Abstract: The objectives of this study were to assess the impact of the time and methods of liquid hog manure (Nm) applications and the ammonium nitrate (N34) on the forage hay dry matter yields and the forage N use efficiency with regards to the groundwater quality. Nm, either overhead (NmO) or low-level broadcast (NmL), and N34 were applied on the forage hay in the spring and after the first cut upon the N fertilizer recommended rates. The results indicated that the effect of Nm was not significantly different from that of N34 with regards to the dry matter yields for the first cut. The soil organic matter (SOM) provided up to 80 kg N ha-1 in addition to the applied N fertilizer amounts, regardless of the N source. NmL effects applications were not significantly different from that of NmO. The second cut yields and the corresponding N removals were very low probably due to reduced rain precipitation. Consequently, increased amounts of residual N were left in the soil and may have represented potential risks to groundwater contamination. A model based on the target dry matter yield of the forage is suggested to predict and prevent potential risks to groundwater contamination. The results also suggest SOM should be taken into account in the forage N recommendations. The study should be continued over the next two years in order to validate these preliminary results.