One of the biggest issues is going to be silt but there are several practical ways this can be tackled.
Silt is generally low in organic matter, phosphorus and nitrogen but this can vary widely so getting a starting point is essential to ensuring only the required inputs are applied. In the current climate nobody wants to put on more than they need.
It is recommended to collect more soil samples than normal because much of the sample will consist of floodwater and debris. Tackling regrassing on flood-damaged pasture requires careful planning. There are so many variables a farmer must consider, such as how long pastures were under water, how deep the silt deposits are and how well-grazed a paddock was before the flood.
One vital thing is waiting until floodwaters recede and the ground dries before tackling damaged pastures, because compacting saturated soils could do more harm than good.
After the 2004 summer floods in the lower North Island, MD Wilson and I Valentine interviewed 52 farmers about the techniques they used to regrass flood-damaged pastures. They came up with five key points:
- The more thoroughly the paddock was prepared the more successful the grass establishment.
- The use of a roller drill was found to be the most successful way of regrassing and increased clover establishment – this aligns well with ensuring the paddock is prepared well.
- Direct drilling was not very successful with the thought being effective direct drilling techniques require good soil structure with silt having very little structure.
- Weeds become more of an issue in pastures that had no or very little silt deposited. Weeds with rhizomatous roots such as couch-twitch or creeping buttercup seemed to thrive after floods and became more of an issue. With the reduction in pasture vigour because of waterlogging other weeds also recovered more quickly and required control strategies.
- There was no difference in success between different cultivar-species sown because it depended on the whole farm strategy as to which to use. There is a theory that the quicker the seed gets on the ground the quicker the paddock will be back into the grazing round. However, the paper found broadcasting seed on to unprepared ground was not a reliable method for successful pasture establishment. If the soil is too wet, the seed will not get into the soil profile, reducing its germination. If the silt is too dry, the seed can be blown into the cracks or into wind rows. Those who had success with broadcasting seed used double the normal sowing rates.
Weed and nutrient management of existing pasture is vital to maximise production while damaged pastures are recovering. With increased weed germination controlling those weeds when they are small is vital to ensure their impact on pasture production is minimal. The advantage of cooler soil temperatures is clovers aren’t as active so chemicals such as 2, 4-D and MCPA are control options.
More? Go to www.regrassingsolutions.co.nz to access Ravensdown’s Silt Buster tool or talk to your Ravensdown agri manager. Call 0800 100 123 for more help.
- Kay Nitschke, Ravensdown technical agronomy manager
