Managing Dietary Phosphorus in Lactating Cows

When it comes to feeding phosphorus to your dairy herd, research studies show, you can have too much of a good thing. An essential dietary mineral for livestock and a valuable fertilizer component, phosphorus becomes a potential environmental pollutant when overfed.

In your dairy herd’s diet, phosphorus, or just P as it’s often known, is necessary for bone strength, energy metabolism and milk production. It’s found in every cell of a cow’s body. But feed too much and the excess phosphorus ends up in manure. That can harm the environment—an issue becoming more crucial as Ontario gets closer to implementing the Nutrient Management Act. Moreover, feeding more phosphorus than needed can add costs to your feed bill.

Concern about over-feeding phosphorus has arisen in many jurisdictions. This has prompted research to more accurately determine phosphorus requirements that ensure cow health and productivity while minimizing nutrient excretion in manure.

Results of two U.S. studies are among the research findings that prompted the National Research Council (NRC) to revise phosphorus recommendations for dairy cattle in 2001. The new recommendations, at 0.36 to 0.4 per cent of dry matter, are lower than previous guidelines—almost 0.5 per cent of dry matter.

Short-term nutritional deficiencies of P can complicate phosphorus research. The cow’s biological process draws on her body’s phosphorous reserves to maintain normal levels in blood. This can hide a dietary deficiency, making long-term studies an important part of determining phosphorus requirements.

Many dairy farmers and nutritionists have historically associated optimal reproductive performance with phosphorus levels much higher than the new NRC recommendations. Reproductive performance was, however, an important area of investigation before the NRC released its revised guidelines.

Larry Satter of the U.S. Dairy Forage Research Center and his colleagues recently summarized phosphorus research from the last three decades. The table on page 30 shows some of their findings. These data revealed that higher dietary levels of phosphorus, at 0.39 to 0.55 per cent of dry matter, didn’t improve reproductive performance.

Satter’s group has also published results of a two-year experiment using 65 Holstein cows, including 30 over two lactations. The cows were fed a diet containing 0.38 per cent P or, with the addition of monosodium phosphate and dicalcium phosphate, a diet containing 0.48 per cent P. All cows in both years were allowed to graze for part of the year. During the grazing period, the phosphorus content of the diets was estimated at 0.31 per cent for the low-P group and 0.44 per cent for the high-P group.

Blood phosphorus measurements found no significant difference between the two groups of cows. Values for both groups were within the normal range. Results also indicated no difference in milk production or reproductive measures as a result of the different dietary phosphorus levels.

The researchers noted that having more cows in an experiment would’ve been useful. However, their results agreed with other studies looking at phosphorus levels and reproductive performance.

A second experiment reported by Satter’s group in 2000 measured milk production, reproductive performance and fecal excretion from three diets having different phosphorus levels. The diets were similar to many Ontario dairy rations—mainly alfalfa silage, corn silage and high-moisture corn. The three diets were formulated to have 0.31, 0.40, or 0.49 per cent phosphorus on a dry matter basis. Researchers added monosodium phosphate to raise the phosphorus content of the diet to medium and high levels. The low-P diet had no supplement added.

Phosphorus balance was estimated between weeks two and 40 of the experiment. Researchers subtracted total phosphorus output in milk, feces and urine from total phosphorus intake. Milk production averaged more than 11,000 kg for 308 days for the experiment.

A dietary phosphorus deficiency can affect milk production, feed consumption and animal performance. In this experiment, the 0.31 per cent P diet supported milk production equal to the higher P diets for only the first two-thirds of the lactation. Milk production from the low-phosphorus group was about 3.3 kg per day lower in the remainder of lactation. This result was consistent with other studies that showed reduced milk yield when dietary phosphorus was intentionally less than required, at 0.24 to 0.3 per cent on a dry matter basis.

As dietary phosphorus levels are lowered in a cow’s diet, she can increase the amount of P absorbed from her gut. This improves the efficiency of phosphorus digestibility. But there’s always a proportion of phosphorus in her diet that a cow can’t absorb. So it’s important to make efficient use of digestible dietary phosphorus.

It’s possible for early-lactation cows to be temporarily deficient in dietary phosphorus because they can access P stored in tissues and eventually from bone. This type of short-term early-lactation deficiency isn’t a problem when it’s corrected later, similar to calcium metabolism.

Implementing a strategy to feed more accurately to a cow’s phosphorus requirements has been shown to benefit the environment. Reducing dietary phosphorus to 0.4 from 0.5 per cent for a cow consuming 25 kg of dry matter per day would save 25 grams of P per cow per day.

Besides benefiting the environment, reducing dietary P can benefit your bottom line. U.S. studies have shown savings on supplemental mineral purchases of $15 to $20 per cow per year.

We can apply some principles from this research and the new NRC recommendations on Ontario dairy farms. While phosphorus is an important part of overall dairy cow nutrition, dairy rations typically feed P at about 20 per cent above the requirement. This provides no benefit to reproductive performance or higher milk production.

The new NRC guidelines contain an adequate safety margin because of conservative values for P absorption. By feeding to these recommendations you can benefit the environment and, possibly, modestly boost your bottom line at the same time.

For further reading:

Wu, Z. and L. D. Satter. 2000. Milk production and reproductive performance of dairy cows fed two concentrations of phosphorus for two years. Journal of Dairy Science. 83:1052-1063.

Wu, Z., L. D. Satter, and R. Sojo. 2000. Milk production, reproductive performance, and fecal excretion of phosphorus by dairy cows fed three amounts of phosphorus. Journal of Dairy Science. 83:1028-1041.

Table 1. Summary of research trials comparing dietary phosphorus intake and reproduction in dairy cows based on the new NRC range and diets above that new range.

Reproductive Measure	NRC Range	Above NRC Range
Days to first heat	46	48
Days to first AI	73	76
Days Open	96	100
Services/Conception	1.8	1.9
Pregnancy Rate	0.87	0.86

Adapted from Wu and Satter (2000)

Note: This article was originally published in the Ontario Milk Producer ("Waste Not, Pollute Not"), March 2003, pages 28-31.

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