Reduced Phosphorus Feeding Makes Sense

Feeding more phosphorus (P) than recommended won't improve your milking herd's milk yield or reproductive performance. There is, however, a major benefit to your pocket book and the environment when you don't over-feed this nutrient to your dairy cows.

According to the Canadian Food Inspection Agency's Feed Act, the minimum dietary P requirement for dairy cattle is 0.30 per cent of dry matter intake (DMI). The U.S. National Research Council (NRC) calculates that P should make up 0.33 to 0.38 per cent of the dietary dry matter. Yet many dairy producers feed high-producing cows as much as 0.5 to 0.6 per cent P.

Research shows you can reduce dietary P by as much as 30 to 40 per cent from those levels without sacrificing milk production or quality. It won't impact reproductive performance either.

A recent California trial, for instance, monitored two groups of cows fed at two different P levels. One group got the NRC recommended level of 0.37 per cent and the other was fed at 0.57 per cent.

Researchers monitored the cows for:

  • days to first increase in progesterone;
  • days to first estrus detection;
  • days to first service detection by the herdsperson;
  • conception rate at first service.

They found no difference in these measurements for the two groups. Both had similar numbers for:

  • conception rate at 30 days;
  • days open;
  • pregnancies lost from 30 to 60 days;
  • multiple ovulation rate;
  • incidence of annovulation by day 71 in milk.

Also, there was no difference in duration of estrus, the average number of mounts per estrus, or the total mounting time during estrus.

A Wisconsin study looked at the same P levels fed to two different groups and monitored milk production. The two groups produced equal amounts of milk (see graph).

Dietary phosphorus intake doesn't affect calcium-to-phosphorus ratio. Cows can adjust to wide ranges in Ca:P ratios as long as the diet you feed them meets individual requirements for each mineral.

The 2001 NRC recommendations adjusted assumptions on P availability to the cow. It was previously assumed that 50 per cent of P was available from all feeds. Now the assumptions indicate 64 per cent of P is available from forages and 70 per cent from concentrates. New research suggests even these assumptions may be low. So, not only do the recommendations lower dietary P requirements, but assumptions on what your cattle are getting from each feed stuff have been adjusted. Research studies show that it's difficult to formulate a P-deficient diet even without P supplementation.

From an economic standpoint, cutting back on your P feeding from to 0.37 from 0.57 per cent could cut P investment by $716 yearly for a 100-cow herd. This is based on dicalcium phosphate costing $13 per 25 kg bag, and each cow consuming 13.8 kg per year.

As well as saving you money, reducing P in diets to recommended levels helps protect the environment. By lowering P in the ration, you also decrease P output in the manure. For example, say you fed a lactating dairy cow averaging 9,100 kilograms milk over 305 days a diet with 3.8 grams of P per kg of dietary dry matter. You'd need one acre of crop land to recycle P excreted in the manure. If you fed P at the rate of 5.7 grams, you'd need two acres per cow.

Cows consuming a high-phosphorus diet at 0.57 per cent can excrete as much as 50 grams of P per day in their manure. This translates into over 18 kg of manure P per year. Cows consuming at a lower rate around 0.30 per cent excrete 22.7 per cent less P in their manure. Generally researchers conclude that for each gram-per-day decrease in P intake, there's a 0.55-gram reduction per day of P excreted.

Nutrients such as P, excreted in animal manure and over-applied to your crop land, can potentially run off into waterways. This causes surface or groundwater pollution.

Completing a nutrient management plan, which looks at balancing your farm's nutrient needs, can help you assess your herd's nutritional program from an environmental perspective. Regardless of your farm's environmental risks, adopting improved P nutrition strategies may reduce production costs.

This article first appeared in the Ruminations column of The Milk Producer Magazine, May, 2005

Graph showing impact of reducing phosphorus on lactation

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