Initial Farm Nutrient Balance
The P balance for the beef case-study farm, as currently operated, suggests that for every three units of P entering the farm (59500 kg P yr–1 of inputs in Table 3, Balance 1), only one unit leaves (19200 kg P yr–1 of managed output in Table 3, Balance 1). The remaining two units are either lost to the environment or stored on the farm (40400 kg P yr–1 imbalance in Table 3, Balance 1). If this imbalance remains constant, this farm will add 40000 kg of elemental P to the soil reservoir or the feedlot surface each year. Note that the primary source of P arriving on this farm is feed (46300 kg) and that fertilizer is the smallest P input (2400 kg). Best management practices that address this primary P input will potentially produce the more significant environmental benefits.
The observed P imbalance can represent an increased inventory of soil P after all crop removal of P has been discounted. If we assume that 4 kg of elemental P beyond crop removal will raise soil P levels by 1 ppm, the imbalance will increase soil test values by about 25 ppm per year. In spite of the potential errors inherent to this assumption, the observed P imbalance still suggests the potential for continuing increases in soil P levels for this farm if all manures are applied to the existing land base. In reality, some excess manure will likely not be harvested from the feedlot.
Achieving a ratio closer to 1 unit of P input to 1 unit of P as managed outputs is our goal if stable soil P levels are to be attained. Thus, the farm needs to identify BMPs that either reduce P inputs or increase P outputs by roughly the amount of the imbalance.
Effect of Agronomic Manure Application Rates
The most common strategy for addressing nutrient issues is to implement an on-farm plan based on agronomic application of manure and associated adjustments in commercial fertilizer applications (Balance 2 of Table 3). Currently, application of manure based on N is required in all situations and will be assumed for this situation. Application of manure nutrients at agronomic rates based on N should eliminate the need for all fertilizer purchases, with the possible exception of a starter N fertilizer for corn. For our case-study farm, elimination of all purchased fertilizer would eliminate only 2400 kg of a 40400 kg P imbalance. For this specific farm, a significant P imbalance would be anticipated to remain after implementation of the nutrient management plan based on agronomic manure applications. Some implications to consider include:
- The mandatory nutrient management plan focused on manure management has not solved the accumulation of nutrients on this farm. A producer's good-faith effort to meet the regulatory standards does not ensure correction of the underlying environmental problems associated with nutrient concentration observed by many animal feeding operations. Will the regulatory community be forced to set higher standards at a future time if a lack of environmental progress is observed?
- If manure is applied at agronomic rates on this farm according to a USEPA-required nutrient management plan, no use is available for significant portions of the manure. Manure application planning at an N-based rate leaves a 25% manure excess for our case-study farm. If manure is applied at a P-based rate, approximately 90% of the harvested manure will not be utilized on-farm. Implementation of a nutrient management plan without an associated program to export manure commonly results in a growing accumulation of manure within the feedlot. Decomposition of the dry manure solids within the feedlot minimizes any visible accumulation of manure and results in greater C and N release to the atmosphere and P accumulation in the lot. Many feedlots have found that it is possible to operate open lots without harvesting all manure, probably with additional odor and dust emissions and accumulation of additional P within the feedlot. Thus, for many open-lot production systems, a strategy based on on-farm agronomic application of manure may have only minimal effect on overall farm P imbalance and encourages less harvesting of manure solids.
Effect of Setback Requirements
The 30.5-m (100-foot) mandatory setback from waters of the United States and agricultural well heads will reduce the available land base for manure application by about 12 ha on the case-study farm (Balance 3 results shown in Table 3). This assumes that "waters of the United States" will not include intermittent streams and that "conduits to surface waters" do not include road ditches and grassed waterways. Inclusion of some or all of these additional sites from which setbacks would be needed could substantially increase the acreage lost for manure application. Regulatory interpretation of situations to which the setback must be applied will have substantial effect on losses of land for manure application. As an alternative, a vegetated buffer could be established, removing about 2 ha of land from production, requiring an increase in purchases of feed from off-farm sources.
