Centre Pivot Irrigation

Technological Leap at New Yarmouth

New Yarmouth has set itself the ambitious task of producing cane yields of well over 100tc/ha. This is not just a whim. Economic analysis suggests it must produce at those levels to reap the kinds of profits the company demands. Records show that this has been done before, the last time in 1977, since when the estate has expanded. Between 1990 and 1999 for instance, that was never achieved, the estate averaging approximately 68tc/ha (range 56-79). Unlike the other J Wray and Nephew farms, which receive abundant rainfall in St. Elizabeth, New Yarmouth lies in the dry area of southern Clarendon where an economic crop cannot be grown without irrigation.

Traditionally, the estate, like most other farms in the irrigated belt, has used furrow irrigation. In the last few years, water seems hardly enough to go around. Instead of the desired 2-3 week irrigation cycle, a 4-5 week cycle has become the norm. Yet, the total quantity of water ought to be adequate for the farm. The problem is that the water delivery system and the twig and main method of application are grossly inefficient resulting in much wastage and poor crop response.

While recognising that water might not be the only limitation, in efforts to achieve its goal, the estate has taken the bold step to invest in the more efficient system of centre pivot irrigation. The first centre pivot, covering 74ha, was installed in February 2001. By August, with early indications of superior cane growth, the estate was about to set up a further two and was contemplating eventually converting the whole farm to this system of irrigation.

Centre Pivot

Centre pivot, a form of sprinkler irrigation, was first developed in the USA in 1954. It consists of a long water pipe supported by towers high enough to pass over the mature crop. Sprinklers are suspended from this pipe at intervals and heights depending on the type and age of crop being irrigated. Each tower has a drive motor so that the entire line rotates about a central fixed point, the centre pivot, at which point water is forced into the pipe and sprinklers. The assembly travels slowly on wheels, completing one revolution in 24 hours. The system may complete a full circle, like the arm of a clock, or could be adjusted to complete a portion of a circle as may be necessary when coming up against obstacles such as powerlines or dwellings.

Operation

The centre pivot installed at New Yarmouth is designed to deliver a maximum of 8mm of water per hectare each day and should be operated continuously, unless rain falls. The radius is 466m and an end gun attached extends the wetting reach to 490m. If desired, the rotation time may be reduced to as low as 9 hours by speeding up the motion of the machine.

Once programmed to operate at a given speed, delivering a certain volume of water, the system operates automatically. Provided the area is properly laid out to accommodate the machine, one man can do all the work required.

Taking care of the wheel’s path is important for successful operation. While the machine can ride over banks and furrows, the hard high ridges of old ratoon fields need to be cut down (a level track is ideal). Generally, no special track has to be laid down, as the wheels create their own paths through the cane field, handling modest variations in slope quite easily. Naturally, deep drains or canals have to be filled in or bridged. However experience has shown that the wheel tracks themselves can become water traps (especially during rains) in which wheels can become stuck. If the system comes to a halt, water emission is automatically stopped.

Advantages

As a system of irrigation, only drip irrigation surpasses the centre pivot in minimising water loss. Furrow irrigation is the most wasteful in this regard. Compared with other sprinkler systems, centre pivot beats both the hand moved and big gun systems. The latter two suffer greater evaporation losses as the spray is usually directed in the air and is subject to more wind drift. Drop nozzles in the centre pivot system can be set close to ground level and below canopy level as the crop grows.

Water is applied so gently with the centre pivot that there is not the usual puddling or surface run-off associated with the hand moved or big gun systems. With drip irrigation, unless there is some damaged tube, puddling and run-off would not be a factor at all. Soil erosion is greatest with the furrow system.

Management at New Yarmouth seemed particularly pleased with the superior results obtained in pre-emergence weed control where the chemical receives just enough water for activation but not too much to wash it away, as tends to occur with furrow irrigation.

Somewhat more pressure is required to operate a centre pivot compared with a drip system of equivalent size, 60 as against 50 psi, but both could be considered low pressure systems.

Water coverage under the centre pivot is very uniform, much more so than with furrow irrigation or other sprinkler systems. Nozzles are so designed that they rarely get clogged although the system does not require the sophisticated filters demanded by drip irrigation.

The expected life of the centre pivot, 20 years, is however much greater than that of drip tubes buried underground which would last only the 6-8 years of ratooning. Water mains for the drip system would however remain intact for subsequent use.

Centre pivot offers the possibility of "chemigation” - application of chemicals such as fertilizers and ripeners to the crop.

While several of the above features show an advantage towards drip irrigation, in the critical area of capital cost, centre pivot, despite its seeming complexity, is often appreciably cheaper. The furrow system is cheapest.

Disadvantages

The main disadvantages surrounding centre pivots may be summarised as follows:

• Centre pivots, because of their circular wetting areas, result in considerable waste of arable land between adjoining circles;
• Fitting centre pivots to a land area often demands a radical re-blocking, rerouting canals and roadways, felling trees;
• A centre pivot system could be more prone to vandalism;
• Centre pivot is more exposed to potential damage during hurricanes.

Technological Leap

Installation of this system represents a great technological leap for the estate. Positive results are immediately obvious. Canes under the centre pivot are growing at a dramatically faster rate than on the rest of the estate. Stands are so dense, and tillering so profuse from frequent wetting, that six-month-old cane looks far more advanced.

This comes on the heels of the farm reaping all of its canes mechanically and opens the possibility of the canes being reaped without pre-harvest burning. A centre pivot would not be hampered in any way by the trash blanket in delivering water, as is the case with furrow irrigation. Further benefits should therefore accrue from the trash mulch, improved soil fertility and reduced moisture loss.

From a management standpoint, the area under a centre pivot is an obvious block that can be more or less treated the same way, not just in irrigation but in drying off for harvesting, ripener application, actual harvesting, fertilizing and various cultivation activities.

Up to this point, New Yarmouth is on a learning curve with this new system. There are the inevitable bugs to be ironed out such as how best to avoid the equipment getting stuck occasionally. Exactly how much water is necessary, how much is too much, what is the ideal frequency of wetting, how soon to restart following rains are all questions that will be answered eventually. Growers are invited to view the system which may well be an essential plank to profitable cane growing in the irrigated area.