Livestock Farm Phosphorus Import/Export Analysis, and Strategies for Phosphorus Management.

The objective of this project is to measure farm-gate phosphorus (P) import/export balances on Minnesota dairy and beef farms, identify sources of imbalances, and examine strategies for reducing excess soil test phosphorus (STP) buildup.  Where livestock farms are land-limited relative to manure-P available, STP can rise, increasing dissolved and particulate P in runoff, and increasing algae growth in rivers and lakes. Detailed spreadsheet analyses of farm-gate imports and exports of P were conducted on 14 dairy and 5 beef farms that were expected to show excess P imports over exports.  Excess P ranged from less than 1 to 130 kg ha^-1 year^-1. The dominant sources of P imports were feed, feed supplements, and fertilizer, with ratios differing considerably by farm, but feed supplements usually the largest. Case studies were developed, identifying strategies for reducing import/export imbalances. Some of the strategies are: 1. Feed management, including reduce P in rations to National Research Council recommendations and reduce feed waste. 2. Herd management, including breed all heifers to freshen at 22-24 months and reduce heifer inventory. 3. Crop management, including increase crop yields and forage quality to substitute for imported feed and base P fertilizer applications (including starter) on university guidelines. 4. Manure management, including increase transport of manure to more distant, low STP fields; export manure by contractual arrangement; and inject or incorporate manure to conserve nitrogen (N), allowing reduced application rates to meet crop N requirements. The case studies were presented at 21 workshops primarily for dairy and beef farmers and agricultural professionals. Almost half of the producer attendees stated that their operations are likely out of balance, and most are considering adopting one or more of the strategies presented. The case studies will be posted at the University of Minnesota Extension website at z.umn.edu/manure.