Managed Grazing as an Alternative
Manure Management Strategy

Jay Dorsey, Jodi Dansingburg, Richard Ness
USDA-ARS, Land Stewardship Project

Background

In 1993 several farm families making the transition to managed grazing asked the Minnesota-based Land Stewardship Project (LSP) and the Minnesota Institute for Sustainable Agriculture (MISA) to help them develop methods for monitoring the impacts of managed grazing systems. The two organizations helped lay the groundwork for the Biological, Financial, and Social Monitoring Project by bringing together a team of 25 people that included six farm families making the transition to managed grazing, university researchers, private consultants, LSP staff and federal, state and local agency officials. Preliminary results from the first four years of the "Monitoring Project's" multi-disciplinary research efforts show managed grazing has the potential to be a major component of an affective strategy for managing livestock wastes.

Managed grazing, also called rotational grazing or management intensive grazing, is a method of milk and meat production that utilizes the natural ability of cattle and other livestock to harvest their own feed directly from pastures, spreading their own manure on the same fields as they graze. Managed grazing systems utilize from a few to several dozen fenced-in paddocks to confine livestock to a restricted area for a limited length of time, usually a few hours to several days. Farmers attempt to design their system of paddocks to balance several key factors including the amount and quality of forage available, the number and type of animals, and nutritional needs.

Monitoring Project farmers gravitated to managed grazing primarily for economic and quality of life reasons. From a financial standpoint, the farmers felt that managed grazing allowed them to focus more directly on maximizing net profits as opposed to maximizing production. Grazing systems are less equipment-, infrastructure- and input-intensive than confined livestock operations, with manure storage and handling a foremost example. Because the animals are distributing their manure as they graze, the volume of stored manure is greatly reduced or eliminated. Managed grazing also assures that nutrients will be distributed relatively uniformly over the entire grazed area.

In addition to reduced manure handling responsibilities, managed grazing has improved the farmers perceived quality of life by reducing crop production responsibilities and the stress that goes with them. More time is available during the traditional row crop field preparation, planting, cultivating, and harvesting windows for livestock and pasture management. The farmers prefer the time spent walking their pastures or on the seat of a 4-wheeler to time spent behind the wheel of a tractor or combine. They also feel good about protecting soil and streams, and improvements to wildlife habitat that has been documented as part of the Monitoring Project.

Because of the forage and livestock management components, one of the great promises of managed grazing is its potential for positively impacting soil quality. We set out to discover what those impacts were. In addition, we decided to address concerns about the potential for leaching of soil nitrates into ground water. Our preliminary results are presented below.

For a given soil type, the structure, or aggregation characteristics, of the soil largely determines the soil's capacity to transmit water and air. In addition to the arrangement of soil aggregates, we are interested in the aggregate's ability to resist breakdown by the action of water. Theoretically, aggregates that are more water-stable will resist breakdown by water and be less susceptible to erosive forces. It is widely known that grass roots tend to build aggregate stability whereas tillage tends to decrease it (see e.g., Frye et al., 1985). Therefore, we would expect water-stable aggregation of soil to be greater under managed grazing when compared to row crop production.

Using the method of Yoder (1936), we found soil aggregate stability to be an average of 53% higher under managed grazing than under row crops for 10 paired comparisons covering six different soil types (Table 1). The one site where the aggregate stability was not noticeably different between managed grazing and row crops, was a cropping system with a three-year rotation which included small grain and hay crops.

Earthworm Populations

Earthworms benefit soil quality in many ways. A description of their positive impacts is detailed by Minnich (1977) and includes: increasing pathways for infiltration, soil aeration, and root growth; breaking up of hardpans; incorporation and mixing into the soil of organic materials; and making soil nutrients more available to plants. Managed grazing systems appear to be an ideal environment for encouraging earthworms because tillage and harmful chemicals are rarely, if ever, used, cooler temperatures and moister soils are maintained by the "permanent" grass/legume cover, and a constant supply of organic matter is available at the soil surface from cowpies, uneaten forage, and manure applications.

