Evaluation of Managed Rotational Grazing of Livestock to Reducing Ground Water Contamination from Soil Nitrates

Richard Leep, Martha Tomecek, Jim Lempke

Department of Crop and Soil Sciences

Michigan State University

Part I.  Effect of Managed Rotational Grazing on Soil Nitrate Concentration

The purpose of this experiment is to evaluate managed rotational grazing (MRG) of livestock to reduce groundwater contamination from soil nitrates.  We have set up sampling sites in pair comparisons of  (MRG) pastures and adjacent cornfields of similar soil type being grown for corn silage.  Two of the sites for cornfields were manured (Lake City and Chatham); the other corn sites utilized commercial fertilizer N as a nitrogen source.  Each site was sampled using a hydraulic soil probe mounted on a truck.  Soil cores were taken up to 60 inches deep in increments based on the ability to penetrate the soil profile with the probe.  Transects were staked out consisting of a two straight lines of 100 yards each from the edge of the corn fields and two straight lines 100 yards each, starting from either a watering or feeding site within a paddock of a (MRG) pasture.  The soil profile was sampled at 0-3, 3-6, 6-12, 12-24, 24-36, 36-48, 48-60 inches.   Samples were taken in October 1995, April 1996, and October 1996.  Sampling was done in the fall and spring to determine soil nitrate loss over the winter in the different agricultural systems.  The data reported is from the October 1995 and April 1996 soil sampling dates.  All soil was dried, ground, extracted with appropriate extracting solutions, and analyzed on a lachet analyzer. 

Site History

Chatham:  The pasture has been rotationally grazed for 7 years.  The species composition is orchardgrass, quackgrass, birdsfoot trefoil, and white clover.  The area sampled starts from a loafing/feeding area and ends in the middle of a paddock.  The corn field was in a small grain in 1995 and planted to corn in 1996.  It had manure applied to the field in the fall of 1995.

Lake City: The pasture has been rotationally grazed for 10 years.  The species composition is mostly common bluegrass, orchardgrass, and white clover.  The sites sampled started from either a watering or a feeding site and ended in the middle of the paddock.  The cornfield was in silage production in 1995 and planted back to corn in 1996.  The cornfield was manured in the spring of 1996.

Kellogg Biological Station: The pasture has been rotationally grazed starting in 1995.  Prior to that it was in alfalfa and irrigated with flush water from the diary barn.  The cornfield was in wheat in 1995 and planted to corn in 1996.

Kalamazoo Farm: The pasture was rotationally grazed in 1995 for the first time.  Prior to that it was in corn production.  The cornfield was in corn in 1995 and soybeans in 1996.

Results

The results in a power point presentation can be found in this website.

Chatham:  The Chatham pasture showed significant (up to 20 PPM nitrates) at the 24 inch depth in the fall sampling near the watering sites.  Levels of nitrates from 100-300 feet from the watering sites were below 10 PPM at the 24-inch depth.  There were higher levels of soil nitrate at the 0-3 inch depth in the first 100 feet from the feeding site in the pasture.  The spring samples showed significant soil nitrate levels (up to 30 PPM) at the 0-3 inch depth in the first 100 feet from the feeding/loafing sites and lesser amounts 5-15 PPM between 100-300 feet from the feeding/loafing sites.  This would indicate mineralization of ammonium from the manure and urine deposited by the cattle near the feeding/ loafing sites.  Most of the spring nitrates found in the top 0-3 inches of soil will be available for plant uptake if there is grass in the sites.  The higher levels of nitrates found in the 24-inch soil depth may have been leached in the winter months.  The nitrate levels found in the cornfield in the spring showed very high amounts (up to 60 PPM) in the 0-3 inch depth of soil.  Most of the soil nitrate in the 0-3 inch soil depth should be available to the corn plants for uptake unless significant rainfall would occur early in the season before much corn growth occurs.  The fall, 1996 samples will determine how much soil nitrate is left in the soil profile.

