Evaluating nitrate toxicity potential in grazed cover crops

Project Overview

Project Type: Graduate Student
Funds awarded in 2017: $11,947.00
Projected End Date: 08/31/2019
Grant Recipient: University of Nebraska — Lincoln
Region: North Central
State: Nebraska
Graduate Student:
Faculty Advisor:
Dr. Mary Drewnoski
University of Nebraska - Lincoln

Information Products


  • Animals: bovine


  • Animal Production: animal protection and health
  • Crop Production: cover crops

    Proposal abstract:

    Grazing cover crops allows producers to obtain short term economic returns on their investment while also obtaining long term environmental benefits. However, one of the potential barriers to grazing cover crops is that high (currently suggested to be toxic) nitrates levels have often been observed. In some cases producers have not grazed these forages due to concerns of risk of toxicity. However, others have grazed without apparent negative effects. This project, “Evaluating nitrate toxicity potential in grazed cover crops” will increase the understanding of and refine guidelines for nitrate toxicity thereby allowing producers to more confidently make decisions about grazing cover crops with various concentrations of nitrates. Nitrate is reduced in the rumen to ammonia following consumption. When high levels of nitrate are ingested, the intermediate nitrite can accumulate in the rumen and can be absorbed into the bloodstream. In the blood, nitrite will convert hemoglobin to methemoglobin, which is unable to carry oxygen. The clinical signs associated with nitrate toxicity are directly related to the lack of oxygen in the blood. There are recommendations for the toxic level of nitrate in feed, however, these recommendations are based on dried hay and supplemental nitrate rather than freshly grazed forages. We hypothesize that the current recommendations do not accurately reflect the risk of nitrate toxicity for fresh, grazed forages. Given that grazed forages have a slower rate of intake, that fresh forages have a slower release of nitrates, and that higher energy diets (such as immature forages) reduce the accumulation of nitrite in the rumen we believe the current recommendations are too conservative. To test this, samples will be taken from herds of cattle grazing cover crops in multiple locations in Nebraska with the goal of obtaining samples for cattle grazing forages with varying levels of nitrate. Blood samples from a subset of cattle at each location will be taken to measure methemoglobin, to use as an indicator of toxicity. Forage samples will also be taken at each location and evaluated for nitrates, as well as water soluble carbohydrates, in vitro digestibility, NDF/ADF, crude protein, and sulfur. This will allow us to evaluate these variables in combination with nitrate concentration for their potential to assist in predicting blood methemoglobin concentrations and thus potential toxicity. Ultimately the success of this project will be based on feedback from producers and extension educators who attend meeting where the updated guidelines are presented.

    Project objectives from proposal:

    Grazing of cover crops can be way to obtain economic value while achieving environmental stewardship. This project addresses one of the potential barriers to grazing cover crops as high nitrates levels have been observed frequently. However, perceived risk when grazing cover crops/annual forages may be greater than actual risk.

    The reduced rate of intake and increased selectivity when grazing, the decreased rate of nitrate release into the rumen with fresh forages, and the high-energy content of the diet may decrease the potential for toxicity.

    This study will:

    1. Increase the understanding of nitrate toxicity potential in grazed cover crops/annual forages
    2. Contribute to improved quality prediction of various commonly planted cover crops/annual forages

    As a result:

    1. University Extension will adjust risk of nitrate toxicity guidelines for grazing cover crops/annual forages
    2. Producers will be more confident in making decisions regarding grazing cover crop/annual forages with varying levels of nitrate.
    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.