Many chemical weed killers have long-lasting residual effects on the applied-to areas. Pet owners should use caution when applying such products to places where their pets are likely to walk or lounge since they could ingest the chemical directly or by licking their paws and fur. Organic weed killer may be a better option since its compounds tend to break down more rapidly and are absorbed by the ground. However, it is still important to read the directions for use and consider the warning label on any products you use since the potential effects can vary based on the active ingredients of each formula.