Do you mean shooting barns, or shooting inside barns? If you are shooting handheld inside barns a faster lens will allow you to use a higher shutter speed to avoid camera shake. Slower lenses with IS (Image Stabilisation) will also work. Most SLR lenses now have IS. If you're regularly taking pictures in low light or just want to get maximum sharpness then a sturdy tripod and head will help to avoid camera shake. Of course you can always brace the camera against something solid.
Canon has an excellent book describing their lenses and their uses: EF Lens Work III, The Eyes of EOS. Worth a read.
Bob Atkins has a beginners guide to Canon lenses here
. SLRGear has lens reviews here
, including those made for Canon mount by other manufacturers. Some non-Canon lenses are very good indeed, e.g. Zeiss, and some cheaper brands still have very respectable image quality, and might meet your needs if you're on a budget.
Most lenses perform best when used at about 2-3 stops down from their maximum aperture, but that does vary from lens to lens. If you're mainly shooting in good light and don't need the shallower depth of field provided by wide apertures, you may not need a faster lens.
I mostly use the EF 24-105/4L IS when shooting at friends' farms. It's designed as a good all-round lens for people who don't need to shoot at wide apertures. 24mm is wide enough for most purposes, although I sometimes wouldn't have minded a 21mm inside a shearing shed. However for static subjects you can always stitch photos together in Photoshop - that's where a tripod with a panning head is useful. 105mm is enough magnification for my needs. I find it an excellent all-round lens for landscape and rural work. Its only drawback is the relatively slow f4 maximum aperture. If you shoot in low light without a tripod or need subject isolation using dof, the f2.8 zooms mentioned in earlier posts might suit you better.
The fewer lenses you carry around the easier life is, and the more you focus on taking pictures rather than deciding which lens to use. Having both crop and full frame sensor bodies has advantages - if you need more reach, a 24-105 on your 40D becomes effectively a 38-168mm lens because the smaller sensor crops the image circle.