Inappropriate fertilization practices in regions where livestock manure exceeds crop requirements may cause pollution by NH4+-N) during snowmelt. Annual and seasonal NH4+-N losses were evaluated for three consecutive years from 45 m2 corn (Zea mays L.) and timothy (Phleum pratense L.) and red and white clover (Trifolium repens L.) erosion plots that had annually received the recommended chemical fertilizer rates of 180 and 55 kg N ha-1, respectively, plus hog manure at twice those rates. Total N applications for corn and forage were 540 and 165 kg ha-1 except for a check plot receiving only fertilizer. The hog manure was surface-applied in three different ways: all in the fall, all in the spring, and in a split application with about one-half in each season. Corn plots receiving fall-applied manure were immediately rototilled while plots receiving spring-applied manure were rototilled prior to planting. Each plot was equipped to collect runoff and drainage water separately throughout the whole year. Total NH4+-N losses in both runoff and drainage water from autumn application were greater than for the two other application methods for corn and the forages. Plots receiving chemical fertilizer plus hog manure had a significant increase in NH4+-N losses as compared with the plots receiving fertilizer only. Due to the fall manure effect, NH4+-N loads increased by three-fold after the first year (1.11-3.43 kg ha-1), by 1.46 times between the first and the second year, and by 1.14 times between the second and the third year for corn when compared with the fertilizer only treatment and exceeded the standard of 0.5 mg L-1 for raw water. Similar trends were measured. for the forages. Most NH4+-N was lost during the autumn and winter periods for corn. For both crops, there was an increase in loss from fall to winter and a decrease both from winter to spring and from spring to summer.