Crop eradication

Objective: To completely kill the old crop

This is to prevent disease transmittance to the new crop and to prevent mixing of cultivars if a new cultivar is to be planted. Ratoon stunting disease (RSD) can be transmitted to the new crop by surviving cane stools as can the diseases smut and mosaic. There would be little point in efforts to utilize disease free seedcane if diseases were allowed to be transmitted to the new crop.

Methods include mechanical, chemical, hand hoeing and a combination of chemical and mechanical eradication methods.

Mechanical

A wide range of machinery and implements have been used worldwide to eradicate sugarcane stools, but this has generally involved ploughing the field in drier times of the year to invert the cane stools, followed by repeated cross cultivations with disced or tined implements to ensure

desiccation of the old stools. Volunteers that subsequently emerged were generally dug out with a hand hoe or killed by repeated passes with discs. This can be very ineffectual and the cost is high, particularly if the operation needs to be continued into the wet season. A major disadvantage of mechanical methods is their potential to allow soil erosion if conducted in wet weather on erodible soils and on steep slopes. In endeavoring to improve the efficiency of mechanical methods of eradication, Dicks et al. (1981) tested a number of implements and showed that satisfactory eradication could be provided by the use of a moldboard plough set to a depth of 100 mm on heavier soils in winter, and by a rotary hoe in sandy soils. However, they found that summer harvested fields were better eradicated with the use of chemicals. Equipment tested included the chizel plough, power harrows and the conventional plough, in addition to the moldboard plough and rotary hoe.

Box 3.8 Implements for cane eradication

The moldboard plough is able to successfully cut the stool away from the roots without inverting the stool if set to a shallow depth (80-120 mm)

Disc ploughs are generally unable to kill the cane satisfactorily due to the difficulty of controlling the depth and this results in portions of the cane row not being affected.

Subsoilers with wings or stool ploughs have been found to be effective if operated in the same way as a mouldboard plough.

Heavy disc harrows are satisfactory when operated at shallow depths and high operating speeds.

All the above implements separate the plant from its root system and they require a further operation to lift the stools out of the soil to allow them to dessicate.

Spring tine rippers and cultivators can be used for the purpose of lifting stools out of the soil and they should be used at a higher operating speed which also results in a finer tilth.

The rotary hoe can be very effective when operated with the back flap open in sandy soils but also tends to pulverize the soil and cause severe damage to soil structure on heavier soils and in dry conditions.

Timing for efficient mechanical stool eradication

This must be conducted in the dry part of the year so that cane stools become desiccated after exposure. It is totally ineffective in the rainy season. This means that fields for plough out and replant should be harvested at the beginning of the harvest season, which normally coincides with the onset of winter in rainfed industries. This also coincides with the period of lowest growth potential and hence least loss of cane production. Also since the soil is most vulnerable to erosion in the ploughed state this is best conducted at this time of year when there is less potential for high rainfall events. In dry areas where irrigation is necessary the period of mechanical eradication can often be extended as the risk of rainfall and erosion is less.

Soil type also has a large influence on the success of mechanical eradication and needs to be taken into account. Examples of recommended procedures for mechanical crop eradication are included in Box 3.9 (SASRI 2002)

Box 3.9 Recommendations for mechanical eradication

Objective: Remove the cane stool from the ground, remove as much soil as possible from around the roots, invert the stool and leave it on the surface to dessicate.

On heavy soils: (loams, clay loams and loamy clays)

1. Moldboard plough with a depth wheel set to a depth of 100 mm. This cuts away the top part of the stool from the roots and inverts the stool allowing it to dessicate.

2. When shoots begin emerging from the old stools (usually about 4 weeks later) use a disc harrow to chop up the remaining live stools

3. A single pass with a spring tine cultivator which brings stools to the surface.

4. Apply a second pass with a disc harrow (4 weeks later)

5. Follow up removal or spot spray of volunteers on a regular basis.

On light soils:

A rotary hoe set at a speed to chop the stool into small sections and fling them on toi the soil surface is recommended. This is facilitated by leaving the flap in the open position, operating at a depth of 100 mm, with a forward speed of 2.5 km/h and using 200 revs/min.

This method is totally unsuccessful during wet times of the year.

Costs of mechanical cultivation

Proper costing can only be done in local situations but some general thoughts include the fact that fossil fuel costs are increasing faster than many other costs, thus placing mechanical cultivation at a disadvantage. Future costings will increasingly focus on the Net Energy Ratio (NER), which is the renewable energy output of biofuel and co-products divided by the fossil energy input in the whole production chain (see Ch 13.1.2). Future mechanical work on farms will increasingly be carried out with non-fossil fuels.

Summary of advantages and disadvantages of mechanical eradication Advantages

Land is fallow during the least productive growth period.

