Paddock Practices: Take all possible steps to minimise harvest contamination by snails

Date: 22 Sep 2020

image of Small pointed snail
Small pointed snail (Prietocella barbara) in barley leaves. (PHOTO: © Evan Collis Photography)

Key points

  • Snail contamination can lead to harvest downgrades or rejection
  • Assessing snail densities will help ensure an efficient, clean harvest
  • To minimise snail problems at harvest:
    • Schedule harvest operations to avoid snails
    • Minimise snail numbers entering the header
    • Maximise separation inside the header
    • Harvest, store and clean contaminated grain separately
  • Contaminated grain may be cleaned with snail crushing rollers
  • Year-round control is essential for minimising snail numbers next harvest

As spring leads into summer, managing snail numbers and keeping snails out of harvested grain is once again a critical seasonal issue.

The key to minimising snail contamination of harvested grain is awareness of snail densities across the farm.

Good awareness will support planning for an efficient harvest, with reduced grain contamination and equipment problems.

Snail control before harvest

In warm weather, snails will climb up plants to avoid heat from the soil. As spring progresses, they increasingly seek shelter in crop canopies, sometimes between grains in the seed head, and become very difficult to dislodge.

Bait applications need to stop at least eight weeks before the planned harvest to adhere to withholding periods, as there is zero tolerance for bait contamination of grain. Any remaining bait should be stored in cool, dry conditions to maintain its efficacy for autumn applications.

Recent studies have shown that baiting is less effective after mid-winter and that baiting programs should focus on autumn and early winter.

Any other chemical treatments must not contravene label or permit conditions, otherwise an entire grain shipment could be rejected on those grounds.

Know where snails are concentrated

While many farmers know the problem areas on their farms, snail densities vary across seasons, making regular monitoring important. High-risk areas may include paddock perimeters, as well near calcareous rock outcrops and unmanaged areas.

Paddocks should be surveyed to identify areas with high snail numbers at least three to four weeks before harvest to allow time to plan harvest strategies accordingly.

Monitoring for live snails is best done with a 30 x 30 centimetre square sampling frame. Move in a straight line from the edge of the paddock to the centre, placing the frame down at 10 metre intervals and counting the snails within it.

Use timing to avoid snails

Strategies for reducing the impact of moderate snail numbers include harvesting early in the season, and at times of day when fewer snails are present in the crop canopy.

Recent camera studies have shown that in spring, snails frequently move up and down plant canopies, particularly overnight and early in the morning, or in response to light moisture events.

Monitoring these snail movements can help growers decide the best times of the day to harvest snail-infested areas, while taking account of the need to meet delivery specifications for grain moisture content.

Growers should try to harvest their most valuable or easily damaged crops first. At early maturity, plants are more resilient, fewer snails are present in the crop canopy and snails will be easier to dislodge.

Harvesting early is also one strategy for reducing snail contamination in more heavily infested areas, but it does increase the risk of delays due to machinery clogging.

Areas with higher snail numbers, such as the perimeters of paddocks, should ideally be harvested and stored separately.

The separated grain can be cleaned later to avoid compromising grain from zones with lower snail numbers.

Keep snails out of the header

Knocking snails off crops in front of the header will help the harvest run smoothly and efficiently.

Large numbers of snails can clog headers and sieves. The combination of snail guts and dust will stick to the inside of machinery and set hard if left to dry.

image of stripper fronts
Stripper fronts have been shown to reduce snail intake into harvesters by up to 50 per cent, compared to an open standard front. Photo Liz Wells

Stripper fronts have been found to reduce the number of snails entering the harvester in cereal crops by up to 50 per cent compared to a standard open front, while still allowing a fast harvest speed. In addition, strippers will crush some snails before they enter the machine.

Alternatively, raising the cutting height can reduce snail intake but will result in more standing stubble that may need to be managed afterwards.

Windrowing can also dislodge snails. Windrowed crops should be picked up as soon as possible before snails can move back up into the windrow. Open raking fronts with crop lifters and PVC covers over the unused width of the cutter bar will help keep snails out of the header.

image of Snails can clog up farm machinery
Snails can clog up farm machinery and contaminate harvested grain. Photo: GRDC

Maximise separation

Where snails do enter the harvester, punched metal and expanded mesh sieves are most effective for separating them from the grain. However, cleaning throughput is lower than with louvre sieves, so the harvest speed may need to be reduced to avoid excessive grain losses.

In areas of high snail contamination, fixing chains to the top of the sieve can help stop snail residue from gumming up the openings. If repeats are low in grain but high in snails, opening the repeats door and wasting onto the ground can help reduce clogging.

Post-harvest cleaning

Keeping grain from heavily infested areas separated from the main bulk of the harvest can reduce the time and cost of post-harvest cleaning.

Snail-crushing rollers are among the most effective post-harvest cleaning methods. Rollers are more effective when snail numbers in the grain have already been minimised through good in-paddock management, harvest tactics and header set-up.

Rolling is effective for all hard grains at their optimum moisture content. Single and double roller machines have been used to clean wheat barley, field peas, beans and lentils to market standard at rates of up to 80 tonnes per hour.

The roller gap must be adjusted carefully to crush snails while avoiding grain damage. When rolling canola, additional care must be taken to adjust plastic coated steel rollers and regulate the flow rate to avoid cracking the grain.

Additional cleaning through aspiration has proven beneficial for lentils.

Practice year-round snail control

Best practice management to reduce snail contamination of the grain harvest involves an integrated, year-round control program.

This begins during the post-harvest fallow period, when killing summer weeds will deny snails shelter and food.

Rolling, cabling and/or grazing stubble have all proven effective at killing snails by knocking them onto the ground when soil temperatures are lethal to snails. In paddock trials, kill rates of 50 per cent to 90 per cent have been achieved.

For best results, rolling or cabling should be done during periods when there are consecutive days with temperatures above 35°C, as long as the fire risk is managed carefully. Low humidity will help prevent snails rehydrating overnight.

Summer rain events can trigger snail activity. Breeding activity typically commences with increased moisture during autumn, when snails are most vulnerable to baiting. Killing snails when they are active but before they lay eggs is crucial to reduce populations. Baits need to be spread evenly and at label rates to maximise the chance of a snail encounter.

It is important to begin baiting before snails commence egg-lay, then continuing to monitor and re-apply bait as necessary until around mid-winter. This will minimise future generations of snails and help reduce problems in the following harvest.

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