You are clearly admitting that less water will be used by using auto steer because you are not overlapping as much. If you are using less water, then you are also using less product.

From this point forward I will say "you did this" like I know this is what you did. I'm not accusing or anything. It's just easier than typing "it could have happened like this".

Let's say that while you're spraying manually, without any sort of guidance other than foam, you're overlapping 4 feet. For kicks and giggles, we'll say that you've got a 60 foot boom.

For the purpose of this illustration, I'm going to create an imaginary field. You can draw it on paper if you want. You said 100 acres so we'll start there. If your 100 acre field is shaped like a rectangle and one side is 1/2 of a mile (2640 feet) then the other side is 1650 feet. Most people spray the long way so we'll say you go 2640 feet on each swath and each swath is 56 feet wide because you're overlapping 4 feet each time. It's going to take you 29.46 swaths to finish the field. If you had not overlapped any (very accurate GPS signal and auto-steer) you would have done the same field in 27.5 swaths. That means in this field you had the equivalent of 1.96 swaths of overlap.

If you're going 60 feet by a half mile, each swath is 3.63 acres. In this 100 acre field you overlapped on 7.13 acres of that field (3.63 x 1.96). So how in the world did you evenly spread 1 pint of product on every acre of the field if some of the field was getting double the product. Answer? You didn't. If you used 100 pints on this 100 acre field then you were putting less than 1 pint on the 56 foot strips and way more than 1 pint on the 4 foot strips. If you wanted to do a little math, you could figure out just how much more or less you were putting on.

100 pints = (92.87 acres)x + 2(7.13 acres)x

100 pints = (92.87 acres)x + (14.26 acres)x

100 pints = (107.13 acres)x

.933 pints = x

Basically what this is saying is that for every acre that you're sprayer covered (remember you covered more than 100 acres because you were overlapping), you put on .933 pints of product. On the acres that you covered twice (the 4 foot swaths) you put on 1.866 pints per acre.

You said you "calibrated" your sprayer so that it would come out a 1 pint per acre by the time you averaged the whole field in. I don't know how you did this but it could be done a number of ways. You could have changed the width of the booms in the controller. you could have adjusted the flowmeter calibration setting. You could have adjusted your speed calibration setting. Assuming you overlapped close to the same amount on every field that you sprayed, you could have kept fiddling with one of these things until the sprayer put on what you wanted it to for every field. You probably had to adjust that number again once you started using autosteer.

The problem with "calibrating" your sprayer that way is that you don't know what you're putting on in the areas that you are not overlapping. In this case, you thought you were putting on a pint but instead you were putting on .933 pints. You were running at a reduced rate in the areas where you did not overlap and you were running at a much higher rate in the areas that you did overlap.

Most people do not do this. They want to know how much product per acre they are putting on in the area that they do not overlap. In this way they can adjust the product rates according to recommendations from people they trust or their own experience.

Apparently the .933 pints was working for you. You must have gotten a good kill because otherwise you'd be complaining about not having a good kill. So that means that you could have put .933 pints on the whole field instead of 1.866 pints in the overlapped area. You were putting an additional .933 pints on 7.13 acres of the field. That's 6.65 pints of product that were wasted. 6.65 pints is .83 gallons. If you were using Roundup, it's what, 35 bucks a gallon (I don't know cause we bought a ton back when it was 14 so we haven't bought any for awhile)? That means you wasted 29 dollars on that field and you were only putting 16 oz of roundup on. Had you been putting 32 oz of roundup on you would have wasted almost 60 dollars of roundup alone. 29 dollars divided by 100 acres is 29 cents an acre. Let's say you spray 10,000 acres a year. You've just wasted 2900 dollars. Most spraying operations are more expensive than 16 oz of Roundup. Let's say the average spraying operation costs you more along the lines of what 32 oz of roundup would cost. Now you've wasted 5800 dollars in one year.

This does not take into account other products that you would be mixing in (AMS, 2,4-D, etc). It doesn't take the extra fuel you burned into account. It doesn't take into account the extra wear and tear on your sprayer due to covering extra acres (with that kind of overlap and spraying 10,000 acres a year, it's like spraying 10,700 acres a year, an extra 700 acres of wear and tear on your sprayer), etc etc. With this kind of math it doesn't take spraying very many acres to make an auto steer system pay for itself.

There are other things to consider in this equation. An auto-steer system does not take your overlap down to 0. Most people put around a 1 foot overlap in (unless they have a really good hydraulic auto-steer and are running on RTK or at least OmniStar HP) so that takes some of the money saved away. Also, you're still going to be overlapping on the ends when you turn your booms on and off unless you have perfect timing or unless you have something that automatically turns them on and off for you. Anyways, this is kindof an idea of how the auto-steer pays for itself.