Disadvantages High risk of erosion.

Only effective in dry conditions.

Regrowing stools dispersed throughout the field and difficult to distinguish from new crop.

Chemical

With the limitations to mechanical eradication in respect of season and soil restrictions, it was fortunate that a chemical became available which was able to kill the crop without having a residual effect in the soil and thus avoiding damage to the subsequent planting. Glyphosate was found to show these properties and was tested for its effect at varying rates, at various growth stages and on a number of sugarcane cultivars and found to provide an acceptable kill (Iggo 1974). The

disadvantage of conventional mechanical eradication was the length of time required to eradicate the volunteers, and the ability to spot spray these with an effective chemical was an added benefit of chemical eradication. Further work on glyphosate confirmed its ability to kill sugarcane and refined the conditions under which performance was acceptable (Turner 1980; Alonso et al. 1984).

Using chemicals to eradicate cane has distinct advantages in avoiding unnecessary soil disturbance and thus preventing erosion on steeper slopes. Glyphosate, which is totally inactivated on contact with soil and has no residual effect, needs to be applied to actively growing cane between 400 and 700 mm in height and with most tillers developed. The disadvantage is that the cane needs to be allowed to re grow after harvest and, under dryland conditions this must be done in spring or early summer thus only allowing the new crop to be planted later in summer. Only clean water can be used for spraying. If canal water containing silt is used, glyphosate is inactivated.

Advantages and disadvantages of chemical cane eradication (SASRI 1996) Advantages

Risk of soil erosion is reduced substantially compared with mechanical eradication Good kill can be achieved under favorable conditions

Volunteers remain in the same lines and are more easily identified for removal Perennial weed problems can be eradicated at the same time.

Disadvantages

Can only be used in summer and potential growth is lost Effectiveness is dependent on weather conditions.

Hand eradication

Sherwell (1990) reported on the use of hand chipping as a means of sugarcane eradication to complement minimum tillage and conventional tillage treatments on a sugar estate in KwaZulu- Natal in South Africa. This involved the development of a suitable implement which is able to split the stool from the roots and lever the stool out of the ground. The main advantage of the implement is the fact that cane harvested in early winter can be planted in spring without having to wait for regrowth before chemicals can be applied. Chipping can however be used at any time of the year.

Advantages and disadvantages of hand eradication Advantages

Can be conducted at any time of the year

Risk of soil erosion is substantially reduced compared with mechanical tillage Can be used on steep slopes with erodible soils.

Disadvantages

Difficult on heavy soils.

Combination tillage

Recognizing the value of planting new crops in spring to allow the best use of summer growing conditions led Butler (1992) to test the use of a combination of mechanical and chemical treatments.

This technique was called combination tillage. The crop is first sprayed with either glyphosate (or fluazifop butyl) and then the stool is sheared mechanically at a depth of 100 to 150 mm below ground surface 3-7 days after spraying. The cane roots are severed and the stool lifted but not inverted , thus leaving the stool in its original position in the cane row. Results indicate that sugarcane can be killed earlier in the season, at a younger age and at lower rates of chemical.

Advantages and disadvantages of combination tillage Advantages

Allows use of chemical eradication earlier in the season More effective than either chemical or mechanical alone Less risk of soil erosion than mechanical alone.

Disadvantages

Unsuitable for steep slopes.

Summary – criteria for selecting a cane eradication method

Table 3.1. An example of a selection chart for eradication methods used in South Africa.

(SASRI, 2002)

Soil type Slope Season Method

High erosion hazard

< 10 % clay

0-20 % Winter Spring Summer

Chipping

Chipping or Fusilade

Chipping or glyphosate/sulphosate Moderate erosion hazard

10-20 % clay

< 15 % Winter Spring Summer

Chipping or shallow MB plough/harrow Combination tillage or Fusilade

Combination tillage or glyphosate/sulfosate

> 15 % Winter Spring Summer

Chipping

Combination tillage or Fusilade

Combination tillage or glyphosate/sulfosate Low erosion hazard

> 20% clay

< 15 % Winter Spring Summer

Plough/harrow

Combination tillage or Fusilade

Combination tillage or glyphosate/sulfosate

> 15 % Winter Spring Summer

Chipping Fusilade

Glyphosate/sulfosate

Ploughing/harrowing (maximum tillage) is not recommended at all on highly erodible soils. No cultivation at all is permitted on these soils on slopes greater than 20 %.

Whatever method is selected, follow up eradication of volunteer plants is required after the initial operation. The choice of method may be restricted by law in some countries to prevent erosion on the most erodible soils and on steep slopes. However, even where mechanical methods are allowed, chemical and hand methods may be preferred purely because of their advantages in limiting erosion even further.