Recovery of pastures after spring floods
Note Number: AG0585
Published: June 1997
There are generally two types of floods. The first is often described as a "flash flood" which usually covers pastures from one to four days. The second is a "prolonged" flood which is best described as five or more days of flood waters covering a pasture.
Assessing flood damage needs to be undertaken on a case by case and paddock by paddock basis. It is most important after the flood recedes, to get the remaining water off the pastures. Once the pasture is accessible, a decision about the various pasture management options will mainly depend on:
- the damage done;
- the pasture composition; and,
- the time of year the flood occurs.
The type of damage done to pastures varies considerably depending on:
- the duration of inundation;
- the depth of water:
- the speed of water flow;
- the water quality;
- the material left on the pasture;
- the pasture length at time of flooding;
- the different pasture species affected; and,
- the soil texture type.
Damage to pasture can range from minor sediment deposition with rapid recovery, through to deep sedimentation of silt, sand or gravel deposits on pastures, erosion of topsoil and pastures, scalding of the pastures, and total loss of the existing pasture.
Timing of the flood during the year
Winter versus spring floods provide different conditions and opportunities for pasture recovery. The rate of recovery of pastures following winter floods may be severely slowed by cold conditions. Winter floods can cause significant damage, but their time of occurrence gives better opportunities for pasture manipulation and immediate resowing during spring.
Mid to late spring floods create a significant problem on dryland pastures as the options for pasture recovery and resowing with the onset of summer are reduced. This Agriculture Note concentrates on pasture recovery following spring floods.
Flash floods (water cover for 1-4 days)
Pastures usually recover from short duration floods, if no erosion or serious deposition has taken place. Most pasture species are likely to survive, regardless of their length at the time of inundation.
Three main pasture problems occur in short duration floods. These are:
- first, small amounts of mud, silt or organic matter covering pastures;
- second, erosion of topsoil and its pasture cover; and
- third, deposition of silt, sand or gravel to varying depths on pastures.
In the first situation, the existing pasture generally recovers quickly as the paddock dries and rain rinses the mud and silt off the leaves. Waterlogging in heavy soils may reduce clover content and some annual grasses, but the perennial grasses will rapidly recover with further rains. It is important to make sure that water can drain away as quickly as possible after the flood and also from any follow up rains. Close inspection of the area is important, to deal with the emergence of weeds such as docks, thistles, couch and fleabane.
The second problem of soil erosion is far more serious in terms of lost valuable topsoil and the financial cost of restoration. The long term aim, which may take several months, is to refill or re-layout the land, and then sow back to permanent pasture. However, in the short term it may be best to sow a fodder crop and apply extra fertiliser to the area to get valuable early feed, stabilise the area and build up the organic matter level, as well as remove some of the weed burden before sowing a permanent pasture the following autumn.
In the third situation the action needed will depend on the depth and type of material deposited on the paddock. The pasture will grow through small deposits of sediment (up to about 5 cm deep), but it may be necessary to clear up debris and run a grader blade or smudger over the area to get an even spread of sediment. Where heavy silt and sand deposits exist, it is often impractical to remove them and pasture resowing will be necessary. Use of a temporary summer fodder crop and extra fertiliser may be used to provide short term feed, build up the soil fertility and provide competition against weeds.
Prolonged floods (water cover for 5 days or more)
Significant long term damage can occur to pastures during prolonged inundation. It is important to determine the survival of the desirable pasture species as soon as possible. It may take two to four weeks and a good rain after the flood recedes to make an accurate assessment of the pasture condition and the potential survival of desirable species. Growth will be depressed for up to four weeks as the plant roots re-establish, and new leaf growth commences.
Pasture species tolerance to floods
There are a number of species tolerant of waterlogging and inundation. The main tolerant grass pasture species include paspalum (Paspalum dilatatum), phalaris (Phalaris aquatica), tall fescue (Festuca arundinacea), tall wheatgrass (Agropyron elongatum) and weeping grass (Microlaena stipoides). Each of these will survive with some degree of waterlogging.
The legume component of a pasture will not tolerate inundation for extended periods and is usually the first to succumb to waterlogging. Prolonged waterlogging will damage clover and annual grasses, but the perennial grasses may recover with late rains.
The degree to which perennial grass species succumb to flooding is dependent on the factors described below. If there is a significant loss, resowing during the following autumn will be required.
Factors affecting pasture recovery
- Duration of Inundation - Water cover by itself is not the only determinant of pasture damage. A combination of factors including: soil texture (drainage); speed of water flow; the water quality; and, water depth are also responsible for varying levels of pasture damage. Obviously the longer the pasture is under water the more potential waterlogging is going to take place.
- Soil Texture -Light textured soils that drain freely will allow a speedy pasture recovery. Heavy soils hold the water for a longer period after the flood has receded and extend the period of waterlogging.
- Speed of Flow - Providing the soil is not eroding, the quicker the water flow rate the better the pasture recovery. The slower the water, the more sedimentation and the slower the pasture recovery.
