new grass comes up and dies

last year in june, I seeded and fertilized my dying lawn using cow manure fertilizer from lowes and some walmart grass seeds (supposedly high traffic and heat resistant). It grew beautifully and then died when the heat came on strong in July. Why did this happen. I am about to seed and fertilize again today and don't want the same thing to happen. What should I do differently.

First you need to find out exactly what type of grass seed you planted.

vigoro tall fescue, drought resistant

It could be the heat coming on too fast for new grass. Here in the midwest, Aug/Sep is really the best time to sow rather than spring. Not sure about CA although we also sow fescue here.

Manure from feed lots (bagged steer manure) can be a bit too salty for use in the garden.
I would add a different sort of organic matter, perhaps redwood soil conditioner, but no manures.

As the grass seed sprouts you will need to water it frequently because the roots are quite shallow, but as the grass gets going water less often, but let the sprinklers run longer. This deep soaks the soil so the roots can go deep. The first summer the seed should develop a good stand, and be fairly deep rooted, but you should check this occasionally. Also check how deep the water is really getting when you run the sprinklers.
The easiest way to see how deep the water is going is to irrigate, then the next day, or at least several hours later go out there with a freshly sharpened pencil and poke it deep into the soil. Then remove it and see it the new wood is dark. It is wet when it is dark.
Here is the minimum I would look for:
New seed: Couple of inches.
Thin stand of grass, some seed still sprouting: 6" deep, but also keep it moist near the surface for those last few seeds.
Better established seedlings: Moist a foot deep, and allowed to dry an inch to 2" deep. The pencil might just show a few dark spots when you only push it 2" deep into the soil.

Here is how to know when to water an established lawn:
Walk across the lawn.
Turn and watch your footprints:
The crushed blades are jumping up so fast you cannot tell where you walked: No water.
The crushed blades are coming back, but you can see your path: Water.
The crushed blades are not coming back: You should have watered yesterday.

If your sprinklers run off instead of soaking in, then set your timer to run only until the water starts running off. Then turn off the sprinks for a few hours. Turn them on again later the same day.
Then go test the next day to see how deep the water has soaked into the soil.

I hope you were able to grow grass.

When we first moved here we planted seeds every year with fescue. Fescue we found out is susceptible to fungus and fungus is caused by watering late PM.The area extension suggest planting fescue in the fall.

We now have zoysia and we love it.

Belle

The best time to do any grass sewing is either early spring before the soil get's toooooo warm and dry, OR late summer after the strong heat of summer has left and the soil is cooling down.
As Diana has mentioned that grass has very shallow roots then it gives you the idea that just a few inches under the soil, tender new roots will just die off as usually ant water wont be getting down where the roots are. The green shoots that you get growing form a mat and this is like laying a carpet on top of your soil preventing the roots getting sufficient moisture in the type of heat you have as summer get's going.

You have not said how much prep you did to the soil where you are sewing the grass seed, and I assume you put the cow fertilizer onto the grass as it began to grow, again as Diana has mentioned, this is far too strong for tender new grass shoot's or roots, next time, use maybe a multi purpose granule feed spread and raked into the bare soil before you scatter the grass seed, and even as you scatter the seed, I would water this area right away but dont use a strong force of water, just a sprinkle, too strong and you will wash all the grass seed away or into one area.
I would say without any experience of lawn making, April is really too late in your area and if I were you I would wait till end of summer, around maybe September / October time as the soil is still warm for germination and you will not be fighting the real hot weather that any new grass cant cope with.
Hope this helps a bit.
Good Luck, WeeNel.

The cheap, generic grass seed sold at WM can't handle the heat of the south. The last time I got some, it was either Poa annua or that entity's doppelganger!