Ground conditions have deteriorated in recent weeks following frequent rain, especially across the south of the country. Up to 25 August, most of the main tillage regions of the country had received between 150mm and 200mm for July and August combined, but Cork Airport was just 250mm, with well over half of that in August so far.

Again, most tillage areas have received 60mm to 80mm in August so far, with southern stations between 130mm and 150mm. These volumes of rain will make ground conditions “soft”, a very different problem to the drought earlier in the year.

Better days may enable harvesting but trafficability is a real risk. If this happens, we are likely to have more compaction and surface damage than normal but our response may influence the extent of its impact. Wet harvests and/or moist soil cause the following difficulties:

  • Picking up lodged crops, resulting in extra losses, poor work rates and increased combine breakdowns/component wear.
  • Trafficability problems where heavy combines and trailers have serious difficulties staying mobile when soils are wet and consequently weak.
  • Soil compaction where combine and trailer traffic cause soil structure damage when the soil is moist.
  • Picking up lodged crops

    This article does not deal with the combine settings required to cut lodged crops. Most operators should know this.

    Settings will vary between combine models. The combine’s instruction book is usually quite good. The following are some important points to consider.

  • Crop preparation: pre-harvest herbicide should reduce the amount of green material and make harvesting easier.
  • Header flotation: the header flotation system should be set to allow the header to operate close to or on the ground, while exerting minimum ground pressure. Springs, nitrogen accumulators and pressure monitoring/control systems should be carefully set.
  • Reel: set tine angle and reel position carefully to pick up laid crops. The approach differs between combine models.
  • Lifters: the fitting of crop lifters will usually assist in getting a laid crop into the combine without bulldozing soil. However, some operators prefer to operate without them on certain models, allowing the reel to pick up the crop in front of the knife.
  • Knife: the knife cutting system must be maintained in top condition to handle laid crops. This is difficult as the conditions will inevitably cause plenty of damage and wear in this area.
  • Note: in difficult harvest conditions, all parts of the combine will be under pressure from the wet crop and the soil and stones that enter the machine. As well as the header, the feed elevator and grain augers (wet grain) will be under particular pressure. Time spent keeping these areas in good condition is all that can be recommended.

    Combine trafficability and soil compaction

    The problems with both trafficability and soil compaction are caused by heavy machinery working in moist/wet conditions. As there is little that can be done to change machine weight (unless you choose a smaller machine), the only option is to try to reduce the ground pressure of the equipment.

    Prolonged wet will inevitably increase the risk of soil damage. \ Donal O'Leary

    Ground pressure is reduced by increasing the contact area between the tyres and the soil by fitting larger tyres, working at lower pressures, or possibly fitting dual wheels.

    Combines are heavy machines and they are getting heavier, as the trend is towards purchase of larger capacity machines (Table 1).

    Inflation pressure

    Ensure the correct inflation pressure with all tyre options. Ideally, the combine axles should be weighed. Exact tyre pressures should then be determined from a tyre manual, taking into account that the axle will only be fully loaded at low speeds (<10 km/h), ie full grain tank. Lower speeds allow lower pressures and give better flotation.

    Tyre size

    Larger tyres are able to work at lower inflation pressures and, consequently, exert lower ground pressures. If conditions are poor, it would be desirable to have tyres large enough to be able to operate at 1.0-bar pressure or less. In dry conditions, tyres big enough to work at 1.5-bar pressure or less would be desirable.

    Tyre options for two sizes of combines are shown in Table 2.

    The first tyre size figure is its width in inches or millimetres. There are other size options available also.

    Tyres fitted as standard to combines typically require 1.6-bar to 3.0-bar inflation pressure (consequently, high ground pressure). Obviously, fitting larger tyres can offer greatly improved flotation but there are problems:

  • It is an expensive option to change, as it involves tyres plus rims. It is vital and less expensive to choose the larger size when the combine is purchased.
  • Wider tyres can increase the road transport width of larger combines.
  • In isolated conditions, eg greasy top surface on a slope with the header weight on the ground, low ground pressure tyres may disimprove traction as they will not cut through the surface
  • The use of newer technology tyres (VF and IF), which allow more side-wall deflection, means lower ground pressures (LGP) can be achieved. It can be difficult to achieve LGP with the large combines available today. The use of rubber tracks helps the situation, as the long contact patch allows decent ground pressures to be achieved (less than 1.0-bar possible, depending on tracks and combines) without having an excessive road width.

