Dietary phosphorus in dairy cattle


Source: Ontario Ministry of Agriculture, Food and Rural Affairs


The livestock industry is facing a number of environmental challenges and there is increased pressure on farmers to manage their nutrients more efficiently. One major area of concern is phosphorus (P) and its role as an environmental pollutant. Manure P, spread over land, has the potential to build up in the soil if the rates applied exceed what the crop is removing. With erosion, or if the soil is highly saturated as a result of continuous and excessive application, P may enter surface water causing algae populations to grow rapidly and impair the survival and productivity of other aquatic life.

According to Table 4, “Range of nutrient guarantees for complete feeds for use in the exemption of feeds from registration”, of the Canadian Food Inspection Agency’s Feed Act, the minimum dietary P requirement for dairy cattle is 0.30% of dry matter intake (DMI). The NRC suggests that P represent approximately 0.33%–0.38% of the dietary dry matter. However, it is not uncommon for producers to feed as much as 0.5%–0.6% P for high producing cows. Research shows that dietary P can be reduced by as much as 30%–40% from the level at which some producers are currently feeding without sacrificing milk production or quality. (See References for web site link.)

Dietary Phosphorus Effects on Manure Application

To manage nutrients more appropriately, producers should reduce the amount of P excreted in the manure. Nutrition plays a key role and may be the most cost-effective approach to reduce P losses from dairy farms. By lowering P content in the diet, P output in manure is also lowered. If a lactating cow, averaging 9,100 kg milk over 305 days, is fed a diet of 3.8 g P/kg of dietary dry matter (DM), approximately 0.71 ha or 1.75 acres of cropland (mixed alfalfa/corn/soybean cropping system) is needed to recycle the manure P excreted. So, if dietary P is raised from 3.8 g P/kg DM to 4.8 g P/kg DM, one cow requires an additional 8.1 kg of supplemental P per year. Furthermore, 0.28 ha/cow or 44% more cropland may be needed to recycle the manure P produced with this type of cropping system (Powell and Satter, 2001). On the production side, a 25 kg bag of dicalcium P costs approximately $13. Therefore if you were to feed 100 cows 8.1 kg less P per day you could save approximately $420 annually. Other research has shown that cows consuming a high phosphorus diet of 0.56% (112g P/d) can excrete as much as 49.6 g P/d in their manure. This translates to 18.10 kg of manure P/year. In comparison, dairy cows consuming 0.30% (60 g P/d) excrete 22.7% less P in their manure. Generally, researchers conclude that for each g/d decrease in P intake, there is a reduction of 0.55 g/d of P excreted. (Morse et al. 1992) Lowering P content in the feed by 52 g/d/head over 305 days could mean $825 in savings for a herd of 100 dairy cows.

Feeding System Types and Nutrient Use

When all lactating cows are fed one Total Mixed Ration (TMR), cows will eat approximately 7% more nitrogen (N) and P, and excrete 10% more nutrient in manure. This is because producers that feed on a herd basis rather than a group basis typically provide nutrients to meet fresh and high-producing cow requirements. Grouping cows and supplementing P appropriately can reduce both P excretion and feed bills. Typically cows are split into groups based on milk production in order to provide the correct amount of nutrients to both high and lower producing cows. Fresh cows (0–3 wks) are capable of mobilizing as much as 500–600 g of P from their bones during the early stages of lactation, so their requirement is lower than high producing cows. Researchers recommend that dietary P be fed above 0.30% for low to medium producing cows (7,500–9,000 kg/lactation) and between 0.38%–0.40% for high producing cows (>10,000 kg/lactation). Generally, fresh cows can be fed a diet containing as little as 0.30% P and still maintain good milk yield, but as they proceed into late lactation dietary P must be increased in order to maximize milk production. The amount of P fed and thus P output in manure can be reduced by phase feeding cattle to their stage of production by matching P supplied with the amount required. This is opposed to feeding all cows at a high P level throughout lactation. Some producers may also group cows by age or parity. When choosing how to group your herd you should take into consideration labour, barn size and layout, and herd size. Additionally, if you choose to feed one TMR to the entire herd rather than group feed, monitor body condition scoring to help with ration formulation.

