beyond 0.45 percent P. A survey of nutritionists provides some insight as to why. The survey, part of a 10-year study of P, indicates that the primary reason excess P is provided to lactating cows is because of a lack of confidence in the bioavailability estimates of P in various feedstuffs and, secondly, cost associated with a lower-P ration.

Knowing this, Dr. Partha Ray and myself, under our adviser, Dr. Katharine Knowlton, in the dairy science department of Virginia Tech , conducted a series of digestibility experiments to address the issue of P bioavailability in dairy cows.

The questions we wanted to answer included: Does P form (phytate, organic P and inorganic P) affect the propensity for P to be absorbed or available?

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Where in the gastrointestinal tract is phytate digested? What is the maximum capacity of the small intestine for P absorption? Can P be absorbed in the large intestine? Does phytase have a place in dairy rations?

Phytate is a form of P that is unavailable in the gut of non-ruminant animals. It is a six-carbon ring structure with a phosphate molecule on each carbon. In ruminants, some microbial populations in the rumen have the capacity to digest phytate to make the phosphate molecules available for absorption.

However, the extent of ruminal degradation of phytate is somewhat dependent upon a variety of factors including grain content of the ration, dry matter intake and passage rate. Phytate content in grain is approximately 70 percent of the total P in grains and similar concentrations have been observed in byproducts as well.

As a result, a significant amount of P is excreted and lost to the environment because it is not available to the animal.

Beginning in the 1990s, the swine industry turned to phytase as an option to reduce P in the ration. Swine feed is comprised mainly of corn, soybean meal and some byproducts, so approximately 70 percent of the P in swine diets is in the phytate form and cannot be utilized by the pig without phytase.

Phytase is an enzyme that liberates phosphate molecules from the phytate compound, making it available to the animal. While rumen microorganisms have some phytase capabilities, phytase can also be made synthetically and included as an additive in swine diets to improve overall P digestibility.

This enzyme is used extensively in swine and poultry production to minimize total P inclusion and maximize utilization of P in feedstuffs.

Research surrounding feeding phytase in lactating cows has had some variable results. We conducted an experiment to investigate the effect of feeding rumen-protected phytase in late-lactation cows fed 50 percent of total dietary P as phytate.

No real benefit was observed; the fact that the animals were in late lactation proved to be a limitation. Phytate digestibility in the rumen of late-lactation cows is much higher than that of early lactation animals. Animals in late lactation tend to have lower dry matter intakes and, therefore, lower passage rates.

Longer feed retention time in the rumen leads to higher ruminal phytate digestibility because phytate is exposed to ruminal phytase much longer than an animal with high dry matter intake and a faster passage rate.

Regardless of stage of lactation, phytate digestibility may not be complete in the rumen and there is some capacity for phytate to be digested in the large intestine. Phosphorus absorption occurs primarily in the small intestine. Other locations of P absorption in the gastrointestinal tract are not clear.

To have a direct answer on phytate digestion and phosphate absorption in the large intestine, we designed an experiment to answer this question. Various levels of phytate were infused into the beginning of the large intestine and phytate and total P concentrations were measured in the feces to determine total phytate digestibility and absorption of P in the large intestine.

We concluded that about 15 percent of the phytate entering the large intestine was digested and 5 grams per day of P was absorbed.This indicates that there is some capacity for phytate digestion beyond the rumen and phosphate absorption beyond the small intestine.

Determining the maximum absorption capacity of the small intestine for P is key to determine necessary P supply needed to meet animal requirements. Because P intake and excretion are tightly linked, we suspected there was some “breaking point” at which efficiency of utilization was reduced.

To clearly answer this question, phosphate solutions of various concentrations were infused into the beginning of the small intestine and digesta was collected at the end of the small intestine. Phosphorus concentration infused in conjunction with P supply from the diet equaled 50, 80, 110 or 140 percent of NRC (2001) requirement.

No negative effects were observed at any of the infusion levels. Milk production was not affected by the P supply in the small intestine. When cows were fed 50 percent of P requirement (no phosphate infused), blood P was reduced; however, the cow did not sacrifice production.

It is important to note that these treatments were short-term and, therefore, at 50 percent of P requirement the animal may not be able to sustain high production throughout an entire lactation.

When cows were supplied with 80, 110 and 140 percent of requirement, there was no alteration in blood P. The amount of inorganic P absorbed increased with increasing infusion of inorganic P, but the percent of infused that was absorbed dropped from 83 percent to 53 percent. Thus, absorption of inorganic P from the small intestine is not complete.

From this information we can conclude that absorption of P from the large intestine does occur at a level that contributes to total P balance of the animal.

Additionally, current NRC (2001) recommendations for P may be exceeding P requirement of the animal. The sweet spot, or optimized efficiency of utilization, in the short term, seems to be at 80 percent of NRC (2001) requirement indicated by the animal’s ability to maintain blood P. How this translates into long-term performance needs further investigation.

Remember, if you decide to make a ration change, animals do not eat percentages – they eat grams, pounds and kilograms of nutrients. So it is important to know dry matter intake of the animals when reducing dietary content of any nutrient to at or below requirement to be confident in the nutrient supply to meet the animal’s needs. PD

Dr. Jamie P. Jarrett is a dairy marketing nutritionist with Land O’Lakes Purina Feed . Click here to contact her via email, or at (651) 375-5579.

References omitted due to space but are available upon request. Click here to email an editor.

Jamie Jarrett