2012 Annual Report 1a.Objectives (from AD-416): To determine the level of ivermectin in the blood serum of pastured horses that are provided with free-access to a solid, cooked protein supplement that is medicated with ivermectin. 1b.Approach (from AD-416): Ten horses will be placed in an approximate 20-acre pasture and allowed to freely range the entire area. A single tub containing an ivermectin-medicated protein block will be placed inside a central penning area supplied with water. The protein block will be weighed prior to allowing equine access. Once the horses are allowed access to the treatment block, the block will be weighed twice per week to determine (by weight) the quantity of material consumed by the horses. Once per week the horses will be haltered and a blood sample obtained from the jugular vein of each animal in separate vacuum tubes. Blood samples will be processed to obtain the serum from the whole-blood, and the serum will be frozen for later analysis. Serum samples will be analyzed by HPLC to determine the concentration of ivermectin in each serum sample. All data collected from the study will be tabulated to establish the amount of medicated material consumed and correlated to the concentration of ivermectin in the blood system. 3.Progress Report: All data collection has been completed on this project and a manuscript is now in preparation. It was determined that the horses consumed enough ivermectin from the feed block to kill ticks. There were some changes in consumption rate due to environmental and pasture conditions. These changes in block consumption resulted in fluctuations in ivermectin concentrations within the horses. We recommend to have a high and a low ivermectin block so that animals can be properly treated depending on environmental and pasture conditions.