Many cost-averse farmers are interested in the new precision ag technology; they are just determined to find a less expensive way to get the job done.

An e-mail from a farmer who identified himself only as Ray is typical of a new type of letter we've been receiving.

I have a question regarding a laptop use and GPS. Is there any way someone could use a laptop for the processor and display for Ag Leader or Greenstar? I talked to someone who said they have seen someone using just a laptop with GPS software to vary air seeding on the go. If this could be done, it would be far cheaper than buying the complete systems. The problem I see is the hookup for the moisture sensor and grain flow meter. I would also like to use it for putting in my tile. I know there is GPS software available with street maps, but I have never seen anything to use it as a yield monitor.

To help answer Ray's question, we called on Daniel Humburg, an ag engineer at South Dakota State University.

It is possible to do some level of variable-rate control with a laptop or even a palmtop device. If someone only wishes to do output control of chemical, or fertilizer, or seeding rate, they can do this with some application machinery and a laptop or palmtop. I will give just one example.

If I wanted to control variable rate of a herbicide across my CRP field, I would require several things. First, my system must know where it is. Differentially corrected GPS will do this part. Second, I must have a plan. This is in the form of an electronic map. This may be addressed by the software I purchase to run on the portable computer. It might also be possible to generate a map on my desktop mapping software that can be loaded onto the portable computer in a compatible format. Third, I must have a computer and software combination that can accept the map, accept the GPS signal, make the current output decision, and communicate that output to an application machine. Last, I must have an application device that will accept the output command from the portable computer.

Field Rover II is a program developed by Farm Works that has utility for GPS-based scouting and has a variable-rate add-on capability.

Let us assume that the desktop portion of the program can help generate the desired application map. That map is then loaded onto the palmtop device. The palmtop is connected through an RS 232 serial connection to the GPS receiver to obtain input location coordinates from that device, while the transmit wire of the RS 232 serial connection must be connected to the RS 232 input of the chemical application system. I will use a Raven system in this example.

Many of Raven's application controllers now accept dynamic application rate inputs through RS 232. So once in the field the palmtop computer (or laptop) will monitor its position and send a command for the current rate to the Raven controller, which will set its current rate to the command.

In order for this to work, you would have to build your own wiring harness to get data into and out of the portable computer. Most have only one RS 232 port and we are using it for both incoming and outgoing information. Also, you would want external power provided to the portable computer as continuous serial port use will drain batteries quickly.

For more ideas on how to use handheld or laptop computers with GPS, visit www.farmindustrynews.com.