Several implications, some often not intended, result from setbacks on which manure cannot be applied but commercial fertilizer can be applied:
- If the setback is maintained in crop production, commercial fertilizer will be required. For our case-study farm, an additional 400 kg of P will be brought onto this farm as commercial fertilizer, increasing slightly the overall farm P imbalance (Table 3). Setback from surface water will add to the nutrient imbalance, but only slightly for the case-study farm.
- Setbacks can create considerable inconvenience in meeting the nutrient requirements of a crop. If those inconveniences become significant, some producers will chose to use commercial fertilizer on entire fields or significant areas beyond the setback area (see Fig. 2). Equipment spread patterns that do not match setbacks, challenges in tracking where manure leaves off and commercial fertilizer begins, and the inconvenience of operating two sets of equipment in smaller fields may significantly reduce land available for manure application. In turn, this will encourage a greater reliance on imported commercial fertilizer resources on agricultural operations with an existing excess of nutrients.
The two mandatory BMPs, on-farm nutrient management plans and manure application setbacks, did not create a nutrient balance situation for this farm. The environmental benefits achieved by currently mandated BMPs will vary among farms, depending on the distribution of nutrients arriving on farm as feed or fertilizer. A reliance on BMPs that focus on improved on-farm manure utilization and reduced fertilizer purchases benefits those farms importing more nutrients as fertilizer. Based on results from 33 Nebraska swine and confined beef operations (Fig. 3) , the environmental benefits will be the greatest for smaller operations. Smaller and medium-sized livestock operations typically have greater relative land base and greater reliance on purchased fertilizers. Larger livestock operations, which typically import most of their feed, gain the least environmental benefit from the currently mandated BMPs. Because of the limited land base associated with the case-study feedlot, the whole-farm nutrient imbalance remains large and the potential environmental value of the two mandatory BMPs is likely to be small.
Manure Transfer to Off-Site Users
The new USEPA CAFO regulations do not explicitly encourage or discourage manure transfer to off-farm users. The current rules only require that large CAFOs maintain records that document the amount of manure transferred, date of transfer, and recipient. As livestock systems, including the case-study farm, implement mandatory NMPs, there will likely be an extended learning curve for many livestock operations relative to the need for manure export. Time will lapse as farmers realize that their land base is no longer sufficient and implement measures necessary to transfer manure to off-farm sites. Gaining the trust and willingness of neighboring crop producers to accept manure as a nutrient source can require many years. Implementation of a NMP will likely not spur the immediate manure export programs that should accompany the NMP for many farms.
If the beef case-study farm transfers 50% of the manure to off-site uses, excess P will reduce from approximately 38000 to 20200 kg yr–1 (Balance 4 of Table 3). This BMP should reduce the farm's nutrient-related risks substantially more than either of the previous two alternatives. Implementation of this practice will require identifying of neighboring crop farms willing to accept 2270 Mg of manure.
An additional practice that can affect feedlot nutrient balance is the level of P in feed programs. Many by-product feeds that are growing in popularity produce a feed ration high in P. Corn gluten feeds and ethanol distilling by-products can produce dietary P levels of 4 to 5 g P kg–1. Corn-based rations are typically –1 P, whereas National Research Council recommendations for beef cattle are –1 P (National Research Council, 1996). Any excess P in the diet beyond minimum requirements is likely to be excreted in the manure. Koelsch and Lesoing (1999) observed significantly greater whole-farm nutrient imbalance for beef cattle feedlots feeding by-products of ethanol production and corn processing.
For the beef case-study farm, an alternative feeding program (feed program Option 1 in Table 2) was proposed that involved removal of the 3150 Mg of corn gluten feed from the diet and rebalancing the diet based on corn, alfalfa, and supplements. This approach represents a common feeding program used in regions without access to by-product feeds. This feed program change would reduce the whole-farm P balance from 37900 kg to 16500 kg of excess P yr–1 (Balance 5 of Table 3). For this farm, the practice would appear to have the single greatest value for addressing P-related environmental risks. However, the environmental benefits of this voluntary practice are commonly not recognized by the producer or regulator as achieving the goals of current regulations. Many producers do not recognize that excess P in the diet produces significantly greater quantities of P in animal manure, and current regulatory focus does not raise this issue as one of importance. Without incentives, adoption rate of this change is likely to be low especially considering the economic benefits of using these by-products.