Reducing phosphorus in dairy cattle manure starts with the feed

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EAST LANSING, Mich. – Scientists at Michigan State University have found that feeding phosphorus to dairy cattle in excess of levels recommended by the National Research Council has no benefit on cows’ performance but a large effect on the cows’ manure.

David Beede, professor of animal science, and graduate students Jill Davidson, Ashley Peterson and Zach Myers have been studying the physiological and production responses of dairy cattle to various levels of phosphorus in their diets.

Their research with cows of various ages and in various stages of the lactation cycle found that overfeeding phosphorus did not benefit the cows’ performance – the NRC dietary recommendation proved to be adequate.

But many producers and nutritionists supplement dairy rations with phosphorus in excess of recommendations. All the phosphorus fed in excess of the animals’ requirements ends up in the manure and, subsequently, on farmland.

“Reducing phosphorus in dairy cattle manure is a serious undertaking in Michigan and elsewhere,” Beede said. “As nutritionists, we can help by reducing the amount that animals consume. That’s where reducing manure phosphorus must start – with the ration.”

Producers have traditionally added phosphorus to the cows’ feed thinking the extra phosphorus would help production and reproduction problems. Veterinarians and nutritionists have often recommended increased dietary phosphorus when problems arose.

But Beede’s research and his review of other research from around the world show that phosphorus fed in excess of NRC requirements did not improve either reproduction or production.

Many producers and nutritionists increase phosphorus levels in the close-up diet before calving to help maintain the cow’s blood phosphorus levels right after calving. Beede found, however, that more than half of the cows studied still had below normal blood phosphorus levels at calving due to natural biological events as the animal’s metabolism adjusts.

Low blood phosphorus was found among cows whether their close-up diet contained 0.44, 0.31 or 0.21 percent phosphorus (this low level is enough to supply the cows’ phosphorus requirement), dry basis.

Cows fed high phosphorus (0.44 percent) had lower blood calcium around the time of calving and so were more prone to milk fever than cows fed 0.21 percent phosphorus in the close-up ration.

Increased dietary levels of phosphorus do not affect the cows’ performance, but they do have substantial effects in the manure. Beede found that a one-tenth of 1 percent increase in the phosphorus in the ration for lactating cows in a 300-head herd resulted in an additional 16,560 pounds of elemental phosphorus in the manure over a year.

This equates to the need for an additional acre of land per cow annually to dispose of the excess phosphorus. Along with the negative environmental effects of overfeeding phosphorus, producers are also incurring unnecessary costs when buying the supplemental mineral.

“Phosphorus is one of the most expensive minerals in the cows’ ration,” Beede observes. “Overfeeding typically costs $10 to $15 per cow annually (approximately $100 million annually in the United States).

“More importantly, producers need to consider the management costs of disposing of the excess phosphorus in the manure as nutrient levels of the land increase.”

He emphasizes that it is the obligation of dairy nutritionists and producers to implement nutritional management practices to achieve efficient utilization of feed phosphorus imported into farms and reduce excretion.

Feeding dietary phosphorus in excess of NRC requirements is costly and unnecessary, he says, and it could get even more costly as nutrient management plans are implemented on dairy farms and more attention is paid to phosphorus flows into cropland soils.

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