Effects of flooding on pasture

Published 28 February 14

Effects of flooding on pasture

The largescale flooding that we have witnessed over previous weeks has significant implications for many farmers, both on grazing infrastructure and on pasture performance and quality. Dr Debbie McConnell, DairyCo R&D manager takes a look at pasture damage and recovery after flooding events.

Flood damage to pasture

Flooding can cause significant damage to pasture. Although water cover is a major component of this damage other factors, including: pasture species, duration of flooding, soil type, amount of silt and debris, and the flow rate of water determine the effects of flooding on pasture damage and the subsequent recovery.

In waterlogged conditions, air spaces in the soil are filled, reducing available oxygen levels by up to 90% and resulting in anoxic stress to the plant. Depending on soil porosity and the volume of air-filled space in the soil, within a few hours of flooding, microorganisms and roots in the soil will have used up the oxygen present in the water or trapped in the soil.

Oxygen diffuses through air roughly 10,000 times faster than through water. Consequently, research has shown that ryegrass plants will start exhibiting signs of anoxic stress after just four hours under waterlogged conditions.

As a rule of thumb, it is thought that after 10 -14 days of submergence under standing water, ryegrass plants will begin to die. However, how individual cultivars respond to anoxic stress varies within and between genotypes. For example, a trial undertaken in Australia to examine the effect of flooding on four different perennial ryegrasses discovered that after submergence in water – for 14-21 days, leaf and root mass were reduced by up to 70% on one variety while other varieties showed no significant reduction in leaf and root mass.

New research is now being conducted across the globe to identify genetic markers in ryegrass plants for flood resistance.

However, flooded soils are not necessarily completely devoid of oxygen. A very small layer near the surface which is in contact with the oxygenated surface water can assist in the translocation of some oxygen into the soil profile. This will depend on the conditions of the flood water. Flowing water tends to contain higher levels of oxygen than standing water, making more oxygen available to the plant.

In contrast, standing water contains less oxygen and, in slow moving or standing water silt and mud, is likely to settle out which can potentially seal the soil surface and have knock-on implications for plant emergence and gas exchange.

Flooding also impacts on gas exchange between the roots, soil and atmosphere. Gases and toxins such as methane, ethylene and carbon dioxide can build up in the soils, restricting plant growth further.

Pasture recovery

Flooding of soil induces a series of physical, chemical and biological changes within the soil but most of these changes will be reversed once the soil begins to dry out. As mentioned earlier, the rate of recovery of a soil and pasture after flood waters have receded will depend on a number of factors.

  1. Soil texture – pasture recovery will be better on light textured soils. Even after the flood waters have receded, heavy soils will retain the water for longer, extending the period of water logging.
  2. Sward height – pastures with lower covers prior to flooding should recover better as they will collect less silt and mud.  Longer grass slows water flow which will cause sediment to settle out.  
  3. Silt and mud – sediment deposits of less than 5cm should allow pasture to regenerate relatively quickly. However, deposits of very fine sediment can cause surface sealing reducing water infiltration and aeration creating anaerobic conditions in the soil.  

Assessing the damage

Once the flood waters have receded it is important to assess:

  1. Pasture

Assessing survival and health of the plants after flooding will help determine whether fields need to be reseeded

The weed burden after flooding is likely to be large, as flood waters can introduce new weeds species to pastures. In addition, thin, slow recovering pastures and bare soils will allow weed infestation and a reduction in the seed bank of desirable species. Stressed plants may also be susceptible to disease at this time.

  1. Soil

Soil structure, health and fertility must all be considered after flood events. Deposits of sediments on the land can be a lucky dip. Typically, finer clay loam deposits will in theory contain more nutrients than the coarser, sandy material which is often deposited closer to the river channel.

Either way, it is essential to undertake soil testing to be able to redress any imbalances in soil chemistry, particularly in situations where salt water has inundated. Following flooding, it is likely that readily mobile nutrients such as nitrogen and sulphur will have been leached from the soil profile. Working with an agronomist to discuss soil recovery is advisable.

  1. Infrastructure

Flood waters can cause extensive damage to grazing infrastructure and flood debris will most likely have to be cleared. More information about the clearance of debris is available from the Environment Agency.