Mitigation Measures to Manage Runoff

Mitigation Measures

A variety of factors including soil type, slope, and weather conditions (e.g., rainfall) can influence volume and intensity of water running off the treated field. The applicator must evaluate all factors and make appropriate adjustments when applying this product. Land management, field condition and application practices that reduce, to the maximum extent practicable, runoff from treated fields, must be implemented by land managers/users of this product. 

Requirements When Using Enlist Herbicides

For land with Hydrologic Soil Groups* A & B: The land manager/applicator must effectively implement measures in the following table to equal a minimum of 4 credits.

For land with Hydrologic Soil Groups* C & D: The land manager/applicator must effectively implement the measures in the following table to equal a minimum of 6 credits.

*Hydrologic Soil Group (HSG) definitions:
A = Sand, loamy sand, or sandy loam
B = Sandy clay loam
C = Silt loam or loam
D = Clay loam, silty clay loam, sandy clay, silty clay or clay

Contour Buffer Strips or Terrace

Contour Buffer Strips: Contour buffer strips are strips of permanent herbaceous vegetation, primarily of perennials such as grass, alternated with wider cultivated strips that are farmed on the contour. Contour buffer strips allow runoff and trap sediment. Because the grass buffer strip is established on the contour, runoff flows evenly across the entire surface of the strip, reducing sheet and rill erosion. The grass slows runoff, helping the water soak into the soil and reducing erosion. Sediment, nutrients and other pollutants are filtered from the runoff as it flows through the strip thereby improving surface water quality.

The specific recommendations for establishing buffers vary from site to site.

Cultivated strip widths must be a minimum of 15 feet wide.

Wider distances may be appropriate based on variables such as slope, soil type, field conditions, climate, and erosion potential Contour buffer strips are unsuitable in fields where irregular, rolling topography makes following a contour impractical.

To ensure maximum performance, the integrity of the buffer must be maintained for the entire width and length, including:

-          The contour buffer must be harvested or mowed, reseeded, and fertilized as necessary to maintain plant density and vigorous plant growth.

-          Vegetation must be kept tall in spring and early summer to help slow runoff flow, maximize disruption of concentrated flow, and reduce the chance of structural damage.

-          Regular maintenance must also include inspection after major storms, removal of trapped sediment, and repair of eroding areas.

Terraces: Terraces are described as a stair stepping technique of creating flat or nearly flat crop areas along a gradient. They can be constructed as earth embankments or a combination of ridge and channel systems. A terrace is an earthen embankment that is built across a slope to intercept and store water runoff. Some terraces are built level from end to end to contain water used to grow crops and recharge groundwater. Others, known as gradient terraces, are built with some slope or grade from one end to the other and can slow water runoff. Both help to reduce soil erosion by slowing the velocity of runoff and increasing the time for water infiltration. On the field, terraces can be used as a part of an overall system based on the topography of the land. Additionally, an earthen ridge or terrace can be constructed across the slope upgrade from a field area to prevent runoff from entering the area or to direct runoff from one area of production to a common runoff collection area. Reduced tillage practices will result in less sediment loading and the best performance.

The ends of terraces, including turnrows, must be structured and maintained to prevent concentrated flow from damaging the function of the terrace. In the event that runoff outflows are necessary, runoff must be directed to a system such as a grassed waterway, a grade-stabilization structure, a filter strip, water or sediment basin, or other suitable outlet with adequate capacity to handle the runoff and prevent gully formation.

Contour Farming

Contour farming is the use of ridges and furrows formed by tillage, planting and other farming operations following the contour to change the direction of runoff from directly downslope to across the slope.  The disruption of downslope flow slows the runoff velocity and allows for more time for runoff to infiltrate the field soils thereby reducing runoff.

The effectiveness of contour farming to reduce soil erosion and increase infiltration of runoff is dependent on several factors including the amount of rainfall, the grade and height of row ridges, steepness and length of the slope, crop residue and surface roughness, and the soil hydrologic group.

Contour farming must only be implemented on slopes between 2 and 10 percent, minimum ridge height of 1 inch, in areas with 10-year rain events less than 6.5 inches/24hrs, and with a length of slope between 100 and 400 feet long.

In areas with heavier rainfall events, and/or fields with steeper or longer slopes, the function of the ridges to hold back the runoff is lessened and may result in structural failure along the contour and therefore the efficacy of this practice is potentially compromised.

Establish and maintain the rows as close to the contour as possible.

Coupling the practice with reduced tillage practices will result in the best performance of contour farming.

Contour Strip Cropping

In strip cropping, a field is managed with planned rotations of row crops, forages, small grains, or fallow in a systematic arrangement of equal width strips following the contour across a field. Crops are typically arranged so that a strip of grass or forage (low erosional risk because of their fibrous root system) is alternated with a strip of row crop (high erosional risk; e.g., corn). The crops are planted across the slope of the land, as in contour buffer strips. This practice differs from contour buffer strips in that it allows for crops to be planted across 100% of the field area.

Less than half the field may be planted in row crops, and at minimum 50% of the slope must be planted with low erosional risk plants (e.g., grass plants because of their fibrous root system).

