The steps to understanding and managing waterlogging are:
- Identify the problem sites.
- Consider drainage (shallow surface drains or raised beds) as the first option for controlling excess water. This will minimise crop losses and land management problems from waterlogging.
- In all cases, control weeds and plant early using appropriate varieties, high seeding rates and nitrogen.
- Adjust nitrogen to growing conditions to maximise fertiliser efficiency.
- Avoid growing crops that are susceptible to waterlogging.
Management options include: choice of crop, seeding, fertiliser, weed and disease control.
Choice of crops
Grain, legumes and canola are generally more susceptible to waterlogging than cereals and faba beans.
Seeding crops early and using long-season varieties help to avoid crop damage from waterlogging. Crop damage is particularly severe if plants are waterlogged between germination and emergence. Plant first those paddocks that are susceptible to waterlogging. However, if waterlogging delays emergence and reduces cereal plant density to fewer than 50 plants/m2, resow the crop.
Increase sowing rates in areas susceptible to waterlogging to give some insurance against uneven germination, and to reduce the dependence of cereal crops on tillering to produce grain. Waterlogging depresses tillering. High sowing rates will also increase the competitiveness of the crop against weeds, which take advantage of stressed crops.
Crops tolerate waterlogging better with a good nitrogen status before waterlogging occurs. Applying nitrogen at the end of a waterlogging period can be an advantage if nitrogen was applied at or shortly after seeding has been lost by leaching or denitrification. However, nitrogen cannot usually be applied from vehicles when soils are wet, so consider aerial applications.
Weed density affects a crop's ability to recover from waterlogging. Weeds compete for water and the small amount of remaining nitrogen, hence the waterlogged parts of a paddock are often weedy. If herbicide resistance is not a problem, spray the weedy areas with a post-emergent herbicide when the paddock is dry enough to allow access, provided the crop is at an appropriate growth stage. Aerial spraying is an alternative when ground-based sprays cannot be used.
Diseases particularly take-all of wheat and barley, are often more severe in waterlogged crops because the pathogens tolerate waterlogging and low oxygen levels better than the crops. Eliminating grasses from the preceding crops or pastures will reduce the severity of take-all in both well-drained and waterlogged areas. Leaf diseases are likely to be more severe in waterlogged crops because the crop is already under stress. Spraying may be an option after the site has dried, but only in crops with a high yield potential.