As expected, our results showed that earthworms fare well in a managed grazing system when compared to row crop production. Table 1 shows total earthworm numbers averaged 131% higher under managed grazing than under row crops for 9 paired comparisons covering five different soil types. These differences were even more striking for the family of worms that includes the nightcrawlers. We found juvenile Lumbricus spp at only two of five row crop sites (at an average of one worm/shovelful at those two sites) whereas we found between 2 and 23 L. spp/shovelful on the 9 managed grazing sites (Table 1).

Organic Matter Content

Managed grazing systems, with their permanent grass/legume cover, show increased organic matter at the soil surface. Figure 1 shows a comparison of organic matter content (%) throughout the top 12" of soil for a managed grazing system, and an across-the-fence row crop comparison, for a Lester soil in southeastern Minnesota. It should be noted that at the time of sampling (fall 1994) the managed grazing system had been in place for only three years. Comparisons between managed grazing and row crop systems on other soils show similar trends. The increasing organic matter content at the surface helps improve and stabilize soil aggregation (see section on water stable aggregates) at the soil surface resulting in increased water infiltration, and reduced runoff and erosion. Keeping more water (and sediment) on fields reduces the threat of contamination of surface waters from runoff. The addition of manure by the grazing animals adds additional organic matter, which is rapidly incorporated into the soil matrix by earthworms and other soil organisms. Observations by Monitoring Team farmers and researchers would suggest that, during the growing/grazing season (May-October) that most cowpies are completely incorporated within two grazing cycles (about 60 days) after deposition.

The increased infiltration potential, in conjunction with the large concentration of animals, in managed grazing systems raised concerns about the potential movement of nitrogen into groundwater through the leaching of soil nitrates. For selected sites, we decided to sample for deep soil nitrates to provide an estimate of the nitrogen leaving the system by leaching into the groundwater. We collected samples at three depths - 0-8", 8-24", and 24-40" (10 samples x 3 depths at each site) to analyze for nitrate content. The samples were collected in late October (1994, 1995) after harvest (row crop system) or after plant growth, and thus nitrate uptake, had all but ceased (managed grazing system). The samples collected below 24" depth were assumed to represent that nitrate that would be leached into the groundwater. For all three soil types, in the fall of 1994 deep soil nitrates were lower in the managed grazing system when compared to row crop production. This trend was again apparent for 1995 data.

In the interest of economy and ease of maintenance, several project farmers began wintering their cattle on pasture. In this wintering system, the animals were typically concentrated in one or two "sacrifice paddocks" from the end of November until April. The livestock were fed either with a systematic grid of big round hay bales and electric fencing, or through daily visits to a feedbunk/cowyard. For the winter of 1995/1996, we collected nitrate samples before (October 1995) and after (April 1996) the wintering period to track any increase in deep nitrates as a result of pasture wintering. Table 2 shows that, for two of three sites, nitrates increased at the soil surface (0-8") but were stable, or even decreased below 8". Both of these sites had finer textured soils. The third site, a coarse-textured alluvial soil, showed an increase in nitrate content at all depths. This would suggest that site selection may be critical for minimizing wintering impact on nitrate movement to groundwater.

Monitoring Project research has documented benefits associated with managed grazing systems on water chemistry, physical habitat, and biological indicators of streams. The results showed that the managed grazing systems produced meaningful improvement in stream quality when compared to the continuously grazed sections of streams. Pastures used in the managed grazing operations involved in the Monitoring Project were also documented to provide beneficial habitat for grassland birds as well as frogs and toads.

The Monitoring Project has documented a number of benefits to soils and streams as a result of transition to managed grazing. These benefits are derived, in part, from the system of "grassed" paddocks and manure management by the livestock themselves. In addition, the large proportion of manure handling performed by the cattle in a managed grazing system reduces the requirements for manure storage and handling adding financial and quality of life incentives for consideration of this strategy as a manure management alternative.

¹ Geometric mean of 10 samples (Maximum in parentheses).

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