The definition of a weld defect is "Any weld irregularity that measures beyond the acceptable limits of a defined criteria".
As an OEM, you have the option of writing your own criteria, or adopting other standards.
Other standards come in a myriad of different requirements meant to adapt to a myriad of differing circumstances.
So, on size does not fit all. GMAW weld quality standards for building fences are much different than those for building submarines, etc.
As you're making agricultural sprayers, I recommend taking a look at AWS D14.3 - Specification for Earthmoving, Construction and Agricultural Equipment.

http://pubs.aws.org/p/1039/d143d143m2010-amd1-specification-for-welding-earthmoving-construction-and-agricultural-equipment

This will provide weld criteria applicable to your product.

Tim

When you say "agricultural sprayer" are you talking about the tank, pump, cart, any or all of the aforementioned? 

They will all fall under differing categories and code applications.  And remember, porosity that would be allowable/acceptable for the cart/frame would not be any good at all for the tank.  Can we say 'LEAKS'?  Especially under pressure.

Now, as the OEM, you have a fair amount of latitude and if you are only after general weld quality standards that the production shop is to maintain then there are shorter, simpler answers to your question. 

Any standards or references mentioned here may be obtained on the AWS Website at the bookstore (notice the button at the top of this page on the left).

Basically, I'm going to say that both the tank and the carrier/cart assembly would be thin enough to be under D1.3 as a basic beginning place.  Tim's suggestion is also a very good place to start.

There are many other books we could recommend but until you give us some more direction it is difficult and with all the various preferences by all of us here you would be buying every book in the bookstore.  You don't need that.  A couple would probably get you by. 

My personal choices would be the applicable code book, which could end up being D1.3, order code: D1.3 and costs $124 for non-members and $93 for members, and would be as good as any; and then the 'Welding Inspection Handbook' which is order code: WI in the bookstore and costs $84 for non-members and $63 for members.

He Is In Control, Have a Great Day,  Brent