The Effectiveness Of Nutrient Management Plans

UPDATED: January 28, 2013

Nutrient management plans (NMPs) are implemented to supply plants with ideal amounts of nutrients, minimize runoff pollutants, and maintain or improve the soil condition. As we continue to understand the effects farming practices have on the environment, NMPs present farmers with a set of conservation practices designed to reduce harmful pollutants while obtaining optimal crop yields.

Many U.S. Animal Feeding Operations (AFOs) produce excess N and P compared with the nutrient requirements of their cropland, and more regulations are being implemented to help reduce the pollution. In 1998, in response to President Clinton’s Clean Water Action Plan, the USDA and USEPA developed a Unified National Strategy to minimize the effects of AFOs on water quality and public health.

Today, each state has a different set of guidelines for NMPs, most of which involve farmers developing field-by-field records of nutrient applications. Currently, only Delaware and Maryland have nutrient management laws requiring all farmers to implement NMPs.

A study published in the March–April 2012 Agronomy Journal evaluates how effectively four dairy farms implemented their own NMPs from 1999 to 2005. The researchers, from the University of Connecticut and the USDA, compared the farmers’ reported practices with the recommended manure and fertilizer management plans and evaluated whether manure and fertilizer management had significant effects on the nutrient status of soil and corn tissue tests.

The majority of the cropland of the four farms selected for this study was used for corn silage, with substantial acreage also used for grass hay. The farmers’ priority for management and nutrient tests was corn, so only corn fields were used in this study. Each of the soils at the four farms had sandy loam to loam texture, a slope of 0% to 8%, organic matter content of 3 to 6%, and were moderately well to well-drained.

The NMPs of each farm consisted of two parts: a baseline year and the implementation years. In the baseline year, the research team collected information about each farm’s nutrient management. The factors considered included the number of livestock, cropland acreage, nutrient status of the cropland, manure production, manure management, and fertilizer management.

The farmers kept field-by-field records about manure and fertilizer applications but made no changes to the management of their nutrients. The information collected in the baseline year was used to develop field-by-field recommendations for manure and fertilizer management to be applied in the subsequent implementation years. The recommendations were based primarily on soil tests.

The rate of manure recommended was based on the amount of P needed by the crop, which researchers estimated from soil test results. If the farm had excess manure after fulfilling the crop’s P needs, recommendations were made to apply manure at a rate equal to the amount of P expected to be removed by the crop. If the farm still had manure leftover, it was to be applied to fields to meet the N requirement for the crop to fields that had a low potential to export P to water bodies. Chemical fertilizer recommendations were made after all manure was allocated.

Soil samples were routinely collected by farmers or crop advisers and analyzed at the University of Connecticut Soil Testing Laboratory. Samples were taken from the surface 30-cm layer from most of the fields in late spring when the corn was 15 to 30 cm tall. The samples underwent pre-sidedress nitrogen tests (PSNTs), which indicate how much nitrogen is available in the soil.

Cornstalk samples were also collected for corn stalk nitrate tests (CSNTs), which provide a retrospective assessment of a season’s nitrogen management. Farmers collected 15 cornstalk samples from each field during the period ranging from one week prior to harvest through one day after harvest.

During the implementation years, farmers were provided the results of the PSNT test and the recommended N sidedress. However, the percentage of times the farmers that actually applied the recommended N rate when the sidedress N recommendation was greater than zero was low, ranging from 6% to 24%.

The percentage of times the farmers applied the recommended N rate ranged from 79% to 90% when the recommended rate was zero. This data implies that farmers were not likely to follow recommendations derived from PSNT tests when the recommendation was to apply N. This behavior is supported by scientific reports showing that PSNT tests are most reliable when the N recommendation is zero.

The farmers applied fertilizer P at the recommended rate in the NMPs in the majority of the fields. In most years, more than 50% of the fields received a recommendation of zero fertilizer P, and farmers tended to follow that recommendation. Across the four farms, the overapplication of P occurred in less than 3% of the field-by-year combinations. This data shows that farmers were willing to adopt P fertilizer recommendations provided in their NMPs.

Only a small percentage of the fields, ranging from 3% to 37%, received the recommended amount of manure N and P. Variability in the amount of residual N available from previous manure applications caused great variance in the PSNT and CSNT tests, indicating that several years of data are needed before these tests can be used to accurately evaluate the performance of a NMP.

The four farmers in this study seemed confident that fertilizer recommendations were appropriate when based on soil tests, especially recommendations for fertilizer P. However, there was no significant change in soil test P values, probably due to the large spatial variation of P in the fields.

The results from this study showed no decreasing trend in the concentrations of nitrate in the PSNT and CSNT tests. The large variation in the amounts of manure and fertilizer applied by the farmers may be responsible for this lack of improvement.

Only a small percentage of fields received the recommended amount of manure application, and no significant decrease in the amount of manure over application compared with the recommended amount was found in this study.

Documenting improvements in N and P management after the implementation of an NMP proved to be a difficult task for the researchers of this study, due to both biological and management factors, such as non-uniform manure spreading and annual variations in weather, especially rainfall.  

Overall, the researchers say that manure and fertilizer management on dairy farms could be improved by using a longer timeframe than the three- to five-year plans typically used by NMPs. Management plans implemented using a process of adaptive management for N, which provides more detailed, continuous evaluations about the N status of corn fields, have also been shown to improve N management.

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