What to expect this year

Spring growth of most perennial grass pastures and alfalfa stands in the western Corn Belt will likely be delayed due to consequences of excessive flooding and slowed growth from late cold soil and air temperatures.  Spring planting of annual forages may be similarly delayed.

The good news

• Few plants have broken winter dormancy, so possible flood damage may be minimized.
• Frost was present in most areas that flooded, so erosion and silting issues may be lessened. 

Recent flooding across much of the western Corn Belt has been widespread and damaging. However, the timing provided two situations that could reduce some of the flood-damage effects for forage producers and pasture managers. 

First, few perennial forage plants have broken winter dormancy. Flood damage to perennial grass pastures and alfalfa stands would have been much greater had the plants been actively growing and submerged or covered with sediment. Secondly, with frost still in the ground, erosion may be reduced compared with similar situations later in the growing season.

While some erosion will likely occur, widespread sheet erosion was lessened and as a result, silt deposits may have been reduced. However, any flood damage remains unknown until flood water recedes and assessments can be completed.

First things first

• Wait at least 2 to 3 weeks after flood waters have receded before assessing flood damage and finalizing a pasture recovery plan.
• Grazing should only be considered after issues such as debris and sediment removal, erosion control, fence repair, possible chemical contamination, and water quality issues have been assessed and mitigated.

Flooding effects on pastures and forages

Forage plant response to a flood event is complex. Every stand will respond differently depending on the degree and duration of flooding, the forage species present, stand age, pasture health and vigor, fertility level, stage of plant development at the time of flooding, and temperature.

Floods that consist of standing water can be more harmful, and likely lethal, than plants flooded by moving water. Also, severe damage is less likely for plants that have leaves protruding from the water.

During a flood event, oxygen uptake by plant roots is eliminated, or at least severely restricted. Living plant roots, even with dormant top growth, still require oxygen to remain healthy. This can also reduce nutrient uptake, photosynthesis, and nitrogen fixation by forage legumes in growing plants.

Perennial forage plants that are dormant or semi-dormant have a reduced likelihood of flood damage. Generally, flood events are more common during late winter to early spring. These early seasonal floods provide warm-season grass species a recovery advantage. 

Additional factors affecting plant response to a flood event include soil type and depth to the water table in flooded areas. Negative forage responses on heavier clay soils with high water tables are more likely than sandier soils with deeper water tables and better drainage.

The major flooding impact on grass pastures may be due to excessive sediment deposits. Most perennial forages can produce new shoots and tillers if sediment deposits are less than 2 inches. Deeper sediment can suffocate plants and result in substantial stand loss. Some pastures will likely have sediment deposits greater than 2 inches. In these cases, mechanical removal is preferred to reduce forage loss and reduce the need for reseeding. With sediment deposits less than 2 inches, crusting may occur with light tillage needed to enhance recovery. 

Forage response to flooding

The number of days actively growing forage crops are likely to tolerate flooding is provided as a guide to relative flooding tolerance of forage grasses and legumes (Table 1). It is based on limited research, together with observations of flooding impacts on common forage plants.

With a few exceptions, perennial grasses and legumes are more flood tolerant than annual grasses and legumes. Similarly, grasses are generally more flood tolerant than legumes.

Grasses

Many grasses can withstand prolonged flooding. The native warm-season grasses have the greatest variation among forage types with indiangrass tolerant of flooding only for 3 to 4 days and eastern gamagrass tolerant of flooding up to 5 to 6 weeks. Switchgrass has greater flood tolerance than big bluestem.

Smooth bromegrass can withstand over 3 weeks of flooding, while reed canarygrass and timothy can withstand 6 weeks or more when still dormant. Most perennial cool-season grasses have flood tolerance exceeding 3 weeks with orchardgrass, Kentucky bluegrass, and tall fescue capable of tolerating flooded conditions for up to 2 weeks. Small grains have minimal flood tolerance.

Legumes

Alfalfa is not very tolerant to flooding, but it can generally withstand 1 to 2 weeks of flooded conditions. Other perennial legumes, such as red clover, white clover, and birdsfoot trefoil are slightly more flood tolerant. Red clover has been reported to withstand 2 weeks of flooding with white clover tolerating flooded conditions for 3 weeks or longer. Similar to the small grains, annual legumes have minimal flood tolerance.

What’s next?

There are several factors, both plant and environmental, responsible for flooding tolerance of different forage species. When evaluating flood damage to pastures and forage stands, the reality is that patience is the proper approach.

Most perennial grass pastures and forages are resilient and can recover from early-spring flooding under good conditions. Two strategies to speed pasture recovery and livestock production are

• Postpone grazing as long as possible to allow new shoot growth and replenish plant root reserves, set seed and for seedlings to establish
• If large pasture areas of pasture have been damaged, concentrating livestock on sacrifice areas may be necessary to rest pastures or replant forages.

Pasture recovery from flooding depends on both the survival and growth of existing plants. This usually occurs more quickly following summer floods than from winter floods. Pastures in good condition going into the fall and winter prior flooding should recover more quickly.

Water Quality Issues

While groundwater wells typically have low risk for harboring pathogens that can impact animal heath, wells inundated with flood water may have been contaminated. While there are no strict values for coliform bacteria in livestock water, a water quality test can provide information on coliform bacteria levels in wells used as livestock water sources.

*Less than 100 CFU (coliform forming units) per 100 milliliters of water should be safe for a livestock water source.

Additional Resources 

• Pasture Recovery Following Flooding
• Recovering Flooded Forages
• Little Good Comes From Flooded Pastures
• Cattle Checklist After a Flood: Issues to Consider 
• Renovating pastures or hay meadows damaged by flooding