Wednesday, 1 July 2015

Five considerations before striking back against silt


As the clean-up in the Manawatu gains momentum, farmers face a stressful time considering the state of their pasture and how much the silt will affect their soil.

However hard it may seem, one of the most important aspects of getting pastures back on track is to plan with a clear mind and with the facts to hand. As the proverb goes “worry is like a rocking chair, it gives you something to do, but gets you nowhere.”

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 ensure 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 flood water and debris.

Tackling re-grassing on a flood-damaged pasture requires careful planning. There are so many variables a farmer must consider, such as how long pastures were underwater, how deep the silt deposits are and how well-grazed a paddock was before the flood.

One vital thing is that farmers should wait until flood waters 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, M.D. Wilson and I. Valentine (2005) published a paper* in which they interviewed 52 farmers about the techniques they used to regrass flood-damaged pastures and came up with the below summaries;

  1. The more thoroughly the paddock was prepared, the more successful the grass establishment.
  2. The use of a roller drill was found to be the most successful way of re-grassing and increased clover establishment – this aligns very well with ensuring the paddock is prepared well.
  3. Direct drilling was not very successful with the thought being effective direct drilling techniques require good soil structure, with silt having very little structure.
  4. 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 post-floods and became more of an issue. With the reducing in-pasture vigour due to waterlogging, other weeds also recovered quicker and required control strategies.
  5. There was no difference in success between different cultivar/species sown as it depends on the whole farm strategy as to which to use.

There is a thought that the quicker the seed gets on the ground, the quicker the paddock will be back into the grazing round. However the paper found that broadcasting seed onto 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 in to the cracks or into wind rows. However those that had success with broadcasting seed, used double the normal sowing rates.

Weed and nutrient management of existing pasture is vital to maximise production, while the 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 the clovers aren’t as active, so chemicals such as 2, 4-D and MCPA are options to control germinating weeds.

Your Ravensdown Agri Manager will be able to advise or try the interactive Silt Buster tool below.

* Regrassing flood-damaged pastures, M.D. Wilson and I. Valentine (2005). Proceedings of the NZ Grasslands Association 67.