In December of 2004, we conducted several intensive sampling events to determine the variation in nutrient content of dairy manure within a flush cycle. In addition, we completed a comparison of methods to estimate whole-farm flow of nutrients.
A 600-cow dairy in northwestern Washington was used to determine the amounts of N, P and K that were imported to and exported from the dairy over a year. Feed imports were determined using both formulated diets and actual feed receipts. In addition, feed samples were collected and analyzed to determine imports of nutrients to the farm. Nutrients exported via milk were determined using daily tank weights or monthly averages. Nutrients excreted in manure were estimated by three methods: 1) sample analysis and mass volume flow, 2) ASAE manure excretion standards and 3) the Dairy WFNBNET model.
The objective of this project was to estimate the whole-farm nutrient balance of nitrogen, phosphorous and potassium on a commercial dairy with various methods of calculating nutrient imports to and exports from the dairy operation.
In 2004, the dairy operation exported approximately 8 million kg of milk and 39,000 kg of milk N. Estimates for export of P and K in milk were 7,200 and 12,000 kg of P and K, respectively. The estimates of nutrients imported in feed were much greater when calculations were made from purchased feed as compared to formulated diets. The study provided insights into what information was most valuable in order to make an estimate of whole-farm balance and emphasized that feed and manure sampling, as well as accurate record keeping, are important to determine nutrient balances for an operation.
Sampling protocols were developed on a commercial dairy operation to determine the best strategy for taking samples from various points within the manure-handling system. The manure-handling system on the dairy is a flush system with sand bedding. A flume was installed prior to the sand-settling basin to determine the volume of manure and sand-laden water entering the settling basin. An in-line flow meter was installed between the sand-settling basin and the solids separator, and a second flume was installed after a screen-solids separator and solids-settling basin. Manure flow from the second flume directly entered a storage lagoon. Flow rates through the flumes and the flow meter were recorded electronically. Samples were taken from the large flume, from the solids separator, and the second flume to determine nutrient contents of manure at each of the individual points in the manure-handling system. Samples were collected to determine if the timing of the sample collection varied within a flush cycle. Samples were also taken to determine if flushes originating from various pens altered the composition of the manure through any of the sampling points in the manure-handling system.
The sampling time within each flush cycle had little effect on N and P contents but there was some variation in the solids contents. Evaluations of sampling strategies are important to determine how and when samples should be taken on commercial operations to provide accurate information for nutrient management decisions.
BENEFITS AND IMPACTS ON AGRICULTURE
The manure flow meter readings allowed us to collect and record actual application of manure as required in our nutrient management plan. In addition, we were able to monitor the efficiency of our manure solids separator and better monitor when the system needed cleaning. The initial evaluation of the manure-nutrient-application data suggests we need to export an additional 30% of manure nitrogen off the farm. The data collected on sample variation within in flush indicate it is best to obtain a sample in the middle of a flush cycle for measurement of nutrient content.
Presentations of data collected from the study were presented at the annual Dairy Science meetings in Cincinnati, Ohio, in July of 2005 as two separate papers.