Time is money

Why optimal tractor speed matters. By Anna Mouton.

South African apple growers are turning to high-density orchards because these promise to increase profits. High-density orchards improve productivity — which we all know improves the bottom line. But is there more that growers could do to maximise the yield of their investment?

Philip Rebel of specialist agricultural consultancy ProCrop has researched the impact of dosing models on the cost of spray programmes. He believes that growers can make significant savings by adapting their approach to spraying high-density orchards.

The tree-row-volume model

Dosing models aim to work out how much tree there is to treat and to then relate this to the quantities of chemicals and water a grower needs to spray. The most common model used in South African apple orchards is the tree-row-volume model.

The tree-row-volume model calculates the three-dimensional volume of tree foliage per hectare and relates this to the water volume needed to spray that specific orchard to the point of runoff. This is called the high-volume water requirement.

Growers using the tree-row-volume model will need to tweak their spray machine setup and the concentration of chemicals in their tank for orchards with different canopy sizes. In practice it can be easier to allocate specific machines to groups of orchards with similar canopy sizes even though this could mean purchasing additional equipment.

A modern model for modern plantings

Rebel evaluated the MABO — short for Marktgemeinschaft Bodenseeobst — dosing model as an alternative dosing model for spray applications in high-density orchards. He was interested in whether this model would help growers save time and money without sacrificing the efficacy of their spraying programs.

The MABO model is related to the tree-row-volume model but it adjusts the dose applied to different orchards by changing tractor speed between orchards with differing canopy volumes. Growers therefore do not need to change the settings on their spray machines or the concentration of chemicals in their tanks between orchards.

Slow tractor speeds and low-profile spray machines used in orchards with old-fashioned open-vase trees will promote better canopy penetration. The MABO model might not hold much advantage over conventional spray application strategies in these orchards. In modern high-density plantings the MABO model can help increase work rate by adjusting tractor speed between orchards and thus increase the efficacy of the spray applications.

The MABO model diverges from most other dosing models in that it doesn’t take tree height into account. Different tree heights are accommodated by adjusting the number of open nozzles on the spray machine.

Rebel compared spray deposition in high-density apple orchards when spray applications were based on the tree-row-volume model and the MABO model. He evaluated orchards with row widths of 3.5 and 4.0 metres and found that the MABO model achieved similar to better spray deposition parameters when compared to the control treatments.

Rebel also examined fan speeds. Fan speeds producing 28 000 cubic metres per hour did not compromise deposition relative to fan speeds producing 36 000 cubic metres per hour when using a high-profile spray machine.

Cashing in on the MABO model

Tractor speeds used in the industry tend to be lower than the output of the MABO model. Slow driving speed may lead to over-spraying of the narrow canopies of high-density plantings but it also has another implication — it wastes time.

Rebel calculated the cost of spraying a theoretical farm with high-density orchards based on the tree-row-volume model and a constant tractor speed of 4.2 kilometres per hour. The resulting work rate including refill time was 1.33 hectares per hour. Rebel used this value to work out the average cost of spraying per hectare taking into account all costs associated with the equipment and the spray operator. He arrived at a total of R576.44 per hectare.

Rebel then repeated these calculations but using tractor speeds and spray volumes generated by the MABO model — anything between 5.0 and 9.6 kilometres per hour. The resulting work rate was 2.09 hectares per hour which translated into a cost of R366.23 per hectare.

The cost of spraying in this study did not include the price of chemicals. Spraying based on the MABO model was more economical simply because it took less time. And everyone knows that time is money. The savings can be significant if the number of hectares and the number of applications per season are considered.

The MABO model can also improve cost-efficiency by facilitating the use of lower water volumes and fan speeds which reduce energy inputs.

Rebel stresses that this research examined the MABO model specifically in the context of high-density plantings and that his cost calculations are based on a theoretical orchard. The next step for this project is to implement the different models in actual orchards to see whether the figures hold up under real-life conditions.

This work was carried out by Rebel under the leadership of Dr Gideon van Zyl and Bekker Wessels from ProCrop and Prof Adéle McLeod of the Department of Plant Pathology at Stellenbosch University. It was funded by Hortgro Pome.