Lake City: There were significant soil nitrate levels at the 24-36 in depth (up to 35 PPM) found in the pasture within the first 100 feet from the watering or feeding sites in the fall sampling.    Spring samples showed most of this was leached as levels found in the spring samples at the same depths showed only levels between 5-10 PPM soil nitrate.   Soil nitrate levels found in the cornfield at all depths ranged from 5 PPM to 20 PPM.  Spring samples showed levels of soil nitrate at the 24-inch depth ranging from 2.5 to 7.5 PPM.  This would indicate losses at the 24-inch depth due to leaching during the winter months. 

Kalamazoo Farm: Relatively low levels of soil nitrates were found at all depths in the pasture (2 to 7.5 PPM) except for the first 30 feet around the watering site where levels of soil nitrates ranged up to 19 PPM at the 24 inch depth.  The subsequent spring sampling showed soil nitrate levels in the same areas at the 24-inch depth up to 6 PPM.  There were similar levels of soil nitrate found at the 0-3 in soil depth in the fall and spring, which would indicate mineralization of soil nitrates and little losses from the top 3 inches.  This was probably due to vegetative cover during the winter months.  However, the lower amounts of soil nitrates in the spring at the 24-inch depth would indicate leaching around the watering site.  Levels of soil nitrates found in the corn field was similar or slightly lower to those found in the pasture.  There appeared to be little leaching of nitrates in the cornfield at this site. 

KBS:  Relatively lower concentrations of soil nitrates were found near the watering sites at this site, especially at the 24-36 in depth.  There was a high level found at the 200-foot mark from the watering site.  This was probably a random urine spot, which was sampled.  The relatively high concentration of soil nitrate found in the spring sampling near the watering site indicates a concentration of manure and urine near the watering site.  However, at distances further than 100 feet from the watering site, the soil nitrate concentration was similar to that found in the fall sampling.  The cornfield had higher concentrations of soil nitrates from 24 to 48 inch soil depths compared to the pasture.  Subsequent spring sampling showed significantly lower soil nitrates at the same soil depths.  This was mostly likely due to leaching of soil nitrates through the soil profile during the winter months. 

Significance

The observations and data found in the paired comparisons show there is potential for ground water contamination from either managed rotationally grazed pastures or row crop corn silage systems.  The only significant areas were soil nitrates appeared to be concentrated and potential for leaching to occur was near or around loafing/ watering/ or feeding sites.  This is probably due to the amount of time the animals frequent these sites thereby increasing the opportunity for increased manure levels and subsequent soil nitrate concentrations.  These problems can be reduced by either moving portable water tanks or by varying the areas used for supplemental feeding.  The relatively low soil nitrate levels found both in the spring and fall in the pastures would indicate low leaching potential.  This is probably due to the permanent vegetative cover, which is growing until freeze up.  Cornfields varied in the amount of soil nitrate concentration found both in the spring and fall.  There was significant leaching which probably occurred in some of the fields sampled, however, some of the fields were managed extremely well and had very little soil nitrates leached during the winter months.

Additional soil samples were analyzed which validate the above results.  In addition, suction samplers which were installed at the KBS site to determine soil nitrate movement within the pasture over time.  Samples are being collected in 1999 after allowing the samplers to equilibrate during 1998.  Soil bulk density sampling has been done this spring to calibrate the samplers with soil moisture holding capacity.  Dr. Mike Russelle, University of Minnesota Soil Scientist will assist in analysis of leachate collected.  These studies will be complimentary to the on-going studies present at the Living Field Laboratory site where Dr. Harwood and students are collecting leachate in field cropping systems.  The studies will show when the critical times is for potential soil nitrate leaching.  We will also be able to determine the effects of nitrogen fertilization or manure applications to pastures in terms of potential soil nitrate leaching. 