The low erosional risk crops reduce erosion, slow runoff water, and trap sediment entering through runoff from upslope areas. The practice combines the benefits of contouring and crop rotation.

Strip cropping is not as effective if the row crop strips are too wide and must only be implemented on slopes <10% slope. Establish and maintain the rows as close to the contour as possible.

Coupling the practice with reduced tillage practices will result in the best performance of contour strip cropping.

Cover Crop

A cover crop is a close-growing crop that temporarily protects the ground from wind and water erosion during times when cropland is not adequately protected against soil erosion. Common cover crops include cereal rye, oats, clover, crown vetch, and winter wheat or combinations of those crops. Cover crops are most often recommended when low residue-producing crops such as soybeans or corn silage are grown on erodible land. Cover crops increase soil stability, reduce runoff, and reduce erodibility of field soils when not under a harvestable production system.

The cover crop must be planted after harvest of the previous season’s crop and remain on the field up to the field preparation for planting the Enlist crop.

Planting directly into a standing terminated, mowed or rolled cover crop will provide the greatest benefit for reducing runoff. Cover crops may be used in conjunction with reduced tillage practices to further reduce surface runoff from production fields.   

Field Border

A field border is defined as a strip of permanent vegetation established at the edge or around the perimeter of a field. A field border can reduce runoff-based erosion and protect soil and water quality by slowing the flow of water, dispersing concentrated flow, and increasing the chance for soil infiltration. Its use can support or connect other buffer practices within and between fields.

Establishment and maintenance of the field border and immediately upslope (typically a cropped area) must aim to eliminate or significantly reduce concentrated flow and promote surface sheet flow runoff.

To prevent gully erosion within a field border, concentrated flow must be broken up or redirected. This may be achieved by aligning the field border and planting rows as closely as possible in a direction that is perpendicular to the slope. Use of water bars or berms to divert concentrated flow back into the field is another useful tool to break up the concentrated flow and promote sheet flow into the border.

The minimum width for the purpose of reducing pesticides in solution must be 30 feet and be comprised of a permanent dense vegetative stand. This stand should be composed of stiff upright grasses. Forbs may also be included in a well-managed border.

Reduced tillage practices, especially in proximity of the field border strip, will result in less sediment loading and the best performance.

Inspect after major storms and repair eroding areas.

Grassed Waterways

Grassed waterways are natural or constructed vegetated channels designed to direct surface water, flowing at non-erosive velocities, to a stable outlet (e.g., another vegetated channel, an earth ditch). Grassed waterways are used to control gully erosion. In concentrated flow areas, grassed waterways can act as an important component of erosion control by slowing the flow of water and filtering sediment. Other benefits of grassed waterways include the safe disposal of runoff water, improved water quality, improved wildlife habitat, reduced damage associated with sediment, and an improvement in overall landscape aesthetics. Grassed waterways are usually planted with perennial grasses, preferably native species where possible. Some common grass species used in waterways are Timothy, tall fescue, perennial ryegrass and Kentucky bluegrass.

The Enlist user must establish a maintenance program to maintain waterway capacity, vegetative cover, and outlet stability. Do not damage vegetation by machinery, herbicides, or erosion. Grassed waterways must be protected from concentrated flow by using diversion of runoff, silt fences, mulching, hay bale barriers, etc. to stabilize grade during vegetation establishment and after disruption or damage. Grassed waterways must be inspected regularly, especially following heavy rains. Any damage or disruptions must be repaired immediately by filling, compacting, and reseeding. Sediment deposits must be removed to maintain capacity of grassed waterway. Maintain a healthy, dense, and functional grass strip. Runoff outflow must be directed to a system such as another grassed waterway, an earthen ditch, a grade-stabilization structure, a filter strip, water or sediment basin, or other suitable outlet with adequate capacity to handle the runoff and prevent gully formation.

Residue and Tillage Management

This category of practices includes conservation tillage practices such as no-till, strip-till, ridge-till, and mulch-till.

Each of these involves management of the amount, orientation and distribution of crop and other plant residue on the soil surface year-round while limiting the soil-disturbing activities used to grow and harvest crops in systems where the field surface is tilled, raked or left undisturbed prior to planting. For each tillage practice below, more than 20 percent of the surface must remain covered with plant residue.

No-till/strip till: In these systems, the soil is left undisturbed from harvest to planting. Planting or drilling is accomplished using disc openers, coulter(s), and row cleaners. Weeds are controlled primarily with crop protection products.

Strip till: In these systems, the soil is left undisturbed from harvest to planting except for strips up to one-third of the row width. (The strips could involve only residue disturbance or could include soil disturbance.) Planting or drilling is accomplished using disc openers, coulter(s), row cleaners, in-row chisels, or rototillers; cultivation can be used for emergency weed control. Other common terms used to describe Strip-till, include row-till, and slot-till.

