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Extension > Agriculture > Manure Management and Air Quality > Manure Application > Reduce phosphorus levels in the lactation ration

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Reduce phosphorus levels in the lactation ration

Case study: dairy operation

The purpose of this case study is to demonstrate the economic and environmental value of reducing imports of feed ration phosphorus into dairy farms when lactation phosphorus levels are above NRC recommendations.

Farm description:

This dairy farm in Central Minnesota milks 60 Holstein cows, and with replacement heifers maintains an average of 124 Animal Units. Lactation cow housing is in a tie-stall barn using oat straw bedding. Barn manure is stored in an earthen liquid pit. Dry cows and replacement heifers are housed on conventional bedding packs with oat straw and chopped corn stalks. The farm has 253 harvested and 221 tilled acres on primarily Sandy Loam soils, with some Loam and Loamy Sand. All fields are in dry-land crop production. The herd's DHIA Rolling Herd Average is 23,000 pounds on 2X milking. They produce all of the forage, grain, and bedding needed for the dairy operation except some of the grass hay for the dry cows. In most years, they sell some small grains and corn grain.

Phosphorus import-export analysis:

This farm purchases feed supplements and the total phosphorus (P) in the lactation ration is 0.40%. They use 4.9 gal/acre of a 10-34-0 liquid fertilizer corn starter, and broadcast the new seeding alfalfa with 10-6-29 fertilizer. All manure application to the fields is based on nitrogen needs and no manure is imported or exported. Most of the farm's soil tests are greater than 50- ppm Bray 1-P. The total annual farm P imports exceed exports by 1,123 lb., which is an excess of 4.4 lb. per harvested acre (see following table). Since the soil tests are already very high it would be desirable to balance P imports with exports to avoid driving test values higher and risk a rise in P in field runoff. The annual per-animal and per acre excess P is not great, so a single strategy of adjusting the P in rations should achieve an import-export balance.

Phosphorus Balance

Phosphorus (lb.)

Ratios

P Source Imports Exports Excess
Animals 0 293 Harvested Acres Animal Units Ratio
Forages 50 0 253 124 2.0
Grains 0 210
Protein/Minerals 1,801 0 Excess P (lb.) Harvested Acres Ratio
Fertilizer 945 0 1123 253 4.4
Milk 0 1,170
Manure 0 0 Excess P (lb.) Animal Units Ratio
Total P 2,796 1,673 1,123 1,123 124 9.1

Dairy ration phosphorus background:

The National Research Council (NRC) phosphorus requirement for lactation Holsteins ranges from 0.32 - 0.36 % dry matter (DM)1 depending on milk production, days in milk, and dry matter intake. P levels in dairy rations have been a controversial subject over the last few decades with many nutritionists and dairy farmers following a "more is better" approach to dietary P. Current scientific data regarding dairy production and reproduction does not support dairy lactation P levels higher than the current NRC recommendations2. Many high producing dairy herds with excellent reproduction results are at or near the NRC P recommendations. Recent work at Pennsylvania State University shows that cows fed P above the NRC requirement excrete more P in the manure than cows fed the recommended amount of P3.

Summary of the reproductive performance of lactating dairy cows fed lower and higher concentrations of phosphorus. Summary of 7 published reports and 785 cows.

Phosphorus concentrations (% diet DM)
Lower (0.32 - 0.40%) Higher (0.39 - 0.55%)
Days to first estrus 46 48
Days to first A.I. 73 76
Days open 76 100
Services per conception 1.8 1.9
Pregnancy rate 0.87 0.86

Wu et al. 2000 J. Dairy Sci. 83:1028

By-products in rations:

Most protein ingredients used in today's dairy rations are bi-products. Soybean meal, canola meal, linseed meal, and sunflower meal are bi-products of the vegetable oil industry. Corn distillers, corn gluten meal, and corn gluten feed are bi-products of the starch, corn syrup, or alcohol industries. Most bi-products are a result of extracting a major nutrient for other uses such as oil or starch in the above examples. This leaves the remaining nutrients more concentrated than in the original raw product making protein levels in the above bi-products higher than protein levels in the original raw products. This process also concentrates the minerals, including P, in the bi-product. Meat and bone meal, another commonly used bi-product from the meat rendering industry, is also high in P.

Depending on the source, the P content of corn distillers is similar or slightly higher than that of soybean meal. When corn distillers, with a protein content of around 30%, is substituted for soybean meal, which has a protein content of 47%, nearly twice as much corn distillers as soybean meal must be used to attain the same protein percent in the final mix, depending on the other ingredients. Higher inclusion of bi-product ingredients usually results in a higher P content of the mix.

Phosphorus content of forages grown on high soil test P fields:

Luxury consumption of potassium (K) by grasses and legumes has long been known4. Recent work also indicates that some forage crops such as alfalfa will also luxury consume P5. Increased forage and supplemental protein P content would reduce the need to add minerals containing P. Most often, attainment of the lactation P requirements is possible without any mineral P added to the ration. This situation can make it challenging to reduce ration P down to the NRC requirement.

Dairy ration P level:

One proposed strategy to move this farm toward import-export P balance would be to reduce P in the lactation ration. Historically, the nutritionist for this dairy herd has been more comfortable calculating the ration P at a minimum of 0.40% on the dry matter basis. Occasionally the ration reached 0.43% P due to least-cost formulation usage of bi-product feeds. An option would be to impose a lactation ration maximum P level of 0.34%, which is close to the NRC minimum of 0.32%. Using 0.34% phosphorus level lowers the P imbalance on this dairy farm from 602 to 521 pounds total, or a net reduction of 2.3 lb. P/acre and 4.9 lbs. P/A.U. Obtaining these lower P levels required eliminating the use of porcine meat and bone meal and adjustment of other ingredients to maintain the desired nutrient profile.

Economic analysis:

For this farm, lowering the P content in the ration resulted in a net savings of $9.60/ton of TMR custom dairy protein. This nets out to be $912 savings on an annual basis. Lower ration P should result in a lower P levels in the manure. Ration ingredient costs vary, so it is possible to have a slight increase in ration cost when adjusting for lower levels of P. Lower ration P levels will reduce the P content in the manure and lower this farm's P import-export excess enough to more closely stabilize soil test P values.

Conclusions:

  1. Feeding P above NRC requirements will not improve reproduction or milk production.
  2. Many dairy herds inadvertently over-feed P because many bi-product feeds contain relatively high levels of P, and forage crops grown on high soil test P fields may luxury-consume P.
  3. Feeding P at higher than NRC requirements will increase P in the manure, contributing to excess soil test P.
  4. Reducing the P in a dairy ration may increase or decrease the net cost of the ration depending on the individual ration, local feedstuffs available, and the current commodity market.
  5. Lowering manure P levels may allow more flexibility with manure application on high P soils and offset any higher feed costs.

References:

  1. Nutrient Requirements of Dairy Cattle. 2001. National Research Council.
  2. Wu, Satter, and Sojo. 2000. Milk Production, Reproductive Performance, and Fecal Excretion of Phosphorus by Dairy Cows fed Three Amounts of Phosphorus. J. Dairy Science 83:1028-1041.
  3. Ishler. October 2013. Fecal Phosphorus levels in dry cow, pregnant heifer groups. Penn State University Extension.
  4. Cherney JH, Cherney DJR. Grass management for dry dairy cows. Accessed 11/13/2015.
  5. Goss and Stewart. May 1979. Efficiency of Phosphorus Utilization by Alfalfa from Manure and Superphosphate. Soil Science Society of America.
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