Reducing Nitrogen Fertilizer Inputs to Irrigated Pastures and Hayfields by Interseeding Legumes

Project Overview

Project Type: Professional + Producer
Funds awarded in 2010: $49,849.00
Projected End Date: 12/31/2012
Region: Western
State: Colorado
Principal Investigator:
Joe Brummer
Colorado State University

Annual Reports


  • Agronomic: general hay and forage crops, grass (misc. perennial), hay
  • Animals: bovine
  • Animal Products: dairy


  • Animal Production: pasture fertility, pasture renovation, feed/forage
  • Crop Production: conservation tillage
  • Education and Training: extension, farmer to farmer, on-farm/ranch research
  • Farm Business Management: budgets/cost and returns
  • Production Systems: general crop production
  • Sustainable Communities: sustainability measures


    Suppressing grasses with glyphosate prior to interseeding resulted in the most consistent legume establishment. Close mowing to simulate heavy grazing generally did not result in improved establishment. Of the five legumes evaluated, alfalfa established the best in the glyphosate treatment in Colorado, increasing yield by over a ton per acre. In Idaho, establishment was more variable, with red clover establishing well regardless of suppression treatment. No legumes established in Oregon due to heavy rodent activity. This study highlighted the importance of suppressing the existing grasses and choosing a vigorous legume species for interseeding to reduce the risk of seeding failure.


    Forage producers that primarily manage irrigated grass pastures or hayfields are struggling with how to maintain yields given the current price of nitrogen fertilizers. The price of nitrogen fertilizers has increased from $0.20 per pound of nitrogen a few years ago to a high of $0.80 to $1.10 per pound in 2008, with current prices in the range of $0.60 to $0.65 per pound. Nitrogen is the number one limiting nutrient for grass production, so it is essential to apply if producers want to maintain yields. Some producers have started to use manures and composts as alternative sources of fertilizer in areas where they are available. The high rates of application coupled with high transportation costs limit the use of these sources.

    Another alternative is to introduce various legumes into grass dominated stands by interseeding. Legumes are known for their unique ability to fix nitrogen from the atmosphere through a symbiotic relationship between the plant and specific bacteria that infect the roots, forming nodules. Legume plants themselves benefit from the fixation of nitrogen, but the associated grass plants can also benefit as the nodules periodically sluff, decompose and release nitrogen into the soil for uptake by other plants. Productivity of grass-legume stands will generally never equal what a grass-only stand that is adequately fertilized with nitrogen will produce. However, the loss in productivity is offset by the higher quality forage that is produced by having a legume in the mix.

    Interseeding generally involves the use of specialized drills that cut through the existing sod layer and place the seed in contact with the soil at the proper depth. Although seed can be broadcast into the existing stand, as a general rule, establishment success of forages is greater with drilling. With interseeding of legumes, the key is suppressing the existing grasses long enough for the new plants to establish. Only minimal establishment of the seeded species can be expected without suppression of the existing vegetation. Various methods have been tried over the years with varying degrees of success. Suppression methods that have been tried include close grazing with livestock, light disking or rototilling, flail mowing and various herbicides such as Roundup or Paraquat. Even with suppression of the existing grasses, it sometimes takes two to three years before the interseeded legumes reach full productivity. On the positive side, the cost of interseeding is considerably less compared to complete renovation of a stand using tillage. Additionally, a full season's production is generally not lost since the existing plants tend to recover quickly following suppression and can be harvested for hay or lightly grazed during the year of seeding.

    Although basic knowledge of how to interseed pastures and hayfields currently exists, producers still have a number of questions on how to successfully implement these techniques. Without answers to their questions, they are reluctant to implement these practices. Questions commonly asked include: 1.) Which brand or type of drill is best to use?, 2.) Which legume species are easiest to establish?, 3.) Once established, which legume species are most persistent?, 4.) What time of year should I interseed to achieve the best results?, 5.) How do I effectively suppress the existing grasses to insure establishment of the legume I am interseeding?, and 6.) How is the yield and quality of my forage stand affected by interseeding a legume? This project attempted to answer a number of these questions posed by producers.

    Project objectives:

    The overall objective of this project is to demonstrate to producers how legumes can be interseeded into existing grass-dominated pastures and hayfields, thereby increasing the quantity and quality of forage produced while reducing nitrogen fertilizer inputs. Both on-farm demonstration and small plot trials will be used to achieve the following specific objectives:

    Objective 1 - Evaluate the establishment success of various legume species, including alfalfa, birdsfoot trefoil, red clover, white clover and sainfoin, interseeded into grass-dominated pastures and hayfields with and without mechanical, herbicidal and/or animal suppression of the existing vegetation.

    Objective 2 - Evaluate the effects of introducing legumes into grass-dominated pastures and hayfields on forage yield and quality compared to fertilizing the existing grass dominated stand with nitrogen.

    Objective 3 - Using inputs of seed, herbicide, fertilizer, labor, machinery, etc. associated with objective 1 and yield and quality outputs from objective 2, conduct a basic economic analysis comparing pastures or hayfields that have adequate (>20% by weight) legume composition to straight grass stands fertilized with nitrogen.

    Objective 4 - Disseminate results of this project to other producers through such means as field days, workshops, conferences, on-line reports and other media, and the production of a how-to manual.

    Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.