Regrassing after the Manawatu floods

April 2005
A survey of 52 farmers who were forced to regrass after the floods in February 2004 highlighted the more successful approaches used. There were three distinct flooding categories no silt, manageable silt (5-20cm), deep silt and each needed a different approach. Breaking the crust on top of the silt was critical to speed drying. Weed control measures were important. Roller drills were best for sowing.

Farmers were very forthcoming in telling about their failures as well as successes, and almost all were resilient, positive, and expectied to end up with a better pasture than they had before the floods, because of new cultivars.

One option suggested by an AgResearch person was to put in a quick growing short term crop such as oats to provide feed and then into permanent pasture in autumn, but the time limit put on the subsidy for new pasture by government ruled that out for most farmers.

The project was carried out from November 2004 to January 2005 by Michael Wilson, a B.Agr.Sc student who had just completed his studies, and was supervised by Dr Ian Valentine, pasture and crop specialist from the Institute of Natural Resources. In conjunction with soil scientists and others they identified what they needed to find out and developed a questionnaire. The aim was to find out exactly how farmers had tackled regrassing under different circumstances, and how successful their approaches had been, what had worked and what hadnt, looking for specific solutions to particular circumstances. Levelling, debris removal, seedbed preparation, seeding, fertiliser, weeds etc.

Usually farmers had more than one scenario in terms of silt cover, and each of those Michael treated as a case, and he ended up with about 110 cases. He took photos of the pastures, estimated cover, dug a hole and took a photo of the profile, looked at rooting depth, recorded any layers that were still smelly and blue, determined whether the silt depth was visible. Sometimes the old pasture was quite clearly seen in the black line.

There were three distinct post flooding scenario:

- Pasture killed but no silt deposit the main problem was in handling the dead organic matter on the surface and breaking the seal that it made. Mulching this organic matter plus light tillage was the most successful to speed drying and decomposition.

- 5-20 cm of silt deposit cultivation to mix the sediment with the underlying soil so that some of the nutrients could get back into the topsoil.

- More than 20cm silt - essentially building up soil from scratch and there is no quick way of doing that, people have put in lots of fertiliser and nitrogen and got some response but it dried off quickly -- in some areas it was like sowing on a beach. Broadcasting seed on top of the goo was largely unsuccessful even at double normal seeding rates blasted on with a helicopter.

With the first two scenarios, the more thoroughly the seedbed was prepared the better that pasture was established, the more nitrogen that was applied the better the pasture growth and the use of a roller drill increased the reliability of establishment as opposed to direct drilling or broadcasting

Many perennial weeds survived flooding, and if pasture was planted amongst them then the weeds came away. More success came from letting the weeds green up after the floods, then spray with glyphosate, then mulching, then sowing grass seed.

Where there was a lot of silt, small-scale surface cultivation decreased drying time dramatically. A small set of discs or tines pulled behind a light tractor or harrows behind a motorbike to break the surface up. The surface of the silt layer forms a crust that seems to seal in moisture and needs to be broken. Heavy machinery would sink, but light machinery opens up the surface and some drying takes place, then further deeper cultivation allows more drying, and so on. In this scenario, weeds were not a problem initially, but selective herbicides were a good idea once the pasture came through.

Michael Wilson says I had an excellent response from farmers, they were more than helpful and accommodating. They were very open about what they had done successfully and also about what hadn't worked and it was very helpful. I notice that the amount of resilience the farmers had was just amazing, they were feeling positive and the feeling was that within a year they would be in a better position than they were before the flood because of the new cultivars that they had put in. They were on the front foot rather than the back foot and most were feeling very positive.

The project was partly funded by Horizons, the regional Council, and by one local dairy farmer. None of the environmental or flood protection organisations were interested in funding the project. Although it is a student project, students often have more success in getting alongside farmers and getting good information from them than researchers do.

Because the flood happened in February when soil temperatures were still warm the conditions were reasonably conducive to pasture establishment. If it had been midwinter it would have been a much longer process in drying out and having to wait until spring to establish pasture, and spring is not as good a time to do that.

Having a plan for the next flood, based on the lessons of the last one, is a very good idea. One farmer had been caught once before, and this time he had a plan of what to do, which included everything relating to his stock and the land both during and after the flood.