Forage Establishment and Restoration: Post Flooding Concerns and Solutions

When flood water recedes, what is left behind can make restoration difficult.  Silt, debris, invasive species, loss of fertility and weed pressure are issues that may have to be addressed prior to re-establishing your forage acres.

Dealing with Debris, Overgrowth of Wetland Species and New Weed Issues

Debris

Small and large debris carried onto your field by the flood waters pose no agronomic issues other than acting as obstacles during any field operations. For example, small debris such as clumps of grass, stones, and garbage will affect seed and fertilizer placement and smother growth, leaving dead patches to deal with later.  Large debris such as fence posts, trees and large stones simply limit field travel and must be removed before renovation can begin. Many producers pull drag bars (multiple I-beams or stacks of old grader blades) behind their tractors to help collect this debris and simultaneously scrape the bark off any small woody growth to control it. If a burn is conducted, it should be recognized that burning will make nutrients more available to immediate plant growth but that up to 90% of nitrogen and sulphur are lost in the process, phosphorus and potassium less so.

Overgrowth of wetland species

• Wetland forage species can be fed to livestock but there are a few concerns to be aware of. Cattails provide a lot of biomass, and have a nutritional value similar to that of cereal straw. Reed canary grass should be fed with caution unless the variety name is known because some varieties have high levels of anti-nutritive alkaloids which can only be fed if blended with other feeds. Excessive amounts of Smart weed (Lady’s Thumb) in the ration is known to cause photosensitivity in livestock (animal skin is sensitive to the sun). Kochia, a common weed in saline areas, has good nutritional value in the early stages of maturity but may be high in salts so intake should be limited.
• Burning is an option to remove these species from a forage field prior to tame forage re-establishment.  On native fields burning will minimize these species and allow for re-growth of native forages. If a burn is planned, seeding should take place as soon as possible to take advantage of the fertile ashes.

Introduction of new weeds

• Unfortunately, flooding and flood mitigation strategies such as building dykes can introduce new weeds on your farm (ie. Leafy Spurge). High risk areas should be monitored repeatedly to ensure new weeds are not able to establish on your farm. Once weeds have been located, you need to determine if they pose any risks if fed to livestock, assess the risk of it spreading across your pasture and determine if it poses a risk to reducing your pasture’s long-term carrying capacity. If this is an issue on your farm, many weeds can be controlled through use of herbicides or cultural management practices such as grazing or tillage. Weed populations should be monitored repeatedly to ensure they are not spreading and controlled as needed – this includes multiple herbicide or tillage practices.

Dealing with New Silt Deposits

• Flood waters can leave behind a coarse layer of silt on forage acres.  It is common practice to pull drag bars on damaged fields to remove trash and level the soil for a more even seed bed.
• If the new silt layer on your field is causing soil crusting issues, consider planting your forage with a nurse crop to maximize emergence. Another option is to till the field in order to mix the silt with the better quality top soil beneath. The effect of this tillage depends on the amount of new silt on your field and the quality of your top soil.
• The silt deposits can bring new nutrients to the field, but excessive flooding can remove water soluble nutrients (nitrogen, sulphur) and soluble portions of phosphorus. It is important to soil test these fields to determine the background level of nutrients before re-establishing a new forage stand.

Managing Salinity

• First of all, determine if salinity is the actual problem, which can be mistaken for sodic soils. Saline soils have a normal pH with high levels of soluble salts (sodium chloride or calcium sulphate).  Sodic soils on the other hand, have a high pH (>8.5) and elevated levels of sodium relative to magnesium and calcium but do not necessarily have salts. Sodic soils have poor soil structure and dense subsoil that restricts root growth – prone to hard pans. Saline soils are found in areas with shallow water tables (less than 6 feet), backed up recharged areas, discharge areas, and places with lateral ground water flows.
• When establishing forages in saline areas, it is important to prepare the area properly. Do not deep till saline areas as this only increases the problem by bringing lower salts to the surface. In fact, moisture and weed control are more influential in establishing forages than tillage. Always control weeds prior to seeding and try to minimize the number of clods in the seed bed. Seed shallow and in early spring because the cool, moist conditions give the seedling an advantage. Double your seeding rate when conditions are not very good to ensure an adequate stand density.  Over seeding into an existing thatch without suppressing it first is not recommended as success is limited.
• Always plant a mix. The most successful establishments are those with mixes designed for saline areas rather than those designed for maximum production. Tall wheatgrass, slender wheatgrass, and tall fescue are staples; others can be added based on price and availability. Deep rooted alfalfas are sometimes added to the mix to establish on the perimeter of the area to contain the moisture. Native grasses can also be used when the seed is available. Remember, production is always reduced in saline soils, even with tolerant species.  You can expect delayed emergence and maturity.