Cost Comparison of Feeding System Types

Table 1. Comparison Between Different TMR Feeding Methods.

1 Group TMR More than 1 Group TMR
# of tests
Feed cost
Kg of milk/cow

(Adapted from Ontario Dairy Herd Improvement (DHI) data, 2003)

Table 1 data shows no significant difference in feed cost between the 1 group TMR and more than 1 group TMR. Other research has found that income can be increased over feed costs as a result of lower concentrate costs. Producers using feed additives may also find the multi-group feeding system more economical as they have the ability to target specific groups rather than the entire herd. Even the added labour may be cost-effective as each batch only takes approximately 20 minutes to mix.


It comes as no surprise that P supplementation needlessly costs the dairy industry millions of dollars each year. Table 2 shows the increase in annual feed costs ($U.S.), for a 100-cow herd, when P is supplemented above 0.40%. According to another study, feeding a diet containing 0.55% P instead of 0.38% to 100 Holsteins producing 8,390 kg milk/cow/year could increase the feed bill by an additional $3,052 (US) annually (Anderson and Magdoff, 2001).

Table 2. Increase in Annual Feed Cost for a 100-Cow Herd.

Average milk yield (kg/d) DMI (kg/d) Dietary P Concentration
0.45 0.5 0.55

Based on American dollars

DMI = Dry Matter Intake

(Adapted from Knowlton and Kohn, 1999).

Cost Minimizing Feed Rations

It is difficult to adjust for least cost and/or P excretion without negatively affecting N excretion. On average, dairy manure contains more N than P relative to crop need. Therefore, when formulating a ration for nutrient excretion consider N first with P being of secondary importance. But, keep in mind that P is the most expensive ingredient in a dairy ration. For further discussion on N, refer to the OMAF Factsheet Feeding Dairy Cattle to Reduce Excess Nitrogen Output, Order No. 03-055. Generally producers meet their crop demands through the use of both manure and fertilizer in order to prevent over-application of one nutrient versus the other. On a dairy farm with a good supply of available land, including forage in the crop rotation or using cover crops is also beneficial as this allows different levels of nutrient uptake and intermittent spreading of manure on a field. Ultimately, the main objective of reducing dietary nutrient levels is to decrease production cost, which may also have environmental benefits. A model was developed, by researchers to balance a dairy cow feed ration for minimum cost, P, and N excretion. The results show that it is not possible to achieve 100% success of all 3 of these objectives. However, P and N excretion can be reduced for a minor increase in cost. See Table 3 illustrating the effects of balancing a ration for one objective on cost, N and P excretion.

Table 3. Formulating a ration for a single objective solution.

Single Objective Solution
Min cost Min N Min P
Cost ($ US/d) 2.84 3.66 3.36
N excretion (g/d) 276.13 269.93 276.13
P excretion (g/d) 33.63 40.47 32.09
DMI (kg) 20.04 18.7 20.04

(Adapted from Tozer and Stokes, 2001).


Livestock operations can affect the environment. Nutrients such as P, which are excreted in animal manure and over-applied to the land, can potentially run-off into waterways and cause surface water pollution. The first line of defence in reducing P losses on dairy farms is through nutrition. Producers are often feeding P above recommended levels as a safety factor. But this strategy can needlessly increase the cost of production as well as increase the risk of environmental harm. Feeding to meet, and not exceed, nutrient requirements lowers feed cost. Fewer nutrients in manure can also mean fewer acres needed for manure distribution for those farms tight for land base and with high soil test levels. The impact of dietary P reduction will vary from farm to farm as different cropping systems and soil types come into play. Completing a nutrient management plan which looks at balancing N and P needs on the farm fields and assesses the environmental risks from these nutrients can also help you assess just how much emphasis you need to place on balancing 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.


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