Ridge-till: Ridge-till is a system in which seeds are planted into a seedbed prepared by scraping off the top of the ridge. The scraped-off ridge usually provides an excellent environment for planting. Ridges are formed during cultivation of the previous year’s crop. Ridge-till operations consist of planting in the spring and at least one cultivation to recreate the ridges for the next year. Rows remain in the same place each year and any crop residue on the ridges at planting is pushed between the rows.

Mulch-till: This system uses full-width tillage involving one or more tillage trips, which disturbs the entire soil surface but leaves a uniform layer on crop residue on the soil surface and is done before or during planting. Tillage tools such as chisels, field cultivators, discs, sweeps, or blades are used. Weeds are controlled with crop protection products or cultivation or both.

Vegetative Barrier

Vegetative barriers are narrow, permanent strips of stiff stemmed, erect, tall and dense vegetation established in parallel rows on the contour of fields to reduce soil erosion and sediment transport. These buffers function similar to contour buffer strips and may be especially effective in dispersing concentrated flow, thus increasing sediment trapping and water infiltration. Because the vegetative barrier, typically comprised of grasses, is established on the contour, runoff is restricted, reducing sheet and concentrated flow-based erosion. The grass slows runoff, helping the water soak into the soil and reducing erosion. The specific recommendations for establishing the vegetative barrier vary from site to site.

Barrier widths are determined by variables such as slope, soil type, field conditions, climate, and erosion potential but at minimum must be 3 feet wide. To ensure maximum performance, the Enlist user must maintain the integrity of the barrier for the entire width and length, including:

-          The barrier must be harvested, mowed, reseeded, and fertilized as necessary to maintain plant density and vigorous plant growth.

-          The maintenance schedule must keep vegetation tall in spring and early summer to help slow runoff flow, maximize disruption of concentrated flow, and reduce the chance of structural damage. 

-          Regular maintenance must also include inspection after major storms, removal of trapped sediment, and repair of eroding areas.

Vegetative filter strips

Filter strips are managed off-field areas of grass or other permanent herbaceous vegetation that intercept and disrupt flow of runoff, trap sediment, and reduce pesticide concentrations in solution. A filter strip can vary in width (typically 20 to 120 feet wide) and is usually planted with native grasses and perennial herbaceous plants. Nutrients, pesticides, and soils in the runoff water are filtered through the grass, absorbed by the soil, and ultimately taken up by the plants. Filter strips should be designed and sized to match site specific characteristics. The effectiveness of filter strips to reduce pesticide loading into an adjacent surface water body depends on many factors, such as topography, field conditions, hydrologic soil group, antecedent moisture conditions, rainfall intensity, properties of the pesticide, application methods, width of the filter strip and types of vegetation within. Its use can support or connect other buffer practices within and between fields. Hydrologic soil groups (HSGs) are generally defined by the depth to a restrictive layer or water table, transmission rate of water, texture, structure, and degree of swelling when saturated. In other terms, they are related to the composition of sand, silt and clay and their capacity to allow for infiltration or result in more surface runoff. For the purposes of implementing this mitigation measure for Enlist products the HSGs are defined by their soil compositions (HSGs: A = Sand, loamy sand, or sandy loam; B = Sandy clay loam; C = Silt loam or loam; D = Clay loam, silty clay loam, sandy clay, silty clay or clay).

For the Enlist products used on land with Hydrologic Soil Groups A & B, the minimum vegetative filter width must be 30 ft.

Vegetative filter strips on hydrologic soil groups C & D are less effective at reducing pesticides in solution.

Therefore, for Enlist products used on land with C &D soils the minimum buffer width must be 100ft. Establishment and maintenance of vegetative filter strips and the area immediately upslope must aim to eliminate or substantially reduce concentrated flow and promote surface sheet flow runoff. The design and maintenance must consider a 10-year lifespan for the vegetative filter strip.  There must be structural elements added to break up or redirect concentrated water flow within the field to prevent gully erosion and promote sheet flow across the filter strip.

This may be most easily achieved by aligning rows as closely as possible so that they are perpendicular to the slope. Use of water bars or berms to break up the concentrated flow and divert concentration flow back into the field is another useful tool to promote sheet flow. Reduced tillage practices, especially in proximity of the field border strip, will result in less sediment loading and the best performance.

Permanent filter strip vegetative plantings must be harvested or mowed (cuttings removed) as appropriate to encourage dense growth, maintain an upright growth habit and remove nutrients and other contaminants that are contained in the plant tissue.

The maintenance program must keep vegetation tall in spring and early summer to help slow runoff flow, maximize disruption of concentrated flow, and reduce the chance of structural damage. Regular maintenance must also include inspection after major storms, removal of excess trapped sediment, and repair of eroding areas.

Water and Sediment Control Basins

These practices are described as an earthen embankment or basin, or a combination ridge and channel constructed across the slope of minor watercourses to form a sediment trap and water detention basin with a stable engineered outlet. The purpose of the practice is to reduce watercourse and gully erosion and detain sediment laden runoff, or other debris, for a sufficient length of time to allow it to settle out in the basin.

Maintenance must include ensuring a healthy grassed or vegetative surface within the interior of the basin, inspections after major storms and repair to damaged areas, as well as removal and redistribution of excess sediment back to the field.