0107 ANM: Developing whole-farm nutrient plans for


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Tuesday, 02 January 2007 04:44

Manure from feedlot cattle is a valuable source of fertilizer nutrients. However, as agriculture, as a whole, continues to move toward becoming a more environmentally aware industry, management of excreted nutrients will receive more attention. Producers will need to be more aware of nutrient production and nutrient use within their own operation and how best to maintain a balance between the two.

Development of a site-specific nutrient management plan is essential in order to prevent the overapplication of fertilizer nutrients, potentially threatening environmental quality. Owners and operators must be able to accurately estimate the amount of nutrients generated on-farm and the quantity of each nutrient the farm is capable of using. A feedlot must keep accurate records of rations and the amounts of nutrients being fed in each diet throughout the year. An accurate method of estimating nutrient flows, including nutrient excretions, is needed.

Feedlots also must consider nutrient losses that occur as a result of decomposition and volatilization during storage and application of manure. Ensuring equality between the quantities of nutrients applied to the crops and the quantity used by the crops completes the cycle.

Following is a brief step-by-step guide for development of a feedlot whole-farm nutrient management plan. By addressing each step and making reasonable estimates of the nutrients at each stage, producers can identify where opportunities exist within their operation to optimize nutrient flows.

Step 1: Feed management
By knowing nutrient quantities fed in rations and level of production, one can estimate quantity of nutrients excreted. A mass balance approach considers nutrient intake, via diet, and subtracts from this nutrients retained in the product, growth or gain as a means of estimating the difference, which represents nutrient excretion. Estimates of nutrient digestibility of common feeds are currently available, from which dietary nutrient absorption can be calculated. The indigestible portion of nutrient intake is excreted, contributing to total excretion.

One option to maintain a whole-farm nutrient balance or reduce the number of acres required to spread manure is to decrease dietary nutrients (i.e., nitrogen (N) and phosphorus (P)) such that animal performance needs are still met with as little dietary N or P excess as possible. Because protein and P are relatively expensive nutrients, oftentimes reducing dietary N and P to just meet animal needs also reduces ration cost, in addition to reducing nutrient excretions.

As a feedlot animal matures, nutrient needs decrease. Nutrient density in grains, in particular, often exceeds the nutrient needs of a finishing animal.

Many of the co-product feeds included in feedlot rations are higher in P than the primary feed. While in many cases, these feeds are less expensive than corn and soybeans, making diet cost cheaper, cost of manure application to more acres should be considered when determining the economics of feed sources. Co-products serve as an important feed source for the animal industry. Producers just need to be aware of their nutrient content and the availability of nutrients in order to manage excreted nutrients appropriately.

Step 2: Nutrient excretion and retention
Nutrients are essential for growth and weight gain in feedlot cattle. First, we must reasonably estimate the nutrient intake. Nutrient composition of gain multiplied by weight gained is subtracted from nutrient intake to provide a difference that represents nutrient excretion.

In cases where the producer is feeding as close to the animal P requirement as possible, equations may be overestimating excreted P (which then has to be managed). However, it is equally important to note that many operations are overfeeding P and other nutrients, including N. Producers should work closely with their feed managers to ensure this practice is minimized, if not ceased.

Step 3: Storage
Long-term open lot manure storage, anaerobic lagoons and surface spreading contribute to ammonia loss to the atmosphere. Typically, 30 percent of excreted N can be lost as ammonia due to volatilization in open feedlots. During storage in lagoons or settling ponds, up to 70 percent of excreted N can be lost. As a result of ammonia volatilization following excretion, manure P becomes more concentrated (relative to N), than when the manure was first excreted prior to any N losses.

Systems that conserve N result in a product with a N to P ratio more closely aligned with crop needs. Conservation of N also increases the fertilizer value of manure. As excreted, feedlot manure contains a N to P ratio of approximately 3.7 to 1 (equivalent to 8.4 N to 1 P2O5). However, as N is lost to the atmosphere the ratio decreases substantially. When this manure is applied to fields based on P application rates, N is deficient, requiring supplemental inorganic N to meet crop needs. In this case, it is to the producer’s advantage to retain as much of the N in the manure as possible to reduce costs for supplemental fertilizer.

As we look down the road, environmental regulations may restrict N losses to the atmosphere, providing further incentive to retain as much of the excreted N in the manure as possible.

Step 4: Land application
Manure N has typically been used to determine manure application rates because of nitrate concerns and potential leaching of nitrates into groundwater. As a result, producers have been over-applying P. Feeding diets containing P greater than that needed by the animal only serves to exacerbate the problem. In the future, priority will be on reducing excretion of P as a means of decreasing needed land base and retention of a higher proportion of excreted N in the excreted manure.

Retaining more manure N can be accomplished by incorporating manure into the soil immediately after application. By broadcasting manure, 15 to 30 percent of N can be lost. However, broadcasting followed by immediate cultivation can reduce N loss to one to five percent for solid manure. Most losses occur within 24 hours of application to the field. N loss during land application is usually greatest during dry, warm, windy days and during the summer and spring months.

Step 5: Crop uptake
To complete the nutrient cycle, manure excreted by the feedlot animals is applied to crops that can be used as a feed nutrient source for the feedlot animals. Manure nutrients available for crop uptake are the nutrients remaining from excretion after losses to the soil and air are subtracted.

The nutrient management plan helps to create a balance between the nutrients consumed, the manure nutrients that are land-applied and the nutrients harvested by crops grown. Nutrient management has become necessary for all livestock producers.

It is important to keep a balance between nutrients produced and nutrients needed. The steps outlined can help a producer become more actively involved in the nutrients his farm produces and how to manage them. To achieve environmentally acceptable nutrient balances, many animal production facilities will have to export manure or manure products in the future or change nutrient production to match nutrient needs. ANM

References omitted due to space but are available upon request.

—Excerpts from Iowa State University Extension website

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