Leather boot or rubber boot?
A leather work boot with laces breathes better, fits more snugly around the foot and walks more comfortably over long distances; it's water-repellent but not waterproof. A rubber or PVC boot in class S5 is fully waterproof, washable and the logical choice in mud, manure and water.
Size, shaft and maintenance
Choose the size based on your work sock and avoid a boot that's too roomy: a sliding heel causes blisters and makes you unstable on slippery ground. Pay attention to calf adjustment if you have a sturdy calf or a narrow one. Let wet boots dry at room temperature with the insole removed, never by a heater.