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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, scalding and total loss of existing pasture Of the number of aspects to consider, first is an assessment of how many of the desired plant species have survived. For some this simply involves making a visual assessment, but for others it may require a count of desired plants still alive per square meter. For ryegrass and clover pastures the target is to have greater than 70 per cent of the plants survive. If plant population gets under 50 per cent then action should be taken. It is also important not to have too many gaps in the pasture as this will greatly reduce productivity and allow weeds to take a foot hold. In some instances, the floodwater may not have killed the plants, but still will have caused serious stress to them. This will often result in poor future growth for a period of time, and these plants will also often succumb to pest and disease outbreaks. As a result, further thinning of the desired plants and increased invasion of weeds will occur. The likely impact of the floodwater on the seed bank will also need to be considered. Sub clover seed survival is very hard to determine. It is likely some of the seed bank will be lost and the sub clover varieties that have survived will probably be the less desirable, older, earlier maturing varieties. Unfortunately it is difficult to assess how much sub clover seed has survived until after the break, or irrigation has commenced. A recommend strategy would be to sow an annual ryegrass into these paddocks, so if the sub hasn't survived in high enough populations, the ryegrass will still be productive and then the sub can be resown the following autumn. Importantly, weeds also need to be considered when assessing the health of remaining pastures. In cases where there are desired species left but a lot of gaps in the pasture, weeds will rapidly fill these spaces. While these pastures may provide useful feed in the short term, developing weed issues will contribute to making the paddock unproductive in the future. For example, where some paspalum has survived the gaps in the paddock are likely to be filling with weeds such as couch. If nothing is done then both the paspalum and the couch will go dormant over the winter and very little will grow over the winter period. Once this happens there is not much in the way of control options and next summer the dormancy will be broken and the undesirable weeds will outcompete desirable species – and the problem continues. If the pasture or crop has survived in acceptable populations, it is likely to benefit from topping with a mower/slasher to remove any rank growth and/or sediment that might affect palatability. A fresh application of fertiliser may be warranted to replace nutrients that may have been leached (especially nitrogen and sulphur). For more flood related topic, check links below: flood auckland city
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