Nitrogen Balances in Dairy Production Systems In

Nitrogen (N) is essential for agricultural production, but excess quantities can be detrimental to both water and air quality. The increase in dairy cattle populations in the Magic Valley of southern Idaho has lead to concerns over the impact of N losses to the environment and the impact on both water and air in the region. This work examines the N flows and balances at the production facility, whole farm and regional scale. At the production facility, approximately 60% of N imported was lost to the environment, this improved to only 41% lost when we considered the whole farm system. At the regional level, 45% of available N (fertilizer and excreted N by cattle) was lost as NH3-N. This loss of NH3-N is greater than the amount of fertilizer N imported to the region. These calculations indicate that there may be ways to improve efficiency of N use on farms in the Magic Valley which will ultimately benefit producers and the environment. INTRODUCTION Approximately 34 percent of the total U.S. population of milk cows is located in the semiarid to arid western U.S., with California and Idaho being two of the top four dairy-producing states. The total 2012 dairy cattle population in Idaho was580,000 milk cows with 82% of those being located in the Magic Valley (USDA, 2014). The counties comprising the Magic Valley include Cassia, Gooding, Jerome, Lincoln, Twin Falls, and Minidoka. There was approximately 1 million harvested acres within these counties in 2012 (USDA, 2014) with Cassia and Twin Falls Counties having the largest harvested acres, while Gooding and Jerome Counties had the largest dairy cattle populations. On average, in the Magic Valley, there is approximately 2.2 harvested acres of cropland per milking cow. The influx of dairy cattle into Idaho over the past two decades has changed cropping systems in the region as well. Cattle populations increased approximately 240% since 1990, along with that there was a 280% increase in corn acres harvested (USDA, 2014). Essentially there have been two acres of corn added for every three milking cows. It is also important to point out that this region is arid and all crop production is accomplished with irrigation and soils in this region tend to be sandy/loamy and in some cases very shallow (~2’). As with any increase in concentrated animal production, the increase of dairy cattle populations brings an influx of nutrients. Nitrogen (N) will be imported in feed ingredients and the majority of this N will end up being excreted by the cattle. Typically, N use efficiency is only about 30%, meaning that for every 1 lb of N ingested by the animal; approximately 0.7 lbs will be excreted (Kebreab et al., 2010). On the production facility this N will then be vulnerable to losses via volatilization of ammonia (NH3), nitrous oxide (N2O), and other nitrous oxides (NOx) a well as leaching losses of nitrate (NO3) from housing (lots) as well as manure storage areas. If we include a larger view of dairy production, then we also have to include the N imported in the form of fertilizer for forage and feed crops, N fixation by forages (alfalfa), other