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Manure management: Know your phosphorous inputs

Tara L. Felix and Virginia A. Ishler for Progressive Cattle Published on 24 September 2019
Manure management

Manure produced from any livestock operation has tremendous fertilizer value. Manure from a beef finishing operation contains all the essential nutrients commonly found in fertilizer: nitrogen (N), phosphorus (P) and potassium (K), or NPK.

According to the Nutrient Requirements of Beef Cattle (2016), the NPK ratio in feedlot cattle manure, when the feed is well balanced to meet nutrient requirements, is typically 16.0:4.0:5.5. If beef cattle manure is land applied, based on meeting the N requirements of the crops, P is generally applied in excess. Because the reality is that manure from feeder operations contains P in excess of the crop’s requirement.

When P is applied at rates exceeding the crop’s needs, it stays where it is applied. In most soils, the P content of surface horizons is greater than that of subsoil. Except in special situations, added P does not move down readily through the soil.

Historically, in reduced tillage systems, fertilizers and manures were applied to the soil surface with little or no mechanical incorporation, thus contributing to P buildup in the top 2 to 5 inches. Although P moves little down through the soil profile, these soils can be the source of P in runoff, especially where plant-available soil P exceeds the agronomic optimum.

In addition, once available P levels have built up, they decline slowly after manure applications have stopped. Studies have shown that without additional applications, 10 to 20 years of corn or soybean production are needed to reduce available soil P (Mehlich-3) levels from 150 parts per million to agronomic threshold levels of 20 parts per million.

Best practices for NPK

More often, the least-cost fertilizer scenario, with the least environmental impact, includes applying beef cattle manure to meet the P needs of the crop and then adding synthetic N. The best management practice should be to evaluate the NPK status on a field-to-field basis and apply manure only to maintain optimum levels of P. Proceeding in this manner ensures that P is not applied to the land in excess. In addition, there are practices beef producers can employ to reduce manure P and, thus, the amount being applied to the soil.

Reducing P feed inputs is one way to reduce manure P outputs. Because P is the second-most abundant mineral element found in the animal’s body, and is essential in almost all metabolic reactions, there has been a tendency for nutritionists to formulate rations with P levels above the National Research Council (NRC) recommendations.

Some reasons ration P may exceed requirements include: (1) providing a safety margin to account for variations in animal requirements and P concentrations in feeds, (2) accounting for accuracy of feed delivery and feedbunk management and (3) accounting for issues with P availability.

Regardless of the reason, these formulated “insurance policies” can lead to excess P excretion and contribute to the imbalance of NPK requirements for manure application to field crops.

Meeting the pen needs

Formulating rations for P levels in excess of animals’ requirements has been a feeding strategy to ensure individual animals within a pen are consuming their minimum P requirements, realizing that some animals may be over requirements. Nutrients are required by individual cattle in grams or pounds per day based on metabolic bodyweight, expectations for growth, etc.

When a pen of cattle is fed, there is a range of individual requirements within the pen due to variations in animal size and daily dry matter intake. However, it is still most practical and economical to feed the entire pen the same diet and not single out individual calves. Thus, some cattle have their needs met; others may be fed in excess.

In addition to the group feeding conundrum, there is considerable variation in P concentrations of both concentrates and forages. In general, cereal grains are uniformly greater in P than forages with byproduct ingredients (i.e., wheat midds) and oilseed meals (i.e., soybean meal) being especially high. Forages tend to be more variable than grains in P concentration, and because of this, book values should not be counted on as an accurate source of information. Producers and nutritionists should formulate rations based on analyzed feed results. Testing ingredients and adjusting rations accordingly can help ensure cattle are receiving nutrients in the proper amounts.

What to feed, how much to feed

Another feed challenge is that there may not be enough cattle at the operation to justify ordering a bulk mineral mix. Some operations that are cow-calf to finish, and not large enough to buy bulk mineral, may feed bagged minerals. Some bagged minerals are generically formulated to meet the needs of all classes of cattle.

Typically, cows and ffeder calves are fed quite differently

Typically, cows and feeder calves are fed quite differently, with cows consuming diets high in forage and calves consuming diets high in grain. Because of the inherent differences between P concentrations in grains and forages, feeder calves may be excessively overfed P if a general bagged mineral is used. Feeder cattle consuming grain-based diets are often consuming P in excess of their requirements due to the inherent P levels in grain and require little to no supplemental P.

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Educational programs focused on feed management or precision feeding are aimed at better control of feed intake P in order to reduce P output in manure. An added benefit of this approach is reduced ration costs by evaluating the mineral program and feeding P specifically to meet the animals’ requirements.

Environmental issues

Environmental concerns with P have forced the reevaluation of feeding practices for cattle. The excess P in beef cattle manure presents a nutrient management challenge that producers must acknowledge. Improving the precision in feeding P will help reduce manure P and ultimately be a better match for the crops’ needs for P.

Application of synthetic N will likely be necessary when limiting manure to meet P demands, depending on the crop being grown. Environmental concerns related to P and surface waters are not diminishing and all animal industries must be diligent in minimizing P excretion and the potential threats they pose to impaired water quality.  end mark

PHOTO 1: Reducing phosphorous feed inputs is one way to reduce outputs of that phosphorous in manure. But other management considerations are worth studying as well.

PHOTO 2: Typically, cows and feeder calves are fed quite differently with cows consuming diets high in forage. 

PHOTO 3: Calves consume diets high in grain. Staff photos.

Virginia Ishler is an extension dairy specialist at Penn State University. Email Virginia Ishler

Tara L. Felix
  • Tara L. Felix

  • Beef Specialist
  • Penn State Extension
  • Email Tara L. Felix

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