Pasture recovery after flooding

Flooding of farms can be traumatic for both farmer and stock and can significantly impact productivity and farm economics. If your farm is affected by floods, you will most likely have many decisions to make. 

We’re here to help you decide on the best course of action for flood-affected pastures, so that your farm recovers as quickly as possible.

Assess the situation

Within a week of the flood, you should be able to start work on getting your farm back on track. The first step is to determine the severity of the flooding – use the flow chart below to assist.

• Minor Flooding: pasture has been covered with water, but with no or little deposition of silt or gravel.
• Moderate Flooding: pasture has been killed and sediment deposited on the pasture is up to a depth of 25 cm. 
• Severe Flooding: significant quantities of sand, gravel or silt have been deposited on the land. Typically, the depth of the deposited material will be so great that it would be very difficult to incorporate it into the underlying topsoil.  

Moderate or severe flooding

If you’ve had moderate or severe flooding, contact your Ballance Nutrient Specialist. They’ll help you to assess the situation and identify options to recover as quickly as possible. 
You’ll want to consider your short-term and long-term feed needs, stock numbers, soil testing, cropping, regrassing and fertiliser strategy.

Addressing minor flooding

If flooding is minor, then the following points will likely apply: 

• Pastures that have been under water for 2 to 3 days will probably recover. 
•  Pastures with leaf tips above water will survive. 
• If there is less than 5 cm of silt or sand then the grass will probably come through the crust, as long as water has drained away within 3 days. 
• If pasture has not shown signs of recovery after a week, then you should consider it dead.

Existing grass will recover much more quickly than sown grass, so if the pasture is green and density is acceptable, you will be better advised to apply fertiliser than to renew the pasture.

Fertiliser strategy 

After minor flooding or waterlogging, soil is likely to be low in nitrogen and sulphur, both of which are vulnerable to loss by leaching. Sulphur losses will be more marked in soils with a low anion storage capacity (ASC <60), such as sands, podzols, organic and South Island recent soils.

Nitrogen should be applied at a rate of about 50 kg N/ha. Products and rates that will deliver this include SustaiN or Nrich Urea. applied at 110 kg/ha.

Sulphur should be applied as sulphate-sulphur, rather than elemental sulphur. Products that will deliver sulphate-sulphur and nitrogen include: SustaiN Ammo 30N, SustaiN Ammo 36N, Nrich SOA, and the PastureSure and Pasturemag product ranges.

Regrassing considerations

The action you need to take to restore your pastures will depend on how much potential feed has been lost, and whether this will create a feed deficit in the coming months.

If you can see a feed pinch coming in the next 6 months, then drill paddocks with rapidly establishing or Italian ryegrass. This will deliver good-quality feed over the following months. 
If there are parts of the pasture that are open, then these areas can be undersown with a perennial ryegrass/clover mix to restore pasture productivity. 

Key points to note:

• Drill the seed with DAP to help with pasture establishment.
• Don’t put any machinery on the land until the soil is dry enough, otherwise you will end up with soil damage (compaction) and ongoing drainage issues, which will reduce pasture productivity. 
• Moist silt can be oversown aerially with species such as short-term ryegrass treated with bird repellent (ideally with phosphate and nitrogen fertiliser e.g. 300 kg/ha DAP). 
• Insecticide treatment to control pests such as stem weevil and baits to control slugs may also be required.