In this section:
Header Modifications
Preventing grain contamination by snails.
Where snail numbers in autumn remain above the control threshold of 20/m2 in cereals or 5/m2 in pulses and canola there is a strong possibility of snails being present at harvest.
To assess the potential need for harvester modifications snail numbers and sizes should be monitored three to four weeks before harvest. Snail sizes can increase between snail monitoring and harvest.
At harvest, it is the snail infestation in the upper canopy, above the targeted crop cutting height that causes contamination of grain.
By harvest, the opportunities to minimise grain contamination by snails are limited to:
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minimising the intake of snails into the harvester (see below)
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maximising snail and grain separation within the harvester (see below)
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The problem of same size snails and grain is the main challenge for separation at harvest. More snails are found in the upper canopy as the harvest season progresses. White and conical snails both migrate up the plant in response to hot and dry weather. |
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Early in the harvest season, snails are more likely to respond to a light rain shower and descend to the ground for a short period of time. Snail movement response to rainfall reduces later in the harvest season.
Minimising the intake of snails into the harvester
Windrowing
Windrowing barley crops is a useful option to reduce round snail numbers in grain. Many white snails on windrowed barley are left in the cut stubble between the windrows. These are not collected when the windrow is harvested. In contrast, most snails in a standing crop are likely to be on the heads and would be collected in the grain.
By contrast windrowing barley crops does not reduce contamination of grain with Conical snails as many remain attached to the barley plants during windrowing. They are then collected along with the grain when the windrow is harvested.
Dislodger bar or Snail duckerSnail dislodger bars are attached to the windrower or header approximately two metres in front of the cutter bar on the harvester. Snail dislodgers are designed to knock snails from standing crops and work well for white snails in wheat and some barley varieties. All designs are a compromise between reduction in snail dislodging and grain loss. Grain losses increase as snail dislodging performances increase. A dislodger bar is not an effective control of Conical snails in crops. Conical snails are much harder to dislodge. |
PVC pipe 1 - 1.5 m in front of reel |
Rotary Brush
A rotary brush prototype was developed in response to seeking more versatile dislodger bar designs suitable for use in a wide range of crop conditions including pulses.
The brush consists of 5 rows of 500mm long and 2.8mm diameter polypropylene bristles, spaced 50mm apart. The rotary brush is fitted to the header front. During testing the rotary brush showed a high potential for dislodging snails whilst being gentle enough for delicate crops like peas and harsh enough in tougher crops like wheat.
Maximising snail and grain separation within the harvester
High threshing intensity
Increasing intensity of threshing is a harvesting technique that can effectively crush snails. Any crushed snails can be removed later with air separation.
This technique is only suitable for larger round snails.
Fixed aperture sieves
Where there is a significant difference in the size of snail and grain, sieve design can effectively reduce the number of snails entering the grain box.
Adjustable louvre sieves are suitable for large grains such as faba beans.
In Australia, harvesters are generally fitted with adjustable louvre sieves in both the chaffer (upper sieve) and shoe (lower cleaning sieve). With these sieves snail and grain separation within the harvester is primarily reliant on air separation, and to a lesser extent on physical screening. Separation is promoted by the action of air over the the whole surface of either standard or air-foil louvre designs. Indications at this stage are that the air-foil louvre design is better able to separate conical snails then standard louvre designs.
To improve separation of snails from small to medium size grain, the chaffer and shoe can be replaced with fixed aperture sieves, with appropriate specifications for both the grain being harvested and the snail contaminants, Louvre sieves are successfully used with bigger size grains such as beans.
For typical harvest applications, fixed aperture sieves have a similar or greater open area then louvre sieves. The change to fixed aperture sieves increases the reliance on physical screening and lowers the reliance on air separation.
Various types of fixed aperture sieve designs:
| Punch hole screen (PHS) | Expanded metal mesh (EMM) |
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| PHS - Commonly circular, hexagonal, oblong hole shapes which are used as screens for reaping vetch, pea, beans and lentils are placed in top of header. The hole size and shape are chosen to suit the crop type and seed size. Effective in removal of snails larger than legume seeds. | EMM - diamond like opening shape, which can be orientated either crosswise or lengthwise on the sieve. EMM screens tend to be more sensitive to clogging but may provide significant improvements with conical snail separation. A cross wise orientation is recommended to separate conical snails. |
Woven or welded wire meshes (WWM)
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| This sieve material is characterised by wire diameter and size of the square aperture. Woven mesh can show large variation in actual aperture size relative to the nominal values. The welded rather then woven material is less likely to trap residue and promote clogging. |
Double Sieves |
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| Double sieves are made of two types of screen with hard rubber balls enclosed in compartments between the screen. They are used with good success in canola and either replace the top or bottom sieve in the harvester. |  |