In 1999, the pasture treatments have been changed to reflect typical on-farm decisions, which need to be made.  After four years of grazing, the legume content of the pasture has been depleted to approximately 15-20% of the forage composition.  The title of the new research initiative is: Evaluation of pasture management strategies upon forage biomass production, nitrate leaching, animal growth, and health.  The research objectives are:

5.      Determine economic paybacks related to a legume/grass based or nitrogen/grass based pasture management.

Rationale:

Michigan has approximately 2 million acres of pastures.  Most of the pastures used in Michigan are marginally improved and contain volunteer or native species.   Many producers are asking if they should improve pastures by establishing legumes or by applying nitrogen fertilizer.  It might appear on the surface that legumes should be the logical choice, however, legumes do not persist under grazing pressure and are somewhat expensive to establish into an existing pasture.  We are proposing to evaluate pasture improvements with either direct drilling red clover or adding nitrogen fertilizer.  We can study the environmental effect of these management decisions on nitrate leaching with the suction samplers installed in each of the existing paddocks at KBS.  In addition, with forage sampling and weighing cattle throughout the grazing season, we can determine the impact of these pasture management regimes upon biomass production and forage quality.  If dairy heifers are utilized in this trial, one can also compare the health of the heifers on pasture compared to those kept in confinement.    We are proposing to minimize labor by allocating enough forage area to last approximately 10 days so the existing pastures will be divided into 4 paddocks of 2 acres each.  This will give each paddock approximately 30 days rest period between grazing cycles. 

Data Collected:

Forage biomass produced

Forage quality sampled from each grazing cycle or every ten days

Animal weight gains

Animal health including body score, lameness, calving difficulties or ease of calving

Nitrate leaching

Outcomes:

Information obtained from research will be presented at the American Forage and Grasslands Conference and published in the proceedings

Results will be published in the Michigan Hay and Grazier Newsletter

Results will be shared with the Michigan Hay and Grazing Council Members via the above newsletter and local pasture walks

Results will be posted on the MSUE forage information systems web site

Economic impacts will be developed for grazing systems

We will be able to answer the question as to what impacts each pasture management system has upon the economics, ecology, and environment

Part II. Evaluation of Cover Crops for Reducing Groundwater Contamination from Soil Nitrates and extending the Grazing Season.

A supplementary grant was awarded to the Michigan Hay and Grazing Council in 1997 to study the potential of cover crops for extending the grazing season reduces soil nitrate losses, and the effect upon soil bulk density (compaction).  The study was done on the Henry Miller farm near Centerville, MI.  Cover crops were planted in large fields following either snap beans or arial seeded into standing seed cornfields in Mid-July.  Cover crops planted for the study included forage turnips, rape, winter rye, oats, and combinations of winter rye and turnips/rape.  The fields were soil sampled in early November 1997 to determine soil nitrate levels in the top two feet of soil profile.  The cover crops were sampled in November, December, and January to determine biomass and nutrient content of the cover crops.   Enclosures were made in four randomized locations within each field which cover crops were grazed in the fall and winter months to determine the effect of animal trampling on soil compaction.

Results

Soil analysis for soil nitrates was done using the Michigan State University soil-testing laboratory in East Lansing, Michigan.  Results of soil analysis showed insignificant amounts of soil nitrogen in the top 24 inches of the soil profile in early November.  This was probably due to good nitrogen management in the snap bean and seed corn production at the Henry Miller farm.  Since there were only small quantities of nitrate nitrogen present at the time of sampling, the soil nitrate data is not shown.    Cover crops were sampled to determine both nutrient content as well as biomass produced.  This is an indicator of nitrogen uptake by the plants.  The results of total biomass from crops sampled from November through January are given in Table 1.  Results of crude protein in the crops are given in Tables 2 and 3.  These data show that the cover crops were still growing in December as indicated by increased biomass from November to December.  The covers of oats/rape showed the greatest biomass produced at slightly less than 4500 lbs/acre by December.  The oat/rape combination removed approximately 86 lbs/acre nitrogen based upon the protein content and total biomass produced.  The turnip crop removed approximately 58-lbs/acre nitrogen by December.  These data would indicate the faster growing combination of oats/rape have greater potential for removing excess nitrogen in the soil during the fall period of growth.  Soil bulk density was not significantly different where animals grazed compared to the enclosures.  This may be due to the freezing/thawing cycles in the late winter and spring, which created a better soil structure and reduced the effect of animal traffic on the soils.