Turf type Tall Fescue is a clumping type of turf grass. It does not have rhizomes or stolons to spread out and fill in your lawn, like you would see with Kentucky blue grass, standard or hybrid Bermuda grass and St. Augustine.
The Tall Fescue turf needs to be over seeded for the pet spots to fill in properly. The bare spots are receiving sunlight, moisture and nutrients, so any weed seeds, including crab grass, could quickly germinate and grow in your bare spots. To prevent weeds, the spots need to be filled in promptly as needed through out the year.
If you apply a weed and feed fertilizer or a typical home center type crab grass preventer prior to reseeding, you are at risk of not having your new seeds germinating, or damaging or stunting the new growth.
I would suggest over seeding your bare spots with a Tall Fescue seed blend mixed with apx. 10- 15% Kentucky blue grass. ‘Scott’s’ carries a few blends with this mix at your home center. Top dress with 1/4″ of a top dressing such as ‘Kellogg’s Top Dressing’. Due to its high salt content, I would avoid plain steer manure. Keep the spots moist with short waterings throughout the day. The Fescue seeds must remain moist for a min of 21 days in most areas of the country. Keep the pets off the seeded spots untill the new grass has fully matured.
To help avoid a slight color difference between the existing and new grass, and to help fill in the existing Tall Fescue lawn overall, I would lightly over seed the entire yard at the same time. Follow the label’s directions, but I think 5 lbs of seed per 1000 square feet is the rate for over seeding existing healthy turf. Again, you need to water through out the day for short intervals. I would fertilize you lawn at this time with a ‘starter fertilizer’ which contains a lower nitrogen content but a higher than normal phosphorus content that promotes root growth.
Cultural practices such as your mowing height and edging make a big difference in how much crab grass or broadleaf weeds your lawn may have. Taller mowing heights and not edging a wide space between your walks and the turf also limit crab grass. Keeping your lawn thick and healthy reduces sunlight energy from reaching the weed seeds, thus it is a natural weed control method too.
Good luck- Desert Greenscapes