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Managing Phosphorus levels in a Dairy cows Diet

Management of phosphorus has emerged as a serious limiting factor for dairy farming for some of our closest neighbours, but what does this mean for Scotland?

While adhering to recommended phosphorus (P) levels in dairy cows diets can benefit farm businesses and the environment, issues can arise when stock are regularly fed too much of this mineral, which happens with a significant proportion of UK stock.

Currently, it is recommended that 0.35-0.38% (on a dry matter basis) of P is fed daily to a lactating cow. For a lactating cow eating 22 kg of dry matter, this means 77-84 gm of P/head/day. However, according to recent findings of AHDB Dairy-funded research carried out by Harper Adams University, diets in the UK often contain about 20% more phosphorus than needed.

AHDB Dairy’s senior scientist, Martina Dorigo, said: “Farmers are not always aware of how much phosphorus their stock is getting but most phosphorus overfeeding is due to mineral supplementation and the frequently concurrent use of distillery co-products, which are very rich in P and, in Scotland, often cheaper than other protein sources.”

Phosphorus is not just a major component of bone and teeth, but it is also the mineral element mostly involved in the correct functioning of the other organs in the body. For this reason, when formulating a diet for lactating cows, it is important to make sure that P requirements are met.

However, if cows are fed too much phosphorus, they will eventually eliminate the excess in their faeces, increasing the P content in manure and when this manure is then spread onto the fields, it will load the land with phosphorus.

She added: “Phosphorus can accumulate in soils for many years, creating a bank of ‘legacy phosphorus’ and, in the meantime, through run-off and erosion, it will slowly but steadily reach surface water.

“This process is the main cause of eutrophication, an abnormal growth of algae that significantly impairs surface water quality and its ability to support aquatic life. In Scotland 90% of water comes from surface sources and as Scotland’s rivers are highly prized by fishermen, the threat posed by phosphorus pollution from agriculture cannot be ignored,” she argued.

Taking into account the cost of phosphorus supplementation, which in the past has seen dramatic surges and wide variations, there is an opportunity for dairy farmers to reduce it in their cows’ diets without any negative impact.

Ms Dorio added: “In terms of health and performance of dairy cows, scientific data has shown that bringing dietary phosphorus levels down to the requirement is not at all harmful, indicating that no extra benefit is derived from P overfeeding of dairy cows.

“For farmers looking to review their P levels in TMR-fed systems, a mineral analysis of the ration will directly provide the phosphorus content level, while for grass-based systems the P content of all feeds, grass included.

“Together with respective daily intakes, these must be taken into account to estimate the total intake of phosphorus of cows. If P levels are higher than the recommended values of 0.35-0.38 % DM, then mineral supplementation should be revised accordingly.”

 

Source: The Scottish Farmer


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