Impacts of Long-Term Dairy Manure Applications to Irrigated Croplands on Crop Production and Soil Quality
The dairy industry has had a significant impact on Idaho, with a standing herd of 574,000 dairy cows in 2012. An estimated 79% of Idaho's dairy cows are located in only 7 counties in Southwestern and South-central Idaho. Cows in the intensive dairy-producing counties produce an estimated 13.2 million tons of manure every year. In this same region, large acreage is dedicated to a variety of high-value crops. Specifically, there are 118,000 acres of sugar beets, 68,000 acres of potatoes, 111,200 acres of barley, and 231,450 acres of wheat. Fields that are nearer dairies commonly receive the highest rates of applications, as a means to dispose of the manure while avoiding expensive transportation costs to move the manure to fields further away from the dairies. Alternatively, many Idaho growers are realizing the benefit of manure as a nutrient
source and soil amendment, and are applying manure to their fields as an affordable source of phosphorus, potassium, and micronutrients. However, there are concerns that manure does not contain enough P and/or K to replace fertilizers at lower application rates. At the higher rates used for waste disposal, there are concerns with degraded soil quality due to salt and copper accumulations, degraded water quality due to phosphorus accumulation and nitrate leaching, crop yield losses, and crop nutrient toxicities. To avoid some of the issues related pest and pathogen pressure, sugar beet and potato growers commonly apply manures prior to establishing the grain crop in their rotation due to the perception that root crops are more susceptible to pathogen and disease issues than grain crops. Some fields near dairies continue to receive manure applications on a yearly basis, regardless of crop.
The majority of the research regarding manure applications and plant growth in southern Idaho has been focused on only one, two, or three annual manure applications. These research efforts have not captured the impact of repeated manure applications over several years on crop production. These types of applications are more realistic to the common practices of a dairy operation, and will better reflect soil quality, soil nutrient, crop yield, crop quality, microbial, and pest pressure responses than shorter-term manure applications. The goal of this project is to determine the impact of repeated manure applications to a wheat-potato-barley-sugar beet irrigated crop rotation at varying rates and timings on yield, crop quality, plant nutrient uptake, soil nutrient accumulations, soil organic matter, and soluble salts. We will achieve this goal by conducting an eight-year field study
focused on dairy manure application rate and timing. Three manure rates (7, 14, and 21 dry ton/acre) and two timings (every year and every other year) will be evaluated. We will observe how these treatments impact crop growth and soil quality. Crops to be evaluated will include wheat, barley, potatoes, and sugar beets. Using information generated by this study, the expected impact is improved crop quality, soil quality, and water quality on crop fields receiving repeated dairy manure field applications.