- Water Quality (temperature and turbidity) -Flowing water also appears to provide more oxygen (a more aerobic condition for the pasture) and is often at a lower temperature than stagnant or slow moving water. Stagnant or slow moving water can rapidly heat-up in the spring, particularly if it is shallow, with scalding and rapid decay of the pasture resulting. Pastures that have cool, highly turbid, and constantly flowing water covering them for 10 days will often make a better recovery, than pastures with stagnant, warm, low turbidity water covering them for only half this time.
- Water Depth - Generally, the deeper the water over the pasture, the longer the period of inundation, the slower the flow rate, the greater the chance of silt and mud deposition, and the slower the recovery of pasture.
- Length of Pasture -The shorter the pasture prior to flooding the quicker the recovery rate of the pasture. If the pasture was longer than 10 - 15 cm during inundation it appears to collect more silt and mud, lodge, breakdown and smother the newly emerging shoots. The longer the pasture, the more mud on the leaves and the more rank rotting vegetation is left in the pasture; the less likely stock are to eat it. Other methods of trash removal (such as slashing or mulching) may need to be found.
Weed issues after a flood
A host of weed issues are likely to occur after a period of waterlogging, causing reduced pasture quality. Where bare ground is an outcome of flooding, many weeds will proliferate as temperatures rise during spring. These weeds include: water couch (Paspalum paspalodes), docks (Rumex spp.), umbrella sedge (Cyperus spp.), fleabane (Conyza spp.), common rush (Juncus usitatus), smart weed (Persicaria spp.), couch (Cynodon dactylon), summer grass (Digitaria spp.) wire weed (Polygonum aviculare), buttercup (Ranunculus spp.), and a range of thistles and noxious weeds.
Chemical control of some of these weeds (eg. couch and umbrella sedge) can be very difficult and costly. Control may not be achieved in spite of the money spent. Pastures that have been covered in sediment and organic matter can inactivate many chemical products (especially glyphosate). For further advise contact your local chemical advisers and pasture advisers.
Topping (slashing or mulching) of either short or longer pastures will encourage faster and more even regrowth. This needs to be undertaken as soon as practical, taking care not to smother regrowth. Windrowing the trash for later burning, may be necessary depending on the amount of rank material. Appropriate weed control and fertiliser applications will assist in the pastures recovery.
Some nitrogen and sulphur is likely to have been leached from the soil with flooding. Nitrogen will also be used in the breakdown of organic matter lying on the pasture. The application of fertilisers containing nitrogen, phosphorus, and sulphur will assist in the recovery of pastures.
Late spring pasture resowing is not an option for dryland pastures. In most cases pasture sowing will not be possible until autumn. The re-establishment of pastures requires considerable planning and preparation and very careful management after sowing.
Planning should start as soon as possible; this will include taking a soil test, appropriate weed and pest control, removal of trash, and the determination of sowing technique, pasture species to be sown, fertiliser program and a suitable grazing management program.
Summer fodder crop option
The use of a temporary summer fodder crop such as millet, sown in the spring, should be considered. This allows some return out of the paddock when it would be otherwise out of production and provides competition against weeds through the summer months.
Millet crops are often cheaper, provide quick feed, usually have better regrowth after grazing and do not have the toxicity problems sometimes encountered with the sorghum's. It is best to sow millet into a cultivated seedbed. Sod seeding of millet has in the past been rather unreliable.
A fertiliser program would need to be designed depending on the area and soil test. One suggestion would be to incorporate a super-potash 3:1 blend before sowing the summer fodder crop, and to sow the seed with single superphosphate. This will quickly build up the soil fertility after the flood, especially the phosphorus level.
Other flood issues
- Livestock management - Livestock welfare, keeping stock high and dry, and feeding are generally the highest priorities depending on how much pasture is inundated by the flood. After assessing the situation, if feed is short, alternative feeding may be essential (eg agistment or supplementary feeding). Contact your local adviser about designing appropriate diets for the livestock, intensive feeding or other animal health issues.
- Fence damage - Conventional fences with many posts and wires tend to be worst affected. Single and dual wire electric fences do not trap the same amount of debris and are more easily repaired. Replacement of fences with electric fences is the quickest and most cost effective for controlled grazing of flood affected pastures.
- Hay stacks - Check on hay that may have been flood affected. Wet hay has the potential to spontaneously combust. Wet hay can also spoil if it becomes mouldy from the damp conditions.
- Equipment - Machinery and pumps caught in the flood waters will need to be thoroughly checked over prior to their use. The water may cause corrosion, or it may have made its way into the engines. Equipment in good order is essential for timely irrigations to keep the pastures going.
- Earth works - Damaged irrigation layouts and check banks may need repair prior to the irrigation of pastures. Levy banks may also need repairs to prevent further damage from follow-up floods.
- Tracks and roads - Eroded tracks, roads, bridges and culverts may need repairs to give access to pastures affected by the floods. The sooner pasture is assessed and appropriate management applied, the more productive and better the results.
For further information on other flood related issues, summer fodder crops or pasture resowing contact your local adviser or Department office.
This Information Note was developed by Warren Vogel. June 1997.