Here on the farm my go-to pants are Carhartt's Double-Front Work Dungarees, which come in both Firm Duck and Washed-Duck variants.
Left: Firm. Right: Washed.
'Both are made from 12-ounce 100% cotton duck canvas, but the Firm Duck are stiff and when you first buy them, water will bead on the surface--they appear to be impregnated with something that renders them water-resistant. The Washed-Duck are softer and, while still tough, seem to wear a bit faster than the Firm. (Also, pro tip: To maintain the stiffness/water-resistance of the Firm, wash them in cold water only, and hang-dry them only, and they will feel the same for years. Something about heat breaks down whatever they're impregnated with.)
The CAT pants are 60/40 cotton/polyester, and while I'm not crazy about the plastic, they'll take knee pads and are boot-cut like the Carhartts, plus they appear to have way more functional pockets. And at roughly $50 they're about the same price. Will report back on these once I've had a chance to try them out. I went with the brown/black pair but they offer a variety of colors including "safety" ones.