Crop Comments: Grass-fed dairying tends to deplete soil phosphorus

Dietitians tell us that a person consuming an 8-oz. glass of cow’s milk is ingesting approximately 232 mg of phosphorus (P), a nutrient vital for bone health, energy metabolism and cellular function. Taking these numbers further, I calculate that approximately 1 lb. of elemental P departs the farm in every 1,000 lbs. picked up by the milk hauler.

At some point, even though this approximately 0.1% level of this element’s presence in milk may not sound like much, it represents a very real measurable net loss of an element that has to be replaced. If it’s not replaced, its deficiency directly impacts cattle performance.

With “regular” (grain-based) dairying, the depletion of P from soils is much less. This is due to at least two reasons: first, homegrown grains tend to require more P to ensure adequate yields, while homegrown forages tend to achieve satisfactory yields with lower P applications. Secondly, dairy farmers importing purchased grains onto their farms generally enjoy a net increase in P introduction. This is because, on average, grains tend to test about twice as high with this element compared to forages.

For example, most forages, on a dry matter basis, test about 0.20% – 0.25% P. Grains commonly fed to cattle are soybean meal with 0.65% P, corn meal at 0.25% P and distillers dried grains at 0.40%.

Folks formulating dairy rations aim for P levels in each ingested pound of dry matter to include between 0.35% and 0.55% P, dependent on targeted milk production levels. Milking cattle not receiving the necessary P in their diets tend to mobilize this mineral (as well as calcium, Ca) from bone tissue. Most dairy nutritionists believe that close-up dry cow diets should be managed to force the animal to mobilize some Ca and P from bone reserves. This happening helps jump-start the cow’s endocrine system, “convincing” it to mobilize Ca and P from these reserves as well as to utilize dietary Ca and P more efficiently in supporting her assigned task: making milk. (This pre-freshening mobilization of these two elements has been shown to reduce milk fever cases.)

Increasingly, manure from grass-fed (zero grain-fed) dairy cattle has been shown by lab analysis to have less P than manure from grain-fed dairy cattle. That said, it’s particularly critical for fields providing roughage (including pasture) to cattle fed zero grain be soil-tested. Soil P deficiencies, if left uncorrected, become cattle dietary deficiencies for that element. Those deficiencies in turn predispose the cow for milk productive – as well as general reproductive – shortfalls.

There are other jobs we’ve assigned to the dairy cow that need P as a “cornerstone.” Hinted at earlier is bone health, which leads to a discussion of bone disorders. These are a significant concern in the dairy industry, affecting the health and well-being of dairy cows. These disorders can lead to bone loss, rickets, osteomalacia and osteoporosis, which affect milk production and livestock management. (Osteoporosis involves bone thinning, while osteomalacia is a problem with bone mineralization, or how the bone hardens in the first place.)

To prevent and manage osteoporosis in dairy cows, farmers and veterinarians should provide a balanced diet, ensure adequate calcium and P intake, monitor cows for signs and symptoms and take steps to identify these conditions.

A frequently asked question directed to livestock nutritionists is how dietary P levels benefit fiber digestion in ruminants. The answer is that dietary P does not directly aid in fiber digestion, but it plays an indirect role by supporting the microbial populations in the rumen that are essential for breaking down fiber. According to Mississippi State University (MSU) Extension, in ruminants, fiber digestion is carried out by rumen microorganisms (bacteria, protozoa and fungi). These produce enzymes to break down cellulose, hemicellulose and other structural carbohydrates. These microbes require P for cellulose digestion, protein synthesis and overall metabolic activity. Phosphorus is also recycled in the rumen via saliva, which helps maintain microbial P supply.

When dietary P is inadequate, microbial P recycling decreases, leading to reduced cellulose digestion efficiency, lower feed digestibility, decreased feed intake and lower production performance. According to MSU Extension, “Research on fiber transit kinetics in cattle shows that phosphorus supplementation can improve fiber digestibility parameters, such as the rate of particle passage from solid to liquid in the rumen-reticulum. This suggests that adequate phosphorus supports the microbial processes that make fiber more accessible and digestible.”

These scientists summarize their findings as follows: A direct effect of P is that it is not a fiber-digesting enzyme; rather, it is a nutrient for microbes. As an indirect effect, adequate P maintains microbial populations and activity, which in turn improves fiber breakdown and digestibility. As a practical implication, even on pasture, P supplementation is often necessary to maintain optimal fiber digestion and production in cattle.

The key takeaway message is that dietary P supports fiber digestion in cattle by sustaining the microbial populations responsible for breaking down plant fiber, but it is not a direct digestive agent itself.

Now, let’s mentally cross the Atlantic to tap into the wisdom of a European NGO called Inorganic Feed Phosphates (IFP). I’ll quote these scientists, as they address the subject of “Sustainability of Inorganic Feed Phosphates.” (“Inorganic” here means non-carbon-based.) They wrote, “Feed phosphates, as most mineral raw materials used in the feed industry, come from rock, which is a finite resource. Only 5% to 10% of this rock phosphate is dedicated, after industrial process, to inorganic feed phosphates, the rest being used for fertilizer, food or by other industrial sectors. Despite the low share of feed phosphates within the use of the rock, the IFP Sector Group of the European Chemical Industry Council pays attention to the sustainability of this essential raw material for animal life. Indeed, feed phosphates bring phosphorus which is indispensable for bones’ mineralization, energetic metabolism, membrane integrity and DNA structure” (feedphosphates.org).

by Paris Reidhead