    Rear wheels should also be upgraded to improve their flotation and to level the combine if larger-diameter front tyres are fitted.

    Dual wheels on combines

    Dual wheels can be considered as an alternative way to reduce ground pressure. While less expensive than replacing single wheels, they have certain problems.

  • Duals can place extra strain on the combine axle, particularly in rough fields or travelling between fields. Check that they do not invalidate your warranty.
  • They must be taken off when moving the combine on the road. Also, the header must be wide enough to cut greater than the total width of the duals (eg medium machine 4.5m+, larger machine 5.5m+).
  • Wheels with different dimensions can be used (must be the same overall diameter) but these may need a suitable attachment mechanism.
  • Grain trailers

    Grain trailers are now bigger and carry much greater loads. Consequently, they can give rise to trafficability problems and they can cause considerable soil compaction, resulting in yield losses in subsequent crops.

    Fitting bigger tyres can help but most farmers and contractors are reluctant to spend money on trailer tyres that will largely be used on the road. This attitude must change as such investment is probably well justified. Alternatively, go for a chaser bin or alter traffic management within the field.

    Trailer options/driver technique

    Limiting the extent of trailer wheel traffic in the field will help reduce compaction problems. Filling the trailer while harvesting results in a lot of trailer traffic but it is 15% to 20% more time efficient. Time is particularly critical in difficult harvests – hence the dilemma. Consider the following:

  • Only fill the trailer to between 50% and 70% of its capacity on the move. Fill the last couple of tanks into a stationary trailer on the headland.
  • If filling on the move, avoid unnecessary traffic, ie offload the combine as quickly as possible. Stop the trailer tractor as soon as loading is completed and only move directly to where the combine will off-load again. Avoid driving the trailer to the headland after each load or following the combine around until it is ready to offload again. If the combine is more than half unloaded on the move when the headland is reached, finish unloading in a stationary position.
  • Have tractor and trailer tyres correctly inflated and get all drivers to drive with sympathy for the soil. Plan driving routes within the field to avoid excessive traffic and getting stuck. Only stop the trailer/tractor where it will be easy to start again (eg facing downhill). Be careful accelerating loads from rest – you will often only get one chance – smooth driving is essential. Plan the approach to gateways, etc, in advance of driving to them.
  • Tractors

    Tractors should similarly be equipped with tyres capable of operating at low pressures to reduce ground pressure and to improve grip in wet conditions.

    Chaser bins

    The use of a dedicated field trailer in the form of a chaser bin avoids the need and cost of having to fit low ground pressure tyres to a number of trailers. The chaser bin transports grain from the combine to a conventional trailers or trucks. This eliminates or restricts conventional trailer traffic in the field. The chaser-bin is a hopper-shaped trailer with a front unloading auger fitted with large tyres to ease ground pressure. Wide-section 800/65 R32 tyres are commonly fitted, allowing an operating pressure of about 1.8-bar with a 15t axle load. Perhaps an even lower target ground pressure should be considered for Irish conditions.

    Tyre selection

    To illustrate the tyre selection process, a modest tandem-axle 5.4m x 2.4m trailer is chosen. With an empty weight of 4t+, and grain capacity of 14t, the rear-axle load would be about 15t.

    Having tyres with more flexible side walls allows for lower inflation pressure to reduce ground pressure but check axle load relative to inflation pressure.

    Tyre options are given in Table 3. The larger of the two options shown here would require the trailer chassis to be modified to allow adequate clearance for these tyres.

    Ideally, tyres requiring more than 2.0-bar inflation pressure should not be allowed on land, unless ground conditions are very firm. In poor conditions, tyres large enough to operate at 1.0-bar pressure, or less, would be preferable. Obviously, almost all of our trailers are under-tyred for use on land.