Soil Fertility

Flooding can severely reduce the fertility of your land through leaching and runoff. Nitrogen and sulphur are water soluble, thus can move readily down the soil profile (nitrogen more so than sulphur). Additionally, nitrogen is lost to denitrification in saturated soils. Under prolonged flooding soil sulphur levels may decrease. Extensive flooding and plant death also imposes stress on beneficial soil fungal populations, specifically mycorrhizae. To remain active, mycorrhizae need to receive carbohydrate nutrition from living host plants. In return mycorrhizal strands extend through the soil and access more phosphorus for the root. If host plants have been dead for some time, mycorrhizal populations will rebuild slowly from their resting state as spores. However, in the meantime, plants may suffer decreased phosphorus availability. Phosphorus fertilization as close-to-seed placed fertilizer will be beneficial. It is important to test your soil to determine nutrient levels and the right amount of fertilizer to add. Good fertility improves establishment and forage quality. Soil testing should be done when soils are dry enough to access, not when saturated.

Inoculating legumes

Prolonged flooding on soils also causes many of the soil’s nitrogen fixing bacteria (rhizobia) to die. Thus all legume seeds must be treated with new inoculants before reseeding. Do not rely on native populations of bacteria to provide the nitrogen for your forage crops as these are always less productive and not as plentiful in the soil. Once treated with inoculants, bacteria can remain on seed and stored for a season if properly stored in a cool dry location; however once seeded, the bacteria will need to be in the soil within days to avoid succumbing to the elements. For this reason, broadcasting on thatch is not a recommended practice. All broadcast applications must be followed by harrowing or other incorporation techniques to ensure the survival of the bacteria.

Seeding Method

The seeding method chosen will determine your stand establishment success. Not all methods are suitable to all environments.

Drill

Using seeding equipment that places the seed directly in the furrow is the most successful method for establishing forages. This ensures proper seed to soil contact and the protection of inoculant applied on legume seeds, a concern for dormant plantings.

Broadcast

Broadcasting is the predominant form of seeding forages in Manitoba and can be successful on tilled soils with proper weed control (with herbicides or tillage) and proper seed incorporation, including harrowing or packing/rolling (the roller can also assist in pushing stones down). Incorporation is especially important if broadcasting on sod or a layer of thatch, as this is the riskiest method for establishing forages. Options are aggressive harrowing or a pass with an Aerway system.  If not incorporated, germination and emergence will be poor and inconsistent.  

Aerial

Aerial seeding is a risky option for seeding fields that remain too wet to access. This is not a recommended practice unless the seeds can be incorporated soon after with a harrowing pass.

Seeding Rate

Seeding rates vary slightly depending on the goal. New stands should have target seedling densities between 30 to 40 seedlings per square foot. To do so, increase the seeding rate to account for poor seed-to-soil contact, hard seed, disease and insect predation, and uneven germination. Thickening up existing stands is very difficult unless the stand is suppressed with either a half rate of herbicide or has been severely overgrazed prior to seeding.

Alfalfa Autotoxicity & Flooding

Alfalfa autotoxicity presents an issue when attempting to seed alfalfa back into a previous alfalfa stand.  Alfalfa produces a toxin called medicarpin which allows the plant to manage its own stand densities. The effects of the toxin lasts for the life of the plant, and includes pruned roots, poor seedling vigour and low yields.   Alfalfa can be reseeded into alfalfa residue following the recommended wait period of 12 months; this time can be used to summer fallow or grow a crop. Summer fallow is not recommended in areas with salinity issues as this can worsen the problem.

There are a number of conditions that affect the amount of toxin in the soil, and your ability to seed alfalfa back into that land.

• Age of the stand: there are no autotoxicity issues in alfalfa stands less than 24 months old.  For example, any alfalfa stand that was 2 years old or less at the time of flooding can be seeded back to alfalfa as soon as conditions allow. Fields with older stannds will need 12 months before reseeding to alfalfa. The 12 month period may either include growing another crop or summer fallow.
• Stand density: the toxin is found in the alfalfa leaves and contaminates the soil under and around the alfalfa plant, as you move away from the center of the plant the effect of the toxin diminishes. For example, a stand with one plant every 16 inches could be reseeded with little effect on germination, but will effect yield. It might be suitable as a short-term fix to carry your stand for one more season. A stand with one plant every 24 inches could be reseeded with no effect on germination and minimal effect on yield. A stand with one plant greater than 24 inches apart would have no effect on germination or yield.
• Flood duration and soil type: the toxin is water-soluble, thus is transported with water down and out of the rooting zone, or in some cases laterally as water flows off of saturated soils. Soils with lots of downward water movement would see less effect from the toxin. Incases where the flooding has lasted more than 2 growing seasons before the field can be worked, it would be ready to reseed to alfalfa.  
• Tillage: zero or minimal till fields may see a longer lasting effect of the toxin as experience has shown that the effect of tillage (introducing air, warming and mixing the soil) dilutes the toxin and increases biological activity and the breakdown of the toxin.
• Delayed seeding: delaying the seeding date can also reduce the effect of the toxin; however, this only improves it slightly and should be used as a temporary fix to carry your stand for one more season.