Estimating Manure Phosphorus Excretion by Dairy Cows

Dr. Bill Weiss

Manure nutrient management plans are required for large dairy operations and eventually may be required for all dairy farms. An important component of nutrient management plans is phosphorous (P) balance. On average, 55 to 65% of the P fed to a lactating cow is excreted via manure; however, using an average excretion rate is not adequate when developing nutrient management plans. To assist producers in developing nutrient management plans, we summarized several digestibility experiments with Holstein cows conducted at the OARDC. In those experiments, we measured the amount of feed consumed, the concentration of P in the diet, output of feces, fecal P concentration, urine production, and milk production of 130 cows fed 30 different diets. The diets fed were extremely diverse. Forage ranged from 40 to 60% of dietary dry matter (DM). The forage portion of the diet ranged from 100% corn silage to 100% hay crop forage (most of the hay crop forage fed was alfalfa silage, but some studies used alfalfa hay and orchardgrass silage). A wide variety of concentrates were fed, including corn grain, soybean meal, and a host of byproducts. The concentration of P in the diets ranged from 0.32 to 0.46% (average was 0.38% of dietary DM). To meet the NRC (National Research Council, Nutrient Requirement of Dairy Cattle, 2001) P requirement, the average cow in this data set should have been fed a diet with about 0.35% P. On average, diets contained about 8% more P than required.

To estimate manure P excretion for the lactating dairy cows on a farm, the dairy producer must enter DM intake, the P concentration of the diet fed, and milk production into the following equation:

Manure P (grams/cow/day) = P intake (grams) - P secreted in milk (grams) - 3.9

P intake (grams/cow/day) = (Pounds of DM intake/2.2) x (% diet P x 10)
P secreted in milk (grams/cow/day) = Pounds of milk/day x 0.41

To convert grams of P excreted/day to pounds divide by 454.

For example, a farm has a single group of 120 lactating cows, the average DM intake is 45 lb/day, the diet contains 0.4% P (DM basis), and average milk production is 65 lb/day.

Phosphorous intake = (45/2.2) x (0.4 x 10) = 81.8 g/day per cow.

Milk P secretion = 65 x 0.41 = 26.7 g/day per cow.

Manure P = 81.8 - 26.7 - 3.9 = 51.2 grams/cow/day. In pounds, 51.2/454 = 0.11 lb/day/cow of P or 0.11 x 120 cows = 13.5 lb/day of P for the farm (lactating cows only).

If multiple groups are fed different diets, the same approach is followed except that you have to calculate P intake and milk P for each group. For example, Group 1 has 100 cows with an average intake of 50 lb of DM, the diet has 0.38% P, and milk production averages 80 lb/day. Group 2 has 35 cows with an average DM intake of 40 lb/day with 0.35% P and milk production averages 45 lb/day. Phosphorous intake per cow for Group 1 is (50 lb of DM/2.2) x (0.38 x 10) = 86.4 g (86.4/454 = 0.19 lb). Milk P per cow for Group 1 is 80 x 0.41 = 32.8 g (32.8/454 = 0.072 lb). Manure P per cow for Group 1 = 86.4 - 32.8 -3.9 = 49.7 g (0.109 lb), and manure P for the entire group (100 cows ) is 0.109 lb x 100 cows = 10.9 lb/day. For Group 2, P intake/cow = (40 lb/2.2) x (0.35 x 10) = 63.6 g and milk P per cow = 45 x 0.41 = 18.5 g. Manure P per cow for Group 2 = 63.6 - 18.5 - 3.9 = 41.2 g (0.091 lb). The entire group is excreting 3.2 lb/day of P (0.091 x 35 cows). For the whole lactating herd (135 cows), manure P excretion = 10.9 + 3.2 = 14.1 lb/day.

If you do not know average milk production per group but know herd average production, that number should be used. Using the example above, herd average milk production = 70.9 lb [((100 cows x 80 lb) + (35 cows x 45 lb))/135 cows]. Average milk P = 70.9 lb of milk x 0.41 = 29.1 g. Average P intake for the herd is [((86.4 g x 100 cows) + (63.6 g x 35 cows))/135 cows] = 80.5 g. Average manure P per cow = 80.5 - 29.1 - 3.9 = 47.5 g (0.105 lb) and for the whole herd manure P = 0.105 x 135 cows = 14.1 lb/day.

We have not collected data on P balance of dry cows; however, manure P should be approximately equal to P intake (grams) - fetal retained P (grams) - 3.9. The NRC estimates that fetal retention of P for a dry cow averages 4.5 g/day. Therefore, estimated manure P for a dry cow (grams) = P intake (grams) - 8.4. For example, if a dry cow consumes 25 lb of DM that has 0.29% P, estimated manure P = (25/2.2) x (0.29 x 10) - 8.4 = 24.5 grams (0.054 lb/day).

Although these equations were developed from data collected from Holstein cows, they should work reasonably well for Jersey cows (the manure P for a lactating Jersey cow may be 1 or 2 g/day less than estimated and 3 or 4 g/day less for a dry Jersey cow). A spreadsheet is available that will do these calculations for herds with 1 to 10 groups of lactating cows and 1 to 2 groups of dry cows. You need to input the number of cows per group, DM intake per group, percent P in the diets, and milk production.

The single most effective way (also the easiest and cheapest way) to reduce excretion of P in manure is to feed only enough P to meet a cow's requirement. For most lactating cows, diets with 0.32 (lower producing cows) to 0.38% (higher producing cows) should be adequate. All P fed in excess of the requirement